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Biology, 6/e
Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

How Cells Harvest Energy

Answers to Review Questions

Chapter 9 (p. 182)

1. An autotroph (such as a plant) is an organism that "feeds itself" by synthesizing its own organic molecules. Heterotrophs (such as animals) must consume their food.

2. Digestion breaks down organic macromolecules into their component amino acids, fatty acids, and monosaccharides. Catabolism goes farther to break the bonds occurring in these smaller molecules. The breaking of the bonds during catabolism generates more ATP and therefore provides more energy than digestion.

3. Glycolysis occurs in the cytoplasm of the cell. Four ATPs are produced, although the net production of ATP is two because two ATPs are used to mobilize glucose.

4. The two mechanisms that can convert NADH back to NAD+ are oxidative respiration and fermentation.

5. At the end of the electron transport chain, 36 molecules of ATP should be produced, theoretically. In actuality, closer to 30 molecules of ATP are generated due to "leaky" mitochondrial membranes and use of ATP to power other processes at the same time.

6. In carbohydrate catabolism, acetyl CoA is produced from the final product of glycolysis--pyruvate. The pyruvate is cleaved into CO2 and acetyl-CoA. Fatty acids from fat molecules are broken down within the mitochondrial matrix to two-carbon acetyl groups, which then bind to coenzyme A, forming acetyl-CoA in a process called b-oxidation. In both cases then, the newly-formed acetyl-CoA enters into the Krebs cycle.

7. Catabolism of fatty acids yields about 20% more ATP than the catabolism of glucose. Fat contains more energy, almost twice as much, on a per-weight basis than glucose.