Biology (Holt)

Production of ATP

The total amount of ATP that a cell is able to harvest from each glucose

molecule that enters glycolysis depends on the presence or absence of

oxygen. As shown in Figure 16,cells use energy most efficiently when

oxygen is present. In the first stage of cellular respiration, glucose is

broken down to pyruvate during glycolysis. Glycolysis is an anaerobic

process, and it results in a net gain of two ATP molecules. In the sec-

ond stage of cellular respiration, the pyruvate passes through either

aerobic respiration or (anaerobic) fermentation. When oxygen is

present, aerobic respiration occurs. When oxygen is not present,

fermentation occurs instead. The NADthat gets recycled during fer-

mentation allows glycolysis to continue producing ATP. Thus, a small

amount of ATP is produced even during fermentation. Most of a cell’s

ATP is made, however, during aerobic respiration. For each molecule

of glucose that is broken down, as many as two ATP molecules are

made directly during the Krebs cycle, and up to 34 ATP molecules

are produced later by the electron transport chain.

110 CHAPTER 5Photosynthesis and Cellular Respiration

Most ATP is produced during aerobic respiration.

Figure 16 Effect of oxygen on ATP production

Anaerobic processes Aerobic respiration

Glucose

Pyruvate

Lactate and COEthanol 2 (Up to)

Without O 2 With O 2

Glycolysis

Fermentation Krebs cycle

Electron

transport

chain

(Net) 2 ATP

2 ATP

34 ATP

Listthe products of glycolysis. What is the

role of each of these products in cellular

respiration? 4B

Summarizethe roles of the Krebs cycle and

the electron transport chain during aerobic

respiration. 4B

Describethe role of fermentation in the second

stage of cellular respiration. 4B

Critical ThinkingComparing Functions

Explain why cellular respiration is more efficient

when oxygen is present in cells. 4B

Critical ThinkingInferring Conclusions

Excess glucose in your blood is stored in your

liver as glycogen. How might your body senses

when to convert glucose to glycogen and glyco-

gen back to glucose?

When oxygen is present,

most of the ATP made in cellular respiration is

produced by 9B

Aaerobic respiration.Calcoholic fermentation.

Bglycolysis. Dlactic acid fermentation.

TAKS Test PrepTAKS Test Prep

Section 3 Review

4B