Question

How many steps of glycolysis are irreversible?
(a)3 steps $\xrightarrow [ ]{ }$ 1st, 3rd and last
(b)3 steps $\xrightarrow [ ]{ }$ 1st, 5th and last
(c)2 steps $\xrightarrow [ ]{ }$ 1st, last
(d)1st step only

Answer 1

Glycolysis is a process in which glucose turns to pyruvate. It is carried out by different enzymes and in 10 steps. Most of them are reversible whereas the others are irreversible.

Complete answer:
The process in which pyruvate is produced from glucose and free energy is released is known as glycolysis. There are ten steps in this process in which 7 are reversible and 2 irreversible. The 1st, 3rd and last one is irreversible.

Additional Information: Glucose is the reactant of glycolysis and the product is pyruvate. This process occurs in a 10 steps sequence. It is seen in both aerobic and anaerobic organisms. The main reaction of glycolysis is
Glycolysis occurs in 10 steps mainly

1. Phosphorylation of glucose: Phosphate from ATP (adenosine triphosphate) to glucose and forms glucose-6-phosphate. The enzymes required in this process are hexokinase and glucokinase.
   2)Glucose-6-phosphate isomerized to fructose-6-phosphate which involves a shift of the carbonyl oxygen from ${ C }_{ 1 }$ to ${ C }_{ 2 }$. The enzymes involved are phospho hex isomerase.
   3)Fructose-6-phosphate turns into fructose-1,6-bisphosphate. Phosphofructokinase is the enzyme involved in this step.
   4)Fructose-1,6-bisphosphate forms unique cleavage of C-C bond which is catalyzed by the enzyme fructose diphosphate aldolase to form two different triose phosphates- glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.
   5)The enzyme triose phosphate isomerase isomerizes dihydroxyacetone phosphate into glyceraldehyde 3-phosphate.
2. Glyceraldehyde 3-phosphate converts into 1,3-bisphosphoglycerate and ${ NAD }^{ + }$ reduces to coenzyme NADH by the ${ H }^{ - }$ from the Glyceraldehyde 3-phosphate.
   7)ADP gains a phosphate group from the 1,3-bisphosphoglycerate and the enzyme is phosphoglycerate kinase and produces ATP and 3-phosphoglycerate.
   8)The shift of phosphoryl group from ${ C }_{ 3 }$ to ${ C }_{ 2 }$ by the enzyme phosphoglycerate mutase, converts 3-phosphoglycerate into 2-phosphoglycerate.
   9)Enolase enzyme dehydrates 2-phosphoglycerate to phosphoenolpyruvate.
   10)An enol of pyruvate is formed from phosphoenolpyruvate. The enzyme involved is pyruvate kinase.
   So, the correct answer is 3 steps $\xrightarrow [ ]{ }$ 1st, 3rd, and last.

Note: Irreversible reactions occur only in one direction i.e. the products of this reaction can not change into the products. But in the case of reversible reactions, the reactants can change to products and products can change back to reactants as well. In the case of glycolysis, the reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase are 1st, 3rd, and last steps of glycolysis are irreversible.