Oxidative phosphorylation vs. substrate-level phosphorylation: Why has no isolatable “high-energy phosphate donor” analogous to 1,3-bisphosphoglycerate been found in mitochondria?
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ABecause biochemical isolation techniques are still inadequate
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BBecause no such phosphate donor exists in oxidative phosphorylation
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CBecause the donor exists but is too short-lived to isolate
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DBecause mitochondrial ATP is made only by glycolytic enzymes
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EBecause the donor is lipid-soluble and escapes detection
Answer
Correct Answer: Because no such phosphate donor exists in oxidative phosphorylation
Explanation
Introduction / Context: ATP can be produced by substrate-level phosphorylation (for example, from 1,3-bisphosphoglycerate in glycolysis) or by oxidative phosphorylation in mitochondria. This question probes why an analogous small-molecule “high-energy phosphate donor” has never been isolated from mitochondria.
Given Data / Assumptions:
- Glycolysis uses discrete metabolites (for example, 1,3-bisphosphoglycerate, phosphoenolpyruvate) to transfer phosphate directly to ADP.
- Oxidative phosphorylation uses an electrochemical proton gradient across the inner mitochondrial membrane.
- ATP synthase catalyzes ADP + Pi → ATP using proton motive force, not a diffusible donor metabolite.
Concept / Approach: Peter Mitchell’s chemiosmotic theory explains that electron transport chains pump protons, building a gradient (Δp). ATP synthase harnesses this gradient to synthesize ATP. There is no requirement for a soluble “high-energy phosphate” intermediate analogous to glycolytic donors.
Step-by-Step Solution: Contrast mechanisms: substrate-level phosphorylation uses a high-energy intermediate; oxidative phosphorylation uses a transmembrane gradient. Identify mitochondrial catalyst: F1F0-ATP synthase converts electrochemical energy directly into the phosphoanhydride bond. Therefore, no isolatable small-molecule phosphate donor exists in mitochondria. Select the option stating nonexistence rather than technical failure.
Verification / Alternative check: Reconstitution experiments show ATP synthesis when purified ATP synthase is embedded in liposomes with a proton gradient, confirming the gradient is the “donor.”
Why Other Options Are Wrong: Technique limitations (a) are not the reason; “too short-lived” (c) misstates the mechanism; (d) is false—mitochondria make most ATP aerobically; (e) has no support.
Common Pitfalls: Assuming every ATP synthesis pathway requires a discrete phosphorylated metabolite.
Final Answer: Because no such phosphate donor exists in oxidative phosphorylation.