Video Transcript
In the link reaction, pyruvate is converted into acetyl-CoA. During this process, what happens to the coenzyme NAD? (A) It is oxidized. (B) It is hydrolyzed. (C) It is phosphorylated. (D) It is reduced.
Cellular respiration is an important process in all living organisms. It is the process in which glucose is broken down to release energy that is then
stored in molecules of ATP. There are four main steps to cellular respiration: glycolysis; the link reaction; the
citric acid cycle, or Krebs cycle; and oxidative phosphorylation, or the electron
transport chain.
For this question, we will look specifically at the link reaction, which happens in
the mitochondria. This is an aerobic reaction, which means it requires oxygen to proceed. Before the link reaction can begin, glycolysis takes place. Here, one molecule of glucose is converted into two molecules of ATP, two molecules
of NADH, and two molecules of pyruvate. In the link reaction, these pyruvate molecules are converted into molecules of acetyl
coenzyme A, or acetyl-CoA for short. Acetyl-CoA can then be used for the citric acid cycle, which is the next step in
cellular respiration.
Let’s look at the link reaction in more detail to see what happens to NAD. Pyruvate is a three-carbon molecule. During the link reaction, pyruvate is decarboxylated, meaning it loses carbon
dioxide, so it becomes a two-carbon molecule. During this reaction, this two-carbon molecule is joined with another molecule called
coenzyme A to make acetyl-CoA. Also during this reaction, a molecule of NAD+ is reduced to form NADH.
Therefore, to go back to our question, the option that best describes what happens to
NAD during the link reaction is given by answer choice (D). It is reduced.
