20.5: The Antenna Complex
Plants and other photosynthetic organisms comprise pigments capable of absorption of direct sunlight. These pigments are present in the reaction center - the main site of photochemical reactions as well as in the antenna complex. Under average light conditions, the rate at which reaction center pigments absorb light is far below the electron transport chain's capacity. As a result, the reaction center alone cannot provide enough energy to drive photosynthesis. The photosynthetic efficiency can be increased with an antenna complex where hundreds of pigment molecules absorb different wavelengths of light. The energy thus accumulated over a larger surface is then transferred to the reaction center.
The antenna complex, also known as the light-harvesting complex (LHC), comprises hundreds of chlorophyll molecules and dozens of carotenoids. The antenna pigments are organized into protein scaffolds that hold the pigments in the right position and orientation. The major function of the antenna complex is to transfer excitation energy to the reaction center. Another important function is to safely dissipate excess absorbed energy as heat.
In plants and most algae, the chlorophyll a and b molecules are integral components of the light-harvesting complex. The relative concentration of chlorophyll a and chlorophyll b is used as an indicator of the antenna size. Seasonal changes in light intensity may cause variation in the ratio of chlorophyll a/b, thus altering the antenna size. For instance, in LHCII (for photosystem II), low light conditions trigger the synthesis of chlorophyll b. As a consequence, the antenna size increases, allowing increased absorption of available light. In contrast, the antenna size must be reduced under high-light intensities to prevent excess light capture.