In the process of photosynthesis, the liberation of oxygen gas takes place mainly due to which specific reaction occurring in the chloroplasts?

Introduction / Context:
Photosynthesis is the fundamental process by which green plants produce food and release oxygen into the atmosphere. A common question in biology asks from which substance the released oxygen actually originates. Understanding the source of this oxygen clarifies the role of water in the light dependent reactions of photosynthesis. This question focuses on identifying the correct step that produces oxygen gas.

Given Data / Assumptions:

• Photosynthesis occurs in two main stages: light dependent reactions and light independent reactions.
• Water and carbon dioxide are the main raw materials in photosynthesis.
• Oxygen is released as a by product of the process.
• Carbohydrates are formed as the main food product.

Concept / Approach:
Experiments using isotopes have shown that the oxygen released in photosynthesis comes from water, not from carbon dioxide. In the light dependent reactions, water molecules are split by the energy of absorbed light in a process called photolysis. This splitting of water produces electrons, protons and oxygen gas. The electrons are used in the electron transport chain, the protons contribute to ATP and NADPH formation, and the oxygen is released as a by product. Hydrolysis of carbohydrates and reduction of carbon dioxide occur at different stages and do not directly liberate oxygen gas. Breakdown of chlorophyll is related to ageing and stress, not to normal oxygen evolution in photosynthesis.

Step-by-Step Solution:
Step 1: Recall that photosynthesis involves light dependent reactions in the thylakoid membranes and light independent reactions in the stroma. Step 2: Understand that in the light dependent reactions, water molecules are split in the presence of light energy. Step 3: Recognise that this splitting, called photolysis of water, produces oxygen gas as one of the products. Step 4: Note that reduction of carbon dioxide produces carbohydrates but does not release free oxygen gas. Step 5: Conclude that liberation of oxygen is due specifically to photolysis of water molecules in the light reactions.

Verification / Alternative check:
Isotope labelling experiments using water containing a heavy isotope of oxygen showed that the oxygen released during photosynthesis carries the isotope label, while oxygen in carbon dioxide does not. This confirms that water, not carbon dioxide, is the source of the evolved oxygen. Detailed diagrams of the Z scheme of photosynthesis also show water splitting at photosystem II with oxygen evolution. These findings support the conclusion that photolysis of water is responsible for oxygen liberation.

Why Other Options Are Wrong:
Option a, hydrolysis of carbohydrates, refers to breaking down complex sugars and would consume water rather than release oxygen gas.

Option c, reduction of carbon dioxide, forms carbohydrates and uses electrons and protons, but it does not produce molecular oxygen.

Option d, breakdown of chlorophyll, occurs mainly during leaf ageing and stress and is not part of the normal mechanism producing oxygen in photosynthesis.

Common Pitfalls:
Students sometimes assume that because carbon dioxide contains oxygen atoms, the released oxygen must come from carbon dioxide. This misconception is corrected by learning about water splitting and by studying the experimental evidence using isotopes. Another pitfall is to mix up hydrolysis and photolysis, which are both chemical terms but refer to different processes. Remembering that photolysis is driven by light and specifically splits water in photosynthesis helps avoid confusion.

Final Answer:
The liberation of oxygen during photosynthesis is due to photolysis of water molecules in the light reactions.