The Totals

How much ATP does cellular respiration generate?

Although glycolysis nets just 2 ATP molecules per molecule of glucose, in the presence of oxygen, everything changes. Together, glycolysis, the Krebs cycle, and the electron transport chain release about 36 molecules of ATP per molecule of glucose. Notice in Figure 9–7 that under aerobic conditions these pathways enable the cell to produce 18 times as much energy as can be generated by anaerobic glycolysis alone (roughly 36 ATP molecules per glucose molecule versus just 2 ATP molecules in glycolysis).

Our diets contain much more than just glucose, of course, but that's no problem for the cell. Complex carbohydrates are broken down to simple sugars like glucose. Lipids and proteins can be broken down into molecules that enter the Krebs cycle or glycolysis at one of several places. Like a furnace that can burn oil, gas, or wood, the cell can generate chemical energy in the form of ATP from just about any source.

How efficient is cellular respiration? The 36 ATP molecules generated represent about 36 percent of the total energy of glucose. That might not seem like much, but it means that the cell is actually more efficient at using food than the engine of a typical automobile is at burning gasoline. What happens to the remaining 64 percent? It is released as heat, which is one of the reasons your body feels warmer after vigorous exercise, and why your body temperature remains 37˚C day and night.

FIGURE 9–7 Energy Totals The complete breakdown of glucose through cellular respiration results in the production of 36 molecules of ATP. Calculate How many times more energy is produced by all three stages of cellular respiration than by glycolysis alone?

d

9.2 Assessment

Review Key Concepts

1. 1. Review What are the products of glycolysis?

   2. Compare and Contrast How is the function of NAD⁺ similar to that of NADP⁺?

2. 1. Review What happens to pyruvic acid in the Krebs cycle?

   2. Interpret Visuals Look at Figure 9–5 and list the products of the Krebs cycle. What happens to each of these products?

3. 1. Review How does the electron transport chain use the high-energy electrons from glycolysis and the Krebs cycle?

   2. Relate Cause and Effect How does the cell use the charge differences that build up across the inner mitochondrial membrane during cellular respiration?

4. 1. Review How many molecules of ATP are produced in the entire breakdown of glucose?

   2. Use Analogies How is the cell like a furnace?

Apply the Big idea

Cellular Basis of Life

5. As you have learned, cellular respiration is a process by which cells transform energy stored in the bonds of food molecules into the bonds of ATP. What does the body do with all of the ATP this process generates? Review the characteristics of life in Chapter 1 and explain why ATP is necessary for each life process.

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