The Primary Steps of Photosynthesis

Abstract

Photosynthesis, the process by which plants convert solar energy into chemical energy, results in about 10 billion tons of carbon entering the biosphere annually as carbohydrate—equivalent to about eight times mankind's energy consumption in 1990. The apparatus used by plants to perform this conversion is both complex and highly efficient. Two initial steps of photosynthesis—energy transfer and electron transfer—are essential to its efficiency: Molecules of the light-harvesting system transfer electronic excitation energy to special chlorophyll molecules, whose role is to initiate the directional transfer of electrons across a biological membrane; the electron transfer, which takes place in a pigment-protein complex called the reaction center, then creates a potential difference that drives the subsequent biochemical reactions that store the energy. (Higher plants use two different reaction centers, called photosystems I and II, while purple bacteria make do with a single reaction center. The difference is that the bacteria do not generate oxygen in the photosynthetic process.) Both the elementary energy transfer and the primary electron transfer are ultrafast (occurring between 10−13 and 10−12 seconds), leading to the trapping of excitation energy at the reaction center (on a 100-picosecond timescale) and subsequent electron transfer in about 3 picoseconds with almost 100% quantum yield.