Multiple Choice
In what organelle do the Krebs cycle and the electron transport chain take place?
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8. Respiration
Krebs Cycle
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The citric acid cycle is directly regulated by which of the following?
A
The availability of NAD$^+$ and ADP
B
The amount of glucose in the cytoplasm
C
The presence of ribosomes in the cell
D
The concentration of oxygen in the mitochondria
Verified step by step guidance1
Understand the citric acid cycle (also known as the Krebs cycle or TCA cycle): It is a series of chemical reactions that occur in the mitochondria to generate energy in the form of ATP, NADH, and FADH$_2$. It is tightly regulated to ensure efficient energy production.
Identify the key molecules involved in the citric acid cycle: NAD$^+$ and ADP are essential for the cycle to proceed. NAD$^+$ is required for oxidation reactions, while ADP signals the need for ATP production.
Eliminate irrelevant options: The amount of glucose in the cytoplasm is not directly related to the citric acid cycle, as glucose is processed in glycolysis before entering the cycle. Ribosomes are involved in protein synthesis, not energy production. Oxygen concentration affects oxidative phosphorylation but does not directly regulate the citric acid cycle.
Focus on the correct regulatory factors: The availability of NAD$^+$ and ADP directly influences the activity of enzymes in the citric acid cycle, such as isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. These enzymes are activated when NAD$^+$ and ADP levels are high, signaling a need for energy production.
Conclude that the correct answer is: The availability of NAD$^+$ and ADP, as these molecules are directly involved in regulating the citric acid cycle to meet the cell's energy demands.
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