ATP SYNTHESIS
ATP synthesis happens as the result of ADP phosphorylation. ATP molecules are formed of ADP and inorganic phosphate at the cost of light energy (in plant chloroplasts), or, the energy, which releases in chemical reactions (in mitochondria).
P. Mitchell set forward the chemiosmotic theory, which explained the formation mechanism of ATP out of ADP. He supposed that cytochromes in membranes are placed in a special way, which causes gradient of hydrogen ions ( protons). The concentration of ions H+ on the outer side of the membranes of mitochindria (chloroplasts) in this case becomes higher than on the inner side. Single cytochrome components are placed in the way that during thransfering of each pair of electrons along the respiratory chain from NAD to molecular oxygen three pairs of protons move outward. It is due to this gradients are formed. Gradients are used then for providing energy to the reaction of bonding of ADP to a phosphate, resulting in ATP formation.