The production and use of ATP in living organisms
Adenosine Triphosphate, also known as ATP, is the molecule responsible for the energy that we, and all other organisms, need to survive. It is produced primarily in the processes of aerobic and anaerobic respiration by oxidative and substrate phosphorylation. 4 molecules of ATP are produced from 4 ADP and 4 inorganic phosphates in glycolysis in the cytoplasm of every cell, by the oxidation of a triose phosphate into two molecules of pyruvate. In anaerobic respiration these are the only 4 ATP molecules produced per molecule of glucose as there is no oxygen available for the link reaction or electron transport chain to occur in the cytoplasm, instead the pyruvate molecules are reduced into either lactate in muscles or ethanol and CO2 in yeast. However 2 ATP molecules are used in the phosphorylation of glucose at the start of glycolysis so the net product of anaerobic respiration is just 2 ATP. In aerobic respiration the pyruvate molecules move into the mitochondrial matrix where they undergo the link reaction, releasing one CO2 molecule and one NADH each. This leaves two acetyl co-enzyme A molecules which enter the Krebs cycle to release another 2 CO2 molecules, 1 ATP, 3 NADH and 1 FADH each. So far we have a net production of 4 ATP (subtracting the 2 used in glycolysis). The electron transport chain is where the majority of ATP is produced. 10 NADH and 2 FADH (produced from glycolysis, link reaction and the Krebs cycle) are oxidised to NAD and FAD, releasing 12 hydrogens. These hydrogens are split into protons and electrons. The electrons are passed from carrier to carrier in the bilayer of the inner membrane of the mitochondrial cristae, releasing energy at each one. This energy is used to pump the protons through the carriers into the intermembrane space, creating a gradient. Due to this gradient, the protons flow from the intermembrane space back into the matrix by ATP Synthase in the inner membrane. This movement of protons allows 28 ADP and 28