Question

Which of the following about glycolysis is correct?

a. each glucose is split into 4 pyruvate molecule

b. glucose is the only monosaccharide that can be broken down by glycolysis

c. NADH is formed from NAD+

d. Energy is NOT needed to initiate glycolysis

Answer 1

option c)NADH is formed from NAD+ during glycolysis..The scheme of glycosis was given by Embden,Meyerhof and Parnas .In anerobic organism ,it is the only process in respiration .Glycolysis occurs in the cytoplasm of the cell and is present in all living organism .Glycolysis is the process of partial oxidation of glucose to form two molecules of pyruvic acid .The first half of the pathway activates glucose ,so it is called glucose activation phase .the second half extracts the energy ,so it is called energy extraction phase.

Step1- Glucose is phosphorylated to glucose-6-phosphateby ATP in the presence of enzyme hexokinase.

step2- Glucoe-6-phosphate isomerises to fructose-6-phosphate by the action of the enzyme phosphohexoisomerase.

step3- Fructose-6-phosphate is phosphorylated to form fructose-1,6-biphosphate.The enzyme which helps in the transfer of phosphate from ATP to Fructose-6-phosphate is phosphofructokinase .During phosphorylation,ATP is converted to ADP ,thus this pathway of conversion of glucose to fructose-1,6-biphosphate uses two ATP molecule .Thus this step is also called rate limiting step .

step4- The fructose-1,6-biphosphate splits to dihydroxyacetone phosphate and 3-Phosphoglyceraldehyde by the action of Aldose .

step5- The two triose phosphate formed are interconvertible .The enzyme that helps in this conversion is phosphotriose isomerase .

step6-  3-Phosphoglyceraldehyde is now converted to 1,3-Biphosphoglycerate .During this conversion NAD+ is converted to NADH and H+. 3-Phosphoglyceraldehyde is oxidised and with inorganic phosphate ,it get converted to 1,3-Biphosphoglycerate .Enzyme that catalyses this reaction is Diphosphoglyceraldehyde dehydrogenase .

step7- Now 1,3-Biphosphoglyceric acid converted to 3-phosphoglyceric acid by the activity of phosphoglycerate transphosphorylase .During this phase ,ADP is converted to ATP .The direct synthesis of ATP from metabolites is called substrate level phosphorylation .

step8-  3-phosphoglyceric acid now by the action of phosphoglyceromutase changes to 2-phosphoglycerate.

step9- Enolase acts on 2-phosphoglycerate. and converts it into phosphophenol pyruvate.

step10- Phosphophenol pyruvate.is acted upon by pyruvate kinase and gets converted to pyruvic acid .This step is also substrate level phosphorylation ,therefore it is a ATP releasing step.

Thus one molecule of NADH is formed from NAD+.