The Correct Answer and Explanation is:
The correct answer is acetyl-CoA.
The molecule called acetyl-CoA (acetyl coenzyme A) enters the citric acid cycle during the aerobic phase of cellular respiration.
Cellular respiration is the metabolic process that cells use to generate energy in the form of adenosine triphosphate (ATP) from nutrients like glucose. This process is broadly divided into several stages. It begins with glycolysis, which takes place in the cell’s cytoplasm and does not require oxygen. During glycolysis, a six-carbon glucose molecule is split into two three-carbon molecules called pyruvate.
For aerobic respiration to proceed, these pyruvate molecules must enter the mitochondria. Inside the mitochondrial matrix, pyruvate undergoes a crucial intermediate step called pyruvate oxidation, or the link reaction. In this step, each pyruvate molecule is converted into a two-carbon acetyl group. This process involves the removal of one carbon atom as carbon dioxide and the transfer of electrons to form NADH. The resulting two-carbon acetyl group is then attached to a carrier molecule, Coenzyme A, to form acetyl-CoA.
This acetyl-CoA molecule is the primary fuel for the citric acid cycle, also known as the Krebs cycle. It initiates the cycle by transferring its two-carbon acetyl group to a four-carbon molecule called oxaloacetate. This reaction forms a six-carbon molecule called citrate, which is the first intermediate of the cycle and the reason it is named the citric acid cycle. The cycle then proceeds through a series of reactions, regenerating oxaloacetate and producing ATP, NADH, and FADH2. Therefore, while pyruvate is the end product of glycolysis, it is the acetyl-CoA derived from it that serves as the direct entry point into the citric acid cycle.
