Question

When an organism that normally does aerobic respiration runs out of oxygen, the process of making ATP will usually continue for a short time, and then will stop both making ATP and stop using Acetyl CoA in the preceding cycle of reaction. What is the molecular mechanism involved that stops both of these sets of reactions, and what does the organism do with the pyruvate?

a) It undergoes lactic acid fermentation, and pyruvate is converted to lactic acid.
b) It undergoes alcoholic fermentation, and pyruvate is converted to ethanol.
c) Pyruvate is converted to acetic acid and enters the Krebs cycle.
d) The organism goes into hibernation.

Answer 1

When an organism lacking oxygen switches from aerobic to anaerobic respiration, it undergoes lactic acid fermentation, where pyruvate is converted to lactic acid, regenerating the NAD+ needed for glycolysis. The correct answer is (a) lactic acid fermentation occurs.

Step-by-step explanation:

When an organism that normally performs aerobic respiration runs out of oxygen, it shifts to a process known as anaerobic respiration or fermentation. In this scenario, the process of making ATP will usually continue for a short time via glycolysis, but it will stop using Acetyl CoA because the Citric Acid Cycle and Oxidative Phosphorylation cannot occur without oxygen. The molecular mechanism involved in the halting of these processes is the lack of oxygen, which is the final electron acceptor in aerobic respiration. Instead, the cell converts pyruvate into either lactic acid or ethanol, depending on the type of fermentation that occurs.

In the case of humans, when oxygen levels are not sufficient, such as during intense exercise, the cells will undergo lactic acid fermentation. Pyruvate, the end product of glycolysis, is converted to lactic acid. This conversion allows for the regeneration of NAD+ from NADH, which is needed for glycolysis to continue, albeit much less efficiently than aerobic respiration. The correct answer to the question is (a) It undergoes lactic acid fermentation, and pyruvate is converted to lactic acid.