Video Transcript
The flowchart provided shows the stages of the Krebs cycle in an incorrect order,
with each stage assigned a number. Order the stages correctly.
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, the
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, this pyruvate is transported into the mitochondria, where it is converted into
acetyl coenzyme A, or acetyl-CoA, in the link reaction. Acetyl-CoA can then enter the Krebs cycle to produce ATP and to reduce NAD+ to NADH
and FAD to FADH2 to be used in oxidative phosphorylation.
Let’s look at the steps of the Krebs cycle in order to answer this question
correctly. In the first step, the four-carbon molecule oxaloacetate reacts with the two-carbon
molecule acetyl-CoA to form the six-carbon molecule citrate. This is indicated in step five in the provided flowchart. So we’ll mark that as the first step in blue.
Then, the six-carbon molecule citrate breaks down and releases a molecule of carbon
dioxide to form the five-carbon molecule ketoglutarate. This also reduces a molecule of NAD+ to NADH and releases a molecule of carbon
dioxide. This is indicated as step one on the left.
Next, the five-carbon molecule ketoglutarate breaks down to form the four-carbon
molecule succinate, releasing a molecule of carbon dioxide in the process. In addition, a molecule of NAD+ is reduced to NADH, and a molecule of ATP is formed
in this step. This step is indicated as step two on the left.
Next, the four-carbon molecule succinate is converted into the four-carbon molecule
malate. This process reduces a molecule of FAD to FADH2 and uses a molecule of water. This is indicated as step four on the left. Then, the four-carbon molecule malate is converted into the four-carbon molecule
oxaloacetate, while reducing a molecule of NAD+ to NADH. This completes one turn of the Krebs cycle and regenerates oxaloacetate. This is indicated in step three on the left.
We can now put the steps in the flowchart in the correct order and answer our
question. The correct order of the stages for the Krebs cycle is five, one, two, four,
three.
