Q.Cyanobacteria and some other photosynthetic bacteria don't have chloroplasts. How do they conduct photosynthesis?

Why do leaves look green?

You already know that plants make their own food using sunlight. But sunlight is just energy — how does a leaf actually catch it? The answer is that leaves contain special molecules called photosynthetic pigments. Think of them as tiny solar panels built into the leaf cells.

A pigment is any substance that absorbs light. Different pigments absorb different colours (wavelengths) of light. The colour you see when you look at a leaf is the colour that the pigment does not absorb — it reflects that colour instead. So a green leaf looks green because its pigments absorb red and blue light strongly, but reflect green light.

The main pigments and what they do

There are four major photosynthetic pigments in higher plants. They work together like a team — each one catches a slightly different slice of the sunlight spectrum.

Chlorophyll a is the star player. Every photosynthetic plant, alga, and cyanobacterium has it. It absorbs light most strongly in the red (~660–680 nm) and blue-violet (~430 nm) regions. This is the only pigment that can directly convert light energy into chemical energy — it sits at the reaction centre of the photosystems.

Chlorophyll b is the backup. It absorbs light in slightly different red and blue bands (around 460 nm and 640 nm). It cannot do the energy conversion itself, but it captures extra light and passes the energy to chlorophyll a. This broadens the range of light the plant can use.

Carotenoids (like beta-carotene) and xanthophylls are the yellow-orange pigments. They absorb blue-green light (around 450–500 nm) — wavelengths that chlorophylls handle poorly. They also have a critical protective role: they safely dissipate excess energy that could otherwise damage the chlorophyll molecules.

How they work together

Imagine a leaf as a solar panel made of many small antennae. Each antenna is a cluster of pigments — mostly chlorophyll b and carotenoids — surrounding a single chlorophyll a molecule at the centre. This whole assembly is called a photosystem.

When a photon of light hits any pigment in the antenna, its energy gets passed from molecule to molecule like a hot potato, until it reaches the chlorophyll a at the reaction centre. There, the energy is used to excite an electron, which starts the electron transport chain — the first step of photosynthesis.