Final answer:
Increased ATP concentration relative to ADP decreases the activity of both phosphofructokinase and pyruvate kinase, slows down the citric acid cycle by decreasing the activity of isocitrate dehydrogenase, and causes the electron transport chain to slow down. The correct answer is that increased activity of pyruvate kinase does not occur as ATP concentration increases; instead, it decreases.
Step-by-step explanation:
The question asks which of the following events does not occur as ATP concentration increases relative to ADP. As ATP concentration increases, it indicates that the cell has sufficient energy, and this leads to various regulatory effects on metabolic pathways. Starting with option a, the activity of phosphofructokinase, an important enzyme in glycolysis, decreases as ATP levels rise. This is a feedback mechanism where high levels of ATP act as an inhibitor to prevent the unnecessary breakdown of glucose when energy is ample. Moving to option b, pyruvate kinase activity also decreases as ATP concentration increases, since it is negatively regulated by high energy indictors in the cell such as ATP and acetyl-CoA. Regarding option c, the decreased activity of isocitrate dehydrogenase does indeed occur due to high ATP levels as it is an enzyme involved in the citric acid cycle, which is down-regulated when energy is not required. Lastly, option d mentions the slowdown of the electron transport chain, which occurs when ATP concentration rises leading to a lower requirement for ATP synthesis, hence it slows down. Therefore, the one event that does not occur as ATP concentration increases relative to ADP is increased activity of pyruvate kinase; instead, the enzyme's activity decreases.