Question

Why are plants unable to fix nitrogen by themselves?
A. It is easily available from the soil.
B. They lack the nitrogenous enzyme complex.
C. The process has a very high requirement of ATP per mole of ${{\text{N}}_2}$ fixed.
D. Hydrogen evolved in the process is deleterious to plants.

Answer 1

Hint: Plants need several types of nutrients for their proper growth and development. They get these nutrients either directly from the environment or absorb them through their root system. Many nutrients are readily available in the form of gases such as oxygen and carbon dioxide while others have to be either absorbed or utilized in different forms.

Complete answer:
Nitrogen is an essential nutrient required for the growth and other biological activities of the plants. It is considered as one of the macronutrients needed by the plants. Nitrogen is a vital component in the chlorophyll pigments of the leaves. The chlorophyll is an important pigment and absorbs the sunlight to continue the process of photosynthesis. Nitrogen is the major part of the amino acids that build the proteins. The proteins are important units in the enzymatic and structural formation of the plants.
Nitrogen is an abundant element that is found on the surface of the earth but it is not directly available to the plants as they cannot fix the nitrogen directly by themselves. Atmospheric nitrogen (${{\text{N}}_2}$) cannot be utilized by plants in this form nor can they fix it due to the lack of necessary enzymes such as the nitrogenous enzyme complex, to convert the nitrogen into biologically useful forms. To solve this problem of nitrogen fixation, plants form symbiotic relationships with several types of bacteria. These bacteria convert and fix the nitrogen into biological active forms which is then used by the plants.
The bacteria reside in the roots of the plants and convert the available nitrogen into useful compounds. Soil nitrogen is found in three general forms which are organic nitrogenous compounds, ammonium( ${\text{N}}{{\text{H}}_4}^ + $) ions and nitrate( ${\text{N}}{{\text{O}}_3}^ - $) ions. The resulting fixation of the nitrogen can be absorbed and transported into the tissue system of the plants and further, it is incorporated into amino acids. Amino acids are then utilized to form proteins and other important compounds.

Therefore, the correct option is option B.

Note: Nitrogen is found on the earth as one of the most abundant elements and is important to the plants as it affects the growth, development as well as other biological functions. It is an important macronutrient but the plant cannot fix the available nitrogen all by themselves as they do not possess the required nitrogenous enzyme complex.