What are the steps involved in glycolysis and why does glycolysis occur?

Both aerobic and anaerobic respiration begin with glycolysis, which is the splitting of the 6 carbon glucose into 2, 3 carbon pyruvate molecules. This happens because glucose cannot be used directly by cells as a source of energy, as the molecule is too large, so must be used to produce ATP during cellular respiration. The reaction takes place in the cell cytoplasm. There are 4 stages in glycolysis: 1) The activation of glucose by phosphorylation - glucose is made more reactive by the addition of 2 inorganic phosphate molecules. These come from the hydrolysis of 2 ATP molecules. This reaction lowers the activation energy for the enzyme controlled reactions that follow. 2) Splitting of phosphorylated glucose into 2 triose phosphate molecules (3 carbon molecules) 3) Oxidation of triose phosphate - hydrogen is removed from each of the 2 triose phosphates and transferred to a proton-carrier molecule called NAD to form reduced NAD. (the reduced NAD is important in the electron transport chain, where they donate electrons of the hydrogen atoms to the first molecule in the chain) 4) The production of ATP - enzyme controlled reactions convert each triose phosphate into pyruvate, by the removal of the inorganic phosphate. The energy released during this reaction is used to combine ADP with inorganic phosphate and produce ATP. Two molecules of ATP are produced, as this reaction happens twice. Overall, 2 molecules of ATP are yielded (4 are produced in the last step, but 2 are used at the start)