the details of each stage are beyond the scope of this
book, we will summarize the most important aspects
of each, and then relate to them the use of amino acids
and fats for energy. This simple summary is depicted
in Fig. 17–3.
Glycolysis
The enzymes for the reactions of glycolysisare found
in the cytoplasm of cells, and oxygen is not required
(glycolysis is an anaerobic process). Refer now to Fig.
17–3 as you read the following. In glycolysis, a six-
carbon glucose molecule is broken down to two three-
carbon molecules of pyruvic acid. Two molecules of
ATP are necessary to start the process. The energy
they supply is called energy of activation and is neces-
sary to make glucose unstable enough to begin to break
down. As a result of these reactions, enough energy is
released to synthesize four molecules of ATP, for a net
gain of two ATP molecules per glucose molecule. Also
during glycolysis, two pairs of hydrogens are removed
by NAD, a carrier molecule that contains the vitamin
niacin. Two NAD molecules thus become 2NADH 2 ,
and these attached hydrogen pairs will be transported
to the cytochrome transport system (stage 3).
If no oxygen is present in the cell, as may happen in
muscle cells during exercise, pyruvic acid is converted
to lactic acid, which causes muscle fatigue. If oxygen is
present, however, pyruvic acid continues into the next
stage, the Krebs citric acid cycle (or, more simply, the
Krebs cycle).
Krebs Citric Acid Cycle
The enzymes for the Krebs cycle (or citric acid
cycle) are located in the mitochondria of cells. This
second stage of cell respiration is aerobic, meaning
that oxygen is required. In a series of reactions, a pyru-
vic acid molecule is “taken apart,” and its carbons are
converted to CO 2. The first CO 2 molecule is removed
by an enzyme that contains the vitamin thiamine.
This leaves a two-carbon molecule called an acetyl
group, which combines with a molecule called coen-
zyme A to form acetyl coenzyme A (acetyl CoA). As
acetyl CoA continues in the Krebs cycle, two more
carbons are removed as CO 2 , and more pairs of hydro-
gens are picked up by NAD and FAD (another carrier
molecule that contains the vitamin riboflavin).
NADH 2 and FADH 2 will carry their hydrogens to the
cytochrome transport system.
During the Krebs cycle, a small amount of energy
is released, enough to synthesize one molecule of ATP
(two per glucose). Notice also that a four-carbon mol-
ecule (oxaloacetic acid) is regenerated after the forma-
tion of CO 2. This molecule will react with the next
acetyl CoA, which is what makes the Krebs cycle truly
a self-perpetuating cycle. The results of the stages of
cell respiration are listed in Table 17–3. Before you
continue, you may wish to look at that table to see just
where the process has gotten thus far.Cytochrome Transport System
Cytochromesare proteins that contain either ironor
copperand are found in the mitochondria of cells.
The pairs of hydrogens that were once part of glucose
are brought to the cytochromes by the carrier mole-
cules NAD and FAD. Each hydrogen atom is then
split into its proton (H+ion) and its electron. The
electrons of the hydrogens are passed from one
cytochrome to the next, and finally to oxygen. The
reactions of the electrons with the cytochromes
release most of the energy that was contained in the
glucose molecule, enough to synthesize 34 molecules
of ATP. As you can see, most of the ATP produced in
cell respiration comes from this third stage.
Finally, and very importantly, each oxygen atom
that has gained two electrons (from the cytochromes)
reacts with two of the H+ions (protons) to form water.
The formation of metabolic water contributes to the
necessary intracellular fluid, and also prevents acido-
sis. If H+ions accumulated, they would rapidly lower
the pH of the cell. This does not happen, however,
because the H+ions react with oxygen to form water,
and a decrease in pH is prevented.
The summary of the three stages of cell respiration
in Table 17–3 also includes the vitamins and minerals
that are essential for this process. An important over-
all concept is the relationship between eating and
breathing. Eating provides us with a potential energy
source (often glucose) and with necessary vitamins and
minerals. However, to release the energy from food,
we must breathe. This is whywe breathe. The oxygen
we inhale is essential for the completion of cell respi-
ration, and the CO 2 produced is exhaled.Proteins and Fats as Energy Sources
Although glucose is the preferred energy source for
cells, proteins and fats also contain potential energy
and are alternative energy sources in certain situations.
As you know, proteins are made of the smaller mol-
ecules called amino acids, and the primary use for the
amino acids we obtain from food is the synthesis ofBody Temperature and Metabolism 403