During the light-dependent reactions of photosynthesis, which molecule provides the electrons that are passed along the electron transport chain in the thylakoid membranes?

Introduction / Context:
Photosynthesis in green plants occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). In the light-dependent reactions, light energy is converted into chemical energy in the form of ATP and reduced coenzymes. Electrons are excited and passed through an electron transport chain in the thylakoid membranes. This question asks you to identify the original donor molecule that supplies these electrons at the start of the light reactions.

Given Data / Assumptions:

• We are dealing specifically with the light-dependent reactions in chloroplasts.
• The options include water, oxygen, carbon dioxide, light, and glucose.
• We assume normal oxygenic photosynthesis as found in green plants and algae.
• We also assume basic knowledge of photosystems and photolysis of water.

Concept / Approach:
In the light reactions, chlorophyll molecules in photosystem II absorb light energy and become excited. These excited chlorophyll molecules lose electrons, which enter the electron transport chain. To replace the lost electrons, water molecules are split in a process called photolysis. This splitting of water releases electrons, protons (hydrogen ions), and oxygen gas. The electrons from water are used to replenish photosystem II and ultimately help reduce NADP+ to NADPH. Oxygen is a by-product, not the donor of electrons. Carbon dioxide participates later in the Calvin cycle, not in the initial electron donation. Light provides energy but not electrons itself. Glucose is a product of photosynthesis rather than a source of electrons in the light reactions.

Step-by-Step Solution:
Step 1: Identify that the question focuses on the beginning of the light-dependent reactions, not the Calvin cycle. Step 2: Recall that photosystem II uses light energy to excite electrons, which are then passed along the electron transport chain. Step 3: Remember that when chlorophyll in photosystem II loses electrons, it must be re-supplied with electrons from another source. Step 4: Recognise that water molecules are split in the thylakoid lumen by the oxygen evolving complex, releasing electrons, protons, and oxygen. Step 5: Understand that these electrons refill the electron deficiency in photosystem II and begin the chain that ultimately produces ATP and NADPH. Step 6: Conclude that water is the original electron donor in the light reactions.

Verification / Alternative check:
Equations for oxygenic photosynthesis are often summarised as: 2H2O + light energy → O2 + 4H+ + 4e-. This clearly shows that water is broken down to release electrons. The Z-scheme diagram of photosynthesis in textbooks shows water at the bottom, feeding electrons to photosystem II. Oxygen appears at the output side as a gas released into the atmosphere. Carbon dioxide appears later in diagrams of the Calvin cycle where it is fixed into sugars. These standard illustrations confirm that water provides electrons to the light reactions.

Why Other Options Are Wrong:
Option B: Oxygen is produced when water is split but does not donate electrons in the light reactions. Option C: Carbon dioxide is the carbon source for sugar formation in the Calvin cycle, not the electron donor in the thylakoid electron transport chain. Option D: Light is the energy source that drives electron excitation but does not itself provide electrons. Option E: Glucose is a product of photosynthesis and may later be used in respiration; it is not the starting electron donor in the light reactions.

Common Pitfalls:
Students sometimes confuse the roles of water and oxygen, thinking that oxygen must donate electrons because it is part of water or is associated with respiration. Others may mistakenly think light somehow provides particles rather than energy. Remember that in photosynthesis, water is the electron donor, oxygen is a by-product, light provides energy to excite electrons, and carbon dioxide is the carbon source used later. Keeping these roles separate helps in correctly answering questions about photosynthetic pathways.

Final Answer:
In the light-dependent reactions of photosynthesis, the electrons passed along the electron transport chain are originally provided by water, which is split during photolysis.