Light reactions take place during the first stage of photosynthesis. During this stage, light energy is converted into chemical energy. It takes place in the thylakoid membranes of the chloroplast and includes photosynthetic pigments arranged into two light-harvesting complexes within photo system I and photo system II.
Light reaction consists of the following four stages – light absorption, water splitting, release of oxygen and formation of high-energy intermediates ATP and NADPH.
In the light-absorption stage, the chlorophyll a in PSII absorbs the 680 nm wavelength of red light. The electrons from PS II are passed on to PS I through the electron transport system. Simultaneously electrons of the PS I reaction centre are excited by absorbing red light of wavelength 700 nm. These are transferred to another acceptor molecule with greater redox potential. These electrons move to a molecule of energy-rich NADP+, thereby reducing it to NADPH.
In water-splitting, water is split into H+, O and electrons. The electrons that were removed from PS II are replaced by electrons formed due to the splitting of water. The electrons needed to replace those removed from PS I are provided by the excited electrons of PS II through the electron transport system.
The process of synthesising high-energy compounds like ATP by cells in the chloroplast and mitochondria is called phosphorylation. In non-cyclic photo, phosphorylation PS II and PS I work in series. These two photo systems are connected through the electron transport chain. ATP and NADPH + H+ are synthesised in this electron flow. The chemiosmotic hypothesis explains the mechanism of ATP synthesis. ATP synthesis is linked to the development of the proton gradient across the membrane. The proton accumulates towards the inside of the thylakoid membrane in the lumen.
Water splitting takes place on the inner side of the membrane, producing protons or hydrogen ions that accumulate within the lumen of the thylakoid. This creates a proton gradient across the thylakoid membrane. The breaking of the proton gradient releases energy. The ATPase enzyme present in the transmembrane channel helps in the breakdown of the proton gradient. The breakdown of the gradient provides energy to cause confirmation change in the F1 particle of the ATPase. This makes the ATPase enzyme synthesis several molecules of ATP. The synthesis of ATP thus requires a membrane, a proton pump, a proton gradient and the enzyme ATPase. Next, the NADPH and ATP produced by the movement of electrons are used in the Calvin Cycle.