Physical mechanisms in photosynthesis: past elucidations and current problems.

Photosynthetic tissues are organized functionally into aggregates of light harvesting pigments (mainly chlorophylls, carotenoids, and phycobilins) associated with photochemical reaction centers (1, 2). The pigments absorb light and deliver the energy to the reaction centers, where an oxido-reductive photochemistry ensues. The primary photoproducts, oxidizing and reducing entities, serve as starting points for electron transport that is coupled to phosphorylation. In known cases, the primary photochemical electron donor, at the reaction center, is a chlorophyll (Chl) or bacteriochlorophyll (BChl). In the specialized context of the reaction center this donor is generally designated P (for pigment) followed by a number signifying the peak of the long wave absorption maximum: P700, P870, etc. (3, 4). This organization defines certain physical problems: (a) How is energy, absorbed by the light harvesting pigments, delivered to the reaction centers? (b) What are the details of the photochemical process? (c) How are the primary photoproducts used effectively and safely, without wasteful recombination or harmful indiscriminate reactions with the surroundings? We shall consider these questions in turn, especially as related to the photosynthetic bacteria.