C h a p t e r 1
Cell Structure and Function
9
diphosphate (ADP) with the loss of one high-energy
bond and to adenosine monophosphate (AMP) with the
loss of two such bonds. The energy liberated from the
hydrolysis of ATP is used to drive reactions that require
free energy, such as muscle contraction and active trans-
port mechanisms. Energy from foodstuffs is used to con-
vert ADP back to ATP. Hence ATP is often called the
energy currency
of the cell; energy can be “saved” or
“spent” using ATP as an exchange currency.
Two types of energy production are present in the
cell: the
anaerobic
(i.e., without oxygen) glycolytic path-
way, occurring in the cytoplasm, and the
aerobic
(i.e.,
with oxygen) pathway, occurring in the mitochondria.
The glycolytic pathway serves as the prelude to the aero-
bic pathway.
Anaerobic Metabolism
Glycolysis is the anaerobic process by which energy
is liberated from glucose. It is an important source of
energy for cells that lack mitochondria. The process also
provides a temporary source of energy for cells that are
deprived of an adequate supply of oxygen. Glycolysis
involves a sequence of reactions that convert glucose to
pyruvic acid, with the concomitant production of ATP
from ADP. The net gain of energy from the glycolytic
metabolism of one molecule of glucose is two ATP mol-
ecules. Although relatively inefficient as to energy yield,
the glycolytic pathway is important during periods of
decreased oxygen delivery, such as occurs in skeletal
muscle during the first few minutes of exercise.
Glycolysis requires the presence of nicotinamide
adenine dinucleotide (NAD
+
), a hydrogen carrier. The
end products of glycolysis are pyruvate and NADH (the
reduced form of NAD
+
) plus H
+
. When oxygen is pres-
ent, pyruvic acid moves into the aerobic mitochondrial
pathway, and NAD
+
is regenerated as NADH delivers
its electron and proton (H
+
) to the oxidative electron
transport system. Under anaerobic conditions, such as
cardiac arrest or circulatory shock, pyruvic acid is con-
verted to lactic acid, which diffuses out of the cells into
the extracellular fluid. Conversion of pyruvate to lactic
acid is reversible, and once the oxygen supply has been
restored, lactic acid is converted back to pyruvic acid
and used directly for energy or to synthesize glucose.
Aerobic Metabolism
Aerobic metabolism, which supplies 90% of the body’s
energy needs, occurs in the cell’smitochondria and requires
oxygen. It is here that the hydrogen and carbon molecules
from dietary fats, proteins, and carbohydrates are broken
down and combined with molecular oxygen to form car-
bon dioxide and water as energy is released. Unlike lactic
acid, which is an end product of anaerobic metabolism,
carbon dioxide and water are relatively harmless and eas-
ily eliminated from the body. In a 24-hour period, oxida-
tive metabolism produces 150 to 300 mL of water.
The citric acid cycle, sometimes called the
tricar-
boxylic acid
(TCA) or
Krebs
cycle, provides the final
common pathway for the metabolism of nutrients. In
the citric acid cycle, which takes place in the matrix of
the mitochondria, an activated two-carbon molecule
of acetyl-coenzyme A (acetyl-CoA) condenses with a
four-carbon molecule of oxaloacetic acid and moves
through a series of enzyme-mediated steps. This pro-
cess produces hydrogen atoms and carbon dioxide.
As hydrogen is generated, it combines with NAD
+
or flavin adenine dinucleotide (FAD) for transfer to
the electron transport system. Besides pyruvate from
the glycolysis of glucose, products of amino acid and
fatty acid degradation enter the citric acid cycle and
contribute to the generation of ATP.
Oxidation of electrons from the hydrogen atoms
generated during glycolysis and the citric acid cycle
takes place in the electron transport system located
on the inner mitochondrial membrane. The elec-
trons are used to reduce elemental oxygen, which
combines with hydrogen to form water. During this
sequence of oxidative reactions, large amounts of
energy are released and used to convert ADP to ATP.
Because the formation of ATP involves the addition of
a high-energy phosphate bond to ADP, the process is
called
oxidative phosphorylation
.
FIGURE 1-8.
Adenosine triphosphate (ATP) is the major source of cellular energy.
(A)
Each molecule
of ATP contains two high-energy bonds, each containing about 12 kcal of potential energy.
(B)
The
high-energy ATP bonds are in constant flux.They are generated by substrate (glucose, amino acid,
and fat) metabolism and are consumed as the energy is expended. ADP, adenosine diphosphate.
A
CH
2
NH
2
N
N
H
H
H
N
OH OH
O -
H
O
N
H
H
Adenine
Ribose sugar
O -
OP
O
O -
O -
OP
O
OP
O
High-energy bonds
A P P
A P P
P
ATP
ADP
Energy used
Nutrient
metabolism
B
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