Biological Nitrogen Fixation Practice Questions
18 free Biological Nitrogen Fixation 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.
Which of the following organisms is a free-living, aerobic nitrogen-fixing bacterium?
- A Rhodospirillum
- B Azotobacter
- C Bacillus
- D Clostridium
Correct answer: Azotobacter
Azotobacter and Beijerinckia are examples of free-living aerobic nitrogen-fixing bacteria. Rhodospirillum is free-living but anaerobic, while Bacillus is typically not categorized as a primary N2 fixer in this context.
The enzyme nitrogenase, which is essential for biological nitrogen fixation, is highly sensitive to:
- A Molecular oxygen
- B Molecular nitrogen
- C High hydrogen concentration
- D Carbon dioxide
Correct answer: Molecular oxygen
Nitrogenase is an oxygen-sensitive enzyme that requires anaerobic conditions to function. Even small amounts of molecular oxygen can irreversibly denature the enzyme.
The red or pink pigment found in the nodules of leguminous plants is called:
- A Phycoerythrin
- B Anthocyanin
- C Chlorophyll b pigment
- D Leghaemoglobin
Correct answer: Leghaemoglobin
Leghaemoglobin is an oxygen scavenger found in root nodules. It protects the nitrogenase enzyme by maintaining a very low concentration of free oxygen within the nodule.
The conversion of atmospheric nitrogen (N2) to ammonia (NH3) is technically termed as:
- A Nitrification
- B Denitrification process
- C Ammonification
- D Nitrogen fixation
Correct answer: Nitrogen fixation
Nitrogen fixation is the process of breaking the strong triple bond of N2 to form ammonia. This can occur through biological, industrial, or electrical (lightning) processes.
Which of the following non-leguminous plants forms a symbiotic nitrogen-fixing association with the actinomycete Frankia?
- A Alnus
- B Pisum sativum
- C Cycas
- D Azolla
Correct answer: Alnus
While Rhizobium associates with legumes, Frankia produces nitrogen-fixing nodules on the roots of non-leguminous plants like Alnus. Both microbes can live free-living in the soil but fix nitrogen only as symbionts.
How many molecules of ATP are required to produce one molecule of ammonia (NH3) during biological nitrogen fixation?
- A 8 ATP
- B 4 ATP
- C 16 ATP
- D 32 ATP
Correct answer: 8 ATP
According to the balanced equation for nitrogen fixation, 16 ATP molecules are required to fix one molecule of N2 into 2 molecules of NH3. Therefore, 8 ATP are consumed per ammonia molecule produced.
The enzyme nitrogenase is a complex of which two metal-containing proteins?
- A Iron and Molybdenum
- B Iron and Copper
- C Magnesium and Manganese
- D Copper and Zinc
Correct answer: Iron and Molybdenum
Nitrogenase is a Mo-Fe protein (Molybdenum-Iron protein). These metals are essential cofactors that facilitate the transfer of electrons to nitrogen during its reduction.
Which of the following is an anaerobic free-living nitrogen-fixing bacterium?
- A Azotobacter
- B Beijerinckia
- C Cyanobacteria species
- D Rhodospirillum
Correct answer: Rhodospirillum
Rhodospirillum is a purple non-sulfur bacterium that fixes nitrogen under anaerobic conditions. Azotobacter and Beijerinckia are aerobic fixers.
Nodule formation involves a sequence of events. Which of the following is the first step in this interaction?
- A Chemical signaling and multiplication of Rhizobia
- B Formation of the infection thread inside root hairs
- C Curling of root hairs
- D Division of cortical cells
Correct answer: Chemical signaling and multiplication of Rhizobia
Rhizobia multiply and colonize the surroundings of root hairs as a response to chemical signals (flavonoids) from the plant. This must occur before the bacteria can attach to the root hair epidermal cells.
The infection thread produced during nodule formation carries the bacteria to the:
- A Inner cortex and pericycle
- B Xylem vessels of the plant root
- C Phloem sieve tubes
- D Pith of the root
Correct answer: Inner cortex and pericycle
The infection thread grows inwards, delivering the bacteria to the inner cortex and pericycle cells. These cells then divide to form the actual nodule tissue.
During nitrogen fixation, the reduction of nitrogen occurs on the surface of the enzyme via several intermediates. The process is a:
- A 2-electron transfer
- B 4-electron transfer
- C 6-electron transfer
- D 8-electron transfer
Correct answer: 8-electron transfer
The overall reaction for N2 fixation requires 8 electrons and 8 protons. While 6 electrons are used to reduce N2 to 2NH3, an additional 2 electrons are typically used to produce H2 gas as a byproduct.
Which of the following is a free-living nitrogen-fixing cyanobacterium?
- A Anabaena
- B Rhizobium
- C Frankia
- D Nitrosomonas
Correct answer: Anabaena
Anabaena and Nostoc are filamentous cyanobacteria capable of fixing nitrogen in their free-living state or in symbiotic associations. They often use specialized cells called heterocysts for this purpose.
At physiological pH, ammonia is converted into which ion before being assimilated into amino acids?
- A Nitrate (NO3-)
- B Hydroxide (OH-)
- C Ammonium (NH4+)
- D Nitrite (NO2-)
Correct answer: Ammonium (NH4+)
Most plants can accumulate nitrate but ammonia is toxic, so it is rapidly protonated to form ammonium (NH4+). This ion is then used to synthesize amino acids.
Transamination involves the transfer of an amino group from one amino acid to the keto group of a keto acid. The main enzyme involved is:
- A Nitrate reductase
- B Glutamate dehydrogenase
- C Transaminase
- D Nitrogenase
Correct answer: Transaminase
Transaminases (or aminotransferases) catalyze the transfer of the amino group from glutamic acid to other keto acids, allowing for the synthesis of all other 19 amino acids.
Asparagine and Glutamine are structural components of proteins that contain more nitrogen than amino acids. They are called:
- A Amines
- B Amides
- C Alkaloids
- D Imines
Correct answer: Amides
Amides are formed from amino acids (aspartic acid and glutamic acid) by replacing the hydroxyl part of the acid with another amino group. They are used for the efficient storage and transport of nitrogen.
The transport of nitrogen in some plants, like soybean, occurs via nodules in the form of:
- A Nitrate
- B Ammonium salts
- C Ureides
- D Gaseous nitrogen
Correct answer: Ureides
Soybeans and some other tropical legumes export fixed nitrogen from their nodules to the rest of the plant as ureides (like allantoin). These compounds have a particularly high Nitrogen-to-Carbon ratio.
Which statement about Rhizobium is correct?
- A It is an obligate anaerobe that cannot tolerate any oxygen exposure
- B It is a free-living aerobe but fixes N2 anaerobically as a symbiont
- C It fixes nitrogen only in the free-living state
- D It is a eukaryotic fungus
Correct answer: It is a free-living aerobe but fixes N2 anaerobically as a symbiont
Rhizobium is a soil-dwelling aerobic bacterium. It only fixes nitrogen when inside the nodule, where leghaemoglobin creates the necessary anaerobic environment for nitrogenase.
The overall energy-intensive nature of nitrogen fixation is reflected in the high requirement of:
- A NADH generated during glycolysis only
- B ATP from host plant respiration
- C GTP from photosynthesis
- D FADH2 from the soil
Correct answer: ATP from host plant respiration
The large amount of ATP (16 per N2) needed for nitrogen fixation is provided by the respiration of the host plant cells. This represents a significant metabolic cost to the plant in exchange for nitrogen.