Multiple Choice
In one turn of the citric acid (Krebs) cycle per acetyl-CoA (not counting oxidative phosphorylation from NADH and FADH2), how many ATP equivalents are produced directly by substrate-level phosphorylation?
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Review 3: Pyruvate & Fatty Acid Oxidation, Citric Acid Cycle, & Glycogen Metabolism
Citric Acid Cycle 1
Multiple Choice
In the citric acid (Krebs) cycle, why is the pathway considered a “cycle”?
A
The last step regenerates oxaloacetate, the molecule that condenses with acetyl-CoA to begin the next turn of the pathway.
B
The cycle runs only when is directly used as a substrate in one of the steps, ensuring repetition.
C
All carbon atoms from acetyl-CoA are immediately released as in the first reaction, returning the pathway to its starting state.
D
The pathway alternates between cytosolic and mitochondrial compartments, creating a repeating loop of transport.
Verified step by step guidance1
Understand that a metabolic cycle is a series of biochemical reactions where the starting molecule is regenerated at the end, allowing the process to repeat continuously.
Identify the key molecule that initiates the citric acid cycle: oxaloacetate. This molecule condenses with acetyl-CoA to form citrate, the first step of the cycle.
Recognize that throughout the cycle, citrate undergoes a series of transformations, eventually leading to the regeneration of oxaloacetate in the last step.
Note that the regeneration of oxaloacetate is crucial because it allows the cycle to start again with a new acetyl-CoA molecule, maintaining the continuous nature of the pathway.
Understand that the cycle depends indirectly on oxygen because oxygen is the final electron acceptor in the electron transport chain, which regenerates NAD+ and FAD needed for the cycle, but oxygen itself is not a direct substrate in the citric acid cycle reactions.
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