Final answer:
Complex II does not pump protons from the matrix to the intermembrane space in the electron transport chain, instead it participates in transferring electrons to Coenzyme Q without contributing to the proton gradient.B) complex II.
Step-by-step explanation:
The complex in the electron transport chain (ETC) that does not pump protons from the matrix to the intermembrane space is Complex II. Unlike Complex I, Complex III, and Complex IV, which pump protons to generate the proton gradient necessary for ATP synthesis, Complex II only participates in electron transfer without contributing to proton pumping. Coenzyme Q (Q) is involved in electron transfer between Complex I and II to Complex III but also does not function as a proton pump. The correct answer to the question, 'Which of the following complexes pumps protons out of the matrix to generate the proton gradient except' is B) Complex II.
Complex II receives FADH2 which does not pass through Complex I and transfers electrons directly to Coenzyme Q without pumping protons. This distinction is important because it partly explains why fewer ATP molecules are created from the oxidation of FADH2 compared to NADH.
Overall, the generation of a proton gradient is crucial in the process of oxidative phosphorylation, where ATP is synthesized. The proton gradient provides the necessary energy for the synthesis of ATP as protons flow back into the matrix through ATP synthase. Complexes I, III, and IV contribute to this gradient by pumping protons, whereas Complex II and Coenzyme Q transport electrons without pumping protons.