Question

Explain the respiratory pathway which is common for both aerobic and anaerobic respiration.

Answer 1

Glycolysis is the common pathway for both aerobic and anaerobic forms of respiration. The process by which the glucose (6C compound) is split into two molecules of pyruvic acid (3C compound) is called glycolysis or EMP pathway. It occurs in the cytoplasm.

Reactions involved in glycolysis are as follows:

(i) The glucose is phosphorylated with ATP to form glucose-6-phosphate. The reaction is catalyzed by the enzyme hexokinase.

(ii) Glucose-6-phosphate is isomerized to form fructose-6-phosphate by phosphoglucoisomerase.

(iii) Fructose-6-phosphate is then phosphorylated using ATP to form fructose 1,6-bisphosphate. This reaction is catalyzed by phosphofructokinase. The ATP is dephosphorylated to ADP.

(iv) Fructose 1, 6-bisphosphate is cleaved by the enzyme aldolase to two molecules of $3C$ compounds-dihydroxy acetone phosphate (DHAP) and 3-phosphoglyceraldehyde. These two trioses are isomers.

(v) DHAP and 3-phosphoglyceraldehyde are interconvertible by the action of triosephosphate isomerase. These five series of reaction constitute hexose phase and produce two molecules of the 3-carbon compound called 3-phosphoglyceraldehyde. In hexose phase, two ATP molecules are consumed.

(vi) A molecule of 3-phosphoglyceraldehyde is phosphorylated and oxidized to 1, 3-bisphosphoglyceric acid in the presence of 3-phosphoglyceraldehyde dehydrogenase. During this reaction, one NADH${}_2$ is formed.

(vii) 1, 3-bisphosphoglyceric acid is dephosphorylated to a molecule of 3-phospho-glycric acid by the phosphoglyceric kinase. During this reaction, one ATP is formed. This type of ATP synthesis is called direct phosphorylation or substrate phosphorylation.

(viii) A molecule of 3-phosphoglyceric acid is then converted into a molecule of 2-phosphoglyceric acid by phosphoglyceric mutase. In this reaction, phosphate molecule is shifted from third carbon to second carbon.

(ix) A molecule of 2-phosphoglyceric acid is dehydrated to a molecule of 2-phosphoenol pyruvic acid by enolase. Removal of a water molecule from the substance is called enolation.

(x) A molecule of 2-phosphoenol pyruvic acid is dephosphorylated to pyruvic acid and ADP is phosphorylated to ATP. This reaction is catalyzed by the pyruvic kinase. Thus, in the triose phase, two molecules of a molecule of 3-phosphoglyceraldehyde produce 2 molecules of pyruvic acid.

In glycolysis, 4ATP and 2NADH${}_2$ molecules are formed and 2ATP molecules are consumed in hexose phase. Hence, the net gain is 2ATP and 2NADH${}_2$.