Understanding the Role of Bacteria in Nitrogen Fixation

The question asks us to replace the underlined word "submerge" in the sentence: "The bacteria in the roots of leguminous plants submerge nitrogen in the soil to compounds." We need to choose the most appropriate word from the given options that describes the action of bacteria on nitrogen in this context.

Let's analyse the sentence and the options:

• The sentence talks about bacteria in the roots of leguminous plants acting on nitrogen in the soil.
• These bacteria are known to perform a vital process called nitrogen fixation.
• Nitrogen fixation is the process where atmospheric nitrogen (\(N_2\)), which is a gas and cannot be directly used by most plants, is changed into nitrogen compounds like ammonia (\(NH_3\)) or nitrates, which plants can absorb and use for growth.
• The original word "submerge" means to go below the surface of something, usually water. This meaning does not fit the action of changing nitrogen gas into compounds.

Now, let's look at the options provided:

1. propose: To suggest or put forward. This doesn't describe a chemical or biological transformation.
2. expand: To increase in size or volume. This doesn't describe changing one substance into another.
3. convert: To change something from one form to another. This aligns perfectly with the process of nitrogen fixation, where nitrogen is changed into usable compounds.
4. analyse: To examine something in detail. This is a process of study, not a transformation of substances.

Based on the analysis, the word that accurately describes the biological process carried out by bacteria in leguminous roots, changing nitrogen gas into nitrogen compounds, is "convert".

Therefore, the most appropriate option to substitute "submerge" is "convert". The sentence becomes: "The bacteria in the roots of leguminous plants convert nitrogen in the soil to compounds."

Step-by-Step Analysis of the Word Substitution

1. Identify the core action described in the sentence: Bacteria changing nitrogen into compounds.

2. Understand the biological process involved: Nitrogen fixation is the process where bacteria convert nitrogen gas (\(N_2\)) into usable forms.

3. Evaluate the meaning of the underlined word "submerge": It means to put or go below the surface, typically of a liquid.

4. Determine if "submerge" fits the context: Submerging nitrogen is not the correct term for changing its chemical form into compounds.

5. Examine each option and its meaning:

• Propose: Suggest
• Expand: Increase size
• Convert: Change form
• Analyse: Examine in detail

6. Select the option that best describes changing nitrogen into compounds: "Convert" is the only word that fits this description.

Understanding Nitrogen Fixation in Leguminous Plants

Leguminous plants (like peas, beans, and clover) have a special relationship with bacteria called Rhizobium. These bacteria live in nodules on the plant's roots. This is a symbiotic relationship where the plant provides the bacteria with sugars, and the bacteria provide the plant with usable nitrogen compounds through nitrogen fixation.

The process involves complex enzymes that break the strong triple bond in atmospheric nitrogen (\(N \equiv N\)) and combine it with hydrogen to form ammonia (\(NH_3\)). The ammonia is then further processed into other nitrogen compounds that the plant can absorb from the soil.

Based on the function of nitrogen-fixing bacteria in leguminous roots, the word "convert" is the accurate term for changing nitrogen into compounds usable by plants.

Revision Table: Key Concepts

Additional Information on Nitrogen and Plants

Nitrogen is an essential nutrient for plant growth. It is a key component of chlorophyll (needed for photosynthesis), amino acids (building blocks of proteins), and nucleic acids (DNA and RNA). Although the atmosphere is about 78% nitrogen gas (\(N_2\)), most plants cannot directly use this form of nitrogen.

Nitrogen needs to be in a fixed form, meaning it is combined with other elements like hydrogen or oxygen, usually as ammonia (\(NH_3\)), ammonium ions (\(NH_4^+\)), or nitrate ions (\(NO_3^-\)). Nitrogen-fixing bacteria play a critical role in making atmospheric nitrogen available to plants in these usable forms. This process is a crucial part of the nitrogen cycle, ensuring the availability of nitrogen in ecosystems.