MCAT® ExamBiological and Biochemical Foundations of Living SystemsHard
A patient presents with symptoms of extreme fatigue, muscle weakness, and a general inability to tolerate physical exertion. Diagnostic tests reveal a defect in the electron transport chain within their mitochondria, specifically affecting Complex IV (cytochrome c oxidase). This defect would most directly impair the cell's ability to:
- AProduce NADH and FADH2 through the Krebs cycle.
- BMaintain a proton gradient across the inner mitochondrial membrane.
- CCarry out glycolysis in the cytoplasm.
- DGenerate ATP via substrate-level phosphorylation.
Show answer & explanationAnswer & explanation
Correct answer: B. Maintain a proton gradient across the inner mitochondrial membrane.
Complex IV is a key component of the electron transport chain (ETC). Its defect would prevent the efficient pumping of protons into the intermembrane space, thus collapsing or severely diminishing the proton gradient essential for ATP synthesis by chemiosmosis.
Why the other options are wrong
- A. The production of NADH and FADH2 occurs in glycolysis and the Krebs cycle, which precede the ETC.
- C. Glycolysis occurs in the cytoplasm and is an anaerobic process, independent of mitochondrial ETC function.
- D. Substrate-level phosphorylation occurs in glycolysis and the Krebs cycle, independent of the ETC.
Electron Transport Chain (ETC)
A series of protein complexes and electron carriers located in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, creating a proton gradient for ATP synthesis.
- Comprises four main complexes (I, II, III, IV) and mobile carriers.
- Pumps protons into the intermembrane space.
- Oxygen is the final electron acceptor.
Memory trick: Glycolysis (glucose split), Krebs (cycles for carriers), ETC (electrons pump protons, make ATP).