Antenna Complexes for Photosynthesis

The capture of light energy for photosynthesis is enhanced by networks of pigments in the chloroplasts arranged in aggregates on the thylakoids. These aggregates are called antennae complexes. Evidence for this kind of picture came from research by Robert Emerson and William Arnold in 1932 when they measured the oxygen released in response to extremely bright flashes of light. They found that some 2500 molecules of chlorophyll was required to produce one molecule of oxygen, and that a minimum of eight photons of light must be absorbed in the process.

The model that emerges is that of some 300 chlorophyll molecules and 40 or so beta carotenes and other accessory pigments acting as a light harvesting antenna surrounding one chlorophyll a molecule that is a part of an action center. A photon is absorbed by one of the pigment molecules and transfers that energy by successive flourescence events to neighboring molecules until it reaches the action center where the energy is used to transfer an energetic electron to an electron acceptor.

The fluorescence model would suggest that each transferred photon has a longer wavelength and lower quantum energy with some energy being lost to heat.

When a photon reaches the chlorophyll a in the reaction center, that chlorophyll can receive the energy because it absorbs photons of longer wavelengths than the other pigments. Two types of chlorophyll centers have been identified, and are associated with two protein complexes identified as Photosystem I and Photosystem II.

Photosystem I

The light absorption processes associated with photosynthesis take place in large protein complexes known as photosystems. The one known as Photosystem I contains a chlorophyll dimer with an absorption peak at 700 nm known as P700.

Photosystem I makes use of an antenna complex to collect light energy for the second stage of non-cyclic electron transport. It collects energetic electrons from the first stage process which is powered through Photosystem II and uses the light energy to further boost the energy of the electrons toward accomplishing the final goal of providing energy in the form of reduced coenzymes to the Calvin cycle.

The above sketch depicts the setting of Photosystem I in the electron transport process which provides energy resources for the Calvin cycle.

Photosystem I is the light energy complex for the cyclic electron transport process used in some photosynthetic prokaryotes.

The protein complex that constitutes Photosystem I contains eleven polypeptides, six of which are coded in the nucleus and five are coded in the chloroplast. The core of Photosystem I contains about 40 molecules of chlorophyll a, several molecules of beta carotene, lipids, four manganese, one iron, several calcium, several chlorine, two molecules of plastoquinone, and two molecules of pheophytin, a colorless form of chlorophyll a .(Moore, et al.)

Photosystem II

The light absorption processes associated with photosynthesis take place in large protein complexes known as photosystems. The one known as Photosystem II contains the same kind of chlorophyll a as Photosystem I but in a different protein environment with an absorption peak at 680 nm. (It is designated P680). The binding protein for PSII is much smaller than that for PSI, about 47,000 compared to 110,000. It resonates from energy transmitted by about 250 chlorophyll a and b in equal numbers. Its core contains xanthophylls but no beta carotene (Moore).

Photosystem II makes use of an antenna complex to collect light energy for the first stages of non-cyclic electron transport.

This sketch shows some of the context of Photosystem II in the electron transport process in the thylakoid membrane. It is part of the process of splitting water and providing the electrons to plastoquinone for further transport to the cytochrome complex and then to Photosystem I.