Biology 12

Chapter 3 Cellular Energy • MHR 71

contains a sulfur-based functional group. This
electronegative sulfur-based functional group binds
to a carbon in the acetyl group to make the reactive
acetyl-CoA.
This product, acetyl-CoA, is a key reactant in the
next step in aerobic respiration, the Krebs cycle. At

this point, you have seen that carbohydrates provide

the energy for ATP production. Lipids and proteins

can be broken down to acetyl-CoA in the cell, and

produce ATP in the mitochondrion. If ATP levels

are high, acetyl-CoA can be directed into other

metabolic pathways, such as the production of fatty

pyruvate

ADP ATP

GTP GDP

NADH

NAD+

NADH

NAD+

NAD+

H++

H 2 O

FADH 2

FAD

NADH+H+

NAD+

NADH+H+

CO 2

CO 2

CO 2

C

CoA

C

S

CoA

C

C

CC

C

C

C

C

CC

C

C

C

C

C

C

C

C

CS

C

C

CoA

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

acetyl-CoA

citrate ( )C 6

aconitate

(intermediate)

isocitrate ( )C 6

α-ketoglutarate ( )C 5

succinyl-CoA ( )C 4

succinate ( )C 4

fumarate ( )C 4

malate ( )C 4

oxaloacetate ( )C 4

8

9 1 2 3 4 5

6

7

Figure 3.9The Krebs cycle, which includes nine (numbered) reactions, oxidizes
acetyl-CoA. Can you identify where substrate-level phosphorylation takes place?

The cycle starts when two-

carbon acetate and four-carbon

oxaloacetate react to form a

six-carbon molecule, citric acid.

A

Dehydrogenation of

malate forms a third

NADH, and the

cycle begins again.

E

Oxidation

of succinate

forms FADH 2.

D A second

oxidative

decarboxylation

forms another

NADH and CO 2.

C

Oxidative

decarboxylation

forms NADH

and CO 2.

B