During photosynthesis in green plants, which of the following is not a correct description of an event in the process?

Introduction / Context:

This question examines understanding of the basic steps and chemistry of photosynthesis. Green plants convert light energy into chemical energy stored in carbohydrates. Several key events occur during this process, some involving oxidation and reduction reactions. The task is to identify which described event does not correctly match what happens in photosynthesis.

Given Data / Assumptions:

• Photosynthesis occurs mainly in chloroplasts of green plant cells.
• Chlorophyll pigments absorb light energy.
• Water molecules are split, releasing oxygen.
• Carbon dioxide is used to build carbohydrates such as glucose.
• Redox reactions involve both oxidation and reduction steps.

Concept / Approach:

In photosynthesis, water is oxidised and carbon dioxide is reduced. Oxidation involves loss of electrons, while reduction involves gain of electrons. Oxygen is released as a by product when water is split in the light dependent reactions. In the Calvin cycle, carbon dioxide is fixed and reduced to form carbohydrates. Therefore, any statement that describes carbon dioxide as being oxidised rather than reduced is incorrect. The approach is to recall which compound is oxidised and which is reduced during photosynthesis.

Step-by-Step Solution:

Verification / Alternative check:

The simplified overall equation for photosynthesis is often written as carbon dioxide plus water yields glucose plus oxygen using light energy. In redox terms, water loses electrons and is oxidised to oxygen, while carbon dioxide gains electrons and hydrogen atoms to form glucose, so it is reduced. Biochemistry texts clearly classify carbon in carbon dioxide as moving from a higher oxidation state to a lower one in carbohydrates, confirming reduction. Therefore, stating that carbon dioxide is oxidised to carbohydrates is not correct.

Why Other Options Are Wrong:

The option about absorption of light energy by chlorophyll is correct because chlorophyll molecules capture photons in the light harvesting complexes. The conversion of light energy to chemical energy is accurate, as light drives the formation of ATP and NADPH. Splitting of water into hydrogen and oxygen, or more precisely into protons, electrons and oxygen gas, is a central light dependent reaction. The reduction of carbon dioxide to form glucose accurately describes the Calvin cycle outcome. These statements are true events in photosynthesis and therefore are not the answer to the question which asks for the incorrect description.

Common Pitfalls:

Some students are confused about which molecule is oxidised and which is reduced because both water and carbon dioxide change during the process. It is easy to mistakenly think that carbon dioxide is oxidised because oxygen appears in both carbon dioxide and produced oxygen gas, but in fact the oxygen released comes from water. Another pitfall is not clearly understanding the redox definition and focusing only on the summary equation. Keeping track of electron flow helps clarify that water is oxidised and carbon dioxide is reduced.

Final Answer:

The statement that is not a correct event in photosynthesis is Oxidation of carbon dioxide to form carbohydrates.