The Citric Acid Cycle: Energy Harvesting in Action
Ever wondered how your body extracts energy from food? The citric acid cycle (also called the Krebs cycle or TCA cycle) is a key metabolic pathway that does exactly this. In eukaryotes like us, this cycle happens inside the mitochondrial matrix, while in prokaryotes, it occurs in the cytoplasm.
The cycle works like a continuous loop with 8 steps that transform molecules and capture energy. It starts when acetyl CoA combines with oxaloacetate to form citrate. As the cycle progresses, two carbon atoms are released as CO₂, and energy carriers like NADH and FADH₂ are produced to power cellular activities.
For each turn of the cycle, your cells generate 3 NADH, 1 FADH₂, and 1 ATP/GTP molecule. Since each glucose molecule produces two pyruvate molecules (which become two acetyl CoA), the cycle runs twice for each glucose that enters cellular respiration.
💡 Think of the citric acid cycle as your cellular power plant - it doesn't directly make much ATP, but the NADH and FADH₂ it produces will later generate lots of ATP in the electron transport chain!
The first two steps of the cycle involve: (1) Acetyl CoA joining with oxaloacetate to form citrate (releasing CoA in the process), and (2) Citrate being converted to isocitrate through a two-part reaction involving the removal and addition of water.