## P/O Ratio and Oxidative Phosphorylation ### Definition of P/O Ratio **Key Point:** The P/O ratio (also called the ATP/O ratio) is the number of ATP molecules synthesized per atom of oxygen consumed (or per NADH/FADH₂ oxidized). ### Current Consensus Values | Substrate | Entry Point | Complexes Used | H⁺ Pumped | P/O Ratio | ATP Yield | | --- | --- | --- | --- | --- | --- | | NADH | Complex I | I → III → IV | 10 H⁺ | **2.5** | 2.5 ATP | | FADH₂ | Complex II | II → III → IV | 6 H⁺ | **1.5** | 1.5 ATP | | Glycerol-3-phosphate | Complex II | II → III → IV | 6 H⁺ | **1.5** | 1.5 ATP | **High-Yield:** The modern consensus (post-1990s) is P/O = 2.5 for NADH and 1.5 for FADH₂, not the older values of 3 and 2. ### Why These Values? 1. **H⁺ Stoichiometry:** - Complex I pumps ~4 H⁺ per 2 electrons - Complex III pumps ~4 H⁺ per 2 electrons - Complex IV pumps ~2 H⁺ per 2 electrons - **Total for NADH:** 10 H⁺ per 2 electrons = 5 H⁺ per electron pair 2. **ATP Synthesis:** - ATP synthase requires ~4 H⁺ per ATP (including the cost of transporting Pi and ADP into the matrix) - 10 H⁺ ÷ 4 H⁺/ATP = **2.5 ATP per NADH** - 6 H⁺ ÷ 4 H⁺/ATP = **1.5 ATP per FADH₂** **Mnemonic:** **"NADH makes 2.5, FADH₂ makes 1.5"** — Remember that FADH₂ bypasses Complex I, so it pumps fewer protons and yields fewer ATP. **Warning:** Older textbooks cite P/O = 3 for NADH and 2 for FADH₂. These are outdated. Modern biochemistry uses 2.5 and 1.5.
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