AP BIOLOGY

Detailed explanation-1: -The Krebs cycle starts with pyruvic acid from glycolysis. Each small circle in the diagram represents one carbon atom. For example, citric acid is a six carbon molecule, and OAA (oxaloacetate) is a four carbon molecule.

Detailed explanation-2: -Answer and Explanation: Oxaloacetate is one of the essential intermediates of the Krebs cycle. It results in the formation of the citrate after reacting with acetyl-CoA. Citrate synthase plays a significant role in catalyzing carrying out this reaction.

Detailed explanation-3: -At the end of the Krebs cycle, the final product is oxaloacetic acid. This is identical to the oxaloacetic acid that begins the cycle. Now the molecule is ready to accept another acetyl-CoA molecule to begin another turn of the cycle.

Detailed explanation-4: -Throughout the citric acid cycle, oxaloacetate is progressively transformed into several different molecules (as carbon atoms are added to and removed from it), but at the end of the cycle it always turns back into oxaloacetate to be used again.