Glycolysis is a pathway in which one 6-carbon molecule of glucose is oxidized to produce two 3-carbon molecules of pyruvic acid. The pathway can be condensed into the following four main steps.
Step 1. Two phosphate groups are attached to one molecule of glucose, forming a new 6-carbon compound. The phosphate groups are supplied by two molecules of ATP, which are converted into two molecules of ADP in the process.
Step 2. The 6-carbon compound formed in step 1 is split into two 3-carbon molecules of glyceraldehyde 3 = phosphate (G3P).
Step 3. The two G3P molecules are oxidized, and each receives a phosphate group. The product of this step is two molecules of a new 3-carbon compound. The oxidation of G3P is accompanied by the reduction of two molecules of NAD+ to NADH. Like NADP+, NAD+ is an organic molecule that accepts electrons during redox reactions.
Step 4. The phosphate groups added in step 1 and step 3 are removed from the 3-carbon compounds formed in step 3. This reaction produces two molecules of pyruvic acid. Each phosphate group is combined with a molecule of ADP to make a molecule of ATP. Because a total of four phosphate groups were added in step 1 and step 3, four molecules of ATP are produced.