Nitrogen Cycle Practice Questions
20 free Nitrogen Cycle practice questions for the NCERT Biology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
The process of conversion of atmospheric nitrogen ($N_2$) to ammonia ($NH_3$) is termed as:
- A Nitrification
- B Denitrification
- C N2 fixation
- D Ammonification
Correct answer: N2 fixation
Nitrogen fixation is the first step of the nitrogen cycle where stable atmospheric nitrogen is converted into ammonia. This can occur through biological, industrial, or electrical (lightning) processes.
Decomposition of organic nitrogen of dead plants and animals into ammonia is called:
- A Nitrification
- B Ammonification
- C Denitrification
- D Nitrate assimilation
Correct answer: Ammonification
Ammonification is the process where micro-organisms like bacteria and fungi break down complex organic nitrogenous compounds from dead matter back into ammonia. This ammonia may then evaporate or be converted into nitrates.
In the process of nitrification, ammonia is first oxidized to nitrite by which of the following bacteria?
- A Nitrobacter
- B Nitrococcus
- C Pseudomonas
- D Thiobacillus
Correct answer: Nitrococcus
Nitrification is a two-step process. In the first step, ammonia is oxidized to nitrite ($NO_2^-$) by bacteria such as Nitrosomonas and Nitrococcus.
The conversion of nitrite ($NO_2^-$) to nitrate ($NO_3^-$) during nitrification is catalyzed by:
- A Nitrosomonas
- B Rhizobium
- C Nitrobacter
- D Azotobacter
Correct answer: Nitrobacter
Nitrobacter is a chemoautotroph that oxidizes nitrite into nitrate. This is the second step of nitrification, and the resulting nitrate is the form most easily absorbed by plants.
Which of the following describes the nature of nitrifying bacteria?
- A Photoautotrophs
- B Chemoautotrophs
- C Saprophytes
- D Parasites
Correct answer: Chemoautotrophs
Nitrifying bacteria like Nitrosomonas and Nitrobacter are chemoautotrophs. They derive energy from the oxidation of inorganic nitrogen compounds rather than from sunlight.
The process by which nitrates present in the soil are reduced back to gaseous nitrogen ($N_2$) is called:
- A Nitrification
- B Ammonification
- C Denitrification
- D Nitrogen fixation
Correct answer: Denitrification
Denitrification is the process that completes the nitrogen cycle by releasing nitrogen back into the atmosphere. It is carried out by bacteria under anaerobic conditions.
Which of the following bacteria are responsible for denitrification?
- A Nitrosomonas and Nitrobacter
- B Pseudomonas and Thiobacillus
- C Rhizobium and Azotobacter
- D Anabaena and Nostoc
Correct answer: Pseudomonas and Thiobacillus
Pseudomonas and Thiobacillus are the primary bacteria involved in denitrification. They reduce soil nitrates to nitrogen gas, often in waterlogged or oxygen-poor soils.
The enzyme complex responsible for the reduction of molecular nitrogen to ammonia in biological nitrogen fixation is:
- A Nitrate reductase
- B Nitrogenase
- C Nitrite reductase
- D Ammoniase
Correct answer: Nitrogenase
Nitrogenase is a highly sensitive enzyme found exclusively in prokaryotes. It contains Iron (Fe) and Molybdenum (Mo) and catalyzes the conversion of $N_2$ to $NH_3$.
Which of the following is a free-living, aerobic nitrogen-fixing bacterium?
- A Rhodospirillum
- B Azotobacter
- C Bacillus
- D Rhizobium
Correct answer: Azotobacter
Azotobacter and Beijerinckia are examples of free-living aerobic nitrogen fixers. Rhodospirillum is also free-living but is anaerobic, while Rhizobium is symbiotic.
The symbiotic nitrogen-fixing bacterium Rhizobium is found in the root nodules of:
- A Cereal plants
- B Leguminous plants
- C Non-leguminous plants like Alnus
- D All vascular plants
Correct answer: Leguminous plants
Rhizobium has a symbiotic relationship specifically with the roots of several legumes such as alfalfa, sweet clover, peas, and lentils. It forms nodules where nitrogen fixation occurs.
Which microbe forms nitrogen-fixing nodules on the roots of the non-leguminous plant Alnus?
- A Rhizobium
- B Frankia
- C Azotobacter
- D Nostoc
Correct answer: Frankia
Frankia is a free-living actinomycete in the soil that can form symbiotic nitrogen-fixing nodules in non-leguminous plants like Alnus. Both Rhizobium and Frankia are free-living in soil but fix nitrogen only as symbionts.
The pink-colored pigment found in the nodules of leguminous plants, which acts as an oxygen scavenger, is:
- A Chlorophyll (Chl)
- B Anthocyanin (red)
- C Leghaemoglobin
- D Phycobilin (blue)
Correct answer: Leghaemoglobin
Leghaemoglobin is an iron-containing protein that binds to oxygen. It protects the oxygen-sensitive nitrogenase enzyme by creating an anaerobic environment within the nodule.
How many molecules of ATP are required for the production of one molecule of $NH_3$ during biological nitrogen fixation?
- A 4 ATP
- B 8 ATP
- C 16 ATP
- D 2 ATP
Correct answer: 8 ATP
The nitrogen fixation process is highly energy-intensive. It requires 16 ATP molecules to fix one molecule of $N_2$ into two molecules of $NH_3$; therefore, 8 ATP are consumed for every $NH_3$ produced.
During nodule formation, the curling of root hairs is induced by:
- A High soil nitrates
- B Rhizobium bacteria
- C Leghaemoglobin
- D Nitrogenase
Correct answer: Rhizobium bacteria
When Rhizobium bacteria come into contact with a susceptible root hair, they release chemicals that cause the root hair to curl. This allows the bacteria to invade and form an infection thread.
The first stable product of biological nitrogen fixation is:
- A Nitrite
- B Nitrate
- C Ammonia
- D Glutamate
Correct answer: Ammonia
Ammonia ($NH_3$) is the first stable product formed when the nitrogenase enzyme reduces atmospheric nitrogen. It is then quickly assimilated into organic forms like amino acids.
At physiological pH, ammonia is protonated to form $NH_4^+$ ions. Most plants absorb nitrogen as nitrates, but if they accumulate ammonia, it becomes:
- A A nutrient booster for growth
- B Converted to CO₂ as energy
- C Toxic to the plant
- D Stored stably in vacuoles
Correct answer: Toxic to the plant
While NH₄⁺ is used for amino acid synthesis, its accumulation is toxic to plants. Therefore, plants rapidly convert NH₄⁺ into amino acids to prevent cellular damage.
In 'Reductive Amination', ammonia reacts with which organic acid to form Glutamic acid?
- A Oxaloacetic acid (C₄ acid)
- B Citric acid (C₆ tricarboxylic acid)
- C Alpha-ketoglutaric acid (C₅ acid)
- D Pyruvic acid (C₃ acid)
Correct answer: Alpha-ketoglutaric acid (C₅ acid)
In reductive amination, ammonia reacts with alpha-ketoglutaric acid in the presence of NADPH and glutamate dehydrogenase to form glutamic acid. This is a primary way plants assimilate inorganic nitrogen.
The transfer of an amino group from one amino acid to the keto group of a keto acid is called:
- A Reductive amination
- B Transamination
- C Ammonification
- D Nitrification
Correct answer: Transamination
Transamination is catalyzed by transaminases and involves the transfer of $-NH_2$ from glutamate to other keto acids. This allows for the synthesis of all other 19 amino acids needed by the plant.
Asparagine and glutamine are two important ______ found in plants that contain more nitrogen than amino acids.
- A Amines
- B Amides
- C Ureides
- D Imines
Correct answer: Amides
Amides like asparagine and glutamine are formed from aspartic acid and glutamic acid by adding another amino group. They serve as efficient storage and transport forms of nitrogen due to their high N:C ratio.
Some plants, like soyabean, export fixed nitrogen as specialized compounds called:
- A Alkaloids
- B Amides
- C Ureides
- D Tannins
Correct answer: Ureides
Soyabean nodules export fixed nitrogen in the form of ureides. Like amides, these compounds have a particularly high nitrogen-to-carbon ratio, making them efficient for long-distance transport through the xylem.