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IB DP Biology — C: Interaction and Interdependence | SL | C1.2 Oxidative Phosphorylation and Mitochondrial Biochemistry
📋 What You Will Learn
- Identify the electron carriers NAD and FAD and explain their roles in transferring hydrogen atoms from substrate oxidation to the electron transport chain
- Explain how the electron transport chain generates a proton gradient across the inner mitochondrial membrane and how this drives ATP synthesis via chemiosmosis
- Describe the structure and function of ATP synthase and the role of the proton motive force in oxidative phosphorylation
- Analyse data on ATP yield from aerobic and anaerobic respiration and evaluate the significance of cellular energy balance
- Apply understanding of chemiosmosis to unfamiliar contexts, including the effect of inhibitors and uncouplers on ATP production
🔍 Guiding Question How does the controlled movement of protons across a membrane allow cells to convert the chemical energy stored in reduced coenzymes into the universal energy currency of ATP?
🌍 Why This Matters Every time your heart beats, every thought you have, every molecule your cells synthesise — all of it is powered by ATP made through chemiosmosis. Poisons like cyanide kill within minutes precisely because they block the electron transport chain, halting this process entirely. Understanding chemiosmosis is understanding the engine of life.
The Problem the Cell Had to Solve
Glycolysis and the Krebs cycle are impressive biochemical achievements. They dismantle glucose, rearrange carbon skeletons, and release energy. But here is the uncomfortable truth: most of that energy does not immediately become ATP. Instead, it is loaded onto carrier molecules — reduced coenzymes — that carry it away like charged batteries waiting to be spent.
The cell's challenge is this: how do you extract the energy stored in those reduced coenzymes and convert it into ATP without wasting it as heat? The answer, arrived at by billions of years of evolution, is elegant beyond expectation. You use electrons. You use membranes. And you use the flow of protons like water through a turbine.
📖 Key Definition — Reduced NAD (NADH) Nicotinamide adenine dinucleotide in its reduced form, carrying two electrons and one proton (a hydride ion, H⁻) gained from substrate oxidation during glycolysis, the link reaction, and the Krebs cycle. Each NADH carries sufficient energy to drive the synthesis of approximately 2.5 ATP via the electron transport chain.
📖 Key Definition — Reduced FAD (FADH₂) Flavin adenine dinucleotide in its reduced form, carrying two electrons and two protons gained from the oxidation of succinate in the Krebs cycle. FADH₂ enters the electron transport chain at a lower energy level than NADH, yielding approximately 1.5 ATP per molecule.
The Electron Transport Chain — Loading the Gradient
Picture the inner mitochondrial membrane as a factory floor. Embedded within it are four large protein complexes — I, II, III, and IV — arranged in sequence. These are the electron transport chain. Their job is to accept electrons from NADH and FADH₂, pass them along like a relay baton, and use the released energy to pump protons from the mitochondrial matrix into the intermembrane space.
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