Some Aspects of the Relationship between Chlorophyll a Fluorescence and Photosynthetic Carbon Assimilation

Abstract

When a photon strikes a chlorophyll molecule it becomes excited, an electron is raised from a ground state in the molecular orbital to an excited state. Blue photons, carrying higher energy, raise electrons to the excited singlet state 2, red photons raise electrons to excited state 1. These of course are the photons which are preferentially absorbed. Green photons are mostly not absorbed and it is the green light which is reflected (from and transmitted through) leaves which gives them their colour. Excited state 2 decays very rapidly to excited state 1 as electrons cascade down the energy gradient, losing energy by radiationless de-excitation. None of this energy is available for photosynthesis. It is the red, more long-lived, excited state created directly by a red photon (or indirectly by a blue photon) which is the starting point of photosynthesis and, therefore, of most of biological energy transduction. It is from here that electrons are transported through the two photosystems to reduce NADP and finally CO₂. In addition the creation of the red excited state is accompanied by the generation of the massive oxidising potential, the positively charged “holes” within PSII, which are refilled by electrons drawn from water. Otto Warburg believed photosynthetic energy transduction to be perfect. It is not. Oxidation is inevitably associated with reduction. Electrons which have been raised to an excited state will fall back into the ground state if there is no acceptor available. Much of the energy dissipated in these circumstances is dissipated as heat. A fraction, released as electrons drop more or less directly back to the ground state, brings about the emission of photons.