Modern Concept of Photosynthesis

The modern understanding of photosynthesis frames it as one coupled oxidation-reduction process split cleanly into two phases. Water is oxidised to release oxygen, and carbon dioxide is reduced to form sugars; the first of these two halves needs light directly and is called the light reaction (or Hill's reaction, after section 13.7.3), while the second does not need light directly and is called the dark reaction. The light reaction is a photochemical reaction - solar energy, trapped by chlorophyll, is converted and stored as chemical "assimilatory power": ATP and the reducing agent NADPH+H+. It takes place in the thylakoid membrane, and it is here that water molecules are split by light, releasing oxygen as a byproduct. The light reaction itself unfolds in two sequential phases: a photo-oxidation phase (section 13.9), covering the absorption of light energy, its relay from accessory pigments to the reaction centre, and the resulting activation of chlorophyll a; and a photochemical phase (section 13.10), covering the actual photolysis of water with oxygen evolution, and the electron transport that synthesises the assimilatory power (ATP and NADPH+H+). The dark reaction (or biosynthetic phase) is, by contrast, a thermochemical reaction - its rate depends strongly on temperature rather than directly on light - in which the assimilatory power generated by the light reaction is …