Glycolysis

Glycolysis (Greek: glykos = glucose, lysis = splitting) is a linear sequence of ten enzyme-catalysed reactions that splits one six-carbon glucose molecule into two molecules of three-carbon pyruvic acid, using enzymes located entirely in the cytoplasm. Because it needs no oxygen, glycolysis is the one stage common to both aerobic and anaerobic respiration. The pathway was worked out in yeast by Gustav Embden, Otto Meyerhoff and J. Parnas, so it is also known as the EMP pathway, and it is organised into two phases. The preparatory (endergonic/hexose) phase covers steps 1-5: glucose entering from sucrose (the end-product of photosynthesis) is first phosphorylated to glucose-6-phosphate by hexokinase, converted through fructose-6-phosphate to fructose-1,6-bisphosphate, and then split by aldolase into two 3-carbon isomers, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (the latter is converted to the former by triose phosphate isomerase so both feed forward); this phase consumes 2 ATP, invested at steps 1 and 3. The pay-off (oxidative/triose) phase covers steps 6-10: each glyceraldehyde-3-phosphate is oxidised and phosphorylated by glyceraldehyde-3-phosphate dehydrogenase, reducing 2 NAD+ to $2NADH + H^+$, and is then processed through several more enzyme steps -- including substrate-level (direct) phosphorylation, where a phosphate group is transferred directly from a substrate molecule to ADP to make ATP -- ending in two molecules of pyruvate, with 2 ATP generated at step 7 and another 2 ATP at step 10, plus a water-releasing 'enolatio …