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Krebs Cycle

A sequence of reactions where most living cells generate energy during aerobic respiration.

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Krebs Cycle

What type of acid do these fruits contain?

Citric acid. Citric acid is also the first product formed in the Krebs cycle, and therefore this acid occurs in the metabolism of virtually all living things.

Cellular Respiration Stage II: The Krebs Cycle

Recall that glycolysis, stage I of cellular respiration, produces two molecules of pyruvate. These molecules enter the matrix of a mitochondrion, where they start the Krebs cycle. The reactions that occur next are shown in Figure below. You can watch an animated version at this link: http://www.youtube.com/watch?v=p-k0biO1DT8&feature=related.

The Krebs cycle starts with pyruvic acid from glycolysis. Each small circle in the diagram represents one carbon atom. For example, citric acid is a six carbon molecule, and OAA (oxaloacetate) is a four carbon molecule. Follow what happens to the carbon atoms as the cycle proceeds. In one turn through the cycle, how many molecules are produced of ATP? How many molecules of NADH and FADH2 are produced?

Before the Krebs cycle begins, pyruvic acid, which has three carbon atoms, is split apart and combined with an enzyme known as CoA, which stands for coenzyme A. The product of this reaction is a two-carbon molecule called acetyl-CoA. The third carbon from pyruvic acid combines with oxygen to form carbon dioxide, which is released as a waste product. High-energy electrons are also released and captured in NADH.

Steps of the Krebs Cycle

The Krebs cycle itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure above). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle. After citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2, another energy-carrying compound. Carbon dioxide is also released as a waste product of these reactions. The final step of the Krebs cycle regenerates OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two pyruvic acid molecules when it splits glucose. Watch the OSU band present the Krebs cycle: http://www.youtube.com/watch?v=FgXnH087JIk&feature=related.

Results of the Krebs Cycle

After the second turn through the Krebs cycle, the original glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its chemical bonds has been stored in a total of 16 energy-carrier molecules. These molecules are:

  • 4 ATP (including 2 from glycolysis)
  • 10 NADH (including 2 from glycolysis)
  • 2 FADH2


  • The Krebs cycle is the second stage of cellular respiration.
  • During the Krebs cycle, energy stored in pyruvate is transferred to NADH and FADH2, and some ATP is produced.

Practice I

Use these resources to answer the questions that follow.

  1. Where does the Krebs cycle occur in the cell?
  2. What is the first product of this cycle?
  3. How many reactions does it take to complete the cycle?
  1. Provide a definition for the Krebs or TCA cycle.
  2. What are the roles of NADH and FADH2?
  3. Will the TCA cycle occur in the absence of oxygen?
  1. What must happen to pyruvate for it to enter the mitochondria?
  2. What is the function of Coenzyme A?
  3. Is reaction 1 reversible? Why or why not?
  4. How many NADH molecules are produced during the Krebs cycle?
  5. How do carbon atoms leave the Krebs cycle?

Practice II


1. What is the Krebs cycle?

2. What are the products of the Krebs cycle?

3. Explain why two turns of the Krebs cycle are needed for each molecule of glucose.

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