In plant biology, which substance present in chloroplasts is responsible for giving most plants their characteristic green colour and capturing light energy for photosynthesis?

Introduction / Context:
Most plants appear green to our eyes, and this colour is closely linked to how they capture energy from sunlight. The green colour comes from a specific pigment located inside chloroplasts, the organelles where photosynthesis takes place. Understanding which substance is responsible for this colour and for absorbing light is a basic topic in biology and is essential for understanding how plants produce food and oxygen.

Given Data / Assumptions:

• The question asks which substance gives plants their green colour.
• Options include chloroform, chlorine, chlorophyll, and caustic soda.
• Chloroplasts are known as the site of photosynthesis in plant cells.
• We assume standard school-level information about plant pigments.

Concept / Approach:
Chlorophyll is the primary photosynthetic pigment in green plants, algae, and cyanobacteria. It absorbs light mainly in the blue and red regions of the spectrum and reflects green light, which is why plants look green to us. The pigment is located in the thylakoid membranes inside chloroplasts. Chloroform is a laboratory solvent, not a plant pigment. Chlorine is a greenish-yellow gas used as a disinfectant and in water treatment, and it is toxic to plants at high concentrations. Caustic soda (sodium hydroxide) is a strong alkali used in cleaning and industry, not a pigment. Only chlorophyll matches the role of giving plants their green colour and capturing light for photosynthesis.

Step-by-Step Solution:
Step 1: Recall that photosynthesis occurs in chloroplasts and requires a pigment to absorb light. Step 2: Remember that chlorophyll is the green pigment that absorbs sunlight, especially red and blue wavelengths. Step 3: Understand that chlorophyll reflects green light, making leaves and stems appear green. Step 4: Examine the options and notice that chloroform, chlorine, and caustic soda are chemicals used in laboratory or industrial contexts, not plant pigments. Step 5: Conclude that chlorophyll is the only option that correctly describes the green pigment in plants.

Verification / Alternative check:
Biology textbooks and diagrams of plant cells identify chlorophyll as the main pigment found in chloroplasts. They explain that chlorophyll a and chlorophyll b are the major types in green plants and that these pigments are crucial for converting light energy into chemical energy during photosynthesis. Experiments that extract chlorophyll from leaves using alcohol produce a green solution, further confirming chlorophyll's role in plant colour. None of the other substances listed (chloroform, chlorine, caustic soda) appear in standard discussions of plant pigments. This confirms that chlorophyll is responsible for the green colour of plants.

Why Other Options Are Wrong:
Chloroform, an organic solvent used in laboratories, is not a natural plant pigment and does not play a role in giving plants their green colour.
Chlorine, a greenish-yellow gas used as a disinfectant, is toxic at high levels and is not involved in photosynthesis or normal plant pigmentation.
Caustic soda, a strong alkali used in industry, is a chemical that can damage living tissues and has nothing to do with plant colour.

Common Pitfalls:
Students may notice that several options begin with “chloro-” and become confused. The prefix “chloro-” means “green,” but only chlorophyll is the plant pigment. Chloroform and chlorine are different chemical substances with very different uses. A helpful memory tip is that chlorophyll ends with “-phyll,” which is similar to the Greek word for “leaf,” reminding you that it is the leaf pigment. Remembering this will help you quickly identify the correct answer in questions about plant colour and photosynthesis.

Final Answer:
The substance that gives plants their green colour is Chlorophyll, the green pigment in plant chloroplasts.