Video Transcript
As phosphoglyceraldehyde is
converted into pyruvate, what happens to the cofactor NAD? (A) It is oxidized. (B) It is broken down into its
component parts. (C) It is reduced. Or (D) it is phosphorylated.
Pyruvate is the product of
glycolysis, which is the first stage of cellular respiration. In order to answer this question,
let’s first review what happens in glycolysis.
In glycolysis, one molecule of
glucose, a six-carbon sugar, is broken down into two molecules of pyruvate. This reaction has a few steps. In the first step, two phosphate
molecules are added to glucose from ATP, which produces a phosphorylated sugar. In the next step of glycolysis,
this phosphorylated sugar is broken down into two three-carbon molecules, known as
phosphoglyceraldehyde, or G3P, which is mentioned in the question. These two molecules are then
converted into pyruvate, the final product of glycolysis, by removing a hydrogen,
adding a phosphate group, and then removing two phosphate groups from each G3P
molecule.
So what happens to the removed
hydrogen and phosphate? This final stage of glycolysis
requires two different cofactors, NAD+ and ADP. When a hydrogen is removed from the
G3P, this is known as a reduction–oxidation reaction. While NAD+ accepts a hydrogen ion
and so is reduced to form NADH, G3P is oxidized as it loses a hydrogen ion. Energy from this reaction allows a
second phosphate molecule to bind to each G3P molecule, which then phosphorylates
ADP, meaning that G3P loses two phosphates, forming two molecules of ATP and a
molecule of pyruvate.
The question is asking us to work
out what happens to the cofactor NAD in this final stage of glycolysis. As NAD has gained a positive
hydrogen ion, it has been reduced. So the correct answer to this
question as to what happens to the cofactor NAD when phosphoglyceraldehyde is
converted into pyruvate is (C). It is reduced.
