Arrange the following sequential physiological and biochemical events describing how a ele… — JAMB UTME

Arrange the following sequential physiological and biochemical events describing how a electrochemical proton gradient is established and utilized to synthesize $ATP$ during the light-dependent reactions of photosynthesis, from initial photon absorption to photophosphorylation.

1Absorption of light energy by antenna chlorophyll molecules transfers excitation energy to the reaction center of Photosystem II ( $P_{680}$ ), exciting electrons to a higher energy level.
2The oxygen-evolving manganese complex oxidizes water molecules ( $2H_2O \rightarrow O_2 + 4H^+ + 4e^-$ ), replenishing the electron deficiency in Photosystem II and releasing protons into the thylakoid lumen.
3High-energy electrons are transferred along a membrane-bound electron transport chain via plastoquinone, releasing free energy used to pump protons ( $H^+$ ) from the stroma into the thylakoid lumen.
4The combined accumulation of protons from water photolysis and active thylakoid pumping generates a steep electrochemical proton motive force across the thylakoid membrane.
5Protons diffuse down their concentration gradient back into the stroma through the $CF_0CF_1$ $ATP$ synthase complex, driving the rotational photophosphorylation of $ADP$ and inorganic phosphate ( $P_i$ ) into $ATP$ .

Answer

The correct sequence of events in thylakoid chemiosmotic photophosphorylation is: (1) Absorption of light energy by Photosystem II chlorophylls → (2) Photolysis of water by the oxygen-evolving complex to replace lost electrons → (3) Transfer of electrons down the transport chain with active proton pumping into the lumen → (4) Generation of a proton motive force across the thylakoid membrane → (5) Passive efflux of protons through ATP synthase driving ATP synthesis from ADP and inorganic phosphate.

The correct order follows the logical cascade of non-cyclic electron transport and chemiosmosis during the light-dependent phase: photo-excitation of Photosystem II chlorophylls must occur first, which triggers the photolysis of water to replace lost electrons. As these electrons travel through plastoquinone and cytochrome complexes, energy is used to pump protons from the stroma into the thylakoid lumen. The resulting accumulation of protons creates a proton motive force, which finally drives the synthesis of ATP as protons flow back into the stroma through ATP synthase.

Step-by-Step Solution

Identify the primary trigger of the light reaction.

Photon absorption at Photosystem II (

P_{680}

) excites pair of electrons to a primary electron acceptor.

Photosynthesis is driven by light energy; electron flow cannot begin until photo-excitation occurs.

Determine the mechanism restoring the oxidized reaction center.

Photolysis of water splits

H_2O

, releasing

O_2

, protons into the lumen, and

e^-

P_{680}^+

Water oxidation must immediately replace the excited electrons lost by

P_{680}

to sustain continuous electron flow.

Trace the movement of excited electrons and active ion transport.

Electrons pass through plastoquinone and the cytochrome

b_6f

complex, which pumps

H^+

into the thylakoid lumen.

Redox energy released during downhill electron transport is coupled to active proton translocation from the stroma to the lumen.

Assess the physical state resulting from proton accumulation.

A high concentration of

H^+

builds up in the lumen relative to the stroma, forming a proton motive force.

Both water photolysis and cytochrome proton pumping contribute to an electrochemical gradient across the thylakoid membrane.

Identify the mechanism converting the potential energy of the gradient into chemical energy.

Protons pass through the

CF_0CF_1

ATP synthase channel into the stroma, catalyzing the reaction

ADP + P_i \rightarrow ATP

Chemiosmosis couples the downhill movement of protons to the phosphorylation of ADP to generate ATP.

Key Concept

Chemiosmotic Photophosphorylation in Chloroplasts

Estimated Time:2m 0s

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