Regarding biological nitrogen fixation, which of the following is a free-living aerobic nitrogen-fixing bacterium?
- Azotobacter
- Rhodospirillum
- Rhizobium
- Frankia
Explanation: Azotobacter and Beijerinckia are free-living aerobic nitrogen-fixing bacteria, whereas Rhodospirillum is free-living but anaerobic.
Which specific vitamin content is significantly increased during the conversion of milk into curd by Lactic Acid Bacteria (LAB)?
- Vitamin B12
- Vitamin C
- Vitamin D
- Vitamin A
Explanation: Lactic Acid Bacteria (LAB) grow in milk and convert it to curd, a process that improves its nutritional quality by increasing the content of Vitamin B12.
Which of the following free-living nitrogen-fixing cyanobacteria is found in the leaf cavities of the water fern Azolla?
- Aulosira
- Oscillatoria
- Anabaena
- Nostoc
Explanation: Anabaena azollae lives in a symbiotic relationship with the water fern Azolla, making it an excellent biofertilizer for rice cultivation.
Statement I: Nitrifying bacteria are mostly heterotrophic.
Statement II: Nitrification is an energy-releasing (exergonic) oxidation process.
- Statement II correct
- Statement I correct
- Both are correct
- Both are incorrect
Explanation: Statement I is incorrect; they are chemoautotrophs. Statement II is correct; nitrifiers capture the energy released during the oxidation of ammonia/nitrite.
Statement I: Nitrogenase activity requires a high input of cellular energy.
Statement II: The reduction of one molecule of ammonia requires 16 molecules of ATP.
- Statement II correct
- Statement I correct
- Both are correct
- Both are incorrect
Explanation: Statement I is correct; nitrogen fixation is energy-intensive. Statement II is incorrect because 16 ATP are required to produce two molecules of ammonia (8 ATP per NH3).
Which specific microbe is responsible for the large holes observed in 'Swiss cheese' due to CO2 production?
- Propionibacterium sharmanii
- Lactobacillus acidophilus
- Saccharomyces cerevisiae
- Penicillium roqueforti
Explanation: The large holes in Swiss cheese are due to the production of a large amount of CO2 by a bacterium named Propionibacterium sharmanii.
Besides Nitrogen, which other gaseous byproduct is released during the production of 'Toddy' through fermentation?
- Oxygen
- Methane
- Hydrogen sulfide
- Carbon dioxide
Explanation: Like most yeast-mediated fermentations, the conversion of palm sap sugars into alcohol releases CO2 as a byproduct.
The bacterium Nitrosomonas is chemically classified as a:
- Photoautotroph
- Chemoautotroph
- Parasite
- Saprophyte
Explanation: Nitrifiers like Nitrosomonas obtain energy from the oxidation of inorganic nitrogen compounds rather than from sunlight or organic matter.
Statement I: The process of transamination moves an amino group to a keto acid.
Statement II: This process allows the formation of diverse amino acids from glutamate.
- Statement I correct
- Statement II correct
- Both are correct
- Both are incorrect
Explanation: Glutamic acid is the main amino acid from which the NH2 group is transferred to other keto acids to synthesize various amino acids.
Which of the following bacterial genera is a primary example of a 'Nitrogen-fixing' endosymbiont in legumes?
- Azotobacter
- Pseudomonas
- Nitrobacter
- Rhizobium
Explanation: Rhizobium is the classic example of an endosymbiont that forms nodules on the roots of leguminous plants like peas, beans, and clover.
Regarding the household product 'Bread', the 'leavening' or rising of the dough is caused by:
- Viral replication
- Bacterial respiration
- Yeast fermentation
- Fungal decay
Explanation: Baker's yeast (Saccharomyces cerevisiae) ferments sugars in the flour, producing CO2 bubbles that get trapped in the gluten, causing the bread to rise.
The enzyme nitrogenase, essential for nitrogen fixation, is chemically characterized as a/an:
- Copper-Zinc protein
- Molybdenum-Iron protein
- Iron-Copper protein
- Magnesium-Iron protein
Explanation: The enzyme nitrogenase, which is capable of reducing nitrogen to ammonia, is a Mo-Fe protein (Molybdenum-Iron protein).
In the absence of a symbiotic partner, how does the bacterium Rhizobium behave in the soil?
- Free-living aerobe
- Parasitic on roots
- Strictly anaerobic
- Dormant endospore
Explanation: Rhizobium is a free-living, aerobic soil bacterium. It only adopts a symbiotic, anaerobic, nitrogen-fixing lifestyle after nodule formation.
Which of the following plants would most likely benefit from a symbiotic association with Frankia?
- Pea plant
- Wheat crop
- Alnus tree
- Rice paddy
Explanation: Frankia is a nitrogen-fixing actinomycete that forms nodules on the roots of non-leguminous plants, most notably the Alnus (Alder) tree.
Which of the following is a result of the metabolic activity of LAB during curd formation?
- Increase in lactose
- Decrease in acidity
- Fat synthesis
- Partial protein digestion
Explanation: As LAB produce lactic acid, the pH drops, which coagulates milk proteins and partially digests them, making curd easier to digest than milk.
Regarding symbiotic nitrogen fixation, the bacterium 'Frankia' produces nitrogen-fixing nodules on the roots of:
- Cereal crops
- Leguminous plants
- Aquatic ferns
- Non-leguminous plants
Explanation: While Rhizobium forms nodules on legumes (alfalfa, sweet clover), Frankia produces nitrogen-fixing nodules on the roots of non-leguminous plants like Alnus.
Which of the following describes the correct sequence of nitrogen transformation during the process of nitrification in soil?
- Ammonia to Nitrite to Nitrate
- Nitrite to Nitrate to Ammonia
- Nitrate to Ammonia to Nitrite
- Ammonia to Nitrate to Nitrite
Explanation: Nitrification is a two-step aerobic biological process. First, ammonia is oxidized to nitrite by bacteria such as Nitrosomonas and Nitrococcus. Subsequently, the nitrite is further oxidized to nitrate by bacteria like Nitrobacter. The chemical sequence is $\text{NH}_3 \rightarrow \text{NO}_2^- \rightarrow \text{NO}_3^-$.
In the nitrogen cycle, the process of 'denitrification' is primarily carried out by which of the following microorganisms?
- Rhizobium
- Nitrobacter
- Pseudomonas
- Nitrosomonas
Explanation: Denitrification is the biological process of reducing nitrate present in the soil back into free nitrogen gas. This process is carried out by denitrifying bacteria, primarily species of Pseudomonas and Thiobacillus.
Which specific enzyme is inhibited by the presence of oxygen during the biological nitrogen fixation process?
- Nitrate reductase
- Amylase
- Nitrogenase
- Transaminase
Explanation: Nitrogenase is the primary enzyme responsible for nitrogen fixation, and it is highly sensitive to molecular oxygen, which irreversibly inactivates it. To protect this enzyme, nitrogen-fixing root nodules contain an oxygen scavenger called leghemoglobin.
The conversion of amino acids into amides in plants is significant because amides contain more:
- Carbon than oxygen
- Nitrogen than acids
- Nitrogen than amino-acids
- Oxygen than nitrogen
Explanation: Amides (like asparagine and glutamine) contain more nitrogen than the amino acids from which they are derived and are used for nitrogen transport.
Which of the following microorganisms is used as a 'starter' for the industrial production of cheese?
- Lactic acid bacteria
- Nitrosomonas
- Aspergillus niger
- Clostridium butyricum
Explanation: Specific strains of LAB and other bacteria are used to acidify milk and begin the curdling process essential for cheese manufacturing.
Which of the following is a free-living anaerobic nitrogen-fixing microbe?
- Beijerinckia
- Rhodospirillum
- Nostoc
- Azotobacter
Explanation: Rhodospirillum is unique because it is a free-living bacterium that fixes nitrogen under anaerobic conditions, unlike Azotobacter which is aerobic.
Consider the following statements:
Statement-I: Lactic Acid Bacteria (LAB) play a beneficial role in the human stomach.
Statement-II: LAB prevent the growth of various disease-causing microbes in the gastrointestinal tract.
Which one of the following is correct in respect of the above statements?
- Both Statement-I and Statement-II are correct and Statement-II is the correct explanation for Statement-I
- Both Statement-I and Statement-II are correct but Statement-II is not the correct explanation for Statement-I
- Statement-I is correct but Statement-II is incorrect
- Statement-I is incorrect but Statement-II is correct
Explanation: Both statements are factually correct. Lactic Acid Bacteria (LAB) are highly beneficial in the human digestive system, and the primary mechanism for this benefit is their ability to check and inhibit the colonization and growth of pathogenic microorganisms. Thus, Statement-II correctly explains the role mentioned in Statement-I.
The conversion of nitrate present in the soil back into atmospheric nitrogen is called:
- Ammonification
- Nitrogen fixation
- Denitrification
- Nitrification
Explanation: Denitrification is the process that reduces nitrates back into nitrogen gas (N2), closing the nitrogen cycle loop.
The initial step of 'Ammonification' in the nitrogen cycle involves the breakdown of:
- Organic nitrogen
- Rock minerals
- Soil nitrates
- Atmospheric nitrogen
Explanation: Ammonification is the conversion of organic nitrogen (from dead plants/animals) into ammonia by decomposer microbes.
What happens to the ammonia produced in plant cells to prevent toxicity?
- Excreted via roots
- Stored in vacuoles
- Released as gas
- Converted to amides
Explanation: At physiological pH, ammonia is protonated to NH4+. Since NH4+ is toxic, it is rapidly used to synthesize amino acids and amides like asparagine.
Which of the following microorganisms is used in the household production of 'Toddy', a traditional drink in Southern India?
- Lactobacillus
- Propionibacterium
- Fermenting yeast
- Acetobacter aceti
Explanation: Toddy is made by fermenting sap from palms using naturally occurring yeast, similar to the fermentation processes used for bread and beer.
The transport of fixed nitrogen from roots to leaves in plants like Soybeans occurs primarily in the form of:
- Amino acids
- Nitrate ions
- Ammonia gas
- Ureides
Explanation: In some legumes like Soybeans, the nodules export fixed nitrogen as ureides (allantoin and allantoic acid) which have a high nitrogen-to-carbon ratio.
The first step in the formation of a root nodule by Rhizobium involves the:
- Cortical cell division
- Curling of roothairs
- Infection thread formation
- Nitrogenase synthesis
Explanation: The infection process begins with the multiplication of Rhizobium near root hairs, followed by the successful curling of the root hair.
The 'pink' color of functional root nodules in leguminous plants is due to the presence of:
- Leghemoglobin
- Anthocyanin
- Carotenoids
- Chlorophyll
Explanation: Functional nodules are pinkish on the inside because they contain a specialized pigment called leghemoglobin (leguminous hemoglobin).
Which of the following microbes is widely used in the production of 'Roquefort cheese'?
- Specific protozoa
- Specific fungi
- Specific algae
- Specific bacteria
Explanation: Roquefort cheese is ripened by growing a specific fungus (Penicillium roqueforti) on them, which gives them a particular flavor.
Rhizobium bacteria, when living freely in the soil, exhibit which type of lifestyle?
- Aerobic
- Symbiotic
- Parasitic
- Anaerobic
Explanation: Rhizobium live as free-living aerobes in the soil but become symbiotic and anaerobic fixers only after infecting the root hairs of legumes.
Which enzyme is responsible for the 'Transamination' process in plants?
- Nitrate reductase
- Transaminase
- Amylase
- Nitrogenase
Explanation: Transamination involves the transfer of amino groups from one amino acid to the keto group of a keto acid, catalyzed by transaminase enzymes.
Which of the following is a common biofertilizer used specifically to improve the yield of pulses (dal)?
- Blue-green algae
- Mycorrhiza
- Rhizobium
- Azospirillum
Explanation: Since pulses are legumes, Rhizobium is the specific biofertilizer used to enhance their nitrogen-fixation capacity and overall yield.
Which of the following is a characteristic of 'Bacteroids' found within root nodules?
- Small and round
- Highly mobile
- Actively dividing
- Irregularly shaped
Explanation: Once Rhizobium enter the cortical cells, they stop dividing and differentiate into large, misshapen, nitrogen-fixing forms called bacteroids.
Which of the following is a common free-living nitrogen fixer in paddy fields?
- Cyanobacteria
- Rhizobium
- Frankia
- Lactobacillus
Explanation: Cyanobacteria (like Nostoc, Anabaena, and Aulosira) serve as an important biofertilizer in paddy fields, fixing atmospheric nitrogen.
The total energy requirement for fixing one molecule of Nitrogen ($N_2$) into two molecules of Ammonia is:
- 2 ATP
- 32 ATP
- 8 ATP
- 16 ATP
Explanation: The reduction of one N2 molecule is extremely energy-intensive, requiring 16 molecules of ATP (8 ATP per ammonia molecule produced).
Besides curdling, LAB bacteria improve the human diet by specifically increasing the intake of:
- Essential vitamins
- Complex proteins
- Saturated fats
- Dietary fiber
Explanation: By enriching curd with Vitamin B12, LAB provide essential nutritional benefits that are not present in raw milk in the same quantity.
Statement I: All microbes in human welfare are bacteria.
Statement II: Fungi and viruses are also used in various beneficial processes.
- Statement II correct
- Statement I incorrect
- Both are correct
- Both are incorrect
Explanation: Statement I is incorrect; 'microbes' include bacteria, fungi, protozoa, and viruses. Statement II is correct (e.g., yeast is a fungus used in baking).
The term 'Diazotroph' refers to organisms that have the unique ability to:
- Produce methane
- Fix atmospheric nitrogen
- Synthesize vitamins
- Digest cellulose
Explanation: Diazotrophs are bacteria and archaea that fix atmospheric nitrogen gas into a more usable form like ammonia.
In the symbiotic association of Rhizobium, the infection thread is formed by the invagination of the:
- Pericycle cells
- Cortical cells
- Vascular bundle
- Root hair
Explanation: After the bacteria attach and cause the root hair to curl, an infection thread is produced by the inward growth of the root hair plasma membrane.
The primary product of biological nitrogen fixation that is subsequently used by plants to form amino acids is:
- Nitrite
- Ammonia
- Nitrate
- Nitrogen dioxide
Explanation: Biological nitrogen fixation is the process where atmospheric nitrogen (N2) is reduced to ammonia (NH3) by the enzyme nitrogenase.
The enzyme nitrogenase requires which of the following to successfully reduce nitrogen to ammonia?
- High oxygen
- Alkaline pH
- Solar energy
- Strong reductant
Explanation: Nitrogenase requires a source of electrons (reductant like ferredoxin) and a massive input of energy (ATP) to break the triple bond of N2.
A small amount of curd added to fresh milk as 'starter' or 'inoculum' contains millions of:
- Viral particles
- LAB bacteria
- Fungal spores
- Yeast cells
Explanation: The inoculum or starter added to milk contains millions of Lactic Acid Bacteria (LAB) that multiply at suitable temperatures.
Nitrogenase can only function in anaerobic conditions. How do free-living aerobic bacteria like Azotobacter protect this enzyme?
- Nitrogen storage
- Acid production
- Thick cell walls
- High respiration
Explanation: Azotobacter maintains an extremely high respiratory rate to consume oxygen so rapidly that the internal environment remains anaerobic for nitrogenase.
Cyanobacteria like Nostoc and Anabaena are capable of fixing nitrogen in specialized cells called:
- Mesosomes
- Heterocysts
- Endospores
- Akinetes
Explanation: Cyanobacteria are free-living nitrogen fixers in both terrestrial and aquatic environments, utilizing specialized thick-walled cells called heterocysts.
In the human diet, curd is considered superior to milk for individuals with 'Lactose Intolerance' because:
- Lactose is converted
- Proteins are higher
- Fats are absent
- Lactose is removed
Explanation: LAB ferment the lactose (milk sugar) into lactic acid, reducing the lactose content and making it tolerable for those with sensitivity.
Which part of the leguminous plant is directly involved in signaling and attracting Rhizobium bacteria?
- Phloem sap
- Root exudates
- Stem nodes
- Leaf stomata
Explanation: Roots secrete chemical signals (flavonoids) into the soil, which attract Rhizobium and trigger the expression of bacterial 'nod' genes.
Which of the following minerals is a critical structural component of the nitrogenase enzyme complex?
- Magnesium
- Iron
- Molybdenum
- Both Iron/Molybdenum
Explanation: The nitrogenase enzyme is a complex protein containing both Iron (Fe) and Molybdenum (Mo) as essential metallic components.
Which of the following is NOT a symbiotic nitrogen-fixing association?
- Rhizobium and Alfalfa
- Azotobacter and Soil
- Frankia and Alnus
- Anabaena and Azolla
Explanation: Azotobacter is a free-living nitrogen fixer in the soil. The other three represent symbiotic relationships where microbes live within plant tissues.
In the process of 'Reductive Amination', ammonia reacts with which organic acid to form Glutamic acid?
- Citric acid
- Malic acid
- Oxaloacetic acid
- Alpha-ketoglutaric acid
Explanation: Ammonia reacts with alpha-ketoglutaric acid to form glutamate (glutamic acid) in the presence of NADPH and glutamate dehydrogenase.
The puffed-up appearance of dough used for making dosa and idli is caused by the release of:
- Methane gas
- Oxygen gas
- Sulfur dioxide
- Carbon dioxide
Explanation: The dough is fermented by bacteria, and the resulting puffed-up appearance is due to the production and release of CO2 gas.
Which household fermented product is traditionally made by the microbial action on fish or soy?
Explanation: Many traditional condiments like soy sauce or fish sauce are produced through long-term microbial fermentation of proteins.
Which of the following compounds is the final electron acceptor in the process of denitrification?
- Nitrate
- Glucose
- Ammonia
- Oxygen
Explanation: In anaerobic conditions, denitrifying bacteria use nitrate ($NO_3^-$) instead of oxygen as the terminal electron acceptor in their respiratory chain.
The production of 'Large-holed cheese' relies on which microbial metabolic pathway?
- Acetic acid fermentation
- Propionic acid fermentation
- Alcoholic fermentation
- Lactic acid fermentation
Explanation: Propionibacterium ferments lactate into propionate, acetate, and large amounts of CO2, which creates the holes in Swiss-type cheeses.
In the nodules of leguminous plants, the role of leghemoglobin is primarily to act as a/an:
- Oxygen scavenger
- Energy carrier
- Catalytic enzyme
- Nitrogen transporter
Explanation: The enzyme nitrogenase is highly sensitive to molecular oxygen. Leghemoglobin protects the enzyme by acting as an oxygen scavenger, ensuring anaerobic conditions.
Which household microbe is used to check the 'Nutritional quality' of milk during its transformation?
- Methanogens
- Acetobacter
- Lactobacillus
- Yeast
Explanation: Lactobacillus not only converts milk to curd but also serves as a biological indicator of nutritional enrichment, specifically for Vitamin B12.
Biological Nitrogen Fixation contributes approximately what percentage of the total nitrogen fixed on Earth annually?
- Thirty percent
- Ten percent
- Sixty percent
- Ninety percent
Explanation: While industrial (Haber-Bosch) and atmospheric (lightning) fixation are significant, biological processes account for roughly 60% of all fixed nitrogen.
In the process of curdling milk, Lactic Acid Bacteria (LAB) produce acids that primarily result in:
- Fat emulsification
- Lactose synthesis
- Protein coagulation
- Sugar oxidation
Explanation: LAB produce acids that coagulate and partially digest the milk proteins (casein), which effectively converts the liquid milk into solid curd.
Consider the following statements:
Statement-I: Rhizobium bacteria act as obligate aerobes within the root nodules during the active nitrogen-fixation phase.
Statement-II: The enzyme responsible for nitrogen fixation requires strictly anaerobic conditions to function.
Which one of the following is correct in respect of the above statements?
- Both Statement-I and Statement-II are correct and Statement-II is the correct explanation for Statement-I
- Both Statement-I and Statement-II are correct but Statement-II is not the correct explanation for Statement-I
- Statement-I is correct but Statement-II is incorrect
- Statement-I is incorrect but Statement-II is correct
Explanation: Statement-I is incorrect because while Rhizobium microbes live as aerobes under free-living conditions in the soil, they become anaerobic during the active nitrogen-fixing events inside the root nodules. Statement-II is correct because the nitrogenase enzyme is highly sensitive to molecular oxygen and requires strictly anaerobic conditions to function properly.