Video Transcript
For each turn of the Krebs cycle,
how many molecules of CO2 are released?
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 in
cellular respiration: glycolysis, the link reaction, Krebs cycle, also known as the
citric acid cycle, and oxidative phosphorylation, also known as the electron
transport chain.
During glycolysis, glucose is
converted into pyruvate in the cytoplasm. Then in the link reaction, this
pyruvate is moved into the mitochondrion where it is converted into acetyl coenzyme
A, or acetyl-CoA. This acetyl-CoA is a reactant that
then enters the Krebs cycle in the mitochondrion. In the first step, the four-carbon
molecule oxaloacetic acid reacts with the two-carbon molecule acetyl-CoA to form the
six-carbon molecule citric acid. Then, the six-carbon molecule
citric acid breaks down and releases a molecule of carbon dioxide to form the
five-carbon molecule ketoglutaric acid. This also reduces a molecule of
NAD+ to NADH.
Next, the five-carbon molecule
ketoglutaric acid breaks down to form the four-carbon molecule succinic acid,
releasing a molecule of carbon dioxide in the process. A molecule of NAD+ is reduced to
NADH and a molecule of ATP is also formed in this step.
Next, the four-carbon molecule
succinic acid is converted into the four-carbon molecule malic acid. This process reduces a molecule of
FAD to FADH2 and uses a molecule of water. Then the four-carbon molecule malic
acid is converted into the four-carbon molecule oxaloacetic acid while reducing a
molecule of NAD+ to NADH. This completes one turn of the
Krebs cycle. You’ll notice that one turn of the
Krebs cycle produces two molecules of carbon dioxide as indicated here.
Therefore, the correct answer is
two molecules.
