NCERT Class 11-Biology: Chapter – 14 Respiration in Plants Part 8 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 4:

Enumerate the assumptions that we undertake in making the respiratory balance sheet. Are these assumptions valid for a living system? Compare fermentation and aerobic respiration in this context.


The assumptions which are taken for the making of the respiratory balance sheet are:

(i) The metabolic pathway for respiration is sequential as:

Glycolysis? Kreb՚s Cycle? Electron transport system.

Each of the above occurs one after the other in sequential order.

(ii) The NADH which is synthesized during the glycolysis process is taken from the cytoplasm to the mitochondria of the cell, where oxidation occurs along with electron transport.

(iii) The intermediate which is formed by the various biochemical respiration pathways is not used as raw material for any other respiration pathway.

(iv) The enzyme functions rate is controlled by various functions, and it is not controlled by only a single mechanism from a single site.

Difference between fermentation and aerobic respiration:

Fermentation and Aerobic Respiration
FermentationAerobic respiration
(i) Partial breakdown of glucose happens during this process into ethanol(i) Complete breakdown of glucose takes place into carbon dioxide.
(ii) There is a net gain of only two ATP molecules.(ii) There can be a net gain of 36 molecules of ATP.
(iii) Oxidation of NADH to is slow(iii) Oxidation of NADH to is very fast.

Question 5:

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.


Glycolysis is the process which involves the splitting of glucose (a 6-C compound) into two molecules of pyruvic acid (a 3-C compound) . It occurs in the cytoplasm of the cell. The glycolysis process can be divided into two phases-

Hexose phase and

Triose phase.

The end products of glycolysis are two pyruvate (pyruvic acid) molecules, a total of four ATP molecules, and two molecules of NADH.

The reactions involved in the process of glycolysis are as follows:

1. The six carbon glucose molecule is phosphorylated with the presence of ATP to form a glucose-6- phosphate enzyme.

Hexokinase enzyme is used to catalyse this reaction.

2. Then Glucose-6- Phosphate is isomerized by using phosphoglucoisomerase to form a fructose-6- phosphate.

3. The Fructose-6- phosphate obtained from the previous reaction is further phosphorylated using ATP to form fructose 1,6-bisphosphate.

Phosphofructokinase is used to catalyze this reaction, and the ATP is dephosphorylated to form ADP.

4. The Fructose 1,6-bisphosphate gets cleaved by utilizing the enzyme aldolase. It gets cleaved into two molecules of 3 Carbon compounds called dihydroxy acetone phosphate (DHAP) and glyceraldehyde-3- Phosphate. These are isomeric triose compounds.

5. Dihydroxy acetone phosphate (DHAP) and the glyceraldehyde-3- phosphate are interconvertible with the use of triose.

6. Glyceraldehyde-3- phosphate is further phosphorylated and then oxidized in the presence of glyceraldehyde-3- phosphate dehydrogenase to form 1,3- bisphosphoglyceric acid.

7. Then 1,3-bisphosphoglyceric acid is dephosphorylated by Phosphoglyceric kinase to form a molecule of 3-phosphoglyceric acid (3-PGA) .

8.3-phosphoglyceric acid (3-PGA) is then further converted by using Phosphoglyceric mutase into a molecule of 2-phosphoglyceric acid.

9.2-phosphoglyceric acid (2-PGA) is further dehydrated by using enolase into a molecule of 2- phosphoenol pyruvic acid.

10. At last 2-phosphoenol pyruvic acid is dephosphorylated to form pyruvic acid and the ADP is also phosphorylated to form ATP. This reaction is catalysed by using pyruvic kinase enzyme.

A simple flowchart of steps involved in glycolysis is shown below:

Glycolysis occurs in both aerobic and anaerobic organisms. After glycolysis, if the cells have oxygen, then it goes on to a process known as a citric acid cycle or Tricarboxylic cycle. And if there is no oxygen, then the fermentation process occurs.