Final answer:
Complex II of the electron transport chain in cellular respiration does not pump a proton because it bypasses the first complex and delivers electrons directly to the electron transport chain. This results in fewer ATP molecules being generated from the electrons derived from FADH₂.
Step-by-step explanation:
Complex II directly receives electrons from the oxidation of FADH₂ to FAD, using the enzyme succinate dehydrogenase (from the citric acid cycle) and does not pass through complex I. This enzyme and FADH₂ form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex. Since these electrons bypass and thus do not energize the proton pump in the first complex, fewer ATP molecules are made from the FADH2 electrons.
Complex II extends to the matrix from the inner membrane but does not cross the membrane to the intermembrane space. Therefore complex II transports electrons but does not pump protons. The energy released by electron transport is utilized to pump protons in complex III and IV.