Krebs cycle

The Krebs cycle (citric acid cycle) is a series of enzyme-driven reactions in the mitochondrial matrix that fully oxidize acetyl-CoA into carbon dioxide, while generating high-energy electron carriers (NADH and FADH2) and a small amount of ATP. It is the second stage of aerobic respiration, following glycolysis, and it runs continuously as long as oxygen and acetyl-CoA are available.

• In each turn of the cycle, the acetyl group (2 carbons) from acetyl-CoA is combined with oxaloacetate (4 carbons) to form citrate (6 carbons). The cycle then releases 2 CO2 molecules and regenerates oxaloacetate to start again.
• Per acetyl-CoA, the cycle produces 3 NADH, 1 FADH2, and 1 ATP (or GTP). Since 2 acetyl-CoA come from one glucose, a full glucose yields 6 NADH, 2 FADH2, and 2 ATP just from the Krebs cycle.
• This cycle requires oxygen indirectly: it doesn-t use O2 in its steps, but it relies on NADH and FADH2 being oxidized in the electron transport chain. If there-s no O2 to accept electrons, NADH/FADH2 build up and the Krebs cycle stops.

• Location clue: If a question asks where in the cell acetyl-CoA is oxidized to CO2 and electron carriers are produced, the answer is the mitochondrial matrix (Krebs cycle in eukaryotes; in prokaryotes it-s in the cytoplasm).
• Yield trap: Be prepared to compute yields. A classic test point: 'How many NADH from one glucose in the citric acid cycle-' Answer: 6 NADH (3 per cycle - 2 cycles per glucose). Similarly, 2 FADH2 and 2 ATP (GTP) per glucose.
• Name game: Krebs cycle = Citric acid cycle = TCA cycle. Exams might use any of these names. 'TCA' stands for tricarboxylic acid (referring to citric acid-s three carboxyl groups).