Chlorophyll

Chlorophyll is the main light-absorbing pigment of photosynthesis. In plants and algae it sits in the thylakoid membranes inside chloroplasts. Each molecule has a large ring built from carbon and nitrogen atoms, holding a single magnesium atom in the middle, and a long hydrocarbon tail that anchors it in the membrane. Plants contain two main forms, chlorophyll a and chlorophyll b, which differ by one small chemical group on the ring.

Chlorophyll absorbs blue and red light strongly but takes up little green light, which is reflected or passes through; that is why leaves look green. In a photosystem, hundreds of chlorophyll and accessory pigment molecules act as an antenna, passing absorbed energy to a special pair of chlorophyll a molecules at the reaction center. There, the energy boosts an electron to a higher level and hands it to an electron acceptor, starting the flow of electrons that makes ATP and NADPH and splits water to release oxygen.

Other pigments, such as yellow and orange carotenoids, absorb additional wavelengths and protect the system from excess light. In autumn, when deciduous trees break down their chlorophyll, these carotenoids become visible.