During cellular respiration, protons are actively transported across which mitochondrial membrane as part of the electron transport chain?

• A. From the cytoplasm into the mitochondrial matrix across the inner mitochondrial membrane
• B. From the mitochondrial matrix to the intermembrane space across the inner mitochondrial membrane
• C. From the mitochondrial matrix to the cytoplasm across the outer mitochondrial membrane
• D. From the intermembrane space to the mitochondrial matrix across the outer mitochondrial membrane

Answer: (B)

Verified step by step guidance

1. Understand the context: Cellular respiration involves multiple stages, and the electron transport chain (ETC) is the final stage where most ATP is produced. The ETC occurs in the mitochondria and involves the movement of protons (H⁺) to create a proton gradient.
2. Identify the key structure: The mitochondrion has two membranes—an outer membrane and an inner membrane. The space between these membranes is called the intermembrane space, and the area inside the inner membrane is the mitochondrial matrix.
3. Recall the function of the ETC: During the ETC, electrons are passed along a series of protein complexes embedded in the inner mitochondrial membrane. This process releases energy, which is used to actively transport protons (H⁺) across the inner mitochondrial membrane.
4. Determine the direction of proton movement: Protons are pumped from the mitochondrial matrix (inside the inner membrane) to the intermembrane space (between the inner and outer membranes). This creates a high concentration of protons in the intermembrane space, establishing an electrochemical gradient.
5. Conclude the answer: The correct description of proton movement during the ETC is 'From the mitochondrial matrix to the intermembrane space across the inner mitochondrial membrane.' This gradient is later used by ATP synthase to produce ATP during chemiosmosis.