Glycolysis

First of all, a glucose carrier allows glucose to travel to the cells via passive transport from the tissue fluid. Sometimes the carrier is gated which means that it’s controlled by hormones to regulate the amount of glucose that can enter, one example being insulin.

Glucose is made into glucose phosphate by the process of phosphorylation. The phosphate comes from ATP. As it no longer fits into the site of the glucose carrier it’s unable to leave the cell. This means that a concentration gradient is maintained by ensuring pure glucose is kept at a low level inside the cell.

Glucose is then phosphorylated a second time, by another ATP molecule, and split into two 3-carbon sugars: triose phosphate. Over a series of stages the triose sugar is changed into the 3-carbon compound pyruvate. During these stages ATP is released. This is the only time in glycolysis in which energy is produced. A hydrogen atom is also produced which is important for later to create more ATP. It’s picked up by the coenzyme NAD and transported to the respiratory chain, becoming reduced during this process to form NADH or reduced NAD.

NAD⁺ + 2H ? NADH + H⁺

Once pyruvate has been formed this is the end of glycolysis.

Remember that if no oxygen is present (anaerobic respiration) pyruvate is stored. In animals and bacteria it’s stored as lactate while in plants and fungi it’s stored as ethanol. The reverse of glycolysis, gluconeogenesis, will turn pyruvate back to glucose.

If oxygen is present then aerobic respiration can take place and the pyruvate moves on into the mitochondrial matrix.