Glycolysis

This lesson covers: 

1. Where glycolysis occurs
2. The main steps in glycolysis
3. The reactants and products of glycolysis

Introduction to glycolysis

Glycolysis is the first stage of aerobic respiration. It occurs in the cytoplasm of cells.

It breaks glucose, a six-carbon molecule, into two three-carbon pyruvate molecules. This happens through a series of reactions involving specific enzymes.

Glycolysis does not require oxygen so it is an anaerobic process. As well as pyruvate, glycolysis also produces small amounts of ATP and an electron carrier called reduced NAD (NADH).

The main steps of glycolysis

The process of glycolysis is divided into four distinct stages:

1. Phosphorylation of glucose - Two ATP molecules donate phosphate groups to glucose, forming hexose bisphosphate.
2. Lysis - The hexose bisphosphate molecule is split into two molecules of triose phosphate (TP).
3. Phosphorylation of TP - A second phosphate group is added to each TP molecule, converting them into two molecules of triose bisphosphate.
4. Dehydrogenation - A hydrogen is removed from each triose bisphosphate molecule (they are oxidised) and used to form two molecules of reduced NAD, two molecules of pyruvate, and four molecules of ATP through substrate-level phosphorylation.

Substrate-level phosphorylation is the formation of ATP without the involvement of an electron transport chain. After glycolysis, if oxygen is available, pyruvate moves through mitochondrial membranes by active transport.

Reactants and products of glycolysis

The reactants required for glycolysis are:

• One glucose molecule
• Two ATP molecules

The products of glycolysis are:

• Four ATP molecules
• Two reduced NAD molecules
• Two pyruvate molecules

The net energy gain from glycolysis is two ATP molecules and two reduced NAD molecules per glucose molecule processed.

The products of glycolysis, namely pyruvate and reduced NAD, have roles in the following stages of cellular respiration that generate a larger yield of energy.