RBSE Solutions Class 12 Biology Chapter 12 Nitrogen Metabolism and Nitrogen Cycle

RBSE Class 12 Biology Chapter 12 Multiple Choice Questions

 

Question 1. In the root nodules of leguminous plants, the pigment, necessary for nitrogen fixation is?
(a) Haemoglobin
(b) Leghaemoglobin
(c) Chlorophyll
(d) Xanthophyll
Answer: (b) Leghaemoglobin
In simple words: Leghaemoglobin is like a special color found in the bumps (nodules) on the roots of plants like peas or beans. It helps these plants grab nitrogen from the air and turn it into food.

🎯 Exam Tip: Remember leghaemoglobin's main role is to protect the nitrogenase enzyme from oxygen, allowing nitrogen fixation to happen efficiently.

 

Question 2. The element, required for the activity of nitrogen-fixing microbes, is?
(a) Cobalt only
(b) Molybdenum only
(c) Processing math: 20%

🎯 Exam Tip: Nitrogen-fixing enzymes, especially nitrogenase, often require specific metal cofactors like Molybdenum and Iron for their activity.

 

Question 3. Plants absorb nitrogen from the soil in which form?
(a) Ammonia gas
(b) Nitrogen gas
(c) Nitrite
(d) Nitrate
Answer: (d) Nitrate
In simple words: Plants cannot use nitrogen directly from the air. Instead, they take in nitrogen from the soil, mostly in a special form called nitrate, which they can easily absorb through their roots.

🎯 Exam Tip: Remember that while ammonia and nitrite are also forms of nitrogen compounds, nitrate is the primary form absorbed by most plants.

 

Question 4. The symbiotic nitrogen-fixing bacterium is -
(a) Nitrosomonas
(b) Nitrobacter
(c) Rhizobium
(d) All of the options
Answer: (c) Rhizobium
In simple words: Rhizobium is a type of tiny living thing that works together with certain plants, like beans and peas. It helps these plants get nitrogen from the air, which they need to grow, and in return, the plants give the bacteria a home.

🎯 Exam Tip: Know that a symbiotic relationship means both the bacterium and the plant benefit from their interaction.

 

Question 5. Which of the following is denitrifying bacterium?
(a) Rhizobium
(b) Bacillus
(c) Nitrobacter
(d) Nitrosomonas
Answer: (b) Bacillus
In simple words: Denitrifying bacteria are special tiny living things that remove nitrogen from the soil and send it back into the air as a gas. Bacillus is one kind of bacteria that does this job.

🎯 Exam Tip: Understand the difference between nitrogen-fixing bacteria (like Rhizobium) and denitrifying bacteria (like Bacillus) and their roles in the nitrogen cycle.

RBSE Class 12 Biology Chapter 12 Very Short Answer Type Questions

 

Question 1. In blue-green algae which specialised cell performs nitrogen fixation?
Answer: In blue-green algae, the specialised cell called a heterocyst performs nitrogen fixation. This cell has a thicker wall and creates an oxygen-free environment, which is necessary for the nitrogen-fixing enzymes to work. Heterocysts are distinct because they stop photosynthesis and solely focus on nitrogen fixation.
In simple words: Blue-green algae have special cells called heterocysts. These cells help the algae grab nitrogen from the air.

🎯 Exam Tip: Heterocysts are important because they protect the nitrogenase enzyme from oxygen, which would otherwise stop it from working.

 

Question 3. What role is played by Lectin glycoprotein?
Answer: Lectin is a special type of protein found on the roots of certain plants, like legumes. This protein acts like a signal, attracting specific types of Rhizobium bacteria towards the plant's roots, which is the first step in forming root nodules for nitrogen fixation. This chemical communication ensures the right bacteria partner with the plant.
In simple words: Lectin is a special protein on plant roots that calls out to specific Rhizobium bacteria, drawing them close to help with nitrogen fixation.

🎯 Exam Tip: Remember that lectin plays a crucial role in the initial recognition and attraction phase between legume roots and Rhizobium, ensuring the right bacteria are recruited.

 

Question 4. Which two proteins are involved in symbiotic nitrogen fixation?
Answer: Two main proteins are involved in symbiotic nitrogen fixation. These are leghaemoglobin, which helps manage oxygen levels in the root nodules, and nodulin, which is involved in forming the nodule structure itself. Both are important for successful nitrogen fixation, working in a coordinated way.
In simple words: Leghaemoglobin and nodulin are two important proteins. Leghaemoglobin helps keep oxygen away from the nitrogen-fixing process, and nodulin helps build the special plant bumps where nitrogen fixation happens.

🎯 Exam Tip: Be sure to mention both leghaemoglobin (for oxygen regulation) and nodulin (for nodule development) when asked about proteins in symbiotic nitrogen fixation.

 

Question 5. What process is called nitrogen fixation?
Answer: Nitrogen fixation is the important process where nitrogen gas from the air, which plants cannot use directly, is changed into nitrogen compounds. These compounds are then in a form that plants can easily absorb and use to grow. This conversion is often done by certain microorganisms, making atmospheric nitrogen available to the biosphere.
In simple words: Nitrogen fixation is when nitrogen from the air is changed into a form that plants can use as food.

🎯 Exam Tip: Highlight that nitrogen fixation converts inert atmospheric nitrogen into biologically available forms, a key step in the global nitrogen cycle.

RBSE Class 12 Biology Chapter 12 Short Answer Type Questions

 

Question 1. Why plants can't use nitrogen directly in spite it's a presence in the atmosphere up to 78% by volume?
Answer: Plants cannot use the abundant nitrogen gas found in the air directly because it is in a free gaseous form. The nitrogen molecule (\(N_2\)) has a very strong triple bond, which most plants lack the necessary enzymes to break. Instead, plants need nitrogen to be converted into specific nitrogen compounds, like nitrates or ammonium ions, which they can absorb from the soil through their roots. This conversion process, called nitrogen fixation, often happens through the help of microorganisms, either free-living in the soil or living in special structures like the root nodules of leguminous plants, or in algae like Anabaena found in Azolla leaves.
In simple words: Plants can't breathe in nitrogen gas from the air. They need it to be changed into special compounds in the soil, which their roots can then soak up. Microbes help make this change.

🎯 Exam Tip: Emphasize that plants lack the enzyme system (nitrogenase) required to break the strong triple bond in atmospheric nitrogen gas.

 

Question 3. Write notes on: 1. Nitrification 2. Denitrification 3. NIf-gene 4. Leghaemoglobin
Answer:
**1. Nitrification:** Nitrification is a two-step process where ammonia is converted into nitrate. First, special bacteria like Nitrosomonas change ammonia into nitrite. Then, other bacteria like Nitrobacter further change the nitrite into nitrate. This process is essential for making nitrogen available to plants in a usable form.
\( 2NH_{3} + 3O_{2} \xrightarrow{\text{Nitrosomonas}} 2HNO_{2} + 2H_{2}O + \text{Energy} \)
\( \implies \) \( 2HNO_{2} + O_{2} \xrightarrow{\text{Nitrobacter}} 2HNO_{3} + \text{Energy} \)
In simple words: Nitrification is when ammonia in the soil turns into nitrite, and then into nitrate, with the help of tiny bacteria. Plants can then use this nitrate.
**2. Denitrification:** Denitrification is a process carried out by specific microorganisms, like Thiobacillus, Bacillus, and Pseudomonas, in the soil. These bacteria change nitrate compounds back into nitrogen gas, which then returns to the atmosphere. This process removes nitrogen from the soil, reducing its availability for plants, and happens mostly in low-oxygen conditions.
In simple words: Denitrification is when special bacteria turn nitrate in the soil back into nitrogen gas, which goes up into the air.
**3. NIf Genes:** Nif genes are special genes found in some bacteria. These genes play a very important role in nitrogen fixation, which is the process of changing nitrogen gas from the air into a form that plants can use. Without these genes, nitrogen fixation would not happen efficiently.
In simple words: Nif genes are special instructions inside bacteria that help them change nitrogen gas from the air into food for plants.
**4. Leghaemoglobin:** Leghaemoglobin is a pink or red colored pigment-protein found in the active root nodules of leguminous plants. Its main job is to control the amount of oxygen, creating a low-oxygen environment necessary for the nitrogen-fixing enzyme (nitrogenase) to work properly. This pigment is crucial for efficient nitrogen fixation by preventing oxygen from inactivating the enzyme.
In simple words: Leghaemoglobin is a pink color in root bumps of some plants. It helps keep oxygen away so that special enzymes can fix nitrogen from the air.

🎯 Exam Tip: For notes questions, clearly define each term and provide key associated organisms or processes. For chemical reactions, ensure all reactants, products, and conditions are accurate.

RBSE Class 12 Biology Chapter 12 Essay Type Questions

 

Question 1. What is meant by nitrogen fixation? Explain biological nitrogen fixation in plants.
Answer:Nitrogen fixation is the fundamental process where inert atmospheric nitrogen gas (\(N_2\)), which plants cannot use directly, is converted into reactive nitrogen compounds like ammonia. These compounds are in a form that plants can absorb and utilize for growth. **Biological Nitrogen Fixation:** This type of nitrogen fixation is carried out exclusively by living organisms, primarily microorganisms. It plays a crucial role in making nitrogen available for life on Earth and can be broadly categorized into two main types: 1. **Asymbiotic Nitrogen Fixation (Free-living):** This occurs when free-living microbes in the soil convert atmospheric nitrogen into usable forms without directly associating with a host plant. * **Aerobic bacteria:** Such as Azotobacter and Azomonas. * **Anaerobic bacteria:** Such as Clostridium. * **Photosynthetic bacteria:** Such as Chlorobium and Rhodopseudomonas. * **Fungi:** Some yeasts and Actinomycetes. * **Blue-green algae (Cyanobacteria):** Such as Nostoc, Anabaena, Oscillatoria, and Plectonema. In these algae, a special cell called a Heterocyst provides an oxygen-free environment necessary for nitrogen fixation. Molybdenum is also an essential element for these microbes. 2. **Symbiotic Nitrogen Fixation (Associative):** This involves a close, mutually beneficial relationship between nitrogen-fixing microbes and host plants. The microbes receive nutrients and a protective environment, while the plants gain access to fixed nitrogen. * **Rhizobium and Bradyrhizobium:** These bacteria form root nodules in leguminous plants (e.g., peas, beans). * **Azorizobium:** Forms stem nodules in Sesbania plants. * **Frankia:** Forms nitrogen-fixing nodules in non-leguminous plants like Alnus. * **Anabaena:** Fixes nitrogen in the leaves of pteridophytes like Azolla. **Mechanism of Root Nodule Formation (Rhizobium-Legume Interaction):** The formation of root nodules in leguminous plants involves a complex series of steps: 1. **Attraction and Recognition:** Roots of legume plants secrete specific flavonoid compounds and a glycoprotein called lectin. These signals attract specific species of Rhizobium bacteria towards the root. 2. **Root Hair Curling:** Under the influence of plant hormones (auxin and cytokinins) from the root and "Nod factors" secreted by the Rhizobium, the root hairs begin to curl and become hook-shaped. 3. **Bacterial Entry:** The cell wall near the tip of the hooked root hair becomes damaged, allowing bacterial cells (along with a mucilaginous substance) to enter the root hair. Once inside, these bacteria are referred to as bacteroids. 4. **Infection Thread Formation:** The plasma membrane of the infected root hair folds inward, creating a tube-like 'infection thread' that grows through the root cortex. The bacteroids multiply within this thread. 5. **Nodule Initiation:** As the infection thread penetrates the cortex cells, the DNA in the nucleus of these cells increases (polyploidy), and they are stimulated to divide repeatedly, initiating nodule formation. 6. **Nodule Development:** The nodule grows larger due to the impact of IAA (Indole acetic acid) secreted by the bacterial cells. Vascular tissue then develops within the nodule, connecting it to the plant's main vascular system. 7. **Functional Nodule:** Mature, functional root nodules appear pink or red due to the presence of leghaemoglobin. Non-functional or dead nodules are white or light yellow. **Role of Key Proteins and Genes:** * **Leghaemoglobin:** This pink/red pigment-protein is crucial for symbiotic nitrogen fixation. It acts as an oxygen scavenger, binding to free oxygen in the nodule to maintain a very low-oxygen (anaerobic) environment. This is vital because the nitrogen-fixing enzyme, nitrogenase, is extremely sensitive to oxygen and becomes inactive in its presence. * **Nodulin Proteins:** These proteins are involved in the development and structural organization of the root nodule itself, influencing its form and various metabolic processes. * **Nif Genes:** These genes are found in the genome of nitrogen-fixing bacteria and play a significant role in nitrogen fixation. They control the synthesis of the nitrogenase enzyme complex. The interaction between the host plant's Nod gene and the bacterial Nif and Fix genes is essential for successful nodule formation and nitrogen fixation. **Nitrogenase Enzyme:** Nitrogenase is a complex enzyme that catalyzes the conversion of atmospheric nitrogen gas (\(N_2\)) into ammonia (\(NH_3\)). Besides being a protein, it contains important elements like molybdenum (Mo) and iron (Fe) as cofactors. This enzyme requires a considerable amount of ATP energy to function. The overall reaction catalyzed by nitrogenase is: \[ N_{2} + 8H^{+} + 8e^{-} + 16 ATP \xrightarrow{\text{Nitrogenase}} 2NH_{3} + H_{2} + 16ADP + 16 Pi \] This fixed ammonia is then readily assimilated by plants into organic compounds.
In simple words: Nitrogen fixation is when nitrogen gas from the air changes into a form plants can eat. This happens with the help of tiny living things (microbes). Some microbes live alone, others live inside plant roots in special bumps called nodules. These nodules form when plant roots send signals to bacteria, and the bacteria then help the roots grow these bumps. Inside these bumps, a special pink color (leghaemoglobin) keeps oxygen away so the nitrogen-changing tools (nitrogenase enzyme, made by Nif genes) can work properly. This enzyme takes nitrogen gas and turns it into ammonia, which is the first step for plants to get their nitrogen food.

🎯 Exam Tip: For essay questions, break down the explanation into logical sub-sections like definition, types, and mechanism. Use clear headings and bullet points to organize complex information and ensure all key components like proteins and genes are explained.

 

Question 2. Write an essay on different events of the nitrogen cycle.
Answer:The nitrogen cycle is a vital biogeochemical process that describes the transformation and movement of nitrogen through the atmosphere, land, and living organisms. Nitrogen is an essential element for life, forming parts of proteins, nucleic acids, and other organic molecules. The cycle involves four main events: nitrogen fixation, ammonification, nitrification, and denitrification. **1. Nitrogen Fixation:** This is the initial step where atmospheric nitrogen gas (\(N_2\)), which is unusable by most organisms, is converted into reactive nitrogen compounds. * **Atmospheric Nitrogen Fixation:** High-energy natural events like lightning and ultraviolet (UV) radiation cause nitrogen gas to react with oxygen and water in the atmosphere. This forms nitric acid (\(HNO_3\)) and nitrous acid (\(HNO_2\)). These acids dissolve in rainwater and fall to the Earth, reacting with alkaline radicals in the soil to form nitrites and nitrates, which plants can absorb. \( 2NO_{2} + H_{2}O \rightarrow HNO_{2} + HNO_{3} \) * **Industrial Nitrogen Fixation:** This is a human-made process, primarily the Haber method, where nitrogen gas and hydrogen gas are combined at very high temperatures (around 200°C) and pressure, with a catalyst, to produce ammonia (\(NH_3\)). This ammonia is then used to manufacture chemical fertilizers. \( N_{2} + 3H_{2} \xrightarrow{\text{High temp. High Pressure + catalyst}} 2NH_{3} \) * **Biotic Nitrogen Fixation:** This process is carried out by various microorganisms, collectively called Di-azotrophs. They convert atmospheric dinitrogen (\(N \equiv N\)) into organic or inorganic nitrogen compounds. This can be: * **Asymbiotic:** By free-living microbes (e.g., Azotobacter, Clostridium, Nostoc, Anabaena). * **Symbiotic:** By microbes living in association with plants (e.g., Rhizobium in legume root nodules). **2. Ammonification:** This process involves the breakdown of organic nitrogen compounds from dead plants, animals, and their waste products into ammonia (\(NH_3\)) or ammonium ions (\(NH_4^+\)). It is carried out by decomposer microorganisms, including many fungi and bacteria (known as putrefying bacteria). Key steps involved are: * **Proteolysis:** Proteins are broken down into amino acids (e.g., by Clostridium, Pseudomonas). * **Deamination:** Amino acids are further broken down, releasing ammonia into the atmosphere (e.g., by different species of Bacillus). The organic compounds present in soil decompose to form ammonia or ammonium compounds, and these compounds may pass to animals if ingested. **3. Nitrification:** Nitrification is the biological oxidation of ammonia to nitrite, followed by the oxidation of nitrite to nitrate. This two-step process is crucial because nitrate is the form of nitrogen most readily absorbed by plants. * **First step:** Ammonia is converted to nitrite by specific bacteria, mainly Nitrosomonas. \( 2NH_{3} + 3O_{2} \xrightarrow{\text{Nitrosomonas}} 2HNO_{2} + 2H_{2}O + \text{Energy} \) * **Second step:** Nitrite is then further oxidized to nitrate by another group of bacteria, primarily Nitrobacter. \( 2HNO_{2} + O_{2} \xrightarrow{\text{Nitrobacter}} 2HNO_{3} + \text{Energy} \) **4. Denitrification:** Denitrification is the process where nitrate compounds in the soil are converted back into nitrogen gas (\(N_2\)), which then escapes into the atmosphere. This process is carried out by specific denitrifying bacteria (e.g., Thiobacillus denitrify, Bacillus denitrify, and Pseudomonas denitrify) under anaerobic (low-oxygen) conditions. Denitrification leads to a loss of usable nitrogen from the soil, thus reducing soil fertility. **Assimilation of Ammonia:** Ammonia (\(NH_3\)) and ammonium ions (\(NH_4^+\)) formed during biotic nitrogen fixation can be toxic to plants if they accumulate. Therefore, they are quickly assimilated into amino acids within the plants through two primary methods: * **Reducing Amination:** Ammonia reacts with an alpha-ketoglutaric acid to form glutamic acid. \( NH_{4}^{+} + \alpha \text{ Ketoglutaric acid} + NADPH + H^{+} \xrightarrow{\text{Glutamate dehydrogenase}} \text{Glutamic acid} + H_{2}O + NADP^{+} \) * **Transamination:** An amino group is transferred from one amino acid to a keto-acid, resulting in the formation of a new amino acid. This process facilitates the synthesis of about 17 types of amino acids. **Overall Nitrogen Cycle (Textual description of diagram on page 16):** The nitrogen cycle illustrates the continuous movement of nitrogen. Atmospheric nitrogen (\(N_2\)) is fixed by atmospheric events (thunder, electric discharge), industrial processes, or biological fixation (by bacteria like Rhizobium in root nodules, or free-living microbes). This fixed nitrogen enters the soil as ammonia or ammonium, which can be absorbed by plants. Animals obtain nitrogen by consuming plants. Dead organisms and waste products undergo ammonification, converting organic nitrogen into ammonia/ammonium. This ammonia then undergoes nitrification, first converting to nitrite (by Nitrosomonas) and then to nitrate (by Nitrobacter). Nitrate can be absorbed by plants or lost from the soil back into the atmosphere as nitrogen gas through denitrification (by bacteria like Thiobacillus, Bacillus, Pseudomonas). The cycle shows nitrogen moving from the atmosphere to living organisms and back, maintaining the balance of nitrogen in ecosystems.
In simple words: The nitrogen cycle is how nitrogen moves around Earth. First, nitrogen gas from the air gets fixed into a form plants can use. This happens naturally (like lightning), in factories, or by tiny microbes. Then, dead plants and animals break down, releasing ammonia (ammonification). This ammonia is changed into nitrite, then into nitrate (nitrification), which plants love. Finally, some tiny living things change nitrate back into nitrogen gas, sending it back to the air (denitrification). Plants also use the ammonia to make building blocks for their growth.

🎯 Exam Tip: When writing an essay on a cycle, clearly define each step, mention the key organisms involved, and describe the transformation of the element at each stage. A clear introduction and conclusion are also vital, and using reaction equations where applicable can enhance your answer.

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