The Role of Electrochemistry in the Transduction of Light Energy by the Chloroplast Thylakoid Membrane

Abstract

This paper outlines current understanding of energy transduction in photosynthesis, including the primary photochemical reactions where light energy is converted into electrochemical energy, and the mechanisms by which the electrochemical energy is converted into chemical energy in the form of adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH). In the Mitchell chemiosmotic hypothesis, ATP formation is driven by the difference in the chemical potential of protons which is established across the chloroplast thylakoid membrane during the energy transduction process. On the other hand, Williams suggested that protons are accumulated within the membrane during electron transport, and intramembrane protons are the immediate pool for ATP synthesis. We have attempted to distinguish between the transmembrane and intramembrane mechanisms by illuminating chloroplasts with flashes of light and measuring the formation of the transmembrane proton gradient in relation to ATP synthesis. The onset of a measurable rate of ATP synthesis on illumination of chloroplasts with single flashes was too slow to distinguish between the two mechanisms.