BIOLOGY 101 - M02_REEC5174_09_IE_10_15. Questions and Answers

BI4. The cell’s supply of ADP, Pi, and NAD+ is finite (limited). What happens to cellular respiration when all of the cell’s NAD+ has been converted to NADH? If NAD is unavailable, the cell is unable to conduct any processes that involve the conversion of NAD+ to NADH. Because both glycolysis and the Krebs cycle produce NADH, both of these processes shut down when there is no available NAD+ . 5. If the Krebs cycle does not require oxygen, why does cellular respiration stop after glycolysis when no oxygen is present? When no oxygen is present, oxidative phosphorylation cannot occur. As a result, the NADH produced in glycolysis and the Krebs cycle cannot be oxidized to NAD+ . When no NAD+ is available, pyruvate cannot be converted to the acetyl CoA that is required for the Krebs cycle. 6. Many organisms can withstand periods of oxygen debt (anaerobic conditions). Yeast undergoing oxygen debt converts pyruvic acid to ethanol and carbon dioxide. Animals undergoing oxygen debt convert pyruvic acid to lactic acid. Pyruvic acid is fairly nontoxic in even high concentrations. Both ethanol and lactic acid are toxic in even moderate concentrations. Explain why this conversion occurs in organisms. As noted in question 4, when no NAD+ is available, even glycolysis stops. No ATP will be produced and the cell (or organism) will die. The conversion of pyruvic acid (pyruvate) to lactic acid (or ethanol) requires the input of NADH and generates NAD+ . This process, called fermentation, allows the cell to continue getting at least 2 ATP per glucose. 7. How efficient is fermentation? How efficient is cellular respiration? Remember that efficiency is the amount of useful energy (as ATP) gained during the process divided by the total amount of energy available in glucose. Use 686 kcal as the total energy available in 1 mole of glucose and 8 kcal as the energy available in 1 mol of ATP. Efficiency of fermentation Efficiency of aerobic respiration kcal/mole of ATP × 2 ATP = 16 kcal 16 kcal / 2 moles of ATP = 2.3% 686 kcal / mole of glucose 8 kcal/mole of ATP × 38 ATP (maximum) = 304 kcal 304 kcal / 38 moles of ATP = 44.3% 686 kcal / mole of glucose 8. a. Why can’t cells store large quantities of ATP? (Hint: Consider both the chemical stability of the molecule and the cell’s osmotic potential.) ATP is highly reactive at normal body temperatures and therefore difficult for cells to store for any period of time. (In the lab, ATP is usually stored at very low temperatures, for example, at -20°C.) In addition, ATP is a relatively small molecule. As a result, if cells could store high concentrations of ATP, their osmotic potential would change. This is also why cells don’t store glucose. The cells would become hypertonic to the fluid around them and could pick up enough water to burst. b. Given that cells can’t store ATP for long periods of time, how do they store energy?OLOGY 101 - M02_REEC5174_09_IE_10_15. Questions and Answers