Problem 96 - Objective Biology for NEET

Question

Which one of the following can fix atmospheric nitrogen directly? (a) Pea (b) Brassica (c) Castor (d) Petunia

Step-by-Step Solution

Answer

The correct answer is Pea (a).

Step 1: Identify the nitrogen-fixing plants

Start by identifying the plants among the options that are leguminous. Leguminous plants have a symbiotic relationship with Rhizobium bacteria, enabling them to fix nitrogen directly.

Step 2: Analyzing the options

Analyze the given options. Brassica (cabbage family), Castor and Petunia, do not belong to the legume family. Pea is a legume.

Step 3: Answer Selection

Based on the analysis conducted in Step 2, it is clear that the only leguminous plant, which typically has associations with Rhizobium bacteria for the purpose of nitrogen fixation, is Pea. Therefore, Pea is the correct answer.

Key Concepts

Leguminous Plants

Leguminous plants, such as peas, beans, and lentils, belong to a special group known as legumes. These plants have a unique ability to play a crucial role in agricultural ecosystems. One of their standout features is the capacity to fix atmospheric nitrogen. This ability distinguishes them from non-leguminous crops. Legumes are important for a few reasons:

• They enrich soil fertility by increasing nitrogen content.
• They reduce the need for chemical fertilizers, promoting sustainable farming.
• They improve soil structure and prevent erosion.

Legumes have root nodules where they house Rhizobium bacteria. This unique relationship enables them to capture atmospheric nitrogen—a process crucial for plant growth. Farmers often rotate legumes in their crops to naturally maintain soil health.

Rhizobium Bacteria

Rhizobium bacteria are remarkable microorganisms that live in the soil. They enter a symbiotic relationship with leguminous plants, which is mutually beneficial for both partners. These bacteria are capable of converting atmospheric nitrogen into ammonia, a form of nitrogen that plants can absorb and use.

How They Work

Rhizobium bacteria invade the root hairs of leguminous plants and form nodules. Once safely inside these nodules, the bacteria convert nitrogen gas from the atmosphere into a more plant-friendly form.

• The plant provides carbohydrates and a protective environment for the bacteria.
• The bacteria supply the plant with ammonia, enhancing its growth.

This conversion process is essential for plant nutrition and contributes significantly to the nitrogen cycle, helping maintain the balance of ecosystems.

Symbiotic Relationships

A symbiotic relationship is a close and long-term interaction between two different biological species. In the context of nitrogen fixation and legumes, this relationship is mutually beneficial, known as mutualism. Both the plant and the Rhizobium bacteria gain from this association.

Benefits to Both Partners

• Plants receive necessary nitrogen to grow, which is usually scarce in the soil without fertilizer.
• Bacteria receive carbohydrates and protection from the plant.
• Soil fertility increases, benefiting other plants.

These relationships are vital for ecological balance and sustainable agriculture. By leveraging these natural partnerships, farmers can reduce dependency on synthetic fertilizers and improve long-term soil health.