HUMAN BIOLOGY

2 ATP produced

by substrate-level

phosphorylation

2 ADP

2 NAD+ + 2 Pi

2 reduced coenzymes

NADH

ADP

ADP

glucose

glucose-6-phosphate

fructose-1,6-bisphosphate

2 PEP

2 pyruvate

to second stage

2 PGA

2 PGAL

2 ADP

2 ATP produced

by substrate-level

phosphorylation

Net 2 ATP + 2 NADH

Glycolysis

ATP

ATP

ATP

ATP

P

P P

P

P

P P

PP

P

P

Second stage of the aerobic pathway

When pyruvate molecules formed by glycolysis leave

the cytoplasm and enter a mitochondrion, the scene

is set for both the second and the third stages of the aerobic

pathway. Figure A.6 diagrams these steps in detail.

Figure A.6 Second stage of aerobic respiration: the Krebs cycle and reaction steps that precede it. For each three-carbon pyruvate

molecule entering the cycle, three CO 2 , one ATP, four NADH, and one FADH 2 molecules form. The steps shown proceed twice, because

each glucose molecule was broken down earlier to two pyruvate molecules. (© Cengage Learning)

b The Krebs cycle starts as

one carbon atom is transferred

from acetyl–CoA to oxaloace-

tate. Citrate forms, and coen-

zyme A is regenerated.

c A carbon atom is removed

from an intermediate and

leaves the cell as CO 2. NAD+

combines with released

hydrogen ions and electrons,

forming NADH.

d A carbon atom is removed

from another intermediate and

leaves the cell as CO 2 , and

another NADH forms.

In summary, pyruvate’s three

carbon atoms have now

exited the cell, in CO 2.

F The coenzyme FAD com-

bines with hydrogen ions and

electrons, forming FADH 2.

A An enzyme splits a pyru-

vate molecule into a two-

carbon acetyl group and CO 2.

Coenzyme A binds the acetyl

group (forming acetyl–CoA).

NAD+ combines with released

hydrogen ions and electrons,

forming NADH.

E One ATP forms by substrate-

level phosphorylation.

Acetyl–CoA

Formation

coenzyme A

pyruvate

acetyl–CoA

CO 2

NADH

NAD+

Krebs

Cycle

coenzyme A

citrate

CO 2

oxaloacetate

NAD+

NADH

NAD+

NADH

FADH 2

NADH

FAD

NAD+

ADP + Pi

CO 2

ATP

G NAD+ combines with

hydrogen ions and elec-

trons, forming NADH.

H The final steps of the

Krebs cycle regenerate

oxaloacetate.

Appendix i A-5

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