glycolysis

Glycolysis is the first stage of cellular respiration, and it happens in the cytoplasm without oxygen. One six-carbon glucose is split into two three-carbon molecules of pyruvate, in a sequence of steps that spends two ATP to start and produces four, for a net gain of two. It also loads two electron carriers, NADH, which go on to the next stage. Because it needs no oxygen, glycolysis is the whole of the energy story for a cell under anaerobic conditions — and it runs the same way in a cell with plenty of oxygen, as the doorway to the rest of respiration. The rest is the difference: with oxygen, pyruvate enters the mitochondrion and the Krebs cycle takes over, and the yield per glucose rises roughly fifteen-fold. Without it, pyruvate is converted to lactate and the two ATP per glucose is the total. The name is worth parsing: glyco- for sugar, -lysis for splitting.