Light harvesting complexes

Containing pigment molecules of chlorophyll a, b and carotenoids (such as beta carotene, shown), the light harvesting complexes are found in the thylakoid membrane and funnel light into the reaction centre by florescence. The molecules absorb photons which excites an electron inside the molecule to a higher energy level. Note that the absorption spectra for each pigment - chlorophylls a and b, and carotenoids - are non overlapping to broaden the range of light that can be absorbed for photosynthesis. When the electron returns to the ground state it releases another photon with a longer wavelength. The molecules of chlorophyll and carotene are angled at such a way as to insure the photons which are released due to fluorescence are directed toward the reaction centre, transferring the energy to a special pair of chlorophyll a molecules in the reaction centre of a photosystem.

from Rhodopseudomonas acidophila, a purple bacteria (Note: Very large molecule! Please wait for it to load).

the light harvesting complex in complex with, and surrounding, a reaction centre, from the closely related purple bacteria Rhodopseudomonas palustris

In plants, light harvesting complexes - or 'antenna complexes' - are found surrounding the reaction centres, to channel energy inwards. The combination of reaction centres and light harvesting complexes constitutes a photosystem. We shall discuss reaction centres and the photosystems later in this tutorial.

light harvesting complexes surrounding photosystem I

a single light harvesting complex from photosystem I (chlorophyll A heads shown in light green; chlorophyll B heads shown in dark green)

light harvesting complexes in photosystem II; reaction centres are shown in magenta, light harvesting complexes in green, and beta carotene in orange.