NCERT Class 11-Biology: Chapter – 14 Respiration in Plants Part 4 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 5:

Oxygen is an essential requirement for aerobic respiration but it enters the respiratory process at the end? Discuss.


Oxygen plays an essential for aerobic respiration at the end of ETC, i.e.. , respiratory process because it removes the hydrogen from the electron transport system. Oxygen acts as the final hydrogen acceptor in the electron transport chain. If oxygen is not present, then the electrons could not pass through the co-enzymes. Oxygen is responsible for the formation of the proton pump in the ETS process as it gives energy by the oxidation-reduction process. If oxygen is absent, then there will be no formation of proton pump and ATP will not be produced by oxidative phosphorylation.

Question 6:

Respiration is an energy releasing and enzymatically controlled catabolic process which involves a step-wise oxidative breakdown of organic substances inside living cells.


(1) Step-wise oxidative breakdown:

During cellular respiration, all the energy which is contained in the respiratory substrate is not released all at once. Instead, it is released in a step-wise manner.

The energy is released by a series of slow steps which are controlled and moderated enzymatically. It is synthesized in the chemical energy in the form of ATP. Therefore, the energy release cannot occur directly, and it follows a series of steps of catabolic breakdown only when energy is required.

(2) Organic substances (used as substrates) :

Organic substances like carbohydrates used as substrates and oxidized during the respiratory process for releasing energy. Organic substances like protein, fats and organic acids are used as organic respiratory substances in some organisms and plants.

Question 7:

Comment on the statement – Respiration is an energy producing process but ATP is being used in some steps of the process.


Although respiration is an energy producing process but ATP is also being used in some steps of the process. For example, the phosphorylation reactions in the glycolysis process in which glucose is converted to glucose – 6 – phosphate which consumes one ATP.

Although the overall ATP produced by the complete oxidation of the substrate is more than the amount of energy spent on some metabolic processes.

Question 8:

The figure given below shows the steps in glycolysis. Fill in the missing steps A, B, C, D and also indicate whether ATP is being used up or released at step E?

Steps in Glycolysis


At step E, energy is being used up.

Question 9:

Why is respiratory pathway referred to as an amphibolic pathway? Explain.


The amphibolic pathway is defined as a biochemical pathway which involves both anabolic and catabolic process.

The respiration biochemical process has been conventionally considered to be a catabolic pathway as it comprises of the breakdown of substrates. But the breakdown of substrates serves as a part of the synthesis of other substrates.

The fatty acids when respired are broken down into Acetyl CoA. Similarly, glycerol also enters a biochemical pathway and gets broken down into PGAL (3-phosphoglyceraldehyde) .

But when there is a need for synthesizing fat in an organism. The Acetyl is taken from the respiratory pathway. Respiration can cause both the breakdown as well as the synthesis of fatty acids, i.e.. , it is involved in both catabolism and anabolism. Therefore, it is to be an amphibolic pathway.