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The first step of the citric acid cycle joins a two-carbon acetyl group to a four-carbon molecule, forming:
AMalate
BFumarate
CCitrate
DSuccinate
Answer & Solution
Correct answer: C. Citrate
1. The cycle opens with a condensation step.
2. Two carbons arrive on acetyl CoA and four are already present in the cycle.
3. This is a condensation step, combining the two-carbon acetyl group with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate.
4. This step is irreversible because it is highly exergonic.
5. Malate, fumarate and succinate all appear later in the same turn.
_Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 7 'Cellular Respiration', sections 7.1-7.7_
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