In plant physiology, what is the first basic step that initiates the process of photosynthesis in green plants?

Introduction / Context:
Photosynthesis is the fundamental process by which green plants convert light energy into chemical energy. The overall process has many steps, but there is a very first event at the molecular level that begins the light reactions. Examinations often ask you to identify this initial step in the light dependent stage of photosynthesis.

Given Data / Assumptions:

• The question focuses on the first basic step in photosynthesis.
• The plant is assumed to have chlorophyll in its chloroplasts.
• Options mention ATP formation, water ionisation, electron excitation and release of oxygen.

Concept / Approach:
In the light reactions, chlorophyll molecules in the thylakoid membranes absorb photons of light. When a photon is absorbed, one of the electrons in the chlorophyll molecule becomes excited to a higher energy level. This excited electron is then transferred through an electron transport chain, leading to ATP and NADPH formation. Water splitting, or photolysis, occurs to replace lost electrons, but it is not the very first event. Thus, the earliest step is the excitation of chlorophyll electrons by light.

Step-by-Step Solution:
Step 1: Recall that photosynthesis begins with absorption of light by pigment molecules, mainly chlorophyll a. Step 2: Understand that absorption of light energy raises an electron of chlorophyll to a higher energy state. Step 3: Recognise that this excited electron is passed to an electron acceptor, starting the electron transport chain. Step 4: ATP formation and glucose synthesis occur later, powered by the energy from these electrons. Step 5: Therefore, excitation of a chlorophyll electron by a photon of light is the first basic step.

Verification / Alternative check:
Look at standard diagrams of the Z scheme or light reactions in any biology text. The sequence begins with a photosystem absorbing light and exciting electrons. Water splitting is shown as a supporting process that supplies new electrons, and ATP and glucose formation are later steps. This confirms that the first event is excitation of an electron in chlorophyll by light.

Why Other Options Are Wrong:
Option A, formation of ATP, happens after electrons move through the electron transport chain and is not the first step. Option B, joining of three carbon compounds to form glucose, occurs in the Calvin cycle, which is a later dark reaction stage. Option C, ionisation or splitting of water, occurs to replace electrons lost from chlorophyll but is triggered after light is absorbed. Option E, release of oxygen gas, is also a later result of water splitting and not the initiating event.

Common Pitfalls:
Students sometimes mix up the sequence of light and dark reactions or assume that visible outputs like oxygen release are the starting point. The most reliable way to remember is to think of light first, then electron excitation, then a chain of events leading to ATP, NADPH and finally glucose. Light absorption by chlorophyll and the resulting excitation of electrons is the true starting trigger.

Final Answer:
The first basic step in photosynthesis is Excitation of an electron of chlorophyll by a photon of light.