[Biology Class Notes] on Nitrogen Fixation and Nitrogen Metabolism Pdf for Exam

There are several elements present on our earth like oxygen, carbon, hydrogen, etc. which are vital for many living organisms. Nitrogen is one of them. These are present in various vitamins, proteins, amino acids, and hormones. Consequently, these elements and their effects must be studied too. As per NASA, the atmospheric air comprises 78% nitrogen, 21% oxygen gas, 0.04% carbon, 0.93% argon, while the other gases are present in a small amount. 

Notably, despite its colossal concentration, the amount of usable nitrogen gas available for living organisms is less. Therefore, it becomes essential to learn nitrogen fixation definition to get an idea about its various aspects. 

What is Meant by Nitrogen Fixation?

The nitrogen present in the atmosphere is a di-Nitrogen molecule which is non-reactive. Resultantly, plants and other living organisms cannot use it. Therefore, conversion of non-usable form of nitrogen into usable form is necessary. This process is termed as nitrogen fixation. In this process, this molecular nitrogen is converted into usable forms or compounds like ammonia or other nitrogenous elements that are mixed in the soil. 

To explain nitrogen fixation, it is one of the early stages in the nitrogen cycle, and the conversion takes place using bacterial species such as azotobacter, rhizobium, etc. or via natural phenomena. 

Types of Nitrogen Fixation

Nitrogen fixation can be broadly categorised into two parts – 

1. Physical Nitrogen Fixation – About 10% of nitrogen fixation is done by using physical methods. One such physical process is lightning wherein a di-Nitrogen molecule reacts with oxygen to form Nitric oxide. This is oxidised to form NO2 (Nitrogen Peroxide). Along with the rain, it forms nitrous and nitric acid, which gets mixed in the soil. 

2. Biological Nitrogen Fixation – About 90% of nitrogen fixation examples are from biological processes. Here the conversion takes place with the help of a few living organisms. 

One such process of biological nitrogen fixation mechanism (BNF) is when the molecular nitrogen is converted to ammonia in the presence of a biological catalyst known as nitrogenize. This compound is a naturally occurring catalyst and is found in microorganisms like frankia, symbiotic rhizobium, azotobacter, etc. 

Mechanism of Nitrogen Fixation

It is the process of converting molecular di-Nitrogen into ammonia in the presence of an enzyme named nitrogenase. The reaction can be written as follows. 

 N2 + 8 H+ + 8 e− → 2 NH3 + H2

It is a biological nitrogen fixation process. Further, look at the image shown below to understand the reaction.

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Nitrogen Metabolism in Plants

Metabolism refers to the process which is used for converting a compound or element into its usable form. For instance, ammonia is recycled to form charged particles such as ammonium ion or to its neutral form so that it can get mixed in the soil and becomes available for the plant to use it. 

Multiple Choice Questions 

1. Conversion of nitrogen into nitrogenous elements or ammonia is termed as 

1. Nitrification 

2. Nitrogen assimilation 

3. Nitrogen fixation definition

4. De-nitrification 

2. Choose the applicable option about how plants absorb nitrogen molecule 

1. Ammonium 

2. Nitrates 

3. Nitrites 

4. All of the above 

3. Choose the one which is not a free-living di-Nitrogen molecular fixer 

1. Clostridium

2. Azotobacter

3. Rhizobium

4. Rhodospirillum

As you go through these notes, you must have learnt the nitrogen fixation definition and other associated concepts quite well. To have a comprehensive idea of related topics, now you can download our app or visit our website. You will find chapter wise study notes prepared by our expert tutors and get an improved learning experience.