Give an account of glycolysis. Where does it occur? What are the end products? Trace the fate of these products in both aerobic and anaerobic respiration?

* Division of Glucose into two pyruvic acid molecules is called glycolysis.
* It occurs in the cytosol of a cell
* The various steps of glycolysis pathway were worked out by German scientist. Emben,
Mayerhoff & paranas, hence it is called 'EMP' - pathway.
Reactions of Glycolysis :
* Phosphorylation :
Glucose accepts one ATP molecule and forms glucose-6- phosphate.
* This reaction is catalysed by the enzyme
Hexokinase.
Glucose + ATP $\stackrel{\mathrm{Hexokinase}}{\to }$ Glucose 6 phosphate + ADP
Isomerisation : Glucose - 6- phosphate isomerised to Fructose-6-phosphate in the presence of Phospho hexose isomerase.
Glucose- 6phosphate $\stackrel{\mathrm{Phosphohexose} \mathrm{isomerase}}{\to }$ Fructose-6-Phosphate
Phosphorylation :
* Fructose-6-phosphate accepts one ATP and forms Fructose 1-6-bis phosphate in the presence of phosphofructokinase enzyme.
Fructose-6- Phosphate+ATP $\stackrel{\mathrm{Phospho} \mathrm{Fructokinase}}{\to }$ Fructose 1,6 bis- phosphate + ADP
Cleavage : Fructose -1-6- bis phosphate is devided into two trioses glyceraldehyde - 3- phosphate and dihydroxy acetone phosphate, in the presence of Aldolase enzyme.
Fructose-1-6-bis phosphate
Fructose-1-6-bis phosphate $\stackrel{\mathrm{Aldolase}}{\to }$G - 3P + DHAP
Isomerisation : G3P & DHAP are isomers. Among two trioses only G3P directly participates in the subsequent reactions. Hence DHAP converts into G-3P in the presence of 'triose phosphate isomerase' enzyme.
DHAP $\stackrel{\mathrm{Triose} \mathrm{phosphate} \mathrm{isomerase}}{\to }$G3P
Oxidation : G-3P undergoes oxidation as well as phosphorylation in the presence of G-3 P dehydrogenase enzyme resulting in the formation of 1,3 - bis phosphoglyceric acid and
NADH.
G-3P+ H₂PO₄ +NAD⁺ $\stackrel{\mathrm{G}-3\mathrm{P} \mathrm{dehydrogenase}}{\to }$ 1,3 bis PGA+ NADH+ H⁺ 
Dephosphorylation :Removal of an inorganic phosphate from 1,3 bis PGA results in the formation of 3 PGA in the presence of Phosphoglycerokinase.
1,3 bis PGA $\stackrel{\mathrm{Phosphogycerokinase}}{\to }$ 3PGA+ATP
Inorganic phosphate released in this process accepted by ADP to form ATP. This process of
ATP formation is called as 'Substrate level phosphorylation'.
Isomerisation (Intramolecular shift) : Inorganic phosphate in 3PGA transfers from its 3rd carbon to 2nd carbon and forms 2 PGA in the presence of phosphoglycero mutase.
3PGA $\stackrel{\mathrm{Phosphoglyceromutase}}{\to }$ 2PGA
Dehydration : Removal of one water molecule from 2 PGA resulting in the formation of PEP, (Phospho enolpyruvic acid) in the presence of 'enolase'.
2PGA $\stackrel{\mathrm{Enolase}}{\to }$ PEP+ H₂O
De Phosphorylation : Removal of an inorganic phosphate from PEP results in the formation of
pyruvic acid in the presence of Pyruvic kinase. Inorganic phosphate accepted by ADP to form
ATP. Hence it is substrate level phosphorylation.
PEP+ADP $\stackrel{\mathrm{Pyruvic} \mathrm{Kinase}}{\to }$ Pyruvic acid + ATP
End products of Glycolysis :
2 Pyruvic acid molecules.
4 ATP molecules are formed but 2 are utilized in 1st & 3rd steps. So net gain of 2 ATP.
2 NADH+ H+.
Fate of Pyruvic acid :
Pyruvic acid fate depends upon the availability of oxygen.
In the presence of oxygen it is completely oxidized into CO2 & H2O.
In the absence of oxygen pyruvic acid is converted to ethyl alcohol by fermentation.