Question

The first reaction in photosynthesis is:

• A. Photolysis of water
• B. Excitation of chlorophyll molecule
• C. Reduction of CO\(_2\)
• D. Utilisation of assimilatory power \vspace{0.5cm

Hint:

Understanding the initial steps in photosynthesis is crucial, starting with the excitation of chlorophyll by light, which drives the entire series of subsequent reactions.

Answer 1

The Correct Option is B

In photosynthesis, the first reaction that occurs involves the excitation of chlorophyll molecules by photons of light. This process takes place within the light-harvesting complexes of the thylakoid membranes of the chloroplasts. When photons strike chlorophyll molecules, their energy excites electrons to a higher energy state. This excitation is the primary trigger for the subsequent reactions in the light-dependent phases of photosynthesis. Here’s a breakdown of the options:
- Excitation of the chlorophyll molecule is the correct answer because it initiates the chain of reactions in the light-dependent processes by providing excited electrons.
- Photolysis of water, although a crucial early step, actually occurs after the excitation of chlorophyll as it provides the electrons that replace those excited in the chlorophyll.
- Reduction of CO\(_2\) occurs later in the Calvin cycle, which is part of the light-independent reactions.
- Utilisation of assimilatory power involves the use of ATP and NADPH generated in the light reactions to fix CO\(_2\), which occurs even later in the process. Thus, the first reaction in photosynthesis is the excitation of the chlorophyll molecule by light.