chlorophylla

Chlorophyll a, also written chlorophyll-a, is a green pigment that is essential for oxygenic photosynthesis in plants, algae, and cyanobacteria. It is the primary photosynthetic pigment, participating directly in the conversion of light energy into chemical energy.

Chlorophyll a molecules absorb light mainly in the blue-violet (around 430 nm) and red (around 660–665 nm) portions of the spectrum and transmit or reflect green light, which is why plants appear green. Molecularly, chlorophyll a consists of a chlorin ring with a central magnesium ion and a long hydrophobic phytol side chain that anchors the molecule in the thylakoid membranes of chloroplasts or cyanobacterial membranes.

In the photosystems of the thylakoid membranes, chlorophyll a molecules act as primary electron donors after photon absorption. In photosystem II, excited chlorophyll a (P680) donates electrons to the electron transport chain, while in photosystem I, excited chlorophyll a (P700) does similarly for the other branch of the chain. Accessory pigments such as chlorophyll b expand the range of absorbed light.

Chlorophyll a is synthesized via the tetrapyrrole biosynthesis pathway, beginning with porphyrin precursors and culminating in the magnesium-chelated chlorin ring. It is degraded when tissues are damaged or exposed to light and heat, releasing smaller compounds that may contribute to leaf senescence.

Besides photosynthesis, chlorophyll a and related pigments are used as natural colorants; chlorophylls are sometimes processed into chlorophyllin, a water-soluble copper complex used as a food additive and cosmetic dye.