Apentix/AP Biology/Unit 3: Cellular Energetics/Question
AP Biologymediummcq1 pt

How do the proton gradients that power ATP synthesis differ between mitochondria and chloroplasts?

A.Mitochondria pump protons into their matrix while chloroplasts pump them into the cytosol.
B.Neither organelle stores energy as a proton gradient; both make ATP by direct substrate-level phosphorylation.
C.Chloroplasts pump protons from the lumen into the stroma to build their gradient, the reverse of mitochondria.
D.Both organelles pump protons across a membrane into a confined space, but mitochondria fill the intermembrane space while chloroplasts fill the thylakoid lumen.

Explanation

Core Concept

Mitochondria and chloroplasts both use chemiosmosis: electron-transfer reactions establish an electrochemical proton gradient across a membrane, and ATP synthase uses proton flow down that gradient to make ATP. Mitochondria accumulate protons mainly in the intermembrane space and return them to the matrix, while chloroplasts accumulate protons in the thylakoid lumen and return them to the stroma. The shared principle is the same, but the organelles also differ in energy source, electron donors/acceptors, and electron-transfer components.

Correct Answer

DBoth organelles pump protons across a membrane into a confined space, but mitochondria fill the intermembrane space while chloroplasts fill the thylakoid lumen.

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