Preview
IB DP Biology — C: Interaction and Interdependence | SL | C1.2 Cell Respiration
📋 What You Will Learn
- Identify the location and components of the electron transport chain in the inner mitochondrial membrane
- Explain how the proton gradient generated by the electron transport chain drives ATP synthesis via chemiosmosis
- Describe the role of oxygen as the final electron acceptor and explain why its absence forces anaerobic alternatives
- Compare lactate fermentation and ethanol fermentation in terms of substrates, products, and biological purpose
- Analyse data on ATP yield, oxygen consumption, and metabolic products to evaluate aerobic versus anaerobic respiration
- Apply understanding of oxidative phosphorylation to explain the effects of metabolic inhibitors and physiological conditions
🔍 Guiding Question How do cells transfer the chemical energy stored in organic molecules into a form that can drive cellular work, and what strategies do they use when the most efficient pathway is unavailable?
🌍 Why This Matters When a sprinter explodes off the starting blocks, their muscle cells consume ATP faster than aerobic respiration can supply it. Within seconds, lactate fermentation kicks in — a molecular emergency measure that keeps ATP production alive at the cost of accumulating lactate. The burning sensation in exhausted muscles is the direct consequence of a cell choosing survival over efficiency.
The Problem the Cell Had to Solve
Glycolysis and the Krebs cycle are extraordinary — they systematically dismantle glucose and extract electrons from it, loading those electrons onto carrier molecules. But here is the problem: those carriers, NADH and FADH₂, are now saturated. They cannot accept any more electrons. If the cell cannot offload those electrons somewhere, the entire upstream machinery grinds to a halt.
The electron transport chain is the cell's solution. It is a molecular relay system embedded in the inner mitochondrial membrane that passes electrons from NADH and FADH₂ down a series of protein complexes, releasing energy at each step. That energy is used to do something elegant: pump protons across a membrane, creating a gradient that drives the synthesis of ATP. This is oxidative phosphorylation — the most productive ATP-generating process in aerobic respiration.
📖 Key Definition — Oxidative Phosphorylation The synthesis of ATP using energy released by the transfer of electrons along the electron transport chain, coupled to the pumping of protons across the inner mitochondrial membrane and their return through ATP synthase. It is called "oxidative" because the electrons are ultimately transferred to oxygen, and "phosphorylation" because ADP is phosphorylated to form ATP.
The Electron Transport Chain
Picture the inner mitochondrial membrane as a factory floor. Embedded within it are four large protein complexes (I, II, III, and IV) and a small mobile carrier called ubiquinone, plus a protein called cytochrome c. Together, these form the electron transport chain. The matrix side of this membrane is where the Krebs cycle runs; the intermembrane space is where protons accumulate.
Step 1 — Electron Donation from NADH and FADH₂
NADH delivers its electrons to Complex I (NADH dehydrogenase). FADH₂ delivers its electrons to Complex II (succinate dehydrogenase). This oxidises the carriers — they become NAD⁺ and FAD again, ready to return to glycolysis and the Krebs cycle to pick up more electrons. This regeneration is why the upstream processes can