Energy required for ATP synthesis in PSII comes from (a) Proton gradient (b) Electron gradient (c) Reduction o

NCERT Exemplar · Q6

Q. Energy required for ATP synthesis in PSII comes from

(a) Proton gradient

(b) Electron gradient

(d) Oxidation of glucose

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Concept understanding — ATP Synthase Complex

The Problem ATP Synthase Solves

Imagine a water wheel. Water flows downhill, and the wheel captures that flow to turn a millstone. The cell faces a similar problem: it has a steep proton gradient across the inner mitochondrial membrane — many protons (H⁺) crowded in the intermembrane space, few in the matrix. That gradient is potential energy, like water behind a dam. The cell needs to use that energy to make ATP, the universal energy currency.

The machine that does this is ATP synthase, also called Complex V or the F₀F₁ complex. It is a rotary molecular motor.

The Two Parts: F₀ and F₁

The complex has two distinct functional halves.

F₀ (the "o" stands for oligomycin-sensitive, a poison that blocks it) is embedded in the inner membrane. It contains a channel through which protons flow down their gradient, from the intermembrane space into the matrix. This is the "water wheel" part — the proton flow provides the mechanical energy.

F₁ sticks out into the matrix. It is the catalytic head where ADP and inorganic phosphate (Pᵢ) are bound and squeezed together to form ATP. This is the "millstone" part — it does the chemical work.

Important

The key insight: F₀ and F₁ are physically connected by a rotating shaft (the central stalk). Proton flow through F₀ turns the shaft, and that rotation drives conformational changes in F₁ that make ATP.

How It Works: The Binding Change Mechanism

The F₁ head has three identical catalytic sites, arranged like the lobes of a clover. At any moment, each site is in one of three states:

Open (O) — binds ADP and Pᵢ loosely
Loose (L) — traps the bound substrates
Tight (T) — squeezes ADP and Pᵢ together, forming ATP

As the central shaft rotates (driven by proton flow through F₀), it forces each site to cycle through these three states. One full rotation (360°) produces three ATP molecules — one from each site.

Note

The rotation is not continuous. Each proton passing through F₀ causes a small angular step. In most organisms, 3–4 protons are needed per ATP made, because the shaft has multiple subunits (usually 8–12 c-subunits in the rotor ring).

The Proton Path Through F₀

Protons enter F₀ from the intermembrane space, bind to specific sites on the rotor ring (the c-ring), and are carried partway around the ring before being released into the matrix. This movement turns the ring, which is attached to the central shaft. The shaft then rotates inside the stationary F₁ head.

Watch out

Do not confuse the direction of proton flow with the direction of rotation. Protons flow down their gradient (high concentration → low concentration), but the rotation they drive is mechanical — it is the shaft turning, not the protons themselves moving in a circle.

The Final Picture …

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ATP synthesis during the light reaction is powered by the proton gradient that builds up across the thylakoid membrane — the chemiosmotic mechanism.

The chemiosmotic hypothesis explains how the light reaction converts the energy of excited electrons into the chemical bond energy of ATP. As in cellular respiration, ATP synthesis here is linked to a proton gradient across a membrane — in this case, the thylakoid membrane — rather than being a direct product of electron flow itself.

Three processes work together to build this gradient. First, the splitting of water occurs on the inner (lumen) side of the membrane, so the protons produced accumulate inside the lumen. Second, as electrons move through the photosystems, the primary electron acceptor located toward the outer side hands its electron to a carrier that removes a proton from the stroma and releases it into the lumen. Third, NADP reductase, positioned on the stroma side, consumes protons from the stroma when it reduces NADP+ to NADPH. The combined effect is that protons in the stroma decrease while protons in the lumen accumulate, creating a steep proton gradient and a me …

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Q.Energy required for ATP synthesis in PSII comes from

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