Human Physiology, 14th edition (2016)

Cell Respiration and Metabolism 111

The aerobic respiration of glucose (C 6 H 12 O 6 ) is given in the
following overall equation:

C 6 H 12 O 6 1 O 2 → 6 CO 2 1 6 H 2 O
Aerobic respiration is equivalent to combustion in
terms of its final products (CO 2 and H 2 O) and in terms of

the total amount of energy liberated. In aerobic respiration,

however, the energy is released in small, enzymatically con-

trolled oxidation reactions, and a portion (38% to 40%) of

the energy released is captured in the high-energy bonds

o f AT P.

The aerobic respiration of glucose begins with glycolysis.

Glycolysis in both anaerobic metabolism and aerobic respira-

tion results in the production of 2 molecules of pyruvic acid,

2 ATP, and 2 NADH 1 H^1 per glucose molecule. In aerobic

respiration, however, the electrons in NADH are not donated

to pyruvic acid and lactic acid is not formed, as happens in the

lactic acid pathway. Instead, the pyruvic acids will move to a

different cellular location and undergo a different reaction; the

NADH produced by glycolysis will eventually be oxidized, but

that occurs later in the story.

In aerobic respiration, pyruvic acid leaves the cell cyto-

plasm and enters the interior (the matrix) of mitochondria.

Once pyruvic acid is inside a mitochondrion, carbon dioxide

is enzymatically removed from each three-carbon-long pyru-

vic acid to form a two-carbon-long organic acid—acetic acid.

The enzyme that catalyzes this reaction combines the acetic

acid with a coenzyme (derived from the vitamin pantothenic

acid) called coenzyme A. The combination thus produced is

called acetyl coenzyme A, abbreviated acetyl CoA ( fig. 5.5 ).

Glycolysis converts 1 glucose molecule into 2 molecules

of pyruvic acid. Since each pyruvic acid molecule is con-

verted into 1 molecule of acetyl CoA and 1 CO 2 , 2 molecules

of acetyl CoA and 2 molecules of CO 2 are derived from each

glucose. These acetyl CoA molecules serve as substrates for

mitochondrial enzymes in the aerobic pathway, while the car-

bon dioxide is carried by the blood to the lungs for elimination.

It is important to note that the oxygen in CO 2 is derived from

pyruvic acid, not from oxygen gas.

Citric Acid Cycle

Once acetyl CoA has been formed, the acetic acid subunit

(2 carbons long) combines with oxaloacetic acid (4 carbons

long) to form a molecule of citric acid (6 carbons long).

5.2 Aerobic Respiration

In the aerobic respiration of glucose, pyruvic acid is formed

by glycolysis and then converted into acetyl coenzyme A.

This begins a cyclic metabolic pathway called the citric

acid (Krebs) cycle. As a result of these pathways, a large

amount of reduced NAD and FAD (NADH and FADH 2 ) is

generated. These reduced coenzymes provide electrons

for a process that drives the formation of ATP.

Clinical Investigation CLUES

Andrea experienced muscle pain and fatigue during

exercise, and later experienced chest pain that required

medical attention.

• What caused muscle pain during exercise, and why
  did the trainer say it was normal?


• What might her chest pain indicate, how might it
  have been produced, and why did Andrea need
  medical attention?

| CHECKPOINT

1a. Define the term glycolysis in terms of its initial

substrates and products. Explain why there is a net

gain of 2 molecules of ATP in this process.

1b. What are the initial substrates and final products of

anaerobic metabolism?

1c. Describe the physiological functions of lactic

acid fermentation. In which tissue(s) is anaerobic

metabolism normal? In which tissue is it abnormal?

LEARNING OUTCOMES

After studying this section, you should be able to:

2. Describe the aerobic cell respiration of glucose
   through the citric acid cycle.


3. Describe the electron transport system and oxidative
   phosphorylation, explaining the role of oxygen in this
   process.

Figure 5.5 The formation of acetyl coenzyme A in

aerobic respiration. Notice that NAD is reduced to NADH in

this process.

Pyruvic acid

+

OHO

CH H

H

C O

C

CH H

H

C O +CO 2

S CoA

Coenzyme A Acetyl coenzyme A

S CoA

H

NAD NADH + H+