Q.Photosynthetic organisms occur at different depths in the ocean. Do they receive qualitatively and quantitatively the same light? How do they adapt to carry out photosynthesis under these conditions?

Accessory Pigment Function

Imagine a solar panel that only works with red light. On a cloudy day, when red light is scarce, that panel is nearly useless. But what if the panel had a second coating that could capture green and blue light too, and then funnel that energy into the same working parts? That second coating is exactly what accessory pigments do in photosynthesis.

The core problem

Chlorophyll a — the primary pigment — absorbs light most strongly in the red and blue-violet regions of the spectrum. It reflects green, which is why leaves look green. But sunlight contains a broad range of wavelengths. If a plant relied only on chlorophyll a, it would waste a huge fraction of the available light energy, especially in the green-yellow range.

Nature solved this by adding accessory pigments: other molecules that absorb light at wavelengths chlorophyll a cannot use well, and then pass that captured energy to chlorophyll a.

The precise statement

Accessory pigments are light-absorbing molecules (such as chlorophyll b, carotenoids, and xanthophylls) that broaden the absorption spectrum of photosynthesis. They capture photons at wavelengths not efficiently absorbed by chlorophyll a and transfer that excitation energy to the reaction-center chlorophyll a molecules, where the actual conversion of light energy into chemical energy begins.

How it works, step by step

1. A photon of green light strikes a molecule of carotene (an orange accessory pigment). The carotene absorbs it, raising an electron to a higher energy level.
2. That excited energy does not stay on the carotene. Through a process called resonance energy transfer, the excitation hops from molecule to molecule — like a bucket brigade — until it reaches a chlorophyll a molecule in the reaction center of a photosystem.
3. Once the energy arrives at chlorophyll a, it drives the electron-transfer reactions that ultimately produce ATP and NADPH.

The accessory pigment itself never participates in the electron-transport chain. It is purely an antenna — a collector that hands over its catch.

The key consequence

Without accessory pigments, a plant would absorb only about 40–50% of the visible light hitting it. With them, that figure rises to roughly 80–85%. The plant does not need to invent a new kind of solar cell for each colour — it just adds different collectors that all feed into the same central machine. …