What must happen to pyruvate after it enters the citric acid cycle (Krebs cycle)?

Question

Once glycolysis is finished, pyruvate is created as the end product. If oxygen is present, pyruvate doesn’t just stay in the cytoplasm—it’s moved into the mitochondria, where it goes through more steps to release energy. But before and during the citric acid cycle, something must happen to pyruvate. What changes does pyruvate go through? What molecules are made as it's broken down further? And how does this set the stage for the next big step in energy production?

Answer 1

Background:

Once glycolysis is complete, each molecule of glucose has been split into two molecules of pyruvate. But glycolysis happens in the cytoplasm, and in order for energy production to continue efficiently, the pyruvate needs to be sent into the mitochondria, where the citric acid cycle (Krebs cycle) takes place.However, pyruvate can't enter the citric acid cycle as it is. It must first be chemically altered through a process called oxidation.

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What does oxidation mean?

In biology, oxidation means the loss of electrons. In this case, pyruvate loses electrons, which are picked up by the electron carrier NAD⁺, turning it into NADH. (These electrons are important—they’ll eventually be used to help make a lot of ATP in the electron transport chain!)

So, what exactly happens to pyruvate?

Each pyruvate molecule (there are 2 per glucose) undergoes a reaction called pyruvate oxidation.

During this step:

1. One carbon atom is removed from the 3-carbon pyruvate, and released as carbon dioxide (CO₂) — this is one of the first waste products of cellular respiration.
2. The remaining 2-carbon piece is combined with a molecule called Coenzyme A (CoA) to form Acetyl-CoA.
3. At the same time, NAD⁺ picks up electrons and becomes NADH.

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Summary:

So, per molecule of pyruvate, you get:

1. 1 Acetyl-CoA – which goes into the citric acid cycle.
2. 1 NADH – which goes to the electron transport chain.
3. 1 CO₂ – which is a waste product that you breathe out.

(Since you get 2 pyruvate molecules from 1 glucose, all of these numbers would be doubled for each glucose molecule).