Question

In mitochondria, the proton gradient required for ATP synthesis develops across
A. Inner membrane
B. Inter membrane space
C. Outer membrane
D. F0-F1 particles

Answer 1

Hint: Within the mitochondrial membrane, a high-energy electron is passed along an electron transport chain. The energy released pumps hydrogen out of the vacuum of the matrix. The gradient created by this pushes hydrogen back through the membrane, through ATP synthase. When this occurs, ATP synthase synthesizes ATP from ADP by the action of enzymes.

Complete answer:
Mitochondria comprise two main membranes, the outer membrane and the inner membrane. The outer mitochondrial membrane entirely covers the inner membrane, with a small intermembrane gap in between. The outer membrane has several protein-based pores that allow ions and molecules as large as a small protein to pass through. On the other hand, the inner membrane has a much more narrow permeability like the plasma membrane of the cell.

The inner membrane also has proteins involved in electron transport and ATP synthesis. This membrane covers the mitochondrial matrix, where the citric acid cycle creates electrons that migrate from one protein complex to another in the inner membrane. At the end of this electron transport chain, the final electron recipient is oxygen, which eventually forms water $(H_{2}O)$. At the same time, the electron transport chain releases ATP. During electron transport, the participating protein complexes force the protons out of the matrix into the inter membrane space. This generates a concentration gradient of protons that is used by another protein complex, called ATP synthase, to produce the synthesis of the energy carrier molecule ATP.

Thus, the correct answer is option A i.e., inner membrane.

Note: The electrochemical proton gradient across the inner mitochondrial membrane is used to guide ATP synthesis during the crucial step of oxidative phosphorylation. This is facilitated by the enzyme ATP synthase bound to the membrane. The enzyme produces a hydrophilic pathway that enables the protons to migrate down their electrochemical gradient through the inner mitochondrial membrane.