Biology Class 11 NCERT Solutions: Chapter 12 Mineral Nutrition Part 2

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Q: 8. What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium. What is their role in fixation?


Rhizobium is a symbiotic bacteria present in the root nodules of leguminous plants. The basic requirements for Rhizobium to carry out nitrogen fixation are as follows:

Basic requirement of nitrogen fixation

Basic Requirement of Nitrogen Fixation

Basic requirement of nitrogen fixation

(A) Presence of the enzyme nitrogenase

(B) Presence of leg-haemoglobin

(C) Non-heme iron protein, ferredoxin as the electron-carrier

(D) Constant supply of ATP

(E) as co-factors

Rhizobium contains the enzyme nitrogenase protein - that helps in the conversion of atmospheric free nitrogen into ammonia.

The reaction is as follows:

The Rhizobium bacteria live as aerobes under free-living conditions, but require anaerobic conditions during nitrogen fixation. This is because the enzyme nitrogenase is highly sensitive to molecular oxygen. The nodules contain leg haemoglobin, which protects nitrogenase from oxygen.

Image shows the nitrogen fixation

Image Shows the Nitrogen Fixation

Image shows the nitrogen fixation

Q: 9. What are the steps involved in formation of a root nodule?


Multiple interactions are involved in the formation of root nodules.

The Rhizobium bacteria divide and form colonies. These get attached to the root hairs and epidermal cells.

The root hairs get curled and are invaded by the bacteria.

This invasion is followed by the formation of an infection thread that carries the bacteria into the cortex of the root. The bacteria get modified into rod-shaped bactericides.

As a result, the cells in the cortex and pericycle undergo division, leading to the formation of root nodules.

The nodules finally get connected with the vascular tissues of the roots for nutrient exchange increases in region C.

Image of the formation of a root nodule

Image of the Formation of a Root Nodule

Image of the formation of a root nodule

Q: 10. Which of the following statements are true? If false, correct them:

(A) Boron deficiency leads to stout axis.

(B) Every mineral element that is present in a cell is needed by the cell.

(C) Nitrogen as a nutrient element is highly immobile in the plants.

(D) It is very easy to establish the essentiality of micronutrients because they are required only in trace quantities.


(A) True

(B) All the mineral elements present in a cell are not needed by the cell. For example, plants growing near radioactive mining sites tend to accumulate large amounts of radioactive compounds. These compounds are not essential for the plants.

(C) Nitrogen as a nutrient element is highly mobile in plants. It can be mobilised from the old and mature parts of a plant to its younger parts.

(D) True