ABSTRACT
The light-dependent reactions of photosynthesis therefore underpin the production of reduced biomolecules that are re-oxidized during respiration to release energy. The light reactions of photosynthesis also produce adenosine triphosphate, which is the primary source of chemical energy for metabolic reactions in terrestrial ecosystems. Light energy trapped during photosynthesis therefore powers both unmanaged and managed terrestrial ecosystems. Photosystems consist of a reaction center core surrounded by light-harvesting complexes. The fluorescence properties of the light reactions provide an insight into their functioning and into the stress caused by variation in light levels. Photosynthetic organisms are subjected daily to a dark–light transition, so upon first light there are significant changes in the photosynthetic machinery that prepare photosystems for increased light. Excitation energy can be emitted in the form of light—chlorophyll molecules excited by absorption of light can fluoresce as they revert to their ground state.
