Biology 12

76 MHR • Unit 1 Metabolic Processes

2

2

6

2

Krebs

cycle

4

cytoplasm

mitochondrion

ATP Yield 4

4 ATP

6 ATP

18 ATP

4 ATP

2 ATP

32

36 ATP

electron transport chain

2 ATP

NADH

NADH

NADH

glucose

2 pyruvate

glycolysis

2 acetyl-CoA

ATP + ATP

O 2

CO 2

2 CO 2

H 2 O

FADH 2

Figure 3.12Number of ATP

molecules produced for each

molecule of glucose used in

aerobic cellular respiration

The charge difference creates an electrical gradient,

while the concentration difference creates a

chemical gradient. The H+ions can leave the

intermembrane space through a special channel in

the ATP synthase complex, as shown in Figure 3.11.

Chemiosmosis, the movement of these H+ions,

creates an electric current of positively charged

particles. This current provides the energy needed

to phosphorylate ADP with inorganic phosphate

ions in the matrix, forming ATP.

The multienzyme ATP synthase complex is not

part of the electron transport chain. The electrons

moving along the electron transport chain are not

directly involved in the production of ATP. These

electrons activate the proton pumps that move

the H+ions into the intermembrane space. ATP

synthase complex is both a collection of enzymes

that phosphorylate ADP, and a tunnel that allows

the H+ions to move from the intermembrane space

back into the matrix. This flow of charged particles

provides energy that is used by the enzymes to

phosphorylate ADP, and release H+ions back into

the matrix for other reactions. These reactions

include the formation of water at the end of the

electron transport chain. This process produces

about 90 percent of the ATP molecules in a cell.

Most ATP production takes place during the

reactions of the electron transport chain and

chemiosmosis, because of the input of NADH and

FADH 2 molecules (see Figure 3.12). NADH that is

produced by glycolysis in the cytosol can donate

electrons to the electron transport chain by way of

the outer membrane of the mitochondrion. This

additional step has a cost, however, and the

electrons from the cytosol NADH can only pump

enough protons for two ATP. In contrast, the NADH

molecules that are produced in the matrix can

produce three ATP.

Substrate-level phosphorylation during glycolysis

and the Krebs cycle produces four ATP for

each molecule of glucose oxidized. Oxidative

phosphorylation from the electron transport chain

and chemiosmosis accounts for the remaining

32 ATP molecules. The total number of ATP

molecules produced from one molecule of glucose

is 36, as shown in

Figure 3.12. The concept

organizer, shown in

Figure 3.13 on the next

page, summarizes the

important concepts

you have learned that

are involved in ATP

synthesis. Refer to the

chapters and sections

listed to review these

concepts.

http://www.mcgrawhill.ca/links/biology12

Since Boyer and Walker’s work on ATP synthase, biochemists

worldwide have been studying the workings of ATP synthase

complex. To find out more about the intricacies of ATP

synthase, go to the web site above, and click on Web Links.

Prepare an abstract about research into one aspect of the

ATP synthase complex.

WEB LINK