Chloroplast Structure and Pigments

A chlorophyll molecule works like a tiny antenna tuned to certain wavelengths. When a photon of the right energy hits it, an electron inside the molecule absorbs that energy and jumps to a higher energy level, becoming excited. That excited electron is then passed along to other molecules, eventually driving the reactions of photosynthesis. Chlorophyll a absorbs best in the blue and red parts of the spectrum but barely touches green light, which bounces off or passes through the leaf. That is why leaves look green: you are seeing the color chlorophyll rejects, not the color it uses.

Relying on chlorophyll a alone would waste a lot of usable sunlight, since its absorption bands are narrow. Accessory pigments, chlorophyll b, xanthophylls, and carotenoids, absorb wavelengths chlorophyll a misses, then transfer that captured energy over to chlorophyll a. This widens the range of light a leaf can actually use, which is why plants evolved a mix of pigments instead of just one.

All of this happens inside the chloroplast, organized for efficiency. The stroma is the fluid interior, where sugar-building reactions later take place. Suspended in the stroma is the thylakoid membrane, a folded system of flattened sacs that holds chlorophyll, accessory pigments, and the protein complexes of the light reactions. In many spots these sacs stack into grana, piles that pack huge amounts of pigment and membrane surface into a small space. This structure sets up the next piece: the thylakoid membrane is also where protons get pumped to build the gradient that powers ATP synthesis.