Light Gathering Structures

The thylakoid membranes of the chloroplast hold the chlorophyll and some of the accessory pigments. Their membranes contain only a small fraction of the common phospholipids, but are rich in glycolipids. They also contain much protein, and some of the photosynthetic pigments are attached to certain of these proteins.

Chlorophylls a and b are not attached to proteins, but interact with both membrane lipids (through their phytol tails) and proteins.

The assemblies of light-harvesting pigments in the thylakoid membrane together with their associated proteins, are organized into well-defined photosystems, structures that absorb light photons and convert some of it into a chemical form.

The first part of the process occurs in light-harvesting complexes. Each multisubunit protein complex contains multiple antenna pigment molecules, chlorophylls and some accessory pigments, and two chlorophyll molecules that act as the reaction center. The reaction center traps energy quanta excited by the absorption of light.

Absorption of light by a molecule excites it from the ground state to a higher electronic state. With photosynthetic pigments, the excited electron occupies a orbital in the system of conjugated double bonds of the porphyrin. The excitation energy in photosynthesis is harvested in two ways:

> 1. Resonance or exciton transfer (Figure 17.9a) - transfer of energy to an identical molecule. In this case, the first molecule retains the photon and returns to the ground state after the energy is transferred.

> 2. Electron transfer (Figure 17.9b) - transfer of an electron to a nearby molecule with a slightly lower excited stated, making molecule I a cation and molecule II an anion.

Resonance transfer is important for the way chlorophylls function in cells. Only about 1 O 2 molecule is produced for every 2500 chlorophylls. Instead, most of the chlorophyll molecules act as antenna molecules of the light-harvesting complexes. Antenna molecules absorb photons and the energy is passed by resonance transfer to specific chlorophyll molecules in a relatively few reaction centers. The path the energy takes to arrive at the energy center is random (Figure 17.11).

Chlorophylls in the reaction center are in a somewhat different environment, so the excited state energy of the reaction center is a bit lower. Excitation of the reaction center begins the actual photochemistry of the light reactions via a series of electron transfers.

See also: Light Absorbing Pigments, Chloroplast Anatomy, Photosystem II, Photosystem I, The Chloroplast, Chlorophyll, Chlorophyll a, Chlorophyll b