When pyruvate, generated from glycolysis is converted to Acetyl CoA, it enters a cycle called the Tricarboxylic Acid cycle(Krebs cycle).The TCA cycle was worked out by Sir Hans Krebs It occus in most plants, bacteria, fungi and animals.This process, except in bacteria, occurs in the matrix of the mitochondrion. The cycle is called the TCA cycle because of the acids it contain, which exists as salts in the cell.
1) Acetyl CoA, which contains 2 carbons, enters the cycle by combining with oxaloacetate, a 4 carbon compound to form a 6C compound called citrate.
2) Citrate is rearranged to form isocitrate.
3) Isocitrate loses a carbon molecule and is oxidized to form alpha-ketoglutarate.
4) Alpha-ketoglutarate loses another carbon and is oxidized to form succinyl CoA.
5) Succinyl is converted to succinate
6) Succinate converted to fumarate
7) fumarate is hydrated to produce malate
8) malate oxidized to form oxaloacetate
As the acetyl groups pass around the cycle, the two carbon atoms are lost as carbon dioxide. This process is known as decarboxylation. The hydrogen is added to hydrogen carriers in four dehydrogenation reactions, forming 3 reduced NAD and one reduced FAD molecules. One ATP molecule is made for every turn of the cycle. Oxaloacetate is regenerated for every turn of the cycle.