IBDP Biology HL: Advanced Metabolic Regulation

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Allosteric Regulation in Phosphofructokinase (PFK)

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PFK is the rate-limiting enzyme of glycolysis. It is allosterically inhibited by high ATP (signaling high energy) and citrate (signaling abundant biosynthetic precursors), while activated by high AMP (signaling low energy). This negative feedback prevents unnecessary glucose consumption when energy charge is high.

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Chemiosmotic Theory & Proton Motive Force

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Proposed by Peter Mitchell, this theory states that electron transport chains pump protons (H+) across a membrane, creating an electrochemical gradient (proton motive force). The exergonic flow of protons back through ATP synthase drives the endergonic phosphorylation of ADP to ATP.

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Maximal ATP Yield (Theoretical vs. Actual)

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Theoretical max is ~38 ATP (30-32 ATP in eukaryotes due to costs). Actual yield is lower because: 1) The inner mitochondrial membrane is leaky to protons (uncoupling); 2) The proton gradient is used for other work (pyruvate transport, heat generation); 3) NADH from glycolysis requires shuttle energy to enter mitochondria.

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Photorespiration: The Oxygenase Activity of Rubisco

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Rubisco can act as an oxygenase, adding O2 to RuBP instead of CO2, especially in hot, dry conditions when stomata close and O2/CO2 ratio rises. This produces a toxic 2-carbon compound, consuming ATP and releasing CO2 without sugar production, significantly reducing photosynthetic efficiency in C3 plants.

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C4 Pathway: Spatial Separation of Steps

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C4 plants (e.g., maize) minimize photorespiration by fixing CO2 into 4-carbon oxaloacetate in mesophyll cells (using PEP carboxylase, which has no affinity for O2). The 4C acids are transported to bundle-sheath cells, where CO2 is released at high concentration for the Calvin cycle, suppressing Rubisco's oxygenase activity.