What are the main phases of glycolysis?

The Embden-Meyerhof-Parnas pathway is more widely known as Glycolysis. In the glycolysis pathway, it involves two distinct phases: the energy-investment phase in which ATP is consumed, and the energy-payoff phase in which ATP is produced. 

Energy-Investment Phase

In this phase, the cell consumes 2 ATP molecules to convert glucose into high-energy intermediates. 

The key steps in this phase include:

• Glucose is phosphorylated to form glucose-6-phosphate (G6P). This reaction is catalyzed by the enzyme hexokinase and consumes 1 ATP molecule. 
• Glucose-6-phosphate is converted into fructose-6-phosphate through an isomerization reaction catalyzed by glucose-6-phosphate isomerase.
• Fructose-6-phosphate is further phosphorylated to form fructose-1,6-bisphosphate. This phase consumes another ATP molecule. It is catalyzed by the enzyme phosphofructokinase-1. But at the end of this phase, 2 ATP molecules have been consumed. 
• Fructose-bisphosphate aldolase converts fructose-1,6-bisphosphate into 2 triose sugars - glyceraldehyde-3-phosphate (GA3P) and dihydroxyacetone phosphate (DHAP). This reaction is catalyzed by the enzyme aldolase.
• DHAP is broken down into a second molecule of GA3P through the action of the enzyme triose phosphate isomerase. 
• Both GA3P molecules enter the second phase of glycolysis – the energy payoff phase. 

Energy-Payoff Phase

In this phase, the high-energy intermediates produced in the energy-investment phase are metabolized to yield ATP and other energy-rich molecules. Each GA3P molecule that enters this phase generates 2 ATP molecules and 1 NADH molecule, resulting in the total yield of 4 ATP + 2 NADH. 

The key steps in this phase include:

• Each G3P molecule is oxidized and phosphorylated to form 1,3-bisphosphoglycerate (1,3-BPG) accompanied by the reduction of NAD+ to NADH. This reaction is catalyzed by glyceraldehyde-3-phosphate dehydrogenase. 
• The enzyme phosphoglycerate kinase converts 1,3-BPG to 3-phosphoglycerate (3PG). 1 ATP molecule is generated in this reaction. 
• 3-Phosphoglycerate (3-PG) undergoes further conversion to form 2-phosphoglycerate (2-PG) and then phosphoenolpyruvate (PEP). These two reactions are catalyzed by the enzymes phosphoglycerate mutase and enolase respectively. 
• The conversion of PEP to pyruvate results in the production of the second ATP molecule. This is an irreversible reaction catalyzed by pyruvate kinase.
• The net yield of glycolysis is 4 ATP molecules and 2 NADH molecules, providing the cell with energy for various cellular processes.