The Krebs Cycle

What happens during the Krebs cycle?

In the presence of oxygen, pyruvic acid produced in glycolysis passes to the second stage of cellular respiration, the Krebs cycle. The Krebs cycle is named after Hans Krebs, the British biochemist who demonstrated its existence in 1937. During the Krebs cycle, pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions. Because citric acid is the first compound formed in this series of reactions, the Krebs cycle is also known as the citric acid cycle.

Citric Acid Production The Krebs cycle begins when pyruvic acid produced by glycolysis passes through the two membranes of the mitochondrion and into the matrix. The matrix is the innermost compartment of the mitochondrion and the site of the Krebs cycle reactions. Once inside the matrix, 1 carbon atom from pyruvic acid becomes part of a molecule of carbon dioxide, which is eventually released into the air. The other 2 carbon atoms from pyruvic acid rearrange and form acetic acid, which is joined to a compound called coenzyme A. The resulting molecule is called acetyl-CoA. (The acetyl part of acetyl-CoA is made up of 2 carbon atoms, 1 oxygen atom, and 3 hydrogen atoms.) As the Krebs cycle begins, acetyl-CoA adds the 2-carbon acetyl group to a 4-carbon molecule already present in the cycle, producing a 6-carbon molecule called citric acid.

Energy Extraction As the cycle continues, citric acid is broken down into a 4-carbon molecule, more carbon dioxide is released, and electrons are transferred to energy carriers. Follow the reactions in Figure 9–5 and you will see how this happens. First, look at the 6 carbon atoms in citric acid. One is removed, and then another, releasing 2 molecules of carbon dioxide and leaving a 4-carbon molecule. Why is the Krebs cycle a “cycle”? Because the 4-carbon molecule produced in the last step is the same molecule that accepts the acetyl-CoA in the first step. The molecule needed to start the reactions of the cycle is remade with every “turn.”

Next, look for ATP. For each turn of the cycle, a molecule of ADP is converted to a molecule of ATP. Recall that glycolysis produces 2 molecules of pyruvic acid from 1 molecule of glucose. So, each starting molecule of glucose results in two complete turns of the Krebs cycle and, therefore, 2 ATP molecules. Finally, look at the electron carriers, NAD⁺ and FAD (flavine adenine dinucleotide). At five places, electron carriers accept a pair of high-energy electrons, changing NAD⁺ to NADH and FAD to FADH₂. FAD and FADH₂ are molecules similar to NAD⁺ and NADH, respectively.

What happens to each of these Krebs cycle products—carbon dioxide, ATP, and electron carriers? Carbon dioxide is not useful to the cell and is expelled every time you exhale. The ATP molecules are very useful and become immediately available to power cellular activities. As for the carrier molecules like NADH, in the presence of oxygen, the electrons they hold are used to generate huge amounts of ATP.

In Your Notebook List the electron carriers involved in the Krebs cycle. Include their names before and after they accept the electrons.

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