Glycolysis is the primary stage of cellular respiration that happens when the form of sugar molecules breaks to release energy. Glucose is the most common form of sugar, from which the term glycolysis originates. The metabolic pathway helps in breaking and converting one molecule of glucose- C6H12O6 into two molecules of pyruvate – CH3COCOO− + H+. The total amount of energy, released in the glycolysis process is used to produce energy molecules called ATP (Adenosine triphosphate) along with reduced NADH (nicotinamide adenine dinucleotide).

In most living organisms, the glycolytic enzymes are produced in the form of the cytosol in cells.
This metabolic pathway was discovered by three German biochemists- Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas in an early 19th century and known by EMP pathway (Embden–Meyerhof–Parnas), named after its discoverers.

Stages of Glycolysis  

The complete glycolysis consists of two stages:

Stage one: The Preparatory Phase

  • Phase1–In this phase, glucose is converted into glucose-6-phosphate, which is caused by the presence of hexokinases enzyme. Here, ATP molecule is consumed and the phosphate group from ATP molecule is transferred to the glucose so as to produce glucose-6-phosphate. The process is known as phosphorylation.
Glucose (C6H12O6) + Hexokinase + ATP  →  
               Glucose-6-phosphate (C6H13O9P)   + ADP                                     
  • Phase2–The Glucose 6-phosphate is replaced into fructose 6- phosphate in the presence of glucose phosphate isomerase enzyme. This is an isomerization reaction, which could be reversed under normal cell condition.
Glucose-6-phosphate (C6H13O9P) + Phosphoglucose isomerase →    
  Fructose-6-phosphate (C6H13O9P)
  • Phase3– In this stage, phosphorylation reaction from phase 1 is repeated. The second molecule of ATP is used for donating its phosphate group to Fructose 6-phosphate, which is then converted to fructose 1, 6-bisphosphate.
Fructose 6-phosphate  (C6H13O9P) + phosphofructokinase + ATP → 
              Fructose 1, 6 bisphosphate  (C6H14O12P2)+ ADP                  
  • Phase4– An enzyme aldolase breaks fructose 1, 6-bisphosphate into two sugar molecules, and get isomers to produce dihydroxyacetone phosphate and glyceraldehyde phosphate. This phase is known as destabilization.
Fructose 1, 6-bisphosphate (C6H14O12P2)+ aldolase →
 Dihydroxyacetone Phosphate (C3H7O6P)  + Glyceraldehyde phosphate (C3H7O6P) 
  • Phase5– An enzyme triose phosphate isomerase and interconverts the sugar molecules to Dihydroxyacetone phosphate and Glyceraldehyde phosphate. This is where interconversion reaction occurs.
Dihydroxyacetone phosphate (C3H7O6P) → Glyceraldehyde phosphate  (C3H7O6P)  
  • Phase6–The triose phosphate dehydrogenase enzyme helps glyceraldehydes-3-phosphate to dehydrogenase (removing a hydrogen atom) and adds an inorganic phosphate to glyceraldehyde 3-phosphate, finally to produce 1, 3- bisphosphoglycerate. This would be the dehydrogenation phase of glycolysis.
2 Glyceraldehyde phosphate 2(C3H5O3P) + triose phosphate dehydrogenase + 2H- + 2P + 2 NAD+   → 
                            2 molecules of 1, 3-bisphosphoglycerate 2(C3H4O4P2) + 2 NADH + 2H+ 

The production of the 6-carbon glucose molecule begins when they are split into 3-carbon molecules by consuming two molecules of ATP thus ending the glycolysis process.

Stage two: Production of ATP

  • Phase1– In this phase, the 1, 3- bisphosphoglycerate is converted to 3-phosphoglycerate by transferring 1 phosphate group in the presence of enzyme phosphoglycerate kinase. Thus the phosphate group is transferred to ADP to produce the first molecule of ATP. As a result, the 3-phosphoglycerate molecules are rearranged in the position of phosphate converting them into 2-phosphoglycerate. This phase is known by the name substrate-level phosphorylation.
1, 3 bisphosphoglycerate (C3H4O4P2)  + phosphoglycerol kinase +ADP  → 
                            3-phosphoglycerate (C3H5O4P)+ ATP                  
  • Phase2– An enzyme enolase dehydrates the 2-phosphoglycerate to convert it into phosphoenolpyruvate (PEP).
2 molecules of 2-phosphoglycerate (C3H5O4P) + enolase → 
           2 molecules of phosphoenolpyruvic acid PEP (C3H3O3P) + H2O                        
  • Phase3 — The glycolysis converts phosphoenolpyruvate into pyruvate thus generating another molecule of ATP in presence of pyruvate kinase enzyme. At this point, if there is enough oxygen present, the pyruvate further breaks down to form carbon dioxide and yield many molecules of ATP. This phase is called the lyase reaction.
2 molecules of  phosphoenolpyruvic acid PEP (C3H3O3P) +  pyruvate kinase + 2  ADP  →  
         2 molecules of pyruvic acid (C3H4O3) + 2 ATP

Finally, two molecules of ATP per glucose molecules are produced thus ending the glycolysis process.

Learn more about glycolysis through engaging video lectures at BYJU’S.

Practise This Question

What enzyme is used to catalyze the first primary reaction of glycolysis?