ABSTRACT
This reaction is linked or coupled to yet another reaction resulting in the formation of a highly energetic compound called adenosine triphosphate (ATP). As this reaction involves the addition of a phosphate group (labelled
as Pi) to a compound called adenosine diphosphate (ADP) during the light reaction, it is called photophosphorylation. ADP + Pi → ATP Think of the light reaction, as a process by which organisms ‘capture and store’ radiant energy as they produce oxygen gas. This energy is stored in the form of chemical bonds of compounds such as NADPH and ATP. The energy contained in both NADPH and ATP is then used to reduce carbon dioxide to glucose, a type of sugar (C6H12O6). This reaction, shown below, does not require light, and it is often referred to as the ‘dark reaction’. 6CO2 + 24H
+ + 24e-→ C6H12O6 + 6H2O The chemical bonds present in glucose also contain a considerable amount of potential energy. This stored energy is released whenever glucose is catabolised (broken down) to drive cellular processes. The carbon skeleton in glucose also serves as a source of carbon for the synthesis of other important biochemical compounds such as, lipids, amino acids and nucleic acids. In simplest terms, the process of photosynthessis can be viewed as onehalf of the carbon cycle. In this half, energy from the sun is captured and transformed into nutrients which can be utilised by higher organisms in the food chain. The release of this energy during the metabolic re-conversion of glucose to water and carbon dioxide represents the second half of the carbon cycle and it may be referred to as catabolism or ‘oxidative processes’.
