Respiration in Plants

The Krebs cycle (citric acid cycle / TCA cycle) takes place in the mitochondrial matrix. Each turn of the cycle starts with one acetyl-CoA (from pyruvate) and produces:

• 3 NADH (high-energy electron carrier)
• 1 FADH₂ (lower-energy electron carrier)
• 1 GTP/ATP (substrate-level phosphorylation)
• 2 CO₂ (released as byproduct)

Per glucose (2 acetyl-CoA enter), Krebs gives:

• 6 NADH
• 2 FADH₂
• 2 ATP
• 4 CO₂

These NADH and FADH₂ feed the electron transport chain (ETC), which ultimately produces ~26-28 ATP via oxidative phosphorylation.

Total ATP per glucose (aerobic respiration):

• Glycolysis: 2 ATP + 2 NADH (in cytoplasm)
• Link reaction (pyruvate → acetyl-CoA): 2 NADH
• Krebs: 2 ATP + 6 NADH + 2 FADH₂
• ETC: ~28 ATP from those NADH/FADH₂

Grand total: ~32 ATP per glucose (older textbooks say 36-38; modern measurements account for proton leak).

Key enzymes (a JEE/NEET favourite):

• Citrate synthase (start of cycle)
• Aconitase
• Isocitrate dehydrogenase (rate-limiting, NADH-producing)
• α-ketoglutarate dehydrogenase
• Succinyl-CoA synthetase (GTP made here)
• Succinate dehydrogenase (FADH₂ made here; also part of ETC complex II)
• Fumarase
• Malate dehydrogenase

Anaerobic alternatives: in absence of O₂, pyruvate becomes lactate (animals) or ethanol + CO₂ (yeast). Net ATP drops to just 2 per glucose.