HUMAN BIOLOGY

Third stage of aerobic cellular respiration

Most ATP is produced in the third stage of the aerobic path-

way. Electron transport systems and neighboring proteins

called ATP synthases serve as the production machinery.

They are embedded in the inner membrane that divides

the mitochondrion into two compartments (Figure A.7).

They interact with electrons and H+ ions, which coenzymes

deliver from reaction sites of the first two stages of the aero-

bic pathway. Typically, a cell harvests thirty-six ATP mol-

ecules for each molecule of glucose that enters the aerobic

pathway (Figure A.8).

Figure A.7 Electron transport phosphorylation, the third and final stage of aerobic respiration. (© Cengage Learning)

H+ H+ H+ H+ H+ H+ H+ H+

H+

H H H+

e–

ADP + Pi

oxygen (O 2 )

ATP

e–

NADH

FADH 2

O

O

O

Electron Transfer Phosphorylation

outer membrane

cytoplasm

matrix

inner membrane

intermembrane space

1

2

3

4

5

1

2

3

4

5

NADH and FADH 2 deliver electrons to

electron transfer chains in the inner

mitochondrial membrane.

Electron flow through the chains causes

hydrogen ions (H+) to be pumped from the

matrix to the intermembrane space.

The activity of the electron transfer chains

causes a hydrogen ion gradient to form across

the inner mitochondrial membrane.

Hydrogen ions flow back to the matrix

through ATP synthases, which drives the

formation of ATP from ADP and phosphate (Pi).

Oxygen (O 2 ) accepts electrons and hydrogen

ions at the end of mitochondrial electron transfer

chains, so water forms.

Electron Transfer

Phosphorylation

glucose

2 pyruvate

2 acetyl–CoA

2 NADH

6 NADH

2 FADH 2

4 CO 2

oxygen

2 CO 2

(2 net)

cytoplasm

outer membrane

intermembrane space

inner membrane

matrix

2

32

4

2

2 NADH

2 NADH

2 NAD+

e–

e–

Glycolysis

Krebs

Cycle

ATP

ATP

ATP

ATP

ATP

ATP

ATP

H+H+ H+H+H+H+

A First stage: Glucose is

converted to two

pyruvate; two NADH and

four ATP form. An energy

investment of two ATP

began the reactions, so

the net yield is two ATP.

B Second stage:

ten more

coenzymes

accept electrons

and hydrogen ions

during the

second-stage

reactions. All six

carbons of

glucose leave the

cell (as six O 2 ),

and two ATP form.

C Third stage:

Coenzymes that

were reduced in the

first two stages give

up electrons and

hydrogen ions to

electron transfer

chains. Energy lost

by the electrons as

they flow through

the chains is used

to move H+ across

the membrane. The

resulting gradient

causes H+ to flow through

ATP synthases, driving

synthesis of ATP.

A-6 Appendix i

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