Krebs cycle

The Krebs cycle, or citric acid cycle, is a part of cellular respiration. It is a series of chemical reactions used by all aerobic organisms to generate energy. Its central importance to many biochemical pathways suggests that it was one of the earliest parts of cellular metabolism to evolve.

The Krebs cycle comes after the link reaction and provides the hydrogen and electrons needed for the electron transport chain. It takes place inside mitochondria. It was discovered by Hans Adolf Krebs

Summary

Kreb Cycle bring about the conversion of pyruvic acid, fatty acids, fats and amino acids into carbon dioxide and water by oxidation. It is the common path for oxidation of carbohydrates, fats and proteins.

It starts with acetyl coenzyme A which is then converted into several intermediate compounds with the release of ATP, hydrogen atoms (NADH₂ and FADH₂) and then acetyl coenzyme A is regenerated back.

The diagram below shows how this part of respiration is an ever-repeating cycle which produces ATP and gives off CO₂. The ATP is a molecule which carries energy in chemical form to be used in other cell processes. To summarize:

• Two molecules of carbon dioxide are given off
• Two molecules of ATP are formed
• Three molecules of NAD+ are combined with hydrogen (NAD+ → NADH)
• One molecule of FAD+ combines with hydrogen (FAD+ → FADH)

Because two acetyl-CoA molecules are produced from each glucose molecule, two cycles are required per glucose molecule. In simple words, for each glucose molecule, the Kreb's Cycle occurs twice. Therefore, at the end of two cycles, the products are: two ATP, six NADH₂, two FADH₂ two QH₂ (ubiquinol) and four CO₂.

Therefore the cycle accounts for total 24 ATP molecules.

Overview of the citric acid cycle

Related pages

• Cellular respiration
• Glycolysis
• Link reaction
• Electron transport chain

Other websites

• Krebs cycle Animation
• Another Krebs cycle animation