When electrons flow along the electron transport chains of mitochondria, which of the following changes occurs? a. The pH of the matrix increases. b. ATP synthase pumps protons by active transport. c. The electrons gain free energy. d. NAD+ is oxidized.

Verified step by step guidance

1. Identify the process involved: The question refers to the electron transport chain (ETC) in mitochondria, which is a series of complexes that transfer electrons from electron donors to electron acceptors via redox reactions, and couples this electron transfer with the transfer of protons (H+ ions) across the mitochondrial membrane.
2. Understand the role of the electron transport chain: As electrons move through the ETC, they lose energy. This energy is used by the complexes in the chain to pump protons from the mitochondrial matrix to the intermembrane space, creating a proton gradient.
3. Analyze the options: Option a suggests that the pH of the matrix increases, which implies that the concentration of H+ ions in the matrix decreases. This is consistent with the movement of protons from the matrix to the intermembrane space, thus increasing the pH of the matrix.
4. Evaluate other options: Option b is incorrect because ATP synthase does not pump protons by active transport; it uses the proton gradient created by the ETC to drive the synthesis of ATP. Option c is incorrect as electrons lose energy in the ETC, not gain. Option d is incorrect because NAD+ is reduced to NADH during the Krebs cycle, not oxidized in the ETC.
5. Conclude the correct answer: The correct answer is a. The pH of the matrix increases as protons are pumped out of the matrix, decreasing the concentration of H+ ions in the matrix.

Key Concepts

Electron Transport Chain (ETC)

The electron transport chain is a series of protein complexes located in the inner mitochondrial membrane that facilitate the transfer of electrons from electron donors like NADH and FADH2 to electron acceptors. This process generates a proton gradient across the membrane, which is essential for ATP production through oxidative phosphorylation.

Proton Gradient and pH Changes

As electrons move through the ETC, protons (H+) are pumped from the mitochondrial matrix into the intermembrane space, creating a proton gradient. This gradient leads to a decrease in pH in the intermembrane space and an increase in pH in the matrix, which is crucial for ATP synthesis as protons flow back into the matrix through ATP synthase.

ATP Synthase Function

ATP synthase is an enzyme that synthesizes ATP from ADP and inorganic phosphate, utilizing the energy from the proton gradient established by the ETC. It operates through a process called chemiosmosis, where protons flow down their concentration gradient through ATP synthase, driving the conversion of ADP to ATP, rather than pumping protons against the gradient.