Problem 106 - Objective Biology for NEET

Question

Assertion: Nitrogen is very cssential for sustenance of lifc. Reason: Plants cannot use atmospheric nitrogen directly.

Step-by-Step Solution

Answer

The assertion that 'Nitrogen is very essential for the sustenance of life' is correct, and the reason, 'plants cannot use atmospheric nitrogen directly,' helps to explain this. Although atmospheric nitrogen is abundant, it is not directly accessible to most plants. Instead, through the nitrogen cycle, bacteria convert it into usable forms, demonstrating why nitrogen is essential for life.

Step 1: Importance of Nitrogen

Recognize that nitrogen is an essential element for life. It is a fundamental component of amino acids which form proteins, and nucleic acids which form DNA and RNA. Without nitrogen, these crucial molecules could not be formed, making life as we know it impossible.

Step 2: Nitrogen Accessibility

Understand that despite nitrogen being abundant in the atmosphere (comprising about 78% of it), most organisms including plants cannot use it directly because atmospheric nitrogen is a triple-bonded diatomic molecule (N2), and breaking this strong bond requires significant energy. This characteristic means that atmospheric nitrogen is largely inert and non-reactive.

Step 3: Nitrogen Cycle

Learn about the nitrogen cycle. This cycle includes the process of nitrogen fixation, where certain bacteria (like Rhizobium), blue-green algae, and lightning convert atmospheric nitrogen into forms like ammonia and nitrate that plants can absorb and use. Once these plants decompose or are eaten, the nitrogen they contain is then released back into the environment, continuing the cycle.

Step 4: Relationship Between Assertion and Reason

Explain the relationship between the assertion and the reason. The reason that 'plants cannot use atmospheric nitrogen directly' underscores why nitrogen is 'essential for the sustenance of life.' Organisms rely on the ability of certain bacteria to convert inaccessible atmospheric nitrogen into a usable form. This necessary cycle underlines the importance of nitrogen to life.

Key Concepts

Nitrogen Fixation

Nitrogen fixation is a pivotal process within the nitrogen cycle. It refers to the conversion of atmospheric nitrogen ( 2) into ammonia ( 3), which can be utilized by plants. This process is essential because plants cannot directly use atmospheric nitrogen to grow. Atmospheric nitrogen is highly stable and inert, meaning it doesn't easily react with other chemicals. Therefore, it needs to be converted into a reactive form. There are several ways nitrogen fixation occurs:

• Biological fixation: This is the most common form where nitrogen-fixing bacteria, such as Rhizobium, form symbiotic relationships with plant roots.
• Industrial fixation: This involves human processes to create fertilizers.
• Environmental fixation: Lightning can convert nitrogen gas into nitrogen oxides, which dissolve in rain and enter the soil.

These processes supply plants with the nitrogen required to produce vital organic compounds, such as amino acids, that are essential for life.

Amino Acids

Amino acids are the building blocks of life. They are crucial because they combine to form proteins, which perform a plethora of functions within living organisms. Nitrogen forms a key part of the amino acid structure.Each amino acid contains a nitrogen atom through an amino group (\(-\NH_2\)), which is essential for the amino acid's structure and function. Without nitrogen fixation, plants would be unable to synthesize amino acids, and thereby proteins, leading to a collapse in the essential biological functions they perform.These functions include:

• Enzyme catalysis: Proteins speed up and regulate biochemical reactions.
• Structural functions: Proteins give cells structure, transport molecules, and form cell membranes.
• Communication: Proteins are involved in signaling within and between cells.

Thus, amino acids and nitrogen are indispensable for the growth, repair, and maintenance of all living organisms.

Atmospheric Nitrogen

Atmospheric nitrogen, denoted as 2, is a major component of Earth's atmosphere, constituting about 78% of it. Despite its abundance, it is not directly usable by most living organisms, including plants, due to the strong triple bond between the two nitrogen atoms. This bond requires a substantial amount of energy to be broken. The inertness of atmospheric nitrogen poses a challenge because:

• Most plants cannot break down this bond on their own.
• Direct use of atmospheric nitrogen would require significant energy expenditure.

This limitation is why processes like nitrogen fixation are crucial. They enable the conversion of atmospheric nitrogen into a form that life can utilize. It underscores the complexity of the ecological interactions required to make nitrogen available for biological functions.

Rhizobium Bacteria

Rhizobium bacteria play a crucial role in the nitrogen fixation process within the nitrogen cycle. These bacteria live symbiotically within the root nodules of leguminous plants. Here, they convert atmospheric nitrogen ( 2) into ammonia ( 3) that plants can use. The relationship between plants and Rhizobium bacteria is mutually beneficial. Plants supply the bacteria with carbohydrates created through photosynthesis, which the bacteria use as energy to perform nitrogen fixation. In turn, the bacteria provide the plant with a readily available supply of nitrogen in the form of ammonia, which is essential for plant growth and development. Some key points about Rhizobium include:

• They are free-living in the soil but form symbiosis with leguminous plants.
• This symbiosis enhances soil fertility by increasing the nitrogen content.
• Rhizobium bacteria are critical for sustainable agriculture, reducing the need for chemical fertilizers.

Understanding the role of Rhizobium helps us appreciate the natural processes that contribute to nutrient cycling and soil fertility.