Q.17 ATP biosynthesis takes place utilizing the H+ gradient in mitochondria and chloroplasts. Identify
the correct sites of H+ gradient formation.
(A) Across the outer membrane of mitochondria and across the inner membrane of chloroplast
(B) Across the inner membrane of mitochondria and across the thylakoid membrane of chloroplast
(C) Within the matrix of mitochondria and across the inner membrane of chloroplast
(D) Within the matrix of mitochondria and within the stroma of chloroplast

ATP biosynthesis relies on a proton (H+) gradient generated during electron transport, driving ATP synthase in both mitochondria and chloroplasts. The correct sites are across the inner membrane of mitochondria and the thylakoid membrane of chloroplasts. Option (B) is accurate, as confirmed by standard cellular biology principles.​

Option Analysis

Option (A) states across the outer membrane of mitochondria and inner membrane of chloroplast. The outer mitochondrial membrane is permeable due to porins, preventing any sustained H+ gradient there, while chloroplasts form the gradient specifically across thylakoids, not the inner envelope membrane. This option is incorrect.​

Option (B) identifies across the inner membrane of mitochondria and thylakoid membrane of chloroplast. In mitochondria, electron transport pumps H+ from matrix to intermembrane space across the inner membrane; in chloroplasts, it pumps H+ into thylakoid lumen from stroma across thylakoid membrane. This matches the chemiosmotic mechanism for ATP production.​

Option (C) claims within the matrix of mitochondria and across the inner membrane of chloroplast. Gradients do not form “within” the matrix (a single compartment); mitochondrial gradient spans matrix and intermembrane space, and chloroplast inner membrane (envelope) lacks the key electron transport for H+ pumping. Incorrect.​

Option (D) suggests within the matrix of mitochondria and within the stroma of chloroplast. Neither matrix nor stroma supports a transmembrane H+ gradient; gradients require distinct membrane-bound compartments separated by H+-pumping complexes. This is wrong.​

Mechanism Overview

Electron transport chains in mitochondrial inner membrane complexes (I-IV) and chloroplast thylakoid photosystems (PSI, PSII) expel H+ , creating ΔpH and membrane potential (Δψ) for ATP synthase rotation. This proton motive force powers oxidative phosphorylation and photophosphorylation, respectively.​

Key Comparisons