Carbohydrate Metabolism- Post digestion, food material is digested into the amino acid, fatty acid and glucose. All these final digestion products absorbed by intestine and enter into the blood stream. Glucose enters into blood and distribute to the different organs for storage purpose but liver is the prime site for storage. Glucose is converted into the glycogen with the help of an enzyme glycogen synthase. Glucose is oxidized into the glycolysis and kreb cycle to produce ATP and other reducing equivalent to produce energy.  

Glycolysis- Glycolysis is central to carbohydrate metabolism and it is the universal pathway found in prokaryotic or eukaryotic cells. It is a breakdown of 6 membered glucose into two 3 membered carbon suger to feed kreb cycle (in the presence of oxygen) or to send for anaerobic oxidation (in the absence of oxygen). Hence, it plays a crucial role for adopation of a living organism under differet types of stress conditions. The glycolysis is a 10 step chemical reaction to enable glucose for its optimal oxidation. All these reactions are given in Figure 36.4.    

 
STEP-1: Phosphorylation of glucose-Glucose produced after glycogen breakdown is phosphorylated by glucokinase (in liver) or hexokinase in all other tissues especially in muscles. In the phosphorylation reaction, phosphate (γ-phosphate) group of ATP is transferred to glucose to form glucose-6-phosphate. The phosphorylation reaction of glucose to produce glucose-6-phosphate marks the molecule for glycolysis. One molecule of ATP is utilized in this step.      

STEP 2: Conversion of glucose-6-phosphate to fructose-6-phosphate-Phosphorylated sugar produced in step-1 is converted into the fructose-6-phosphate by the action of phospho-hexose isomerase.

STEP 3: Phosphorylation of fructose-6-phosphate- In this step, sugar is further phsophorylated at carbon 1 to produce fructose-1,6 bis phosphate by the action of Phosphofructokinase. In the phosphorylation reaction, phosphate (γ-phosphate) group of ATP is transferred to phosphorylated sugar to form fructose-1,6 bis phosphate. One molecule of ATP is utilized in this step.

STEP 4: Clevage of fructose 1,6-bis phosphate-This step is catalyzed by enzyme aldolase or fructose 1,6 bis aldolase to generate glyceraldehyde-3 phosphate (aldose) and dihydroxy acetone phosphate (ketose).

STEP 1-4: First 4 reactions of enzymatic conversion of glucose (6 carbon sugar) to glyceraldehydes-3 phosphate (aldose) and dihydroxy acetone phosphate (ketose) are considered as preparative phase of glycolysis and during this phase, two major event happened: