Glucose is the source of nearly all energy used by organisms. The first step of converting glucose into usable energy is called glycolysis. Glycolysis occurs in the cytosol of the cell over two phases: an energy-requiring phase and an energy-releasing phase. Over the first three steps, glucose is converted into different forms and attached to two phosphate groups donated by two ATP molecules, resulting in an unstable sugar. In the next two stages, the unstable sugar splits into two sugar isomers that are either converted or used directly in the next phase of glycolysis.
First, glucose is phosphorylated by adding a phosphate group from ATP, converting it into a more reactive form (glucose 6-phosphate). The negatively-charged phosphate cannot cross the hydrophobic cell membrane, so the addition of the phosphate group traps glucose inside the cell.
Next, glucose 6-phosphate is converted into one of its isomers (fructose 6-phosphate), which is required for subsequent energy-requiring steps of glycolysis.
Fructose 6-phosphate is phosphorylated by adding a phosphate group from a second ATP molecule. This converts fructose 6-phosphate into fructose 1,6-bisphosphate, an unstable sugar.
This unstable sugar splits into two distinct three-carbon sugar isomers: glyceraldehyde 3-phosphate and DHAP. Glyceraldehyde 3-phosphate can be directly used in the next stage of glycolysis, whereas DHAP is converted into glyceraldehyde 3-phosphate.
