Where does glycolysis occurs?

Glycolysis is a process where glucose is broken down into two molecules of pyruvic acid, hence called glycolysis (glucose-breaking). It is common to both aerobic and anaerobic respiration. It occurs in the cytoplasm of the cell. It involves ten steps. 

Glycolysis consists of two major phases: 

1. Preparatory phase (1-5 steps). 

2. Payoff phase (6-10 steps). 

1. Preparatory phase: 

a. In this phase, glucose is phosphorylated twice by using two ATP molecules and a molecule of fructose 1,6-bisphosphate is formed. 

b. It is then cleaved into two molecules of glyceraldehyde-3-phosphate and dihydroxy acetone phosphate. These two molecules are 3-carbon carbohydrates (trioses) and are isomers of each other. 

c. Dihydroxy acetone phosphate is isomerised to second molecule of glyceraldehyde-3-phosphate. 

d. Therefore, two molecules of glyceraldehyde-3- phosphate are formed. 

e. Preparatory phase of glycolysis ends. 

2. Payoff phase: 

a. In this phase, both molecules of glyceraldehyde-3-phosphate are converted to two molecules of 1,3- bisphoglycerate by oxidation and phosphorylation. Here, the phosphorylation is brought about by inorganic phosphate instead of ATP. 

b. Both molecules of 1, 3-bisphosphoglycerate are converted into two molecules of pyruvic acid through series of reactions accompanied with release of energy. This released energy is used to produce ATP (4 molecules) by substrate-level phosphorylation.