The two molecules that enter the citric acid cycle are
A) two 3-carbon compounds and 38 ATPs.
B) a 4-carbon compound and an acetyl CoA.
C) a 2-carbon compound and 4-carbon acetyl CoA.
D) a pyruvic acid and a glucose.
E) a glucose and a fructose.

Question

Quizplus · Accepted Answer

The two molecules that enter the citric acid cycle are a 4-carbon compound (oxaloacetate) and an acetyl CoA molecule. These two molecules combine to form a 6-carbon compound (citrate), which begins the citric acid cycle. Choice A is incorrect because 3-carbon compounds are not the molecules that enter the citric acid cycle. Choice C is incorrect because acetyl CoA is already a 2-carbon compound, so it doesn't make sense to say that another 2-carbon compound enters the cycle. Choice D is incorrect because glucose and pyruvic acid are products of glycolysis, which occurs before the citric acid cycle. Choice E is incorrect because fructose is also a product of glycolysis and is not one of the molecules that enters the citric acid cycle.

The Two Molecules That Enter the Citric Acid Cycle Are