Trace Electron Transport and Proton Pumping
Follow the electron route, mark which complexes pump protons, then predict what changes when a carrier or ATP synthase is blocked.
Living-system questions are solved by matching a structure or organism to its function, process and stated condition.
Trace two entry routes that converge
NADH donates electrons to complex I; FADH2 donates through complex II. Both routes reduce ubiquinone (coenzyme Q), a small hydrophobic, non-protein carrier. Reduced ubiquinone passes electrons to complex III, then mobile cytochrome c carries them to complex IV, where oxygen is the final electron acceptor and is reduced to water.
Mark the proton pumps
Complexes I, III and IV pump H+ from the matrix to the intermembrane space; complex II does not. Complex III is shared by electrons arriving from either NADH or FADH2 and translocates four protons per pair of electrons. The proton gradient powers complex V (F0F1 ATP synthase); F0 provides the proton channel and F1 catalyses ATP formation. Blocking F0 stops gradient-driven ATP synthesis.
Predict a block or artificial-acceptor result
Blocking complex I reduces the NADH route but does not necessarily stop all electron flow, because electrons can still enter through complex II. Blocking ubiquinone stops both routes downstream; NADH-linked complex I can still pump before the block, while FADH2-linked complex II contributes no proton pumping. DCPIP is blue when oxidised and colourless when reduced, so decolourisation means it has accepted electrons from the chain.
Keep respiration comparisons straight
Electron transport creates the proton gradient and oxidative phosphorylation uses it to make ATP; the coupled events occur together. Fermentation does not use the mitochondrial electron-transport chain. NADH is reoxidised more slowly in fermentation than in vigorous aerobic respiration, using an organic electron acceptor rather than oxygen.
Chemiosmosis in bacteria
Bacterial electron transport occurs across the plasma membrane. Electron transfer builds a proton gradient, and ATP synthase uses the proton-motive force to phosphorylate ADP; the chain does not directly make ATP without this gradient.
Read mitochondrial pH-gradient graphs
The respiratory chain pumps H⁺ out of the matrix, building a pH gradient. Blocking ATP synthase reduces proton return to the matrix, so the gradient initially rises; blocking electron transport reduces pumping and lets the gradient fall.
OlympIQ exam lens
- ✓NADH → complex I; FADH2 → complex II; both feed electrons to ubiquinone.
- ✓Complex III oxidises ubiquinol and transfers electrons to cytochrome c; oxygen is reduced at complex IV.
- ✓Proton pumps: I, III and IV. Complex II does not pump; III is common to both entry routes.
- ✓Block complex I: NADH entry falls but complex-II entry can continue. Block ubiquinone: both routes stop downstream.
- ✓Blue DCPIP → colourless = reduction by accepted electrons; DCPIP is an artificial acceptor, not fuel.
- ✓F0 block prevents proton flow and gradient-driven ATP synthesis at F1.
Do not count complex II as a proton pump. And when a blocker stops one entry route, ask whether the other route can still feed electrons into the shared chain.
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