Water serves as the source of electrons in the light-dependent reactions of photosynthesis, specifically through photolysis at Photosystem II (PSII). This process replaces electrons excited from chlorophyll and drives the production of ATP and NADPH. The correct answer is (B) Water.

Question Breakdown

This multiple-choice question tests understanding of the electron transport chain in photosynthesis, a key topic for exams like CSIR NET Life Sciences.

• (A) Carbohydrates: Incorrect. Carbohydrates like glucose are products of the Calvin cycle (light-independent reactions), not electron sources. They store energy from prior photosynthesis but do not donate electrons in the light reactions.

• (B) Water: Correct. In PSII, light energy splits water (H₂O) into oxygen (O₂), protons (H⁺), and electrons via the oxygen-evolving complex. These electrons replenish those lost from the reaction center chlorophyll (P680), fueling non-cyclic electron flow to PSI and NADPH formation.

• (C) CO₂: Incorrect. CO₂ acts as a carbon source in the dark reactions, fixed into sugars using ATP and NADPH. It accepts electrons indirectly but is not the initial donor.

• (D) NADH: Incorrect. NADH operates in cellular respiration (mitochondria) as an electron carrier from glycolysis/TCA cycle. Photosynthesis uses NADPH instead, and neither donates electrons to PSII.

Introduction
In photosynthesis, the source of electrons originates from water during the light-dependent reactions, powering the electron transport chain in chloroplasts. This process, vital for converting sunlight into chemical energy, replaces electrons excited in Photosystem II (PSII) and leads to oxygen release. Understanding the source of electrons in photosynthesis clarifies why water—not carbohydrates, CO₂, or NADH—drives ATP and NADPH production.

Light Reactions Overview

Photosynthesis splits into light-dependent reactions (thylakoid membrane) and light-independent reactions (stroma). Light excites electrons in chlorophyll of PSII (P680) and PSI (P700), creating high-energy carriers.

• Electrons flow non-cyclically: PSII → plastoquinone → cytochrome b₆f → plastocyanin → PSI → ferredoxin → NADP⁺ (forming NADPH).

• Proton gradient from electron transport powers ATP synthase for ATP synthesis.

• Water photolysis (2H₂O → 4H⁺ + 4e⁻ + O₂) replenishes PSII electrons, confirmed by ¹⁸O-labeling experiments.

Why Water as Electron Donor?

PSII’s oxygen-evolving complex (OEC) catalyzes water oxidation, a four-electron process requiring light quanta. This evolved in oxygenic photosynthesis, distinguishing plants from bacteria using H₂S. Without water, electron flow halts, halting photosynthesis.

Common Misconceptions

Examinees often confuse roles:

This table highlights precise functions for exam recall.