– light reaction take place on the grana of the chloroplast in photosynthetic units known as quantosomes
– quantosomes are each made up of about 230 chlorophyll molecules
– incoming photon of light strike a chlorophyll molecule and raises the energy level of electron
– molecule becomes excited
– electron is taken up by carriers and passed further through a series of carrier
– the last hand it back to the chlorophyll molecule
– stability is restored when the electron returns
– as chlorophyll loses electron it becomes charged and unstable
– as electron passed from carrier to carrier at different energy level, energy is removed from it and used for synthesis of ATP
– typically, two ATP are formed every time a single electron goes through the cycle
– ATP is used to power reaction of the dark stage
– ADP and inorganic phosphate are required
– if these are absent, the electron emitted from the chlorophyll is returned uncharged
– the energy being released as heat and fluorescence.
– process called photosynthetic phosphorylatic (photophosphorylation)
– As distinguished from oxidative phosphorylation during cellular respiration
– during the process water dissociates into hydrogen and hydrogen and hydroxyl ions
– some of the electrons released from chlorophyll combine with hydrogen ions to form a hydrogen molecule (H2)
– this is used to reduce NADP+ (giving NADPH2)
– reduction power is used to fix carbon (iv) oxide into carbohydrate
– the hydroxyl ion donates an electron to chlorophyll
– the OH resulting from this forms water and oxygen
– the oxygen is released as by-product which other aerobic organisms use in respiration.