Glycolysis (from Greek glyk meaning sweet and lysis meaning dissolving; also known as the Embden-Meyerhof pathway) is the inital stage of glucose metabolism, and converts one molecule of glucose into two molecules of pyruvate. Glycolysis takes place within the cytosol of the cell, and is completely anerobic; oxygen is not required. In prokaryotes, the pyruvate is anaerobically metabolized to either lactic acid or ethanol. In eukaryotes, the pyruvate enters the citric acid cycle.
The first step in glycolysis is the phosphorylation of glucose by hexokinase. This reaction consumes 1 ATP molecule, but the energy is well spent; while the cell membrane is freely permeable to glucose, it is impermeable to glucose 6-phosphate. Glucose 6-phosphate is then rearranged into fructose 6-phosphate by phosphoglucose isomerase. (Fructose can also enter the glycolytic pathway at this point).
Phosphofructokinase then consumes 1 ATP to form fructose 1,6-biphosphate. The energy expenditure in this step is justified in two ways; the glycolytic process (up to this step) is now irreversible, and the energy supplied to the molecule allows the ring to be split by aldolase into two molecules, dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. (Triosephosphate isomerase converts the molecule of dihydroxyacetone phosphate into a molecule of glyceraldehyde 3-phosphate.) Each molecule of glyceraldehyde 3-phosphate is then oxidized by a molecule of NAD+ in the presence of glyceraldehyde 3-phosphate dehydrogenase, forming 1,3-biphosphoglycerate.
In the next step, phosphoglycerate kinase forms a molecule of ATP while forming 3-phosphoglycerate. Note that at this step, glycolysis has now reached the break-even point; two molecules of ATP were consumed, and two new molecules have been synthesized. This step, one of the two substrate-level phosphorylation steps, requires ADP to proceed; thus, when the cell has plenty of ATP (and little ADP) this reaction will not proceed. Since ATP decays relatively quickly when it is not metabolized, this is an important regulatory point in the glycolytic pathway. Phosphoglyceromutase then forms 2-phosphoglycerate; enolate then forms phosphoenolpyruvate. Another substrate-level phosphorylation then forms a molecule of pyruvate while generating a molecule of ATP. This serves as an additional regulatory step.
Something to note is that after the formation of fructose 1,6 biphosphate, many of the reactions are energetically unfavorable. The only reactions that are favorable are the two substrate-level phosphoylation steps that result in the formation of ATP. These two reactions pull the glycolytic pathway to completion.
So, for prokaryotes, the metabolism of one molecule of glucose has a net yield of 2 molecules of ATP. A eukaryote, which has mitochondria will produce an additional 34 molecules of ATP for each glucose molecule.