Photosynthetic Electron Transfer and Energy Transduction in Plants

Abstract

The photosynthetic membranes of plants perform a remarkable feat. They convert a portion of the energy available in light into the chemical energy of ATP and NADPH. In this way photosynthetic membranes provide a stable form of energy that can be used at later times for energy-requiring biochemical processes, such as the reduction of CO2 to carbohydrate. The first step in photosynthetic energy transformation is the absorption of light by the antenna array, resulting in the conversion of the transient energy stored in electromagnetic radiation into the excited state of pigment molecules. The excited state energy residing in the antenna system is short lived and must migrate rapidly to reaction center complexes, where it drives primary charge separation. The energy stored in the reaction centers by charge separation drives a series of oxidation/reduction reactions within the thylakoid membrane that ultimately convert the energy into the chemical free energy of ATP and NADPH. These energy conversion reactions are achieved by the cooperative interaction of four major protein complexes located in the thylakoid membrane. Three of these complexes, photosystems I and II (PS I and PS II) and the cytochrome bf complex (Cyt bf) are involved in light-driven electron and proton transfer. The fourth protein complex (ATP synthase) produces ATP from ADP and phosphate. In this introductory overview we have two goals. First is to introduce the players, that is the components of the chloroplast thylakoid membrane that are responsible for the basic reactions of photosynthetic electron transfer and energy transduction. Second we will track the energy transformations that ultimately result in the conversion of light energy into stable chemical forms. For the sake of clarity, we focus on general concepts and reference mostly review articles from which the interested reader can launch into the primary literature.