chemistry

CHAPTER 2
Some Basic Chemistry
21
CHAPTER 2
Chapter Outline
Elements
Atoms
Chemical Bonds
Ionic Bonds
Covalent Bonds
Disulfide Bonds and Hydrogen Bonds
Chemical Reactions
Inorganic Compounds of Importance
Water
Water Compartments
Oxygen
Carbon Dioxide
Cell Respiration
Trace Elements
Acids, Bases, and pH
Buffer systems
Organic Compounds of Importance
Carbohydrates
Lipids
Proteins
Enzymes
Nucleic Acids
DNA and RNA
ATP
BOX 2–1 BLOOD GASES
BOX 2–2 NITRIC OXIDE
BOX 2–3 LIPIDS IN THE BLOOD
BOX 2–4 A PROTEIN MYSTERY: PRIONS
Student Objectives
• Define the terms element, atom, proton, neutron, and
electron.
• Describe the formation and purpose of ionic
bonds, covalent bonds, disulfide bonds, and hydrogen
bonds.
• Describe what happens in synthesis and decomposition
reactions.
• Explain the importance of water to the functioning
of the human body.
• Name and describe the water compartments.
• Explain the roles of oxygen and carbon dioxide in
cell respiration.
• State what trace elements are, and name some,
with their functions.
• Explain the pH scale. State the normal pH ranges
of body fluids.
• Explain how a buffer system limits great changes
in pH.
• Describe the functions of monosaccharides, disaccharides,
oligosaccharides, and polysaccharides.
• Describe the functions of true fats, phospholipids,
and steroids.
• Describe the functions of proteins, and explain
how enzymes function as catalysts.
• Describe the functions of DNA, RNA, and ATP.
22
Some Basic Chemistry
23
New Terminology
Acid (ASS-sid)
Amino acid (ah-MEE-noh ASS-sid)
Atom (A-tum)
Base (BAYSE)
Buffer system (BUFF-er SIS-tem)
Carbohydrates (KAR-boh-HIGH-drayts)
Catalyst (KAT-ah-list)
Cell respiration (SELL RES-pi-RAY-shun)
Covalent bond (ko-VAY-lent)
Dissociation/ionization (dih-SEW-see-AYshun/
EYE-uh-nih-ZAY-shun)
Element (EL-uh-ment)
Enzyme (EN-zime)
Extracellular fluid (EKS-trah-SELL-yoo-ler)
Intracellular fluid (IN-trah-SELL-yoo-ler)
Ion (EYE-on)
Ionic bond (eye-ON-ik)
Lipids (LIP-ids)
Matter (MAT-ter)
Molecule (MAHL-e-kuhl)
Nucleic acids (new-KLEE-ik ASS-sids)
pH and pH scale (pee-h SKALE)
Protein (PROH-teen)
Salt (SAWLT)
Solvent/solution (SAHL-vent/suh-LOO-shun)
Steroid (STEER-oyd)
Trace elements (TRAYSE EL-uh-ments)
Related Clinical Terminology
Acidosis (ASS-i-DOH-sis)
Atherosclerosis (ATH-er-oh-skle-ROH-sis)
Hypoxia (high-POK-see-ah)
Saturated fats (SAT-uhr-ay-ted)
Unsaturated (un-SAT-uhr-ay-ted) fats
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
When you hear or see the word chemistry, you may
think of test tubes and Bunsen burners in a laboratory
experiment. However, literally everything in our physical
world is made of chemicals. The paper used for
this book, which was once the wood of a tree, is made
of chemicals. The air we breathe is a mixture of chemicals
in the form of gases. Water, gasoline, and diet
soda are chemicals in liquid form. Our foods are
chemicals, and our bodies are complex arrangements
of thousands of chemicals. Recall from Chapter 1 that
the simplest level of organization of the body is the
chemical level.
This chapter covers some very basic aspects of
chemistry as they are related to living organisms, and
most especially as they are related to our understanding
of the human body. So try to think of chemistry
not as a complicated science, but as the air, water, and
food we need, and every substance that is part of us.
ELEMENTS
All matter, both living and not living, is made of elements,
the simplest chemicals. An element is a substance
made of only one type of atom (therefore, an
atom is the smallest part of an element). There are 92
naturally occurring elements in the world around us.
Examples are hydrogen (H), iron (Fe), oxygen (O),
calcium (Ca), nitrogen (N), and carbon (C). In nature,
an element does not usually exist by itself but rather
combines with the atoms of other elements to form
compounds. Examples of some compounds important
to our study of the human body are water (H2O), in
which two atoms of hydrogen combine with one atom
of oxygen; carbon dioxide (CO2), in which an atom of
carbon combines with two atoms of oxygen; and glucose
(C6H12O6), in which six carbon atoms and six
oxygen atoms combine with 12 hydrogen atoms.
The elements carbon, hydrogen, oxygen, nitrogen,
phosphorus, and sulfur are found in all living things. If
calcium is included, these seven elements make up
approximately 99% of the human body (weight).
More than 20 different elements are found, in varying
amounts, in the human body. Some of these are
listed in Table 2–1. As you can see, each element has a
standard chemical symbol. This is simply the first (and
sometimes the second) letter of the element’s English
or Latin name. You should know the symbols of the
elements in this table, because they are used in text-
24 Some Basic Chemistry
Table 2–1 ELEMENTS IN THE
HUMAN BODY
Percent of
Atomic the Body
Elements Symbol Number* by Weight
Hydrogen H 1 9.5
Carbon C 6 18.5
Nitrogen N 7 3.3
Oxygen O 8 65.0
Fluorine F 9 Trace
Sodium Na 11 0.2
Magnesium Mg 12 0.1
Phosphorus P 15 1.0
Sulfur S 16 0.3
Chlorine Cl 17 0.2
Potassium K 19 0.4
Calcium Ca 20 1.5
Manganese Mn 25 Trace
Iron Fe 26 Trace
Cobalt Co 27 Trace
Copper Cu 29 Trace
Zinc Zn 30 Trace
Iodine I 53 Trace
*Atomic number is the number of protons in the nucleus of
the atom. It also represents the number of electrons that
orbit the nucleus.
books, articles, hospital lab reports, and so on. Notice
that if a two-letter symbol is used for an element, the
second letter is always lowercase, not a capital. For
example, the symbol for calcium is Ca, not CA. CA is
an abbreviation often used for cancer.
ATOMS
Atoms are the smallest parts of an element that have
the characteristics of that element. An atom consists of
three major subunits or particles: protons, neutrons,
and electrons (Fig. 2–1). A proton has a positive electrical
charge and is found in the nucleus (or center) of
the atom. A neutron is electrically neutral (has no
charge) and is also found in the nucleus. An electron
has a negative electrical charge and is found outside
the nucleus orbiting in what may be called an electron
cloud or shell around the nucleus.
The number of protons in an atom gives it its
atomic number. Protons and neutrons have mass and
weight; they give an atom its atomic weight. In an
atom, the number of protons ( ) equals the number of
electrons ( ); therefore, an atom is electrically neutral.
The electrons, however, are important in that
they may enable an atom to connect, or bond, to other
atoms to form molecules. A molecule is a combination
of atoms (usually of more than one element) that
are so tightly bound together that the molecule
behaves as a single unit.
Each atom is capable of bonding in only very specific
ways. This capability depends on the number and
the arrangement of the electrons of the atom.
Electrons orbit the nucleus of an atom in shells or
energy levels. The first, or innermost, energy level
can contain a maximum of two electrons and is then
considered stable. The second energy level is stable
when it contains its maximum of eight electrons. The
remaining energy levels, more distant from the
nucleus, are also most stable when they contain eight
electrons, or a multiple of eight.
A few atoms (elements) are naturally stable, or
uninterested in reacting, because their outermost
energy level already contains the maximum number of
electrons. The gases helium and neon are examples of
these stable atoms, which do not usually react with
other atoms. Most atoms are not stable, however, and
tend to gain, lose, or share electrons in order to fill
their outermost shell. By doing so, an atom is capable
of forming one or more chemical bonds with other
atoms. In this way, the atom becomes stable, because
its outermost shell of electrons has been filled. It is
these reactive atoms that are of interest in our study of
anatomy and physiology.
CHEMICAL BONDS
A chemical bond is not a structure, but rather a force
or attraction between positive and negative electrical
charges that keeps two or more atoms closely associated
with each other to form a molecule. By way of
comparison, think of gravity. We know that gravity is
not a “thing,” but rather the force that keeps our feet
on the floor and allows us to pour coffee with consistent
success. Molecules formed by chemical bonding
often have physical characteristics different from those
of the atoms of the original elements. For example,
the elements hydrogen and oxygen are gases, but
atoms of each may chemically bond to form molecules
of water, which is a liquid.
The type of chemical bonding depends upon the
tendencies of the electrons of atoms involved, as you
will see. Four kinds of bonds are very important to the
chemistry of the body: ionic bonds, covalent bonds,
disulfide bonds, and hydrogen bonds.
IONIC BONDS
An ionic bond involves the loss of one or more electrons
by one atom and the gain of the electron(s) by
another atom or atoms. Refer to Fig. 2–2 as you read
the following.
An atom of sodium (Na) has one electron in its outermost
shell, and in order to become stable, it tends to
lose that electron. When it does so, the sodium atom
has one more proton than it has electrons. Therefore,
it now has an electrical charge (or valence) of 1 and
is called a sodium ion (Na ). An atom of chlorine has
seven electrons in its outermost shell, and in order to
become stable tends to gain one electron. When it
does so, the chlorine atom has one more electron than
it has protons, and now has a charge (valence) of 1.
It is called a chloride ion (Cl ).
When an atom of sodium loses an electron to an
atom of chlorine, their ions have unlike charges (positive
and negative) and are thus attracted to one
another. The result is the formation of a molecule of
sodium chloride: NaCl, or common table salt. The
bond that holds these ions together is called an ionic
bond.
Some Basic Chemistry 25
Second energy level
First energy level
Proton [+]
Neutron
Nucleus
Electrons [--]
Figure 2–1. An atom of carbon. The nucleus contains
six protons and six neutrons (not all are visible here). Six
electrons orbit the nucleus, two in the first energy level
and four in the second energy level.
QUESTION: What is the electrical charge of this atom as
a whole?
Another example is the bonding of chlorine to calcium.
An atom of calcium has two electrons in its outermost
shell and tends to lose those electrons in order
to become stable. If two atoms of chlorine each gain
one of those electrons, they become chloride ions.
The positive and negative ions are then attracted to
one another, forming a molecule of calcium chloride,
CaCl2, which is also a salt. A salt is a molecule made
of ions other than hydrogen (H ) ions or hydroxyl
(OH ) ions.
Ions with positive charges are called cations. These
include Na , Ca 2, K , Fe 2, and Mg 2. Ions with
negative charges are called anions, which include Cl ,
SO4
2 (sulfate), and HCO3
(bicarbonate). The types
of compounds formed by ionic bonding are salts,
acids, and bases. (Acids and bases are discussed later in
this chapter.)
In the solid state, ionic bonds are relatively strong.
Our bones, for example, contain the salt calcium carbonate
(CaCO3), which helps give bone its strength.
However, in an aqueous (water) solution, many ionic
bonds are weakened. The bonds may become so weak
that the bound ions of a molecule separate, creating a
solution of free positive and negative ions. For example,
if sodium chloride is put in water, it dissolves, then
ionizes. The water now contains Na ions and Cl
ions. Ionization, also called dissociation, is important
to living organisms because once dissociated, the ions
are free to take part in other chemical reactions within
the body. Cells in the stomach lining produce
hydrochloric acid (HCl) and must have Cl ions to do
so. The chloride in NaCl would not be free to take
part in another reaction since it is tightly bound to the
sodium atom. However, the Cl ions available from
ionized NaCl in the cellular water can be used for the
synthesis, or chemical manufacture, of HCl in the
stomach.
COVALENT BONDS
Covalent bonds involve the sharing of electrons
between atoms. As shown in Fig. 2–3, an atom of oxygen
needs two electrons to become stable. It may
share two of its electrons with another atom of oxygen,
also sharing two electrons. Together they form a
molecule of oxygen gas (O2), which is the form in
which oxygen exists in the atmosphere.
An atom of oxygen may also share two of its electrons
with two atoms of hydrogen, each sharing its
single electron (see Fig. 2–3). Together they form a
molecule of water (H2O). When writing structural
formulas for chemical molecules, a pair of shared electrons
is indicated by a single line, as shown in the formula
for water; this is a single covalent bond. A double
covalent bond is indicated by two lines, as in the formula
for oxygen; this represents two pairs of shared
electrons.
The element carbon always forms covalent bonds;
an atom of carbon has four electrons to share with
other atoms. If these four electrons are shared with
four atoms of hydrogen, each sharing its one electron,
a molecule of methane gas (CH4) is formed. Carbon
may form covalent bonds with other carbons, hydrogen,
oxygen, nitrogen, or other elements. Organic
26 Some Basic Chemistry
Na + Cl = NaCl
+ –
Figure 2–2. Formation of an ionic bond. An atom of sodium loses an electron to an
atom of chlorine. The two ions formed have unlike charges, are attracted to one another,
and form a molecule of sodium chloride.
QUESTION: Why is the charge of a sodium ion 1?
compounds such as proteins and carbohydrates are
complex and precise arrangements of these atoms
covalently bonded to one another. Covalent bonds are
relatively strong and are not weakened in an aqueous
solution. This is important because the proteins produced
by the body, for example, must remain intact in
order to function properly in the water of our cells and
blood. The functions of organic compounds will be
considered later in this chapter.
DISULFIDE BONDS AND
HYDROGEN BONDS
Two other types of bonds that are important to
the chemistry of the body are disulfide bonds and
hydrogen bonds. Disulfide bonds are found in some
proteins. Hydrogen bonds are part of many different
molecules.
A disulfide bond (also called a disulfide bridge) is a
covalent bond formed between two atoms of sulfur,
usually within the same large protein molecule. The
hormone insulin, for example, is a protein that must
have a very specific three-dimensional shape in order
to function properly to regulate the blood glucose
level. Each molecule of insulin has two disulfide bonds
that help maintain its proper shape and function (see
Box Fig. 10–A). Other proteins with shapes that
depend upon disulfide bonds are antibodies of the
immune system (see Fig. 14–8) and keratin of the skin
and hair.
Some Basic Chemistry 27
Figure 2–3. Formation of covalent bonds. (A) Two atoms of oxygen share two electrons
each, forming a molecule of oxygen gas. (B) An atom of oxygen shares one electron with
each of two hydrogen atoms, each sharing its electron. A molecule of water is formed.
QUESTION: Which of the bonds shown here is a double covalent bond?
O + O = O2 O=O
8+ + 8+ = 8+ 8+
A
O + H+H = H2O O
H H
8+ + =
1+
1+
1+ 1+
8+
B
A strand of hair maintains its shape (a genetic characteristic)
because of disulfide bonds. When naturally
curly hair is straightened, the disulfide bonds in the
keratin molecules are broken. When naturally straight
hair is “permed” or curled, the disulfide bonds in the
keratin are first broken, then re-formed in the curled
hair. Neither process affects the living part of the hair,
the hair root, so the hair will grow out in its original
shape. We would not want such a process affecting our
insulin or antibody molecules, for that would destroy
their functioning.
A hydrogen bond does not involve the sharing or
exchange of electrons, but rather results because of a
property of hydrogen atoms. When a hydrogen atom
shares its one electron in a covalent bond with another
atom, its proton has a slight positive charge and may
then be attracted to a nearby oxygen or nitrogen atom,
which has a slight negative charge.
Although they are weak bonds, hydrogen bonds are
important in several ways. Large organic molecules
such as proteins and DNA have very specific functions
that depend upon their three-dimensional shapes. The
shapes of these molecules, so crucial to their proper
functioning, are often maintained by hydrogen bonds.
Hydrogen bonds also make water cohesive; that is,
each water molecule is attracted to nearby water molecules.
Such cohesiveness can be seen if water is
dropped onto clean glass; the surface tension created
by the hydrogen bonds makes the water form threedimensional
beads. Within the body, the cohesiveness
of water helps keep blood a continuous stream as it
flows within the blood vessels, and keeps tissue fluid
continuous around cells. These hydrogen bonds are
also responsible for the other important characteristics
of water, which are discussed in a later section.
CHEMICAL REACTIONS
A chemical reaction is a change brought about by the
formation or breaking of chemical bonds. Two general
types of reactions are synthesis reactions and decomposition
reactions.
In a synthesis reaction, bonds are formed to join
two or more atoms or molecules to make a new compound.
The production of the protein hemoglobin in
potential red blood cells is an example of a synthesis
reaction. Proteins are synthesized by the bonding of
many amino acids, their smaller subunits. Synthesis
reactions require energy for the formation of bonds.
In a decomposition reaction, bonds are broken,
and a large molecule is changed to two or more
smaller ones. One example is the digestion of large
molecules of starch into many smaller glucose molecules.
Some decomposition reactions release energy;
this is described in a later section on cell respiration.
In this and future chapters, keep in mind that the
term reaction refers to the making or breaking of
chemical bonds and thus to changes in the physical
and chemical characteristics of the molecules
involved.
INORGANIC COMPOUNDS
OF IMPORTANCE
Inorganic compounds are usually simple molecules
that often consist of only one or two different elements.
Despite their simplicity, however, some inorganic
compounds are essential to normal structure and
functioning of the body.
WATER
Water makes up 60% to 75% of the human body, and
is essential to life for several reasons:
1. Water is a solvent; that is, many substances (called
solutes) can dissolve in water. Nutrients such as
glucose are dissolved in blood plasma (which is
largely water) to be transported to cells throughout
the body. The sense of taste depends upon the solvent
ability of saliva; dissolved food stimulates the
receptors in taste buds. The excretion of waste
products is possible because they are dissolved in
the water of urine.
2. Water is a lubricant, which prevents friction where
surfaces meet and move. In the digestive tract,
swallowing depends upon the presence of saliva,
and mucus is a slippery fluid that permits the
smooth passage of food through the intestines.
Synovial fluid within joint cavities prevents friction
as bones move.
3. Water changes temperature slowly. Water has a high
heat capacity, which means that it will absorb a
great deal of heat before its temperature rises significantly,
or it must lose a great deal of heat before
its temperature drops significantly. This is one of
the factors that helps the body maintain a constant
temperature. Water also has a high heat of vaporization,
which is important for the process of
28 Some Basic Chemistry
sweating. Excess body heat evaporates sweat on the
skin surfaces, rather than overheating the body’s
cells, and because of water’s high heat of vaporization,
a great deal of heat can be given off with the
loss of a relatively small amount of water.
WATER COMPARTMENTS
All water within the body is continually moving, but
water is given different names when it is in specific
body locations, which are called compartments (Fig.
2–4).
Intracellular fluid (ICF)—the water within cells;
about 65% of the total body water
Extracellular fluid (ECF)—all the rest of the water
in the body; about 35% of the total. More specific
compartments of extracellular fluid include:
Plasma—water found in blood vessels
Lymph—water found in lymphatic vessels
Tissue fluid or interstitial fluid—water found in
the small spaces between cells
Specialized fluids—synovial fluid, cerebrospinal
fluid, aqueous humor in the eye, and others
The movement of water between compartments in
the body and the functions of the specialized fluids
will be discussed in later chapters.
OXYGEN
Oxygen in the form of a gas (O2) is approximately
21% of the atmosphere, which we inhale. We all know
that without oxygen we wouldn’t survive very long,
but exactly what does it do? Oxygen is important to us
because it is essential for a process called cell respiration,
in which cells break down simple nutrients such
as glucose in order to release energy. The reason we
breathe is to obtain oxygen for cell respiration and to
exhale the carbon dioxide produced in cell respiration
(this will be discussed in the next section). Biologically
useful energy that is released by the reactions of cell
respiration is trapped in a molecule called ATP
(adenosine triphosphate). ATP can then be used for
cellular processes that require energy.
CARBON DIOXIDE
Carbon dioxide (CO2) is produced by cells as a waste
product of cell respiration. You may ask why a waste
product is considered important. Keep in mind that
“important” does not always mean “beneficial,” but it
Some Basic Chemistry 29
Figure 2–4. Water compartments, showing the names water is given in its different locations
and the ways in which water moves between compartments.
QUESTION: Which of the fluids shown are extracellular fluids?
Fluid
movement
Intracellular fluid Cell
Capillary
Lymph
capillary
Lymph
Interstitial
(tissue) fluid
Plasma
does mean “significant.” If the amount of carbon dioxide
in the body fluids increases, it causes these fluids
to become too acidic. Therefore, carbon dioxide must
be exhaled as rapidly as it is formed to keep the
amount in the body within normal limits. Normally
this is just what happens, but severe pulmonary diseases
such as pneumonia or emphysema decrease gas
exchange in the lungs and permit carbon dioxide to
accumulate in the blood. When this happens, a person
is said to be in a state of acidosis, which may seriously
disrupt body functioning (see the sections on pH and
enzymes later in this chapter; see also Box 2–1: Blood
Gases).
CELL RESPIRATION
Cell respiration is the name for energy production
within cells and involves both respiratory gases, oxygen
and carbon dioxide. Many chemical reactions are
involved, but in its simplest form, cell respiration may
be summarized by the following equation:
Glucose (C6H12O6) 6O2 → 6CO2 6H2O ATP heat
This reaction shows us that glucose and oxygen
combine to yield carbon dioxide, water, ATP, and heat.
Food, represented here by glucose, in the presence of
oxygen is broken down into the simpler molecules
carbon dioxide and water. The potential energy in the
glucose molecule is released in two forms: ATP and
heat. Each of the four products of this process has a
purpose or significance in the body. The carbon dioxide
is a waste product that moves from the cells into
the blood to be carried to the lungs and eventually
exhaled. The water formed is useful and becomes part
of the intracellular fluid. The heat produced contributes
to normal body temperature. ATP is used for
cell processes such as mitosis, protein synthesis, and
muscle contraction, all of which require energy and
will be discussed a bit further on in the text.
We will also return to cell respiration in later chapters.
For now, the brief description just given will suffice
to show that eating and breathing are interrelated;
both are essential for energy production.
TRACE ELEMENTS
Trace elements are those that are needed by the body
in very small amounts. When they are present in food
or nutritional supplements, we often call them minerals,
and examples are iron, cobalt, and zinc. Although
they may not be as abundant in the body as are carbon,
hydrogen, or oxygen, they are nonetheless essential.
Table 2–2 lists some of these trace elements and their
functions (see also Box 2–2: Nitric Oxide).
30 Some Basic Chemistry
BOX 2–1 BLOOD GASES
oxygen falls below the normal range, oxygen will
be administered; if blood carbon dioxide rises
above the normal range, blood pH will be corrected
to prevent serious acidosis.
Damage to the heart may also bring about a
change in blood gases, especially oxygen. Oxygen
is picked up by red blood cells as they circulate
through lung capillaries; as red blood cells circulate
through the body, they release oxygen to tissues.
What keeps the blood circulating or moving? The
pumping of the heart.
A mild heart attack, when heart failure is unlikely,
is often characterized by a blood oxygen level that
is low but still within normal limits. A more severe
heart attack that seriously impairs the pumping of
the heart will decrease the blood oxygen level to
less than normal. This condition is called hypoxia,
which means that too little oxygen is reaching tissues.
When this is determined by measurement of
blood gases, appropriate oxygen therapy can be
started to correct the hypoxia.
A patient is admitted to the emergency room with
a possible heart attack, and the doctor in charge
orders “blood gases.” Another patient hospitalized
with pneumonia has “blood gases” monitored at
frequent intervals. What are blood gases, and what
does measurement of them tell us? The blood gases
are oxygen and carbon dioxide, and their levels
in arterial blood provide information about the
functioning of the respiratory and circulatory
systems. Arterial blood normally has a high concentration
of oxygen and a low concentration of
carbon dioxide. These levels are maintained by
gas exchange in the lungs and by the proper circulation
of blood.
A pulmonary disease such as pneumonia interferes
with efficient gas exchange in the lungs. As a
result, blood oxygen concentration may decrease,
and blood carbon dioxide concentration may
increase. Either of these changes in blood gases
may become life threatening for the patient, so
monitoring of blood gases is important. If blood
ACIDS, BASES, AND pH
An acid may be defined as a substance that increases
the concentration of hydrogen ions (H ) in a water
solution. A base is a substance that decreases the concentration
of H ions, which, in the case of water, has
the same effect as increasing the concentration of
hydroxyl ions (OH ).
The acidity or alkalinity (basicity) of a solution is
measured on a scale of values called pH (parts hydrogen).
The values on the pH scale range from 0 to 14,
with 0 indicating the most acidic level and 14 the most
alkaline. A solution with a pH of 7 is neutral because
it contains the same number of H ions and OH
ions. Pure water has a pH of 7. A solution with a
higher concentration of H ions than OH ions is an
acidic solution with a pH below 7. An alkaline solution,
therefore, has a higher concentration of OH
ions than H ions and has a pH above 7.
The pH scale, with the relative concentrations
of H ions and OH ions, is shown in Fig. 2–5. A
change of one pH unit is a 10-fold change in H ion
concentration. This means that a solution with a pH
of 4 has 10 times as many H ions as a solution with a
pH of 5, and 100 times as many H ions as a solution
with a pH of 6. Figure 2–5 also shows the pH of some
body fluids and other familiar solutions. Notice that
gastric juice has a pH of 1 and coffee has a pH of 5.
This means that gastric juice has 10,000 times as many
H ions as does coffee. Although coffee is acidic, it is
a weak acid and does not have the corrosive effect of
gastric juice, a strong acid.
The cells and internal fluids of the human body
have a pH close to neutral. The pH of intracellular
fluid is around 6.8, and the normal pH range of blood
is 7.35 to 7.45. Fluids such as gastric juice and urine
are technically external fluids, because they are in
body tracts that open to the environment. The pH of
these fluids may be more strongly acidic or alkaline
without harm to the body.
The pH of blood, however, must be maintained
within its very narrow, slightly alkaline range. A
decrease of only one pH unit, which is 10 times as
many H ions, would disrupt the chemical reactions
of the blood and cause the death of the individual.
Normal metabolism tends to make body fluids more
Some Basic Chemistry 31
Table 2–2 TRACE ELEMENTS
Element Function
Calcium
Phosphorus
Iron
Copper
Sodium and
potassium
Sulfur
Cobalt
Iodine
• Provides strength in bones and teeth
• Necessary for blood clotting
• Necessary for muscle contraction
• Provides strength in bones and teeth
• Part of DNA, RNA, and ATP
• Part of cell membranes
• Part of hemoglobin in red blood
cells; transports oxygen
• Part of myoglobin in muscles; stores
oxygen
• Necessary for cell respiration
• Necessary for cell respiration
• Necessary for hemoglobin synthesis
• Necessary for muscle contraction
• Necessary for nerve impulse transmission
• Part of some proteins such as insulin
and keratin
• Part of vitamin B12
• Part of thyroid hormones—thyroxine
BOX 2–2 NITRIC OXIDE
Nitric oxide is a gas with the molecular formula
NO. You have probably heard of it as a component
of air pollution and cigarette smoke, but it
is synthesized by several human tissues, and
this deceptively simple molecule has important
functions.
Nitric oxide is produced by the endothelium
(lining) of blood vessels and promotes vasodilation
of arterioles, permitting greater blood flow
and oxygen delivery to tissues. It is involved in
nerve impulse transmission in the brain, and may
contribute to memory storage. Some immune
system cells produce nitric oxide as a cytotoxic
(cell-poisoning) agent to help destroy foreign
cells such as bacteria.
Nitric oxide is also being used therapeutically
in clinical trials. It has been found useful in the
treatment of pulmonary hypertension to relax
abnormally constricted arteries in the lungs to
permit normal gas exchange. Other studies
show that nitric oxide helps some premature
babies breathe more easily and efficiently.
Much more research is needed, including a
determination of possible harmful side effects of
greater than normal amounts of nitric oxide, but
the results of some clinical trials thus far are
promising.
acidic, and this tendency to acidosis must be continually
corrected. Normal pH of internal fluids is maintained
by the kidneys, respiratory system, and buffer
systems. Although acid–base balance will be a major
topic of Chapter 19, we will briefly mention buffer
systems here.
Buffer Systems
A buffer system is a chemical or pair of chemicals
that minimizes changes in pH by reacting with strong
acids or strong bases to transform them into substances
that will not drastically change pH. Expressed
in another way, a buffer may bond to H ions when a
body fluid is becoming too acidic, or release H ions
when a fluid is becoming too alkaline.
As a specific example, we will use the bicarbonate
buffer system, which consists of carbonic acid
(H2CO3), a weak acid, and sodium bicarbonate
(NaHCO3), a weak base. This pair of chemicals is
present in all body fluids but is especially important to
buffer blood and tissue fluid.
Carbonic acid ionizes as follows (but remember,
because it is a weak acid it does not contribute many
H ions to a solution):
H2CO3 → H HCO3

Sodium bicarbonate ionizes as follows:
NaHCO3 → Na HCO3

If a strong acid, such as HCl, is added to extracellular
fluid, this reaction will occur:
32 Some Basic Chemistry
Figure 2–5. The pH scale. The pH values of several body fluids are indicated above the
scale. The pH values of some familiar solutions are indicated below the scale.
QUESTION: Describe the pH range of blood compared to the pH range of urine.
HCl NaHCO3 → NaCl H2CO3
What has happened here? Hydrochloric acid, a
strong acid that would greatly lower pH, has reacted
with sodium bicarbonate. The products of this reaction
are NaCl, a salt that has no effect on pH, and
H2CO3, a weak acid that lowers pH only slightly. This
prevents a drastic change in the pH of the extracellular
fluid.
If a strong base, such as sodium hydroxide, is added
to the extracellular fluid, this reaction will occur:
NaOH H2CO3 → H2O NaHCO3
Sodium hydroxide, a strong base that would greatly
raise pH, has reacted with carbonic acid. The products
of this reaction are water, which has no effect on pH,
and sodium bicarbonate, a weak base that raises pH
only slightly. Again, this prevents a drastic change in
the pH of the extracellular fluid.
In the body, such reactions take place in less than a
second whenever acids or bases are formed that would
greatly change pH. Because of the body’s tendency to
become more acidic, the need to correct acidosis is
more frequent. With respect to the bicarbonate buffer
system, this means that more NaHCO3 than H2CO3 is
needed. For this reason, the usual ratio of these
buffers is 20:1 (NaHCO3:H2CO3).
ORGANIC COMPOUNDS
OF IMPORTANCE
Organic compounds all contain covalently bonded
carbon and hydrogen atoms and perhaps other elements
as well. In the human body there are four major
groups of organic compounds: carbohydrates, lipids,
proteins, and nucleic acids.
CARBOHYDRATES
A primary function of carbohydrates is to serve as
sources of energy in cell respiration. All carbohydrates
contain carbon, hydrogen, and oxygen and are classified
as monosaccharides, disaccharides, oligosaccharides,
and polysaccharides. Saccharide means sugar, and
the prefix indicates how many are present.
Monosaccharides, or single-sugar compounds,
are the simplest sugars. Glucose is a hexose, or sixcarbon,
sugar with the formula C6H12O6 (Fig. 2–6).
Fructose and galactose also have the same formula,
Some Basic Chemistry 33
Figure 2–6. Carbohydrates. (A) Glucose, depicting its structural formula. (B) A disaccharide
such as sucrose. (C) Cellulose, a polysaccharide. (D) Starch, a polysaccharide.
(E) Glycogen, a polysaccharide. Each hexagon represents a hexose sugar such as glucose.
QUESTION: What is the chemical formula of glucose?
H
C
OH
CH2OH
C
H
OH
C
H
H
C
OH
H
C
OH
A Glucose
E Glycogen
B Disaccharide
C Cellulose
D Starch
o
but the physical arrangement of the carbon, hydrogen,
and oxygen atoms in each differs from that of glucose.
This gives each hexose sugar a different threedimensional
shape. The liver is able to change fructose
and galactose to glucose, which is then used by cells in
the process of cell respiration to produce ATP.
Another type of monosaccharide is the pentose, or
five-carbon, sugar. These are not involved in energy
production but rather are structural components of
the nucleic acids. Deoxyribose (C5H10O4) is part of
DNA, which is the genetic material of chromosomes.
Ribose (C5H10O5) is part of RNA, which is essential
for protein synthesis. We will return to the nucleic
acids later in this chapter.
Disaccharides are double sugars, made of two
monosaccharides linked by a covalent bond. Sucrose,
or cane sugar, for example, is made of one glucose and
one fructose. Others are lactose (glucose and galactose)
and maltose (two glucose), which are also present
in food. Disaccharides are digested into monosaccharides
and then used for energy production.
The prefix oligo means “few”; oligosaccharides
consist of from 3 to 20 monosaccharides. In human
cells, oligosaccharides are found on the outer surface
of cell membranes. Here they serve as antigens,
which are chemical markers (or “signposts”) that identify
cells. The A, B, and AB blood types, for example,
are the result of oligosaccharide antigens on the outer
surface of red blood cell membranes. All of our cells
have “self” antigens, which identify the cells that
belong in an individual. The presence of “self” antigens
on our own cells enables the immune system to
recognize antigens that are “non-self.” Such foreign
antigens include bacteria and viruses, and immunity
will be a major topic of Chapter 14.
Polysaccharides are made of thousands of glucose
molecules, bonded in different ways, resulting in different
shapes (see Fig. 2–6). Starches are branched
chains of glucose and are produced by plant cells to
store energy. We have digestive enzymes that split the
bonds of starch molecules, releasing glucose. The glucose
is then absorbed and used by cells to produce
ATP.
Glycogen, a highly branched chain of glucose molecules,
is our own storage form for glucose. After a
meal high in carbohydrates, the blood glucose level
rises. Excess glucose is then changed to glycogen and
stored in the liver and skeletal muscles. When the
blood glucose level decreases between meals, the
glycogen is converted back to glucose, which is
released into the blood (these reactions are regulated
by insulin and other hormones). The blood glucose
level is kept within normal limits, and cells can take in
this glucose to produce energy.
Cellulose is a nearly straight chain of glucose molecules
produced by plant cells as part of their cell
walls. We have no enzyme to digest the cellulose we
consume as part of vegetables and grains, and it passes
through the digestive tract unchanged. Another name
for dietary cellulose is “fiber,” and although we cannot
use its glucose for energy, it does have a function.
Fiber provides bulk within the cavity of the large
intestine. This promotes efficient peristalsis, the
waves of contraction that propel undigested material
through the colon. A diet low in fiber does not give
the colon much exercise, and the muscle tissue of the
colon will contract weakly, just as our skeletal muscles
will become flabby without exercise. A diet high in
fiber provides exercise for the colon muscle and may
help prevent chronic constipation.
The structure and functions of the carbohydrates
are summarized in Table 2–3.
LIPIDS
Lipids contain the elements carbon, hydrogen, and
oxygen; some also contain phosphorus. In this group
of organic compounds are different types of substances
with very different functions. We will consider
three types: true fats, phospholipids, and steroids
(Fig. 2–7).
True fats (also called neutral fats) are made of one
molecule of glycerol and one, two, or three fatty acid
molecules. If three fatty acid molecules are bonded to
a single glycerol, a triglyceride is formed. Two fatty
acids and a glycerol form a diglyceride, and one fatty
acid and a glycerol form a monoglyceride.
The fatty acids in a true fat may be saturated or
unsaturated. Refer to Fig. 2–7 and notice that one of
the fatty acids has single covalent bonds between all its
carbon atoms. Each of these carbons is then bonded to
the maximum number of hydrogens; this is a saturated
fatty acid, meaning saturated with hydrogen. The
other fatty acids shown have one or more (poly) double
covalent bonds between their carbons and less
than the maximum number of hydrogens; these are
unsaturated fatty acids. Many triglycerides contain
both saturated and unsaturated fatty acids, and though
34 Some Basic Chemistry
it is not as precise, it is often easier to speak of saturated
and unsaturated fats, indicating the predominance
of one or the other type of fatty acid.
At room temperature, saturated fats are often in
solid form, while unsaturated fats are often (not
always) in liquid form. Saturated fats tend to be found
in animal foods such as beef, pork, eggs, and cheese,
but palm oil and coconut oil are also saturated.
Unsaturated fats are found in other plant oils such as
corn oil, sunflower oil, and safflower oil, but certain
fish oils are also unsaturated, and even pork contains
unsaturated fatty acids.
Unsaturated fats may be changed to saturated fats
in order to give packaged foods a more pleasing texture
or taste, or to allow them to be stored longer
without refrigeration (a longer shelf life). These are
hydrogenated fats (meaning that hydrogens have been
added), also called trans fats. Trans fats contribute significantly
to atherosclerosis of arteries, that is,
abnormal cholesterol deposits in the lining that may
clog arteries, especially the coronary arteries of the
heart. (See also Box 2–3: Lipids in the Blood.)
The triglyceride forms of true fats are a storage
form for excess food, that is, they are stored energy
(potential energy). Any type of food consumed in
excess of the body’s caloric needs will be converted to
fat and stored in adipose tissue. Most adipose tissue is
subcutaneous, between the skin and muscles. Some
organs, however, such as the eyes and kidneys, are
enclosed in a layer of fat that acts as a cushion to
absorb shock.
Phospholipids are diglycerides with a phosphate
group (PO4) in the third bonding site of glycerol.
Although similar in structure to the true fats, phospholipids
are not stored energy but rather structural
components of cells. Lecithin is a phospholipid that is
part of our cell membranes (see Fig. 3–1; each phospholipid
molecule looks like a sphere with two tails;
the sphere is the glycerol and phosphate, the tails are
the two fatty acids). Another phospholipid is myelin,
Some Basic Chemistry 35
Table 2–3 CARBOHYDRATES
Name Structure Function
Monosaccharides—“Single” Sugars
Glucose
Fructose and
galactose
Deoxyribose
Ribose
Disaccharides—“Double” Sugars
Sucrose, lactose,
and maltose
Oligosaccharides—“Few” Sugars (3–20)
Polysaccharides—“Many” Sugars (Thousands)
Starches
Glycogen
Cellulose
Hexose sugar
Hexose sugar
Pentose sugar
Pentose sugar
• Most important energy source for cells
• Converted to glucose by the liver, then used for energy
production
• Part of DNA, the genetic code in the chromosomes of cells
• Part of RNA, needed for protein synthesis within cells
Two hexose sugars • Present in food; digested to monosaccharides, which are
then used for energy production
• Form “self” antigens on cell membranes; important to
permit the immune system to distinguish “self” from
foreign antigens (pathogens)
Branched chains of
glucose molecules
Highly branched chains
of glucose molecules
Straight chains of
glucose molecules
• Found in plant foods; digested to monosaccharides and
used for energy production
• Storage form for excess glucose in the liver and skeletal
muscles
• Part of plant cell walls; provides fiber to promote peristalsis,
especially by the colon
BOX 2–3 LIPIDS IN THE BLOOD
cause in this form cholesterol is more easily
removed from the blood by the liver and excreted
in bile.
A diet low in total fat, with most of it unsaturated
fat, tends to raise HDL levels and lower LDL levels.
The benefit is the delaying of atherosclerosis and
coronary artery disease. A simple blood test called a
lipid profile (or lipid panel) can determine levels of
total cholesterol, triglycerides, HDLs, and LDLs. A
high HDL level, above 50 mg/dL, is considered
good, but some researchers now believe that the
LDL level is more important and should be as low as
possible, below 100 mg/dL.
Other factors contribute to coronary artery disease,
such as heredity, smoking, being overweight,
and lack of exercise. Diet alone cannot prevent atherosclerosis.
However, a diet low in total fat and
high in polyunsaturated fats is a good start.
Triglycerides and cholesterol are transported in the
blood in combination with proteins. Such molecules
made by the small intestine are called chylomicrons.
Those made by the liver are called
lipoproteins and are categorized by their density,
which reflects the proportion of protein to cholesterol.
Low-density lipoproteins (LDLs, which are low in
protein and high in cholesterol) transport cholesterol
to the tissues, where it is used to synthesize
cell membranes or secretions. LDLs are also called
“bad cholesterol,” because in this form the cholesterol
is more likely to be deposited in the walls of
blood vessels, leading to atherosclerosis.
High-density lipoproteins (HDLs, which are
higher in protein and lower in cholesterol than
LDLs) transport cholesterol from the tissues to the
liver. HDLs are also called “good cholesterol,” be-
B
A
Glycerol
Triglyceride
3 Fatty acids
Cholesterol
Figure 2–7. Lipids. (A) A triglyceride made of one glycerol and three fatty acids. (B) The
steroid cholesterol. The hexagons and pentagon represent rings of carbon and hydrogen.
QUESTION: What would a diglyceride look like?
36
which forms the myelin sheath around nerve cells and
provides electrical insulation for nerve impulse transmission.
The structure of steroids is very different from
that of the other lipids. Cholesterol is an important
steroid; it is made of four rings of carbon and hydrogen
(not fatty acids and glycerol) and is shown in Fig.
2–7. The liver synthesizes cholesterol, in addition to
the cholesterol we eat in food as part of our diet.
Cholesterol is another component of cell membranes
and is the precursor (raw material) for the synthesis of
other steroids. In the ovaries or testes, cholesterol is
used to synthesize the steroid hormones estrogen or
testosterone, respectively. A form of cholesterol in the
skin is changed to vitamin D on exposure to sunlight.
Liver cells use cholesterol for the synthesis of bile
salts, which emulsify fats in digestion. Despite its link
to coronary artery disease and heart attacks, cholesterol
is an essential substance for human beings.
The structure and functions of lipids are summarized
in Table 2–4.
PROTEINS
Proteins are made of smaller subunits or building
blocks called amino acids, which all contain the elements
carbon, hydrogen, oxygen, and nitrogen. Some
amino acids contain sulfur, which permits the formation
of disulfide bonds. There are about 20 amino
acids that make up human proteins. The structure of
amino acids is shown in Fig. 2–8. Each amino acid has
a central carbon atom covalently bonded to an atom of
hydrogen, an amino group (NH2), and a carboxyl
group (COOH). At the fourth bond of the central carbon
is the variable portion of the amino acid, represented
by R. The R group may be a single hydrogen
atom, or a CH3 group, or a more complex configuration
of carbon and hydrogen. This gives each of the 20
amino acids a slightly different physical shape. A bond
between two amino acids is called a peptide bond,
and a short chain of amino acids linked by peptide
bonds is a polypeptide.
A protein may consist of from 50 to thousands of
amino acids. The sequence of the amino acids is
specific and unique for each protein, and is called
its primary structure. This unique sequence, and the
hydrogen bonds and disulfide bonds formed within
the amino acid chain, determines how the protein will
be folded to complete its synthesis. The folding may
be simple, a helix (coil) or pleated sheet, called the secondary
structure, or a more complex folding may
occur to form a globular protein, called the tertiary
structure. Myoglobin, found in muscles, is a globular
protein (Fig. 2–8). When complete, each protein has a
characteristic three-dimensional shape, which in turn
determines its function. Some proteins consist of
more than one amino acid chain (quaternary structure).
Hemoglobin, for example, has four amino acid
chains (see Box 3–2). Notice that myoglobin contains
an atom of iron (a hemoglobin molecule has four iron
Some Basic Chemistry 37
Table 2–4 LIPIDS
Name Structure Function
True fats
Phospholipids
Steroids (cholesterol)
A triglyceride consists of three
fatty acid molecules bonded to
a glycerol molecule (some are
monoglycerides or diglycerides)
Diglycerides with a phosphate
group bonded to the glycerol
molecule
Four carbon–hydrogen rings
• Storage form for excess food molecules in
subcutaneous tissue
• Cushion organs such as the eyes and kidneys
• Part of cell membranes (lecithin)
• Form the myelin sheath to provide electrical
insulation for neurons
• Part of cell membranes
• Converted to vitamin D in the skin on exposure to
UV rays of the sun
• Converted by the liver to bile salts, which emulsify
fats during digestion
• Precursor for the steroid hormones such as estrogen
in women (ovaries) or testosterone in men (testes)
atoms). Some proteins require a trace element such as
iron or zinc to complete their structure and permit
them to function properly.
Our body proteins have many functions; some of
these are listed in Table 2–5 and will be mentioned
again in later chapters. And though we usually do not
think of protein as an energy food, if our diet includes
more amino acids than are necessary for our protein
synthesis, those excess amino acids will be converted
to simple carbohydrates or even to fat, to be stored as
potential energy. (See Box 2–4: A Protein Mystery:
Prions, for a discussion of disease-causing proteins.)
One very important function of proteins will be discussed
further here: the role of proteins as enzymes.
Enzymes
Enzymes are catalysts, which means that they speed
up chemical reactions without the need for an external
source of energy such as heat. The many reactions
that take place within the body are catalyzed by specific
enzymes; all of these reactions must take place at
body temperature.
The way in which enzymes function as catalysts is
called the active site theory, and is based on the
shape of the enzyme and the shapes of the reacting
molecules, called substrates. A simple synthesis reaction
is depicted in Fig. 2–9A. Notice that the enzyme
has a specific shape, as do the substrate molecules.
38 Some Basic Chemistry
NH2 C COOH
H
R
•
Amino
group
•
Carboxyl
group
•
Variable portion
• •
Peptide bonds
•
Iron in heme
A Amino acid
B Polypeptide
C Primary structure
D Secondary structurealpha
helix E Tertiary structuremyoglobin
Figure 2–8. Amino acid and protein structure. (A) The structural formula of an amino
acid. The “R” represents the variable portion of the molecule. (B) A polypeptide. Several
amino acids, represented by different shapes, are linked by peptide bonds. (C) The primary
structure of a protein. (D) The secondary structure of a protein. (E) The tertiary structure
of the protein myoglobin. See text for further description.
QUESTION: What mineral is part of myoglobin, and what is its function?
tween them, creating a new compound. The product
of the reaction, the new compound, is then released,
leaving the enzyme itself unchanged and able to catalyze
another reaction of the same type.
The reaction shown in Fig. 2–9B is a decomposition
reaction. As the substrate molecule bonds to the active
site of the enzyme, strain is put on its internal bonds,
which break, forming two product molecules and
again leaving the enzyme unchanged. Each enzyme is
specific in that it will catalyze only one type of reaction.
An enzyme that digests the protein in food, for
example, has the proper shape for that reaction but
cannot digest starches. For starch digestion, another
enzyme with a differently shaped active site is needed.
Thousands of chemical reactions take place within the
body, and therefore we have thousands of enzymes,
each with its own shape and active site.
The ability of enzymes to function may be limited
or destroyed by changes in the intracellular or extracellular
fluids in which they are found. Changes in pH
and temperature are especially crucial. Recall that the
pH of intracellular fluid is approximately 6.8, and that
a decrease in pH means that more H ions are present.
If pH decreases significantly, the excess H ions
will react with the active sites of cellular enzymes,
change their shapes, and prevent them from catalyzing
reactions. This is why a state of acidosis may cause the
death of cells—the cells’ enzymes are unable to function
properly.
Some Basic Chemistry 39
Table 2–5 FUNCTIONS OF PROTEINS
Type of Protein Function
Structural
proteins
Hormones
Hemoglobin
Myoglobin
Antibodies
Myosin and actin
Enzymes
• Form pores and receptor sites in
cell membranes
• Keratin—part of skin and hair
• Collagen—part of tendons and
ligaments
• Insulin—enables cells to take in
glucose; lowers blood glucose level
• Growth hormone—increases
protein synthesis and cell division
• Enables red blood cells to carry
oxygen
• Stores oxygen in muscle cells
• Produced by lymphocytes (white
blood cells); label pathogens for
destruction
• Muscle structure and contraction
• Catalyze reactions
BOX 2–4 A PROTEIN MYSTERY: PRIONS
ing misfolding, which brings about deterioration of
brain tissue. We do not know how to destroy prions.
Prions are not living; they do not contain genetic
material or carry out processes that might be disrupted
by antibiotics or antiviral medications.
Standard sterilization practices that kill bacteria and
viruses do not seem to inactivate prions.
Prevention of prion disease depends upon
keeping animal brain tissue from contaminating
meat destined for human or animal consumption.
In Great Britain, where the human form of madcow
disease emerged and killed nearly 100 people,
butchering practices are now stringently regulated.
The first cases of mad-cow disease in Canada
and the United States were found in 2003, in cattle.
As of this writing, people have not yet been
affected.
Prions are proteinaceous infectious particles, the
cause of lethal diseases of the nervous system in
people and animals. Mad-cow disease is perhaps
the best known; its formal name is bovine spongiform
encephalopathy (BSE). The name tells us about
the disease: Encephalopathy means that the brain is
affected, and spongiform indicates that brain tissue
becomes spongy, full of holes. People may acquire
BSE by eating beef contaminated with infected cow
brain tissue. They develop what is called variant
Creutzfeldt-Jakob disease (CJD). CJD is characterized
by loss of coordination, loss of memory and personality,
and death within a few months. There is no
treatment.
How do prions cause this disease? We do not yet
have the entire answer. We do know that prions
change the structure of other brain proteins, caus-
The active site of the enzyme is the part that matches
the shapes of the substrates. The substrates must “fit”
into the active site of the enzyme, and temporary
bonds may form between the enzyme and the substrate.
This is called the enzyme–substrate complex. In
this case, two substrate molecules are thus brought
close together so that chemical bonds are formed be-
With respect to temperature, most human enzymes
have their optimum functioning in the normal range
of body temperature: 97 to 99 F (36 to 38 C). A temperature
of 106 F, a high fever, may break the chemical
bonds that maintain the shapes of enzymes (see
Fig. 2–9C). If an enzyme loses its shape, it is said to be
denatured, and a denatured enzyme is unable to function
as a catalyst. Some human enzymes, when denatured
by a high fever, may revert to their original
shapes if the fever is lowered quickly. Others, however,
will not. (An example of irreversible denaturation is a
hard-boiled egg; the proteins in the egg white and
yolk will never revert to what they were in the original
egg.) A high fever may cause brain damage or death
because enzymes in the brain have become permanently
denatured.
You already know that metals such as lead and mercury
are harmful to humans and that both may cause
serious damage to the nervous system and other body
tissues. These heavy metals are harmful to us because
they are very reactive and block the actions of our
enzymes. Fig. 2–9D depicts what happens. Notice that
the heavy metal ion bonds with part of the active site
of the enzyme and changes its shape. The substrate
molecule cannot fit, and the enzyme is useless. Many
other chemicals are poisonous to us for the very same
reason: They destroy the functioning of our enzymes,
and essential reactions cannot take place.
40 Some Basic Chemistry
Active site
Enzyme
Substrates Enzyme-substrate
complex
Enzyme
Product
Enzyme
Substrate Enzyme-substrate
complex
Enzyme
Products
Enzyme
Enzyme
Denatured enzyme
Heavy-metal ion
or toxin
Nonfunctional
enzyme
•
A
B
C
D
Figure 2–9. Active site theory. (A) Synthesis reaction. (B) Decomposition reaction.
(C) The effect of heat. (D) The effect of poisons. See text for description.
QUESTION: Which of these four pictures best represents the effect of an acidic pH on an
enzyme, and why?
NUCLEIC ACIDS
DNA and RNA
The nucleic acids, DNA (deoxyribonucleic acid) and
RNA (ribonucleic acid), are large molecules made of
smaller subunits called nucleotides. A nucleotide consists
of a pentose sugar, a phosphate group, and one of
several nitrogenous bases. In DNA nucleotides, the
sugar is deoxyribose, and the bases are adenine, guanine,
cytosine, or thymine. In RNA nucleotides, the
sugar is ribose, and the bases are adenine, guanine,
cytosine, or uracil. DNA and RNA molecules are
shown in Fig. 2–10. Notice that DNA looks somewhat
like a twisted ladder; this ladder is two strands of
Some Basic Chemistry 41
•
Deoxyribose (DNA)
Ribose (RNA)
Phosphate
Adenine
Guanine
Thymine (DNA)
Uracil (RNA)
Cytosine
Chromatin in
the nucleus
•
Cell
Double helix
DNA strands
Hydrogen
bonds
RNA strand
Figure 2–10. DNA and RNA. Both molecules are shown, with each part of a nucleotide
represented by its shape and in a different color. Note the complementary base pairing of
DNA (A–T and G–C). When RNA is synthesized, it is a complementary copy of half the DNA
molecule (with U in place of T).
QUESTION: Why can’t adenine pair with guanine to form a rung of the DNA ladder?
nucleotides called a double helix (two coils). Alternating
phosphate and sugar molecules form the uprights
of the ladder, and pairs of nitrogenous bases form the
rungs. The size of the bases and the number of hydrogen
bonds each can form the complementary base
pairing of the nucleic acids. In DNA, adenine is always
paired with thymine (with two hydrogen bonds), and
guanine is always paired with cytosine (with three
hydrogen bonds).
DNA makes up the chromosomes of cells and is,
therefore, the genetic code for hereditary characteristics.
The sequence of bases in the DNA strands is actually
a code for the many kinds of proteins living things
produce; the code is the same in plants, other animals,
and microbes. The sequence of bases for one protein is
called a gene. Human genes are the codes for the proteins
produced by human cells (though many of these
genes are also found in all other forms of life—we are
all very much related). The functioning of DNA will
be covered in more detail in the next chapter.
RNA is often a single strand of nucleotides (see Fig.
2–10), with uracil nucleotides in place of thymine
nucleotides. RNA is synthesized from DNA in the
nucleus of a cell but carries out a major function in the
cytoplasm. This function is protein synthesis, which
will also be discussed in the following chapter.
ATP
ATP (adenosine triphosphate) is a specialized
nucleotide that consists of the base adenine, the sugar
ribose, and three phosphate groups. Mention has
already been made of ATP as a product of cell respiration
that contains biologically useful energy. ATP is
one of several “energy transfer” molecules within
cells, transferring the potential energy in food molecules
to cell processes. When a molecule of glucose is
broken down into carbon dioxide and water with the
release of energy, the cell uses some of this energy to
synthesize ATP. Present in cells are molecules of ADP
(adenosine diphosphate) and phosphate. The energy
released from glucose is used to loosely bond a third
phosphate to ADP, forming ATP. When the bond of
this third phosphate is again broken and energy is
released, ATP then becomes the energy source for cell
processes such as mitosis.
All cells have enzymes that can remove the third
phosphate group from ATP to release its energy,
forming ADP and phosphate. As cell respiration continues,
ATP is resynthesized from ADP and phosphate.
ATP formation to trap energy from food and
breakdown to release energy for cell processes is a
continuing cycle in cells.
The structure and functions of the nucleic acids are
summarized in Table 2–6.
SUMMARY
All of the chemicals we have just described are considered
to be non-living, even though they are essential
parts of all living organisms. The cells of our bodies
are precise arrangements of these non-living chemicals
and yet are considered living matter. The cellular
level, therefore, is the next level of organization we
will examine.
42 Some Basic Chemistry
Table 2–6 NUCLEIC ACIDS
Name Structure Function
DNA (deoxyribonucleic acid)
RNA (ribonucleic acid)
ATP (adenosine triphosphate)
A double helix of nucleotides;
adenine paired with
thymine, and guanine
paired with cytosine
A single strand of nucleotides;
adenine, guanine, cytosine,
and uracil
A single adenine nucleotide
with three phosphate
groups
• Found in the chromosomes in the nucleus of a cell
• Is the genetic code for hereditary characteristics
• Copies the genetic code of DNA to direct protein
synthesis in the cytoplasm of cells
• An energy-transferring molecule
• Formed when cell respiration releases energy from
food molecules
• Used for energy-requiring cellular processes
Elements
1. Elements are the simplest chemicals, which make
up all matter.
2. Carbon, hydrogen, oxygen, nitrogen, phosphorus,
sulfur, and calcium make up 99% of the human
body.
3. Elements combine in many ways to form molecules.
Atoms (see Fig. 2–1)
1. Atoms are the smallest part of an element that still
retains the characteristics of the element.
2. Atoms consist of positively and negatively charged
particles and neutral (or uncharged) particles.
• Protons have a positive charge and are found in
the nucleus of the atom.
• Neutrons have no charge and are found in the
nucleus of the atom.
• Electrons have a negative charge and orbit the
nucleus.
3. The number and arrangement of electrons give an
atom its bonding capabilities.
Chemical Bonds
1. An ionic bond involves the loss of electrons by one
atom and the gain of these electrons by another
atom: Ions are formed that attract one another (see
Fig. 2–2).
• Cations are ions with positive charges: Na ,
Ca 2.
• Anions are ions with negative charges: Cl ,
HCO3
.
• Salts, acids, and bases are formed by ionic bonding.
• In water, many ionic bonds break; dissociation
releases ions for other reactions.
2. A covalent bond involves the sharing of electrons
between two atoms (see Fig. 2–3).
• Oxygen gas (O2) and water (H2O) are covalently
bonded molecules.
• Carbon always forms covalent bonds; these are
the basis for the organic compounds.
• Covalent bonds are not weakened in an aqueous
solution.
3. A disulfide bond is a covalent bond between two
sulfur atoms in a protein; it helps maintain the
three-dimensional shape of some proteins.
4. A hydrogen bond is the attraction of a covalently
bonded hydrogen to a nearby oxygen or nitrogen
atom.
• The three-dimensional shape of proteins and
nucleic acids is maintained by hydrogen bonds.
• Water is cohesive because of hydrogen bonds.
Chemical Reactions
1. A change brought about by the formation or breaking
of chemical bonds.
2. Synthesis—bonds are formed to join two or more
molecules.
3. Decomposition—bonds are broken within a molecule.
Inorganic Compounds of Importance
1. Water—makes up 60% to 75% of the body.
• Solvent—for transport of nutrients in the blood
and excretion of wastes in urine.
• Lubricant—mucus in the digestive tract.
• Changes temperature slowly, and prevents sudden
changes in body temperature; absorbs body
heat in evaporation of sweat.
• Water compartments—the locations of water
within the body (see Fig. 2–4).
Intracellular—within cells; 65% of total body
water.
Extracellular—35% of total body water
— Plasma—in blood vessels.
— Lymph—in lymphatic vessels.
— Tissue fluid—in tissue spaces between
cells.
2. Oxygen—21% of the atmosphere.
• Essential for cell respiration: the breakdown of
food molecules to release energy.
3. Carbon dioxide
• Produced as a waste product of cell respiration.
• Must be exhaled; excess CO2 causes acidosis.
4. Cell respiration—the energy-producing processes
of cells.
• Glucose O2 → CO2 H2O ATP heat
• This is why we breathe: to take in oxygen to
break down food to produce energy, and to
exhale the CO2 produced.
5. Trace elements—minerals needed in small amounts
(see Table 2–2).
Some Basic Chemistry 43
STUDY OUTLINE
6. Acids, bases, and pH
• The pH scale ranges from 0 to 14; 7 is neutral;
below 7 is acidic; above 7 is alkaline.
• An acid increases the H ion concentration of a
solution; a base decreases the H ion concentration
(or increases the OH– ion concentration)
(see Fig. 2–5).
• The pH of cells is about 6.8. The pH range of
blood is 7.35 to 7.45.
• Buffer systems maintain normal pH by reacting
with strong acids or strong bases to change
them to substances that do not greatly change
pH.
• The bicarbonate buffer system consists of H2CO3
and NaHCO3.
Organic Compounds of Importance
1. Carbohydrates (see Table 2–3 and Fig. 2–6).
• Monosaccharides are simple sugars. Glucose, a
hexose sugar (C6H12O6), is the primary energy
source for cell respiration.
Pentose sugars are part of the nucleic acids
DNA and RNA.
• Disaccharides are made of two hexose sugars.
Sucrose, lactose, and maltose are digested to
monosaccharides and used for cell respiration.
• Oligosaccharides consist of from 3 to 20 monosaccharides;
they are antigens on the cell membrane
that identify cells as “self.”
• Polysaccharides are made of thousands of glucose
molecules.
Starches are plant products broken down in
digestion to glucose.
Glycogen is the form in which glucose is
stored in the liver and muscles.
Cellulose, the fiber portion of plant cells, cannot
be digested but promotes efficient peristalsis
in the colon.
2. Lipids (see Table 2–4 and Fig. 2–7).
• True fats are made of fatty acids and glycerol;
triglycerides are a storage form for potential
energy in adipose tissue. The eyes and kidneys
are cushioned by fat. Fatty acids may be saturated
or unsaturated. Saturated fats and hydrogenated
or trans fats contribute to atherosclerosis.
• Phospholipids are diglycerides such as lecithin
that are part of cell membranes. Myelin is a
phospholipid that provides electrical insulation
for nerve cells.
• Steroids consist of four rings of carbon and
hydrogen. Cholesterol, produced by the liver
and consumed in food, is the basic steroid from
which the body manufactures others: steroid
hormones, vitamin D, and bile salts.
3. Proteins
• Amino acids are the subunits of proteins; 20
amino acids make up human proteins. Peptide
bonds join amino acids to one another (see Fig.
2–8).
• A protein consists of from 50 to thousands of
amino acids in a specific sequence (primary
structure) that is folded into a specific shape (secondary
and tertiary structures). Some proteins
are made of two or more amino acid chains;
some proteins contain trace elements.
• Protein functions—see Table 2–5.
• Amino acids in excess of the need for protein
synthesis are converted to simple carbohydrates
or to fat, for energy production.
• Enzymes are catalysts, which speed up reactions
without additional energy. The active site theory
is based on the shapes of the enzyme and the substrate
molecules: These must “fit” (see Fig. 2–9).
The enzyme remains unchanged after the product
of the reaction is released. Each enzyme is
specific for one type of reaction. The functioning
of enzymes may be disrupted by changes in pH
or body temperature or by the presence of a poison,
which changes the shape of the active sites
of enzymes.
4. Nucleic acids (see Table 2–6 and Fig. 2–10).
• Nucleotides are the subunits of nucleic acids. A
nucleotide consists of a pentose sugar, a phosphate
group, and a nitrogenous base.
• DNA is a double strand of nucleotides, coiled
into a double helix, with complementary base
pairing: A–T and G–C. DNA makes up the
chromosomes of cells and is the genetic code for
the synthesis of proteins.
• RNA is a single strand of nucleotides, synthesized
from DNA, with U in place of T. RNA
functions in protein synthesis.
• ATP is a nucleotide that is specialized to trap
and release energy. Energy released from food in
cell respiration is used to synthesize ATP from
ADP P. When cells need energy, ATP is broken
down to ADP P and the energy is released
for cell processes.
44 Some Basic Chemistry
1. State the chemical symbol for each of the following
elements: sodium, potassium, iron, calcium, oxygen,
carbon, hydrogen, copper, and chlorine.
(p. 24)
2. Explain, in terms of their electrons, how an atom of
sodium and an atom of chlorine form a molecule of
sodium chloride. (p. 25)
3. a. Explain, in terms of their electrons, how an
atom of carbon and two atoms of oxygen form a
molecule of carbon dioxide. (pp. 26–28)
b. Explain the functions of hydrogen bonds
c. Explain the function of disulfide bonds
4. Name the subunits (smaller molecules) of which
each of the following is made: DNA, glycogen, a
true fat, and a protein. (pp. 34, 37, 41)
5. State precisely where in the body each of these fluids
is found: plasma, intracellular water, lymph, and
tissue fluid. (p. 29)
6. Explain the importance of the fact that water
changes temperature slowly. (pp. 28–29)
7. Describe two ways in which the solvent ability of
water is important to the body. (p. 28)
8. Name the organic molecule with each of the following
functions: (pp. 34–36, 42)
a. The genetic code in chromosomes
b. “Self” antigens in our cell membranes
c. The storage form for glucose in the liver
d. The storage form for excess food in adipose
tissue
e. The precursor molecule for the steroid hormones
f. The undigested part of food that promotes
peristalsis
g. The sugars that are part of the nucleic acids
9. State the summary equation of cell respiration.
(p. 30)
10. State the role or function of each of the following
in cell respiration: CO2, glucose, O2, heat, and
ATP. (p. 30)
11. State a specific function of each of the following
in the human body: Ca, Fe, Na, I, and Co. (p. 31)
12. Explain, in terms of relative concentrations of H
ions and OH ions, each of the following: acid,
base, and neutral substance. (p. 31)
13. State the normal pH range of blood. (p. 31)
14. Complete the following equation, and state how
each of the products affects pH: (p. 33)
HCl NaHCO3 → _______ _______.
15. Explain the active site theory of enzyme functioning.
(p. 38–39)
16. Explain the difference between a synthesis reaction
and a decomposition reaction. (p. 28)
Some Basic Chemistry 45
REVIEW QUESTIONS
FOR FURTHER THOUGHT
1. Orange juice usually has a pH of around 4. How
does this compare with the pH of the blood? Why
is it possible for us to drink orange juice without
disrupting the pH of our blood?
2. Estrela, age 7, has cereal with milk and sugar for
breakfast, then walks to school. Explain the relationship
between eating and walking, and remember
that Estrela is breathing.
3. The body is able to store certain nutrients. Name
the storage forms, and state an advantage and a disadvantage.
4. Many “vitamin pills” also contain minerals. Which
minerals are likely to be found in such dietary supplements?
What purpose do they have; that is, what
are their functions?
CHAPTER 3
Chapter Outline
Cell Structure
Cell Membrane
Nucleus
Cytoplasm and Cell Organelles
Cellular Transport Mechanisms
Diffusion
Osmosis
Facilitated Diffusion
Active Transport
Filtration
Phagocytosis and Pinocytosis
The Genetic Code and Protein Synthesis
DNA and the Genetic Code
RNA and Protein Synthesis
Cell Division
Mitosis
Meiosis
Aging and Cells
BOX 3–1 TERMINOLOGY OF SOLUTIONS
BOX 3–2 GENETIC DISEASE—SICKLE-CELL ANEMIA
BOX 3–3 ABNORMAL CELLULAR FUNCTIONING—
CANCER
Student Objectives
• Name the organic molecules that make up cell
membranes and state their functions.
• State the functions of the nucleus and chromosomes.
• Describe the functions of the cell organelles.
• Define each of these cellular transport mechanisms
and give an example of the role of each in
the body: diffusion, osmosis, facilitated diffusion,
active transport, filtration, phagocytosis, and
pinocytosis.
• Describe the triplet code of DNA.
• Explain how the triplet code of DNA is transcribed
and translated in the synthesis of proteins.
• Describe what happens in mitosis and in meiosis.
• Use examples to explain the importance of mitosis.
• Explain the importance of meiosis.
46
Cells
47
New Terminology
Absorption (ab-ZORB-shun)
Active transport (AK-tiv TRANS-port)
Aerobic (air-ROH-bik)
Cell membrane (SELL MEM-brayn)
Chromosomes (KROH-muh-sohms)
Cytoplasm (SIGH-toh-plazm)
Diffusion (di-FEW-zhun)
Diploid number (DIH-ployd)
Filtration (fill-TRAY-shun)
Gametes (GAM-eets)
Gene (JEEN)
Haploid number (HA-ployd)
Meiosis (my-OH-sis)
Microvilli (MY-kro-VILL-eye)
Mitochondria (MY-toh-KAHN-dree-ah)
Mitosis (my-TOH-sis)
Nucleus (NEW-klee-us)
Organelles (OR-gan-ELLS)
Osmosis (ahs-MOH-sis)
Phagocytosis (FAG-oh-sigh-TOH-sis)
Pinocytosis (PIN-oh-sigh-TOH-sis)
Selectively permeable (se-LEK-tiv-lee PER-me-uhbuhl)
Theory (THEER-ree)
Related Clinical Terminology
Benign (bee-NINE)
Carcinogen (kar-SIN-oh-jen)
Chemotherapy (KEE-moh-THER-uh-pee)
Genetic disease (je-NET-ik di-ZEEZ)
Hypertonic (HIGH-per-TAHN-ik)
Hypotonic (HIGH-poh-TAHN-ik)
Isotonic (EYE-soh-TAHN-ik)
Malignant (muh-LIG-nunt)
Metastasis (muh-TASS-tuh-sis)
Mutation (mew-TAY-shun)
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
All living organisms are made of cells and cell
products. This simple statement, called the cell theory,
was first proposed more than 150 years ago. You
may think of a theory as a guess or hypothesis, and
sometimes this is so. A scientific theory, however, is
actually the best explanation of all available evidence.
All of the evidence science has gathered so far supports
the validity of the cell theory.
Cells are the smallest living subunits of a multicellular
organism such as a human being. A cell is a complex
arrangement of the chemicals discussed in the
previous chapter, is living, and carries out specific
activities. Microorganisms, such as amoebas and bacteria,
are single cells that function independently.
Human cells, however, must work together and function
interdependently. Homeostasis depends upon the
contributions of all of the different kinds of cells.
Human cells vary in size, shape, and function. Most
human cells are so small they can only be seen with
the aid of a microscope and are measured in units
called micrometers (formerly called microns). One
micrometer 1/1,000,000 of a meter or 1/25,000 of
an inch (see Appendix A: Units of Measure). One
exception is the human ovum or egg cell, which is
about 1 millimeter in diameter, just visible to the
unaided eye. Some nerve cells, although microscopic
in diameter, may be quite long. Those in our arms and
legs, for example, are at least 2 feet (60 cm) long.
With respect to shape, human cells vary greatly.
Some are round or spherical, others rectangular, still
others irregular. White blood cells even change shape
as they move.
Cell functions also vary, and since our cells do not
act independently, we will cover specialized cell functions
as part of tissue functions in Chapter 4. Based on
function, there are more than 200 different kinds of
human cells. This chapter is concerned with the basic
structure of cells and the cellular activities common to
all our cells.
CELL STRUCTURE
Despite their many differences, human cells have several
similar structural features: a cell membrane, a
nucleus, and cytoplasm and cell organelles. Red blood
cells are an exception because they have no nuclei
when mature. The cell membrane forms the outer
boundary of the cell and surrounds the cytoplasm,
organelles, and nucleus.
CELL MEMBRANE
Also called the plasma membrane, the cell membrane
is made of phospholipids, cholesterol, and proteins.
The arrangement of these organic molecules is
shown in Fig. 3–1. The phospholipids are diglycerides,
and form a bilayer, or double layer, which
makes up most of the membrane. Phospholipids
permit lipid-soluble materials to easily enter or leave
the cell by diffusion through the cell membrane. The
presence of cholesterol decreases the fluidity of
the membrane, thus making it more stable. The proteins
have several functions: Some form channels or
pores to permit passage of materials such as water or
ions; others are carrier enzymes or transporters that
also help substances enter the cell. Still other proteins,
with oligosaccharides on their outer surface, are antigens,
markers that identify the cells of an individual as
“self.” Yet another group of proteins serves as receptor
sites for hormones. Many hormones bring about
their specific effects by first bonding to a particular
receptor on the cell membrane, a receptor with the
proper shape. This bonding, or fit, then triggers
chemical reactions within the cell membrane or the
interior of the cell (see Box 10–3 for an illustration
involving the hormone insulin).
Many receptors for other molecules are also part of
cell membranes. These molecules are part of the
chemical communication networks our cells have. An
unavoidable consequence of having so many receptors
for chemical communication is that some pathogens
have evolved shapes to match certain receptors. For
example, HIV, the virus that causes AIDS, just happens
to fit a particular surface receptor on our white
blood cells. When the virus fits in, the receptor
becomes a gateway into the cell, which begins the
takeover of the cell by the virus.
Most often, however, the cell membrane is a beneficial
structure. Although the cell membrane is the
outer boundary of the cell, it should already be apparent
to you that it is not a static or wall-like boundary,
but rather an active, dynamic one. It is much more like
a line of tollbooths than a concrete barrier. The cell
membrane is selectively permeable; that is, certain
substances are permitted to pass through and others
48 Cells
are not. These mechanisms of cellular transport will
be covered later in this chapter. The cell membrane is
of particular importance for muscle cells and nerve
cells because it carries electrical impulses. This will be
covered in Chapters 7 and 8.
NUCLEUS
With the exception of mature red blood cells, all
human cells have a nucleus. The nucleus is within the
cytoplasm and is bounded by a double-layered nuclear
membrane with many pores. It contains one or more
nucleoli and the chromosomes of the cell (Fig. 3–2).
A nucleolus is a small sphere made of DNA, RNA,
and protein. The nucleoli form a type of RNA called
ribosomal RNA, which becomes part of ribosomes
(a cell organelle) and is involved in protein synthesis.
The nucleus is the control center of the cell
because it contains the chromosomes. Even with a
microscope, the 46 chromosomes of a human cell are
usually not visible; they are long threads called chromatin.
Before a cell divides, however, the chromatin
coils extensively into visible chromosomes. Chromosomes
are made of DNA and protein. Some chromosomal
proteins provide the structural framework for
the coiling of the chromatin into chromosomes so
that cell division can take place. Other chromosomal
proteins help regulate the activity of the DNA.
Remember from our earlier discussion that the DNA
is the genetic code for the characteristics and activities
of the cell. Although the DNA in the nucleus of each
cell contains all of the genetic information for all
human traits, only a small number of genes (a gene is
the genetic code for one protein) are actually active, or
Cells 49
Receptor site
Outside of cell Oligosaccharide antigens
Cholesterol
Inside of cell
Protein forming a pore
Phospholipid
bilayer
Figure 3–1. The cell (plasma) membrane depicting the types of molecules present.
QUESTION: The receptor site shown is probably what type of organic molecule?
“switched on,” in a particular cell. These active genes
are the codes for the proteins necessary for the specific
cell type and its functions. For example, the gene for
insulin is present in all human cells, but only in certain
islet cells of the pancreas is the gene active or switched
on. Only in these cells is insulin produced. How the
genetic code in chromosomes is translated into proteins
will be covered in a later section.
CYTOPLASM AND CELL ORGANELLES
Cytoplasm is a watery solution of minerals, gases,
organic molecules, and cell organelles that is found
between the cell membrane and the nucleus. Cytosol
is the water portion of cytoplasm, and many chemical
reactions take place within it. Cell organelles are
intracellular structures, often bounded by their own
50 Cells
Nuclear
membrane
Nucleolus
Nucleus
Chromatin
Golgi apparatus
Cilia
Microvilli
Smooth endoplasmic
reticulum
Centrioles
Cell membrane
Rough endoplasmic
reticulum
Ribosomes
Cytoplasm
Mitochondrion
Lysosome
Proteasome
Figure 3–2. Generalized human cell depicting the structural components. See text and
Table 3–1 for descriptions.
QUESTION: How do cilia differ in structure from microvilli?
membranes, that have specific functions in cellular
metabolism. They are also shown in Fig. 3–2.
The endoplasmic reticulum (ER) is an extensive
network of membranous tubules that extend from
the nuclear membrane to the cell membrane. Rough
ER has numerous ribosomes on its surface, whereas
smooth ER has no ribosomes. As a network of interconnected
tunnels, the ER is a passageway for the
transport of the materials necessary for cell function
within the cell. These include proteins synthesized by
the ribosomes on the rough ER, and lipids synthesized
by the smooth ER.
Ribosomes are very small structures made of protein
and ribosomal RNA. Some are found on the surface
of rough ER, while others float freely within the
cytoplasm. Ribosomes are the site of protein synthesis.
The proteins produced may be structural proteins
such as collagen in the skin, enzymes, or hormones
such as insulin that regulate cellular processes. These
proteins may function within the cell or be secreted
from the cell to be used elsewhere in the body.
Our protein molecules are subject to damage, and
some cellular proteins, especially regulatory proteins,
may be needed just for a very short time. All such
proteins must be destroyed, and this is the function
of proteasomes. A proteasome is a barrel-shaped
organelle made of enzymes that cut protein molecules
apart (protease enzymes). Proteins that are to be destroyed,
that is, those no longer needed or those that
are damaged or misfolded, are tagged by a protein
called ubiquitin (sort of a cellular mop or broom) and
carried into a proteasome. The protein is snipped
into peptides or amino acids, which may be used again
for protein synthesis on ribosomes. Proteasomes are
particularly important during cell division and during
embryonic development, when great changes are taking
place very rapidly as cells become specialized.
Many of our cells have secretory functions, that is,
they produce specific products to be used elsewhere in
tissues. Secretion is one task of the Golgi apparatus,
a series of flat, membranous sacs, somewhat like
a stack of saucers. Carbohydrates are synthesized
within the Golgi apparatus, and are packaged, along
with other materials, for secretion from the cell.
Proteins from the ribosomes or lipids from the
smooth endoplasmic reticulum may also be secreted in
this way. To secrete a substance, small sacs of the
Golgi membrane break off and fuse with the cell
membrane, releasing the substance to the exterior of
the cell. This is exocytosis, exo meaning “to go out”
of the cell.
Mitochondria are oval or spherical organelles
bounded by a double membrane. The inner membrane
has folds called cristae. Within the mitochondria,
the aerobic (oxygen-requiring) reactions of cell
respiration take place. Therefore, mitochondria are
the site of ATP (and hence energy) production. Cells
that require large amounts of ATP, such as muscle
cells, have many mitochondria to meet their need for
energy. Mitochondria contain their own genes in a
single DNA molecule and duplicate themselves when
a cell divides. An individual’s mitochondrial DNA
(mDNA) is of maternal origin, that is, from the mitochondria
that were present in the ovum, or egg cell,
which was then fertilized by a sperm cell. The mitochondria
of the sperm cell usually do not enter the
ovum during fertilization, because they are not found
in the head of the sperm with the chromosomes (see
Fig. 20–1).
Lysosomes are single-membrane structures that
contain digestive enzymes. When certain white blood
cells engulf bacteria, the bacteria are digested and
destroyed by these lysosomal enzymes. Worn-out cell
parts and dead cells are also digested by these enzymes.
This is a beneficial process, and is necessary before tissue
repair can begin. But it does have a disadvantage in
that lysosomal digestion contributes to inflammation
in damaged tissues. An excess of inflammation can
start a vicious cycle, actually a positive feedback mechanism,
that results in extensive tissue damage.
Many of our cells are capable of dividing, or reproducing,
themselves. Centrioles are a pair of rodshaped
structures perpendicular to one another,
located just outside the nucleus. Their function is to
organize the spindle fibers during cell division. The
spindle fibers are contracting proteins that pull the
two sets of chromosomes apart, toward the ends of the
original cell as it divides into two new cells. Each new
cell then has a full set of chromosomes.
Cilia and flagella are mobile thread-like projections
through the cell membrane; each is anchored by
a basal body just within the membrane. Cilia are usually
shorter than flagella, and an individual cell has
many of them on its free surface. The cilia of a cell
beat in unison and sweep materials across the cell surface.
Cells lining the fallopian tubes, for example, have
cilia to sweep the egg cell toward the uterus. The only
human cell with a flagellum is the sperm cell. The flagellum
provides motility, or movement, for the sperm
cell.
Microvilli are folds of the cell membrane on the
free surface of a cell. These folds greatly increase the
Cells 51
surface area of the membrane, and are part of the cells
lining organs that absorb materials. The small intestine,
for example, requires a large surface area for the
absorption of nutrients, and many of its lining cells
have microvilli. Some cells of the kidney tubules also
have microvilli (see Fig. 1–1) that provide for the efficient
reabsorption of useful materials back to the
blood.
The functions of the cell organelles are summarized
in Table 3–1.
CELLULAR TRANSPORT
MECHANISMS
Living cells constantly interact with the blood or tissue
fluid around them, taking in some substances and
secreting or excreting others. There are several mechanisms
of transport that enable cells to move materials
into or out of the cell: diffusion, osmosis, facilitated
diffusion, active transport, filtration, phagocytosis,
and pinocytosis. Some of these take place without the
52 Cells
Table 3–1 FUNCTIONS OF CELL
ORGANELLES
Organelle Function
Endoplasmic
reticulum (ER)
Ribosomes
Proteasomes
Golgi apparatus
Mitochondria
Lysosomes
Centrioles
Cilia
Flagellum
Microvilli
• Passageway for transport of
materials within the cell
• Synthesis of lipids
• Site of protein synthesis
• Site of destruction of old
or damaged proteins
• Synthesis of carbohydrates
• Packaging of materials for secretion
from the cell
• Site of aerobic cell respiration—ATP
production
• Contain enzymes to digest ingested
material or damaged tissue
• Organize the spindle fibers during
cell division
• Sweep materials across the cell
surface
• Enables a cell to move
• Increase a cell’s surface area for
absorption
expenditure of energy by the cells. But others do
require energy, often in the form of ATP. Each of
these mechanisms is described in the following sections
and an example is included to show how each is
important to the body.
DIFFUSION
Diffusion is the movement of molecules from an area
of greater concentration to an area of lesser concentration
(that is, with or along a concentration gradient).
Diffusion occurs because molecules have free
energy; that is, they are always in motion. The molecules
in a solid move very slowly; those in a liquid
move faster; and those in a gas move faster still, such
as when ice absorbs heat energy, melts, and then evaporates.
Imagine a green sugar cube at the bottom of a
glass of water (green so that we can see it). As the
sugar dissolves, the sugar molecules collide with one
another or the water molecules, and the green color
seems to rise in the glass. These collisions spread out
the sugar molecules until they are evenly dispersed
among the water molecules (this would take a very
long time), and the water eventually becomes entirely
green. The molecules are still moving, but as some go
to the top, others go to the bottom, and so on. Thus,
an equilibrium (or steady state) is reached.
Diffusion is a very slow process, but may be an
effective transport mechanism across microscopic distances.
Within the body, the gases oxygen and carbon
dioxide move by diffusion. In the lungs, for example,
there is a high concentration of oxygen in the alveoli
(air sacs) and a low concentration of oxygen in the
blood in the surrounding pulmonary capillaries (see
Fig. 3–3). The opposite is true for carbon dioxide: a
low concentration in the air in the alveoli and a high
concentration in the blood in the pulmonary capillaries.
These gases diffuse in opposite directions, each
moving from where there is more to where there is
less. Oxygen diffuses from the air to the blood to be
circulated throughout the body. Carbon dioxide diffuses
from the blood to the air to be exhaled.
OSMOSIS
Osmosis may be simply defined as the diffusion of
water through a selectively permeable membrane.
That is, water will move from an area with more water
present to an area with less water. Another way to say
this is that water will naturally tend to move to an area
where there is more dissolved material, such as salt or
sugar. If a 2% salt solution and a 6% salt solution are
separated by a membrane allowing water but not salt
to pass through it, water will diffuse from the 2% salt
solution to the 6% salt solution. The result is that the
2% solution will become more concentrated, and the
6% solution will become more dilute.
In the body, the cells lining the small intestine
absorb water from digested food by osmosis. These
cells have first absorbed salts, have become more
“salty,” and water follows salt into the cells (see Fig.
3–3). The process of osmosis also takes place in the
kidneys, which reabsorb large amounts of water (many
gallons each day) to prevent its loss in urine. Box 3–1:
Terminology of Solutions lists some terminology we
use when discussing solutions and the effects of various
solutions on cells.
Cells 53
BOX 3–1 TERMINOLOGY OF SOLUTIONS
Human cells or other body fluids contain many dissolved
substances (called solutes) such as salts,
sugars, acids, and bases. The concentration of
solutes in a fluid creates the osmotic pressure of
the solution, which in turn determines the movement
of water through membranes.
As an example here, we will use sodium chloride
(NaCl). Human cells have an NaCl concentration of
0.9%. With human cells as a reference point, the
relative NaCl concentrations of other solutions may
be described with the following terms:
Isotonic—a solution with the same salt concentration
as in cells.
The blood plasma is isotonic to red blood cells.
Hypotonic—a solution with a lower salt concentration
than in cells.
Distilled water (0% salt) is hypotonic to human
cells.
Hypertonic—a solution with a higher salt concentration
than in cells.
Seawater (3% salt) is hypertonic to human cells.
Refer now to the diagrams shown in Box
Figure 3–A of red blood cells (RBCs) in each
of these different types of solutions, and
note the effect of each on osmosis:
• When RBCs are in plasma, water moves into
and out of them at equal rates, and the cells
remain normal in size and water content.
• If RBCs are placed in distilled water, more
water will enter the cells than leave, and the
cells will swell and eventually burst.
• If RBCs are placed in seawater, more water will
leave the cells than enter, and the cells will
shrivel and die.
This knowledge of osmotic pressure is used
when replacement fluids are needed for a patient
who has become dehydrated. Isotonic solutions
are usually used; normal saline and Ringer’s solution
are examples. These will provide rehydration
without causing osmotic damage to cells or
extensive shifts of fluid between the blood and
tissues.
Box Figure 3–A Red blood cells in different solutions and the effect of osmosis in each.
FACILITATED DIFFUSION
The word facilitate means to help or assist. In facilitated
diffusion, molecules move through a membrane
from an area of greater concentration to an area of
lesser concentration, but they need some help to do
this.
In the body, our cells must take in glucose to use for
ATP production. Glucose, however, will not diffuse
through most cell membranes by itself, even if there is
more outside the cell than inside. Diffusion of glucose
into most cells requires a glucose transporter, which
may also be called a carrier enzyme. These transporters
are proteins that are part of the cell membrane.
Glucose bonds to the transporter (see Fig. 3–3),
and by doing so changes the shape of the protein. This
physical change propels the glucose into the interior
of the cell. Other transporters are specific for other
organic molecules such as amino acids.
ACTIVE TRANSPORT
Active transport requires the energy of ATP to move
molecules from an area of lesser concentration to an
area of greater concentration. Notice that this is the
opposite of diffusion, in which the free energy of molecules
causes them to move to where there are fewer
of them. Active transport is therefore said to be movement
against a concentration gradient.
In the body, nerve cells and muscle cells have
“sodium pumps” to move sodium ions (Na ) out of
the cells. Sodium ions are more abundant outside the
cells, and they constantly diffuse into the cell (through
specific diffusion channels), their area of lesser concentration
(see Fig. 3–3). Without the sodium pumps
to return them outside, the incoming sodium ions
would bring about an unwanted nerve impulse or
muscle contraction. Nerve and muscle cells constantly
produce ATP to keep their sodium pumps (and similar
potassium pumps) working and prevent spontaneous
impulses.
Another example of active transport is the absorption
of glucose and amino acids by the cells lining
the small intestine. The cells use ATP to absorb these
nutrients from digested food, even when their intracellular
concentration becomes greater than their
extracellular concentration.
FILTRATION
The process of filtration also requires energy, but the
energy needed does not come directly from ATP. It is
the energy of mechanical pressure. Filtration means
that water and dissolved materials are forced through
a membrane from an area of higher pressure to an area
of lower pressure.
In the body, blood pressure is created by the
54 Cells
Table 3–2 CELLULAR TRANSPORT MECHANISMS
Mechanism Definition Example in the Body
Diffusion
Osmosis
Facilitated diffusion
Active transport
Filtration
Phagocytosis
Pinocytosis
Movement of molecules from an area of
greater concentration to an area of
lesser concentration.
The diffusion of water.
Carrier and transporter enzymes move molecules
across cell membranes.
Movement of molecules from an area of
lesser concentration to an area of greater
concentration (requires ATP).
Movement of water and dissolved substances
from an area of higher pressure to an area
of lower pressure (blood pressure).
A moving cell engulfs something.
A stationary cell engulfs something.
Exchange of gases in the lungs or body tissues.
Absorption of water by the small intestine
or kidneys.
Intake of glucose by most cells.
Absorption of amino acids and glucose from food
by the cells of the small intestine.
Sodium and potassium pumps in muscle and
nerve cells.
Formation of tissue fluid; the first step in the formation
of urine.
White blood cells engulf bacteria.
Cells of the kidney tubules reabsorb small proteins.
55
•
A Diffusion
B Osmosis
C Facilitated Diffusion
D Active transport
E Filtration
F Phagocytosis
G Pinocytosis
Small protein
Cell of
kidney
tubule
•
•
•
Lysosome Bacterium
White blood cell
•
• •
•
•
H2O
BP
Glucose
Amino
acid
RBC Capillary in
tissues
•
•
•
•
Cell
membrane
Na+
Tissue fluid
ATP Active
transport
Cytoplasm channel
Diffusion
channel
•
•
•
Cell membrane of intestinal cell
H2O
Na+
Cytoplasm
•
Alveolus of lung
•
Capillary
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
CO2
CO2 CO2
CO2
CO2
CO2
•
Glucose
•
Transporter
Cytoplasm
Tissue fluid
•
Cell membrane
Figure 3–3. Cellular transport mechanisms. (A) Diffusion in an alveolus in the lung.
(B) Osmosis in the small intestine. (C) Facilitated diffusion in a muscle cell. (D) Active transport
in a muscle cell. (E) Filtration in a capillary. (F) Phagocytosis by a white blood cell.
(G) Pinocytosis by a cell of the kidney tubules. See text for description.
QUESTION: Which mechanism depends on blood pressure? Which depends on the movement
of a cell?
pumping of the heart. Filtration occurs when blood
flows through capillaries, whose walls are only one
cell thick and very permeable. The blood pressure in
capillaries is higher than the pressure of the surrounding
tissue fluid. In capillaries throughout the body,
blood pressure forces plasma (water) and dissolved
materials through the capillary membranes into the
surrounding tissue spaces (see Fig. 3–3). This creates
more tissue fluid and is how cells receive glucose,
amino acids, and other nutrients. Blood pressure
in the capillaries of the kidneys also brings about
filtration, which is the first step in the formation of
urine.
PHAGOCYTOSIS AND PINOCYTOSIS
These two processes are similar in that both involve a
cell engulfing something, and both are forms of endocytosis,
endo meaning “to take into” a cell. An example
of phagocytosis is a white blood cell engulfing
bacteria. The white blood cell flows around the bacterium
(see Fig. 3–3), taking it in and eventually
digesting it. Digestion is accomplished by the enzymes
in the cell’s lysosomes.
Other cells that are stationary may take in small
molecules that become adsorbed or attached to their
membranes. The cells of the kidney tubules reabsorb
small proteins by pinocytosis (see Fig. 3–3) so that
the protein is not lost in urine.
Table 3–2 summarizes the cellular transport mechanisms.
THE GENETIC CODE
AND PROTEIN SYNTHESIS
The structure of DNA, RNA, and protein was
described in Chapter 2. We will review some of the
essentials here, and go a step further with a simple
description of how all of these organic molecules are
involved in the process of protein synthesis.
DNA AND THE GENETIC CODE
DNA is a double strand of nucleotides in the form of
a double helix, very much like a spiral ladder. The
uprights of the ladder are made of alternating phosphate
groups and deoxyribose sugar molecules. The
rungs of the ladder are made of the four nitrogenous
bases, always found in complementary pairs: adenine
with thymine (A–T) and guanine with cytosine (G–C).
Although DNA contains just these four bases, the
bases may be arranged in many different sequences
(reading up or down the ladder). It is the sequence of
bases, the sequence of A, T, C, and G, that is the
genetic code. The DNA of our 46 chromosomes may
also be called our genome, which is the term for the
total genetic information in a particular species. The
human genome is believed to contain about 3 billion
base pairs, and the number of our genes is now estimated
to be between 20,000 and 25,000 (or perhaps as
many as 30,000, but much lower than previously
thought).
56 Cells
Table 3–3 PROTEIN SYNTHESIS
Molecule or Organelle Function
DNA
mRNA (messenger RNA)
Ribosomes
tRNA (transfer RNA)
• A double strand (helix) of nucleotides that is the genetic code in the chromosomes
of cells.
• A gene is the sequence of bases (segment of DNA) that is the code for one protein.
• A single strand of nucleotides formed as a complementary copy of a gene in the DNA.
• Now contains the triplet code: three bases is the code for one amino acid (a codon).
• Leaves the DNA in the nucleus, enters the cytoplasm of the cell, and becomes
attached to ribosomes.
• The cell organelles that are the site of protein synthesis.
• Attach the mRNA molecule.
• Contain enzymes to form peptide bonds between amino acids.
• Picks up amino acids (from food) in the cytoplasm and transports them to their proper
sites (triplets) along the mRNA molecule; has anticodons to match mRNA codons.
57
Cell nucleus
Nuclear pore
Nuclear membrane
A
A Codon
A
DNA
Amino acids
Peptide bonds
mRNA
tRNA
Ribosome
Anticodon
A A G C A U A A A G U C U U U
G
U A U U U
Figure 3–4. Protein synthesis. The mRNA is formed as a copy of a portion of the DNA in
the nucleus of a cell. In the cytoplasm, the mRNA becomes attached to ribosomes. See text
for further description.
QUESTION: A tRNA molecule has two attachment sites; what is each for?
Recall that in Chapter 2 you read that a gene is the
genetic code for one protein. This is a simplification,
and the functioning of genes is often much more complex.
We have genes with segments that may be shuffled
or associated in many combinations, with the
potential for coding for many more proteins. A full
explanation is beyond the scope of our book, so for the
sake of simplicity, and in the following discussion, we
will say that a gene is the code for one protein. Recall
too that a protein is a specific sequence of amino acids.
Therefore, a gene, or segment of DNA, is the code for
the sequence of amino acids in a particular protein.
The code for a single amino acid consists of three
bases in the DNA molecule; this triplet of bases may
be called a codon (see Fig. 3–4). There is a triplet of
bases in the DNA for each amino acid in the protein.
If a protein consists of 100 amino acids, the gene for
that protein would consist of 100 triplets, or 300 bases.
Some of the triplets will be the same, since the same
amino acid may be present in several places within the
protein. Also part of the gene are other triplets that
start and stop the process of making the protein,
rather like capital letters or punctuation marks start
and stop sentences.
RNA AND PROTEIN SYNTHESIS
RNA, the other nucleic acid, has become a surprising
molecule, in that it has been found to have quite a few
functions. It may be involved in the repair of DNA,
and it is certainly involved in gene expression. The
expression of a gene means that the product of the
gene is somehow apparent to us, in a way we can see
or measure, or is not apparent when it should be.
Examples would be having brown eyes or blue eyes, or
having or not having the intestinal enzyme lactase to
digest milk sugar. Although these functions of RNA
are essential for us, they too are beyond the scope of
our book, so the roles of RNA in the process of protein
synthesis will be our focus.
The transcription and translation of the genetic
code in DNA into proteins require RNA. DNA is
found in the chromosomes in the nucleus of the cell,
but protein synthesis takes place on the ribosomes in
the cytoplasm. Messenger RNA (mRNA) is the
intermediary molecule between these two sites.
When a protein is to be made, the segment of DNA
that is its gene uncoils, and the hydrogen bonds
between the base pairs break (see Fig. 3–4). Within
the nucleus are RNA nucleotides (A, C, G, U) and
enzymes to construct a single strand of nucleotides
that is a complementary copy of half the DNA gene
(with uracil in place of thymine). This process is transcription,
or copying, and the copy of the gene is
mRNA, which now has the codons for the amino acids
of the protein, and then separates from the DNA. The
gene coils back into the double helix, and the mRNA
leaves the nucleus, enters the cytoplasm, and becomes
attached to ribosomes.
As the copy of the gene, mRNA is a series of triplets
of bases; each triplet is a codon, the code for one
amino acid. Another type of RNA, called transfer
RNA (tRNA), is also found in the cytoplasm. Each
tRNA molecule has an anticodon, a triplet complementary
to a triplet on the mRNA. The tRNA
molecules pick up specific amino acids (which have
come from protein in our food) and bring them to
their proper triplets on the mRNA. This process is
translation; that is, it is as if we are translating from
one language to another—the language of nucleotide
bases to that of amino acids. The ribosomes contain
enzymes to catalyze the formation of peptide bonds
between the amino acids. When an amino acid has
been brought to each triplet on the mRNA, and
all peptide bonds have been formed, the protein is
finished.
The protein then leaves the ribosomes and may be
transported by the endoplasmic reticulum to wherever
it is needed in the cell, or it may be packaged by the
Golgi apparatus for secretion from the cell. A summary
of the process of protein synthesis is found in
Table 3–3.
Thus, the expression of the genetic code may be
described by the following sequence:
Each of us is the sum total of our genetic characteristics.
Blood type, hair color, muscle proteins, nerve
cells, and thousands of other aspects of our structure
and functioning have their basis in the genetic code of
DNA.
If there is a “mistake” in the DNA, that is, incorrect
bases or triplets of bases, this mistake will be copied by
the mRNA. The result is the formation of a malfunctioning
or non-functioning protein. This is called a
genetic or hereditary disease, and a specific example
is described in Box 3–2: Genetic Disease—Sickle-Cell
Anemia.
DNA RNA Proteins:
Structural Enzymes
Catalyze Reactions
Hereditary Characteristics
Proteins
58 Cells
59
BOX 3–2 GENETIC DISEASE—SICKLE-CELL ANEMIA
Normal hemoglobin
Normal red blood cells (RBCs)
Sickle red blood cells
Sickle
hemoglobin
(HbS)
Deoxygenation
α
β
β
α
Iron in heme
Box Figure 3–B Structure of hemoglobin
A and sickle-cell hemoglobin and
their effect on red blood cells.
A genetic disease is a hereditary disorder, one
that may be passed from generation to generation.
Although there are hundreds of genetic diseases,
they all have the same basis: a mistake in DNA.
Because DNA makes up the chromosomes that are
found in eggs and sperm, this mistake may be
passed from parents to children.
Sickle-cell anemia is the most common genetic
disorder among people of African descent and
affects the hemoglobin in red blood cells. Normal
hemoglobin, called hemoglobin A (HbA), is a protein
made of two alpha chains (141 amino acids
each) and two beta chains (146 amino acids each).
In sickle-cell hemoglobin (HbS), the sixth amino
acid in each beta chain is incorrect; valine instead of
the glutamic acid found in HbA. This difference
seems minor—only 2 incorrect amino acids out of
more than 500—but the consequences for the person
are very serious.
HbS has a great tendency to crystallize when
oxygen levels are low, as is true in capillaries. When
HbS crystallizes, the red blood cells are deformed
into crescents (sickles) and other irregular shapes.
These irregular, rigid red blood cells clog and rupture
capillaries, causing internal bleeding and
severe pain. These cells are also fragile and break up
easily, leading to anemia and hypoxia (lack of oxygen).
Treatment of this disease has improved
greatly, but it is still incurable.
What has happened to cause the formation of
HbS rather than HbA? Hemoglobin is a protein; the
gene for its beta chain is in DNA (chromosome 11).
One amino acid in the beta chains is incorrect,
therefore, one triplet in its DNA gene must be, and
is, incorrect. This mistake is copied by mRNA in the
cells of the red bone marrow, and HbS is synthesized
in red blood cells.
Sickle-cell anemia is a recessive genetic disease,
which means that a person with one gene for HbS
and one gene for HbA will have “sickle-cell trait.”
Such a person usually will not have the severe
effects of sickle-cell anemia, but may pass the gene
for HbS to children. It is estimated that 9% of
African-Americans have sickle-cell trait and about
1% have sickle-cell anemia.
CELL DIVISION
Cell division is the process by which a cell reproduces
itself. There are two types of cell division, mitosis and
meiosis. Although both types involve cell reproduction,
their purposes are very different.
MITOSIS
Each of us began life as one cell, a fertilized egg. Each
of us now consists of billions of cells produced by the
process of mitosis. In mitosis, one cell with the
diploid number of chromosomes (the usual number,
46 for people) divides into two identical cells, each
with the diploid number of chromosomes. This production
of identical cells is necessary for the growth of
the organism and for repair of tissues (see also Box
3–3: Abnormal Cellular Functioning—Cancer).
Before mitosis can take place, a cell must have two
complete sets of chromosomes, because each new cell
must have the diploid number. The process of DNA
replication enables each chromosome (in the form of
chromatin) to make a copy of itself. The time during
which this takes place is called interphase, the time
between mitotic divisions. Although interphase is
sometimes referred to as the resting stage, resting
means “not dividing” rather than “inactive.” The cell
is quite actively producing a second set of chromosomes
and storing energy in ATP.
The long, thin, and invisible chromatin molecules
then begin to coil very precisely and extensively, and if
we were looking at the nucleus of a living cell under a
microscope, we would see the duplicated chromosomes
appear. Each would look somewhat like the letter
X, because the original DNA molecule and its copy
(now called chromatids) are still attached.
The stages of mitosis are prophase, metaphase,
anaphase, and telophase. What happens in each of
these stages is described in Table 3–4. As you read the
events of each stage, refer to Fig. 3–5, which depicts
mitosis in a cell with a diploid number of four.
As mentioned previously, mitosis is essential for
repair of tissues, to replace damaged or dead cells.
Some examples may help illustrate this. In several
areas of the body, mitosis takes place constantly. These
sites include the epidermis of the skin, the stomach
lining, and the red bone marrow. For each of these
sites, there is a specific reason why this constant mitosis
is necessary.
What happens to the surface of the skin? The dead,
outer cells are worn off by contact with the environment.
Mitosis of the epidermal cells in the lower living
layer replaces these cells, and the epidermis
maintains its normal thickness.
The stomach lining, although internal, is also constantly
worn away. Gastric juice, especially hydrochloric
acid, is very damaging to cells. Rapid mitosis of the
several kinds of lining cells replaces damaged cells and
keeps the stomach lining intact.
One of the functions of red bone marrow is the
production of red blood cells. Because red blood cells
have a life span of only about 120 days, new ones are
needed to replace the older ones that die. Very rapid
mitosis in the red bone marrow produces approximately
2 million new red blood cells every second.
These dividing cells in the red bone marrow are
among the stem cells present in the body. A stem cell
is an unspecialized cell that may develop into several
different kinds of cells. Stem cells in the red bone marrow
may become red blood cells, white blood cells, or
platelets. These marrow stem cells are often called
adult stem cells, and many, if not all, of the body’s
organs have such cells. Embryonic stem cells will be
described in a later chapter; these are cells in which all
of the DNA still has the potential to be active. They
may become any of the more than 200 different kinds
of human cells. The stem cells found in the umbilical
cords of newborns are between the adult and embryonic
cells in terms of their potential.
It is also important to be aware of the areas of the
body where mitosis does not take place. In an adult,
most muscle cells and neurons (nerve cells) do not
reproduce themselves. If they die, their functions are
also lost. Someone whose spinal cord has been severed
will have paralysis and loss of sensation below the level
of the injury. The spinal cord neurons do not undergo
mitosis to replace the ones that were lost, and such an
injury is permanent.
Skeletal muscle cells are capable of limited mitosis
for repair. The heart is made of cardiac muscle cells,
which, like neurons, seem to be incapable of mitosis. A
heart attack (myocardial infarction) means that a portion
of cardiac muscle dies because of lack of oxygen.
These cells are not replaced, and the heart will be a
less effective pump. If a large enough area of the heart
muscle dies, the heart attack may be fatal.
Some research has found evidence for the potential
for mitosis after damage in both the central nervous
system and the heart. Such cell division may be that of
neurons or muscle cells that were stimulated to divide
60 Cells
by chemicals from the adjacent damaged tissue. Or the
dividing cells may be stem cells that are among the
specialized cells. (A region of the brain called the hippocampus,
which is necessary to form new memories,
seems to have cells capable of division.) At present we
do not have definitive knowledge, but we do know that
for most people with heart damage or central nervous
system injury, mitosis does not take place, or not sufficiently
enough to replace the cells that have died and
preserve or restore normal functioning of the organ.
Research is continuing, and may eventually find the
stimulus necessary to produce extended mitosis that
would bring about true tissue repair.
MEIOSIS
Meiosis is a more complex process of cell division that
results in the formation of gametes, which are egg
and sperm cells. In meiosis, one cell with the diploid
number of chromosomes divides twice to form four
Cells 61
BOX 3–3 ABNORMAL CELLULAR FUNCTIONING—CANCER
the trigger is believed to be infection with certain
viruses that cause cellular mutations. Carriers of
hepatitis B virus, for example, are more likely to
develop primary liver cancer than are people who
have never been exposed to this virus. Research has
discovered two genes, one on chromosome 2 and
the other on chromosome 3, that contribute to a
certain form of colon cancer. Both of these genes
are the codes for proteins that correct the “mistakes”
that may occur when the new DNA is synthesized.
When these repair proteins do not
function properly, the mistakes (mutations) in the
DNA lead to the synthesis of yet other faulty proteins
that impair the functioning of the cell and predispose
it to becoming malignant.
Once cells have become malignant, their functioning
cannot return to normal, and though the
immune system will often destroy such cells, sometimes
it does not, especially as we get older.
Therefore, the treatments for cancer are directed at
removing or destroying the abnormal cells. Surgery
to remove tumors, radiation to destroy cells, and
chemotherapy to stop cell division or interfere
with other aspects of cell metabolism are all aspects
of cancer treatment.
New chemotherapy drugs are becoming more
specific, with very precise targets. For example, the
cells of several types of solid-tumor cancers have
been found to have mutations in the gene for the
cell membrane receptor for a natural growth factor
(epidermal growth factor receptor, or EGFR). These
altered receptors, when triggered by their usual
growth factor, then cause the cell to divide uncontrollably,
an abnormal response. Medications that
target only these altered receptors have already
been developed for some forms of lung cancer and
breast cancer. Not only do they show promise in
treating the cancer, they do not have the side
effects of other forms of chemotherapy.
There are more than 200 different types of cancer,
all of which are characterized by abnormal cellular
functioning. Normally, our cells undergo mitosis
only when necessary and stop when appropriate. A
cut in the skin, for example, is repaired by mitosis,
usually without formation of excess tissue. The new
cells fill in the damaged area, and mitosis slows
when the cells make contact with surrounding cells.
This is called contact inhibition, which limits the
new tissue to just what is needed. Malignant
(cancer) cells, however, are characterized by uncontrolled
cell division. Our cells are genetically programmed
to have particular life spans and to divide
or die. One gene is known to act as a brake on cell
division; another gene enables cells to live indefinitely,
beyond their normal life span, and to keep
dividing. Any imbalance in the activity of these
genes may lead to abnormal cell division. Such cells
are not inhibited by contact with other cells, keep
dividing, and tend to spread.
A malignant tumor begins in a primary site such
as the colon, then may spread or metastasize. Often
the malignant cells are carried by the lymph or
blood to other organs such as the liver, where secondary
tumors develop. Metastasis is characteristic
only of malignant cells; benign tumors do not
metastasize but remain localized in their primary
site.
What causes normal cells to become malignant?
At present, we have only partial answers. A malignant
cell is created by a mutation, a genetic
change that brings about abnormal cell functions
or responses and often leads to a series of mutations.
Environmental substances that cause mutations
are called carcinogens. One example is the
tar found in cigarette smoke, which is definitely a
cause of lung cancer. Ultraviolet light may also
cause mutations, especially in skin that is overexposed
to sunlight. For a few specific kinds of cancer,
cells, each with the haploid number (half the usual
number) of chromosomes. In women, meiosis takes
place in the ovaries and is called oogenesis. In men,
meiosis takes place in the testes and is called spermatogenesis.
The differences between oogenesis and
spermatogenesis will be discussed in Chapter 20, The
Reproductive Systems.
The egg and sperm cells produced by meiosis have
the haploid number of chromosomes, which is 23 for
humans. Meiosis is sometimes called reduction division
because the division process reduces the chromosome
number in the egg or sperm. Then, during
fertilization, in which the egg unites with the sperm,
the 23 chromosomes of the sperm plus the 23 chromosomes
of the egg will restore the diploid number
of 46 in the fertilized egg. Thus, the proper chromo-
62 Cells
Interphase
Nucleus
Prophase
Centrioles
Pair of chromatids
Centromere
Equator of cell
Spindle
fibers
Metaphase
Anaphase
Nuclear membrane re-forms
Telophase
Cytoplasm divides
Figure 3–5. Stages of mitosis in a cell with the diploid number of four. See Table 3–4 for
description.
QUESTION: In prophase, what is a pair of chromatids made of?
some number is maintained in the cells of the new
individual.
AGING AND CELLS
Multicellular organisms, including people, age and
eventually die; our cells do not have infinite life spans.
It has been proposed that some cells capable of mitosis
are limited to a certain number of divisions; that is,
every division is sort of a tick-tock off a biological
clock. We do not yet know exactly what this cellular
biological clock is. There is evidence that the ends of
chromosomes, called telomeres, may be an aspect of it.
With each cell division, part of the telomeres is lost
(rather like a piece of rope fraying at both ends), and
eventually the telomeres are gone. With the next division,
the ends of the chromosomes, actual genes,
begin to be lost. This may be one signal that a cell’s life
span has come to an end (there are probably many different
kinds of signals).
Cellular aging also involves the inevitable deterioration
of membranes and cell organelles. Just as the
parts of a car break down in time, so too will cells.
Unlike cars or machines, however, cells can often
repair themselves, but they do have limits. As cells
age, structural proteins break down and are not
replaced, or necessary enzymes are not synthesized.
Proteins called chaperones, which are responsible
for the proper folding of many other proteins and
for the repair or disposal of damaged proteins, no
longer function as well as cells age. Without chaperones,
damaged proteins accumulate within cells and
disrupt normal cellular processes. Clinical manifestations
of impaired chaperones include cataracts (see
Box 9–1) and neurodegenerative diseases such as
Alzheimer’s disease (see Box 8–6), Parkinson’s disease
(see Box 8–7), and Huntington’s disease (see
Table 21–4).
Much about the chemistry of the aging process
remains a mystery, though we can describe what happens
to organs and to the body as a whole. Each of the
following chapters on body systems provides a brief
discussion of how aging affects the system. Keep in
mind that a system is the sum of its cells, in tissues and
organs, and that all aging is ultimately at the cellular
level.
SUMMARY
As mentioned at the beginning of this chapter, human
cells work closely together and function interdependently.
Each type of human cell makes a contribution to
the body as a whole. Usually, however, cells do not
function as individuals, but rather in groups. Groups
of cells with similar structure and function form a tissue,
which is the next level of organization.
Cells 63
Table 3–4 STAGES OF MITOSIS
Stage Events
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
1. The chromosomes coil up and become visible as short rods. Each chromosome is really two
chromatids (original DNA plus its copy) still attached at a region called the centromere.
2. The nuclear membrane disappears.
3. The centrioles move toward opposite poles of the cell and organize the spindle fibers,
which extend across the equator of the cell.
1. The pairs of chromatids line up along the equator of the cell. The centromere of each pair
is attached to a spindle fiber.
2. The centromeres now divide.
1. Each chromatid is now considered a separate chromosome; there are two complete and
separate sets.
2. The spindle fibers contract and pull the chromosomes, one set toward each pole of the cell.
1. The sets of chromosomes reach the poles of the cell and become indistinct as their DNA
uncoils to form chromatin.
2. A nuclear membrane re-forms around each set of chromosomes.
1. The cytoplasm divides; new cell membrane is formed.
Human cells vary in size, shape, and function.
Our cells function interdependently to maintain
homeostasis.
Cell Structure—the major parts of a cell are
the cell membrane, nucleus (except mature
RBCs), cytoplasm, and cell organelles
1. Cell membrane—the selectively permeable boundary
of the cell (see Fig. 3–1).
• Phospholipids permit diffusion of lipid-soluble
materials.
• Cholesterol provides stability.
• Proteins form channels, transporters, “self” antigens,
and receptor sites for hormones or other
signaling molecules.
2. Nucleus—the control center of the cell; has a
double-layer membrane.
• Nucleolus—forms ribosomal RNA.
• Chromosomes—made of DNA and protein;
DNA is the genetic code for the structure and
functioning of the cell. A gene is a segment of
DNA that is the code for one protein. Human
cells have 46 chromosomes, and their genetic
information is called the genome.
3. Cytoplasm—a watery solution of minerals, gases,
and organic molecules; contains the cell organelles;
site for many chemical reactions.
4. Cell organelles—intracellular structures with specific
functions (see Table 3–1 and Fig. 3–2).
Cellular Transport Mechanisms—the processes
by which cells take in or secrete or
excrete materials through the selectively
permeable cell membrane (see Fig. 3–3 and
Table 3–2).
1. Diffusion—movement of molecules from an area
of greater concentration to an area of lesser concentration;
occurs because molecules have free
energy: They are constantly in motion. Oxygen
and carbon dioxide are exchanged by diffusion in
the lungs and tissues.
2. Osmosis—the diffusion of water. Water diffuses to
an area of less water, that is, to an area of more dissolved
material. The small intestine absorbs water
from digested food by osmosis. Isotonic, hypertonic,
and hypotonic (see Box 3–1).
3. Facilitated diffusion—transporters (carrier
enzymes) that are part of the cell membrane permit
cells to take in materials that would not diffuse by
themselves. Most cells take in glucose by facilitated
diffusion.
4. Active transport—a cell uses ATP to move substances
from an area of lesser concentration to an
area of greater concentration. Nerve cells and muscle
cells have sodium pumps to return Na ions to
the exterior of the cells; this prevents spontaneous
impulses. Cells of the small intestine absorb glucose
and amino acids from digested food by active
transport.
5. Filtration—pressure forces water and dissolved
materials through a membrane from an area of
higher pressure to an area of lower pressure. Tissue
fluid is formed by filtration: Blood pressure forces
plasma and dissolved nutrients out of capillaries
and into tissues. Blood pressure in the kidney capillaries
creates filtration, which is the first step in
the formation of urine.
6. Phagocytosis—(a form of endocytosis) a moving
cell engulfs something; white blood cells phagocytize
bacteria to destroy them.
7. Pinocytosis—(a form of endocytosis) a stationary
cell engulfs small molecules; kidney tubule cells
reabsorb small proteins by pinocytosis.
The Genetic Code and Protein Synthesis (see
Fig. 3–4 and Table 3–3)
1. DNA and the genetic code
• DNA is a double helix with complementary base
pairing: A–T and G–C.
• The sequence of bases in the DNA is the genetic
code for proteins.
• The triplet code: three bases (a codon) is the
code for one amino acid.
• A gene consists of all the triplets that code for a
single protein.
2. RNA and protein synthesis
• Transcription—mRNA is formed as a complementary
copy of the sequence of bases in a gene
(DNA).
• mRNA moves from the nucleus to the ribosomes
in the cytoplasm.
• tRNA molecules (in the cytoplasm) have anticodons
for the triplets on the mRNA.
• Translation—tRNA molecules bring amino acids
to their proper triplets on the mRNA.
• Ribosomes contain enzymes to form peptide
bonds between the amino acids.
3. Expression of the genetic code
64 Cells
STUDY OUTLINE
• DNA → RNA → proteins (structural proteins
and enzymes that catalyze reactions) → hereditary
characteristics.
• A genetic disease is a “mistake” in the DNA,
which is copied by mRNA and results in a malfunctioning
protein.
Cell Division
1. Mitosis—one cell with the diploid number of chromosomes
divides once to form two cells, each with
the diploid number of chromosomes (46 for
humans).
• DNA replication forms two sets of chromosomes
during interphase.
• Stages of mitosis (see Fig. 3–5 and Table 3–4):
prophase, metaphase, anaphase, and telophase.
Cytokinesis is the division of the cytoplasm following
telophase.
• Mitosis is essential for growth and for repair and
replacement of damaged cells.
• Most adult nerve and muscle cells seem unable to
divide; their loss may involve permanent loss of
function.
2. Meiosis—one cell with the diploid number of chromosomes
divides twice to form four cells, each with
the haploid number of chromosomes (23 for
humans).
• Oogenesis in the ovaries forms egg cells.
• Spermatogenesis in the testes forms sperm cells.
• Fertilization of an egg by a sperm restores the
diploid number in the fertilized egg.
Cells 65
REVIEW QUESTIONS
1. State the functions of the organic molecules of
cell membranes: cholesterol, proteins, and phospholipids.
(p. 48)
2. Describe the function of each of these cell
organelles: mitochondria, lysosomes, Golgi apparatus,
ribosomes, proteasomes, and endoplasmic
reticulum. (p. 51)
3. Explain why the nucleus is the control center of
the cell. (p. 49)
4. What part of the cell membrane is necessary for
facilitated diffusion? Describe one way this
process is important within the body. (p. 54)
5. What provides the energy for filtration? Describe
one way this process is important within the body.
(p. 54)
6. What provides the energy for diffusion? Describe
one way this process is important within the body.
(p. 52)
7. What provides the energy for active transport?
Describe one way this process is important within
the body. (p. 54)
8. Define osmosis, and describe one way this process
is important within the body. (p. 52–53)
9. Explain the difference between hypertonic and
hypotonic, using human cells as a reference point.
(p. 53)
10. In what way are phagocytosis and pinocytosis similar?
Describe one way each process is important
within the body. (p. 56)
11. How many chromosomes does a human cell have?
What are these chromosomes made of? (p. 56)
12. Name the stage of mitosis in which each of the
following takes place: (p. 63)
a. The two sets of chromosomes are pulled
toward opposite poles of the cell
b. The chromosomes become visible as short rods
c. A nuclear membrane re-forms around each
complete set of chromosomes
d. The pairs of chromatids line up along the
equator of the cell
e. The centrioles organize the spindle fibers
f. Cytokinesis takes place after this stage
13. Describe two specific ways mitosis is important
within the body. Explain why meiosis is important.
(pp. 60, 62)
14. Compare mitosis and meiosis in terms of: (pp.
60–62)
a. Number of divisions
b. Number of cells formed
c. Chromosome number of the cells formed
15. Explain the triplet code of DNA. Name the molecule
that copies the triplet code of DNA. Name
the organelle that is the site of protein synthesis.
What other function does this organelle have in
protein formation? (pp. 56–58)
1. Antibiotics are drugs used to treat bacterial infections.
Some antibiotics disrupt the process of protein
synthesis within bacteria. Others block DNA
synthesis and cell division by the bacteria. Still others
inhibit cell wall synthesis by the bacteria. If all
antibiotics worked equally well against bacteria,
would any of those mentioned here be better than
the others, from the patient’s perspective? Explain
your answer.
2. A new lab instructor wants his students to see living
cells. He puts a drop of his own blood on a glass
slide, adds two drops of distilled water “so the cells
will be spread out and easier to see,” puts on a
cover glass, and places the slide under a microscope
on high power. He invites his students to see living
red blood cells. The students claim that they cannot
see any cells. Explain what has happened. How
could this have been prevented?
3. A friend asks you how DNA can be used to identify
someone, and why it is called a “DNA fingerprint.”
What simple explanation can you give?
4. A cell has extensive rough ER and Golgi apparatus.
Give a brief explanation of its function. A second
cell has microvilli and many mitochondria. Give a
brief explanation of its function.
5. A bacterial toxin is found to cause harm by first fitting
into a receptor on human cell membranes;
once the toxin fits, the cell will be destroyed. A
medication is going to be made to stop this toxin,
and can work in one of two ways: The drug can
block the receptors to prevent the toxin from fitting
in, or the drug can act as decoy molecules
shaped like the receptors. Which one of these
might be better, and why?
66 Cells