Human Physiology, 14th edition (2016)

Cell Respiration and Metabolism 113

space for every pair of electrons moved along the electron-

transport system. The second pump (the cytochrome c reduc-

tase complex) also transports 4 protons into the intermembrane

space, and the third pump (the cytochrome c oxidase complex)

transports 2 protons into the intermembrane space. As a result,

there is a higher concentration of H^1 in the intermembrane

space than in the matrix, favoring the diffusion of H^1 back out

into the matrix. The inner mitochondrial membrane, however,

does not permit diffusion of H^1 , except through the fourth

respiratory complex of proteins.

This last respiratory complex is a molecular machine, lik-

ened to a rotary turbine, that functions as an ATP synthase.

It contains a channel that permits the passage of H^1 from the

intermembrane space to the matrix, and it is this movement of

Coupling of Electron Transport

to ATP Production

According to the chemiosmotic theory, the electron-transport
system, powered by the transport of electrons, pumps protons
(H^1 ) from the mitochondrial matrix into the space between
the inner and outer mitochondrial membranes. The electron
transport system consists of four respiratory complexes com-
posed of about 90 proteins. Mitochondrial DNA codes for 13
of these proteins, and nuclear DNA codes for the rest. The
first three of these respiratory complexes serve as proton
pumps ( fig. 5.9 ).
The first pump (the NADH-coenzyme Q reductase com-
plex) transports 4 H^1 from the matrix to the intermembrane

Figure 5.7 The complete citric acid cycle. Notice that, for each “turn” of the cycle, 1 ATP, 3 NADH, and 1 FADH 2 are
produced.

FAD

FADH 2

AT P

NADH

NAD

NAD

NADH

GTP GDP

2H

2H

2H

+ H+

+ H+

Oxaloacetic acid (C 4 )

Acetyl CoA (C 2 )

Malic acid (C 4 )

Fumaric acid (C 4 )

Succinic acid (C 4 )

Citric acid (C 6 )

Isocitric acid (C 6 )

훂-Ketoglutaric acid (C 5 )

cis-Aconitic acid (C 6 )

NADH

NAD

2H

+ H+

1

2

3

4

5

6

7

8

HCCSCoA+ H 2 O

HS CoA

HO

H

H

H

H

HOOC H

H

C

C

OH

COOH

COOH

COOH

HH

C

C

O

COOH

COOH

C

C

H

HH

C

C

H

COOH

COOH

H

HO COOH

C

C

H

COOH

HCH

COOH

H

H COOH

C

C

H

COOH

HCOH

COOH

H

H H

C

C

H

COOH

CO

COOH

H

COOH

C

C

H

COOH

CH

COOH

H 2 O

H 2 O

H 2 O

H 2 O

CO 2

CO 2

H 2 O

ADP