Which one of the following substances is a part of both phases A and C of the citric acid cycle?

• A. Succinyl CoA
• B. Oxaloacetate
• C. Acetyl CoA
• D. Citrate

Answer: (B)

Verified step by step guidance

1. Understand the citric acid cycle: The citric acid cycle, also known as the Krebs cycle, is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins.
2. Identify the phases of the citric acid cycle: The cycle begins with the condensation of acetyl-CoA with oxaloacetate to form citrate (Phase A), and ends with the regeneration of oxaloacetate (Phase C).
3. Analyze the role of oxaloacetate: Oxaloacetate is a four-carbon molecule that combines with acetyl-CoA to form citrate at the start of the cycle and is regenerated at the end of the cycle, making it a part of both phases A and C.
4. Consider the other substances: Succinyl CoA, Acetyl CoA, and Citrate are intermediates or starting materials in the cycle, but they do not participate in both the beginning and end phases like oxaloacetate does.
5. Conclude that oxaloacetate is the substance involved in both phases A and C of the citric acid cycle, as it is both a reactant in the formation of citrate and a product regenerated at the end of the cycle.