Biology (Holt)

CCCCCC

CCCCC

CCCC CCCC

CCCC

C

C

BIO

graphic The Krebs cycle produces electron carriers that temporarily store chemical energy.

Krebs Cycle

CC

Acetyl-CoA

6-carbon

compound

5-carbon

compound

4-carbon

compound

4-carbon

compound

4-carbon

compound

CoA

NAD+

NADH + H+

NAD+

NAD+ NADH + H

+

NADH + H+

ADP +

AT P

FAD

FADH 2

CO 2

CO 2

1. Acetyl-CoA combines with a
   four-carbon compound,
   forming a six-carbon
   compound.
   2. CO 2 is released
   from the six-carbon
   compound, leaving a
   five-carbon compound.
   3. CO 2 is released from
   the five-carbon
   compound, leaving
   a four-carbon
   compound.


2. The four-carbon compound
   is converted to a new
   four-carbon compound.


3. The new four-carbon
   compound is converted
   to the four-carbon
   compound that began
   the cycle.

P

1 2

5

3

4

106 CHAPTER 5Photosynthesis and Cellular Respiration

Stage Two: Production of ATP

When oxygen is present, pyruvate produced during glycolysis enters

a mitochondrion and is converted to a two-carbon compound. This

reaction produces one carbon dioxide molecule, one NADH mole-

cule, and one two-carbon acetyl (uh SEET uhl) group. The acetyl

group is attached to a molecule called coenzyme A (CoA), forming a

compound called acetyl-CoA (uh SEET uhl-koh ay).

Krebs Cycle

Acetyl-CoA enters a series of enzyme-assisted reactions called the

,summarized in Figure 12.The cycle is named for the

biochemist Hans Krebs, who first described the cycle in 1937.

Step Acetyl-CoA combines with a four-carbon compound, form-

ing a six-carbon compound and releasing coenzyme A.

Step Carbon dioxide, CO 2 , is released from the six-carbon com-

pound, forming a five-carbon compound. Electrons are

transferred to NAD+, making a molecule of NADH.

Krebs cycle

Figure 12