Biology 12

NADH

reductase

coenzyme

Q

cytochrome

b,

cytochrome c

oxidase

cytochrome

c

ATP

reduction NADH

oxidation

reduction

oxidation

reduction

oxidation

reduction

oxidation

reduction

oxidation

2e−

2e−

2e−

2e−

2e−

2e−

2e− NAD++H+

H+

H+

ADP+

ATP

ADP+

ATP

ADP+

1

2 O^2

2H+

H 2 O

FAD+2H+

FADH 2

c 1

Pi

Pi

Pi

NADH and FADH 2 bring electrons

to the electron transport chain.

ATP is produced at the ATP

synthase complex, but the

diagram shows the power of the

proton pumps for chemiosmosis.

A

Each pair of electrons from

NADH pulls 3 pairs of H+

ions into the intermembrane

space to make 3 ATP with

ATP synthase.

B

Each pair of electrons from

FADH 2 pulls 2 pairs of H+

ions into the intermembrane

space to make 2 ATP with

ATP synthase.

C

Each of the electron carriers

becomes reduced and then

oxidized as the electrons

move down the chain.

D

As a pair of electrons is

passed from carrier to

carrier, proton pumps

carry H+ions into the

intermembrane space.

E

Oxygen is the final

acceptor of the electrons,

and together with hydrogen

becomes water and joins

the general water content

of the cell.

F

74 MHR • Unit 1 Metabolic Processes

catabolized. As Table 3.1 shows, by the end of

the Kreb’s cycle, energy contained in the original

molecule of glucose has been used to form four

ATP molecules and 12 electron carriers.

Figure 3.9 shows that succinate, the ionic form

of succinic acid, is oxidized to produce one

molecule of FADH 2. The enzyme that catalyzes this

reaction is succinic dehydrogenase. You will study

the action of this enzyme in Investigation 3-A.

Figure 3.10Overview of the electron transport chain

Chemiosmosis and

ATP Production

The final stage of energy transformation in

aerobic cellular respiration includes the

electron transport chain and oxidative

phosphorylation of ADP by chemiosmosis.

The reduced coenzymes NADH and FADH 2

shuttle electrons and H+ions from the

Krebs cycle in the matrix to the electron

transport chain embedded on the cristae,

the folds of the mitochondrion’s inner

membrane. The electron transport chain

involves a series of electron carriers and

multienzyme complexes. These carriers

and complexes oxidize NADH and FADH 2

molecules. With their extra electrons

removed, the additional H+ions also leave.

As a result, the oxidized molecules NAD+

and FAD can participate in a redox reaction

in the matrix, such as the Krebs cycle.

The oxidation and reduction of the

electron carriers in the electron transport

chain releases small amounts of energy.

This energy is then used to power proton

pumps that pull H+ions across the inner

membrane into the intermembrane space.

These H+ions are now trapped between

two membranes, building a concentration

gradient between the intermembrane space

and the matrix. The movement of these H+

ions through special channels drives the

production of ATP. (As you learned in the

previous section, this movement of H+ions

Metabolic

process

ATP

produced Energy molecules

glycolysis

oxidation

(decarboxylation)

of pyruvate (× 2)

Krebs cycle (× 2)

Total

2 ATP

2 ATP

4 ATP

2 NADH (in cytosol)

2 NADH

6 NADH 2 FAD

10 NADH 2 FAD

Table 3.1

The output of energy molecules up to the end

of the Krebs Cycle

H 2

H 2