VBQs Class 12 Biology Microbes in Human Welfare

VBQs For Class 12

Please refer to VBQs for Class 12 Biology Microbes in Human Welfare. All value based questions for Biology Class 12 have been provided with solutions. We have provided below important values questions and answers. Students should learn these solved VBQs for Class 12 Biology as these will help them to gain more marks and help improve understanding of important topics.

Microbes in Human Welfare VBQs Class 12 Biology with Answers

Microbes in Household Products

Very Short Answer Type Questions

Question. Name the nutrient that gets enhanced while curdling of milk by Lactobacillus.
Answer. The curdling of milk by Lactobacillus changed milk into curd and its nutritional quality is enhanced due to increase in vitamin B12 content.

Question. How is lactic acid bacteria beneficial to us other than helping in curdling the milk? 
Answer. Lactic acid bacteria is beneficial to us in the following ways other than helping in curdling of milk:
(i) Increases nutritional quality of curd by increasing vitamin B12 content.
(ii) Checks the growth of disease-causing microbes in the gut.

Question. Why do we add an inoculum of curd to milk for curdling it? 
Answer. The starter or inoculum used in preparation of milk products actually contains millions of lactic acid bacteria. Curd is prepared by inoculating cream and skimmed milk with Lactobacillus acidophilus at a temperature of about 40° C or less. Lactobacillus converts lactose sugar of milk into lactic acid which causes coagulation and partial digestion of milk protein casein, and milk gets changed into curd.

Question. Name the gas released and the process responsible for pufing up of the bread dough when Saccharomyces cerevisiae is added to it.
Answer.When Saccharomyces cerevisiae (baker’s yeast) is added to dough it causes its fermentation and releases CO2 gas which is responsible for puffed up appearance of dough.

Short Answer Type Questions

Question. During the production of curd, a small amount of curd is added as a starter to the fresh milk at a suitable temperature. Explain the changes the milk undergoes when it sets into curd.
Answer.  For the production of curd, a small amount of curd is added to fresh milk at suitable temperature. Curd contains millions of lactic acid bacteria (LAB). These bacteria grow in milk and convert the lactose sugar of milk in lactic acid. Lactic acid coagulates and partially digests milk protein casein. This causes curdling of milk and it changes to curd, which also improves its nutritional quality by increasing vitamin B12 content.

Question. Name the bacterium responsible for the large holes seen in “Swiss Cheese”. What are these holes due to? 
Answer.Bacterium responsible for large holes in swiss cheese is Propionibacterium sharmanii. Large holes in swiss cheese are due to CO2 gas produced by these bacteria.

Microbes in Industrial Products

Very Short Answer Type Questions

Question. Which one of the following is the baker’s yeast used in fermentation?
Saccharum barberi, Saccharomyces cerevisiae, Sonalika. 
Answer.Saccharomyces cerevisiae is the baker’s yeast used in fermentation.

Question. Name the enzyme produced by Streptococcus and its action (after being modified).
Answer.  Streptokinase (Tissue Plasminogen Activator or TPA) is an enzyme obtained from cultures of some haemolytic bacterium Steptococcus which is modified genetically to function as clot buster. It has fibrinolytic effect hence, it helps in clearing blood clots inside the blood vessels through dissolution of intravascular fibrin.

Question. Write the scientific name of the microbe used for fermenting malted cereals and fruit juices.
Answer.Yeast or Saccharomyces cerevisiae is used for fermenting malted cereals and fruit juices.

Short Answer Type Questions

Question. Bottled fruit juices are clearer as compared to those made at home. Explain. 
Answer. The fruit juices sold in market or botted juices are treated with pectinases and proteases which makes them clearer than those made at home.

Question. Give the scientific name of the microbes from which cyclosporin A and statin are obtained.Write one medical use of each one of these drugs.
Answer. Cyclosporin A is obtained from fungus Trichoderma polysporum whereas statin is obtained from yeast Monascus purpureus.
Cyclosporin A has immunosuppressive properties. It inhibits activation of T cells and therefore prevents rejection of transplants. Statin inhibits cholesterol synthesis and is therefore used in lowering blood cholesterol.

Question. Name the source of cyclosporin-A. How does this bioactive molecule function in our body?
Answer. Cyclosporin A is an eleven membered cyclic oligopeptide obtained through fermentive activity of fungus Trichoderma polysporum. It has antifungal, anti-inflammatory and immunosuppressive properties. It inhibits activation of T cells and therefore prevents rejection reactions in organ transplantation.

Question. Mention a product of human welfare obtained with the help of each one of the following microbes :
(a) LAB
(b) Saccharomyces cerevisiae
(c) Propionibacterium sharmanii
(d) Aspergillus niger 
Answer.(a) LAB (Lactic acid bacteria)- Curd
(b) Saccharomyces cerevisiae – Bread
(c) Propionibacterium – Swiss cheese sharmanii
(d) Aspergillus niger – Citric acid

Question. Why are some molecules called bioactive molecules? Give two examples of such molecules. 
Answer. Bioactive compounds are those compounds that have an effect on living organisms tissues or cells. Bioactive compounds are found in both plant and animal products or can be synthetically produced. Two examples of bioactive compounds are cyclosporin A and statins.

Question. Name the blank spaces a, b, c and d in the table given below :

VBQs Class 12 Biology Microbes in Human Welfare

Answer.a – Streptococcus lactis, Lactobacillus
b – Trichoderma polysporum
c – Fungus
d – Penicillin

Question. Name the blank spaces a, b, c and d in the table given below :

VBQs Class 12 Biology Microbes in Human Welfare

Answer.a – Streptococcus
b – Fungus
c – Cyclosporin – A
d – Clostridium acetobutylicum

Question. Name the blank spaces a, b, c and d given in the following table

VBQs Class 12 Biology Microbes in Human Welfare

Answer.a – Lactobacillus acidophilus or Streptococcus lactis
b – Trichoderma ploysporum
c – Fungus
d – Penicillin

Question. Identify A, B, C and D in the table given below :

VBQs Class 12 Biology Microbes in Human Welfare

Answer.A – Streptococcus
B – Clears blood clots inside blood vessels
C – Cyclosporin A
D – Immunosuppressive

Question. Name the blank spaces a, b, c and d in the table given below :

VBQs Class 12 Biology Microbes in Human Welfare

Answer.a – Penicillium notatum
b – Acetic acid
c – Fungus
d – Saccharomyces cerevisiae

Short Answer Type Questions

Question. Identify a, b, c, d, e and f in the table given below:

VBQs Class 12 Biology Microbes in Human Welfare

Answer.a – Helps in clearing blood clots inside the blood vessels through dissolution of intravascular fibrin.
b – Trichoderma polysporum
c – Antifungal, anti-inflammatory and immunosuppressive prevents rejection reactions in organ transplantation.
d – Statins
e – Help in lowering blood cholesterol level
f – Lactic acid

Question. Identify a, b, c, d, e and f in the table given below :

VBQs Class 12 Biology Microbes in Human Welfare

Answer.a – Statins
b – Used in lowering blood cholesterol
c – Penicillium notatum
d – Penicillin
e – Trichoderma polysporum
f – Prevents rejection reactions in organ transplantation

Question. What are the properties of an antibiotic?
Answer.  Antibiotics are chemical substances produced by some microbes which in small concentration can kill or retard the growth of harmful microbes without adversely affecting the host. Broad spectrum antibiotic can kill or destroy a number of pathogens that belong to different groups with different structures and wall compositions. Specific antibiotics are effective only against one type of pathogen. Good antibiotics should be harmless to host with no side effects. They should be harmless to beneficial microorganism of alimentary canal and should be effective against all strains of pathogen. They should also be quick in action.

Question. Mention the product and its use produced by each of the microbes listed below :
(a) Streptococcus (b) Lactobacillus
(c) Saccharomyces cerevisiae 
Answer.(a) Streptokinase (Tissue Plasminogen Activator or TPA) is an enzyme obtained from cultures of some haemolytic bacterium Steptococcus which is modified genetically to function as clot buster. It has fibrinolytic effect hence, it helps in clearing blood clots inside the blood vessels through dissolution of intravascular fibrin.
(b) The curdling of milk by Lactobacillus changed milk into curd and its nutritional quality is enhanced due to increase in vitamin B12 content.
(c) Saccharomyces cerevisiae used for breadmaking and commonly called brewer’s yeast, is used for fermenting malted cereals and fruit juices, to produce ethanol.

Microbes in Sewage Treatment

Very Short Answer Type Questions

Question. BOD of two samples of water A and B were 120 mg/L and 400 mg/L respectively. Which sample is more polluted? 
Answer.Water body having high BOD is more polluted as compared to water body having low BOD.Hence water sample having BOD 400 mg/L is more polluted as compared to water sample having BOD, 120 mg/L.

Question. Why is sewage water treated until the BOD is reduced ? Give a reason.
Answer.  Sewage water is highly polluted and contains high amount of organic waste hence biochemical oxygen demand (BOD) of sewage water is high. Sewage is treated in sewage treatment plants to remove polluntants with the reduction in pollution BOD of the water decreases. Hence BOD is the indicator of decrease of pollutants present in sewage. Sewage water is therefore treated till the BOD is reduced to minimal value which ensures that water is free of organic pollutants.

Question. Given below are a few impurities in urban waste water. Select two colloidal impurities : ammonia, faecal matter, silt, bacteria, calcium.
Answer.Two colloidal impurities in urban waste water are faecal matter and bacteria.

Short Answer Type Questions

Question.Distinguish between the roles of flocs and anaerobic sludge digesters in sewage treatments.
Answer. Flocs are masses of aerobic bacteria held together by slime and fungal filaments to form mesh like structures. These microbes digest a lot of organic matter converting it into microbial biomass and releasing a lot of minerals. This reduces biochemical oxygen demand or BOD.
In anaerobic sludge digesters, aerobic microbes present in the sludge get killed. Anaerobic microbes digest the organic mass as well as aerobic microbes of the sludge. During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. These gases form biogas which can be used as source of energy as it is infiammable. The spent sludge of anaerobic sludge digester can be used as manure or part of compost.

Question. Name two groups of organisms which constitute ‘flocs’. Write their influence on the level on BOD during biological treatment of sewage.
Answer.Aerobic bacteria and fungi constitute ‘flocs’.
Flocs are masses of aerobic bacteria held together by slime and fungal filaments to form mesh like structures. These microbes digest a lot of organic matter converting it into microbial biomass and releasing a lot of minerals. This reduces biochemical oxygen demand or BOD.
In anaerobic sludge digesters, aerobic microbes present in the sludge get killed. Anaerobic microbes digest the organic mass as well as aerobic microbes of the sludge. During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. These gases form biogas which can be used as source of energy as it is infiammable. The spent sludge of anaerobic sludge digester can be used as manure or part of compost.

Question. Explain the different steps involved during primary treatment phase of sewage. 
Answer. Primary treatment phase of sewage treatment removes floating and suspended solids from sewage through two processes of filtration and sedimentation. First floating matter is removed through sequential filtration. The filtrate is kept in large open settling tanks where grit settles down. Aluminium or iron sulphate is added in certain places to flocculation and settling down of solids. The sediment is called primary sludge while the supernatant is called effuent. The primary sludge traps a lot of microbes and debris. It is subjected to composting or land fill where anaerobic digestion removes the organic matter.

Question. List the events that reduce the Biological Oxygen Demand (BOD) of a primary effuent during sewage treatment.
Answer. Secondary treatment or biological treatment of sewage considerably reduces the biological oxygen demand (BOD) of primary effuent during sewage treatment. The organic matter present in the effuent is decomposed with the help of microbial flora called as sewage fungus. The effuent is constantly agitated or aerated. This causes the growth of various aerobic microorganisms and sewage fungi. These microbes digest the organic matter thereby reducing the BOD of original sewage by 10-15%.

Question. Explain the process of secondary treatment given to the primary effuent up to the point it shows significant change in the level of biological oxygen demand (BOD) in it.
Answer. During secondary treatment, the primary effluent is taken to aeration tanks. A large number of aerobic heterotrophic microbes grow in the aeration tank. They form flocs which are masses of bacteria held together by slime and fungal filaments to form mesh like structures. The microbes digest a lot of organic matter, converting it into microbial biomass and releasing a lot of minerals. As a result the BOD of the waste matter is reduced to 10-15% of raw sewage, it is passed into settling tank.

Short Answer Type Questions

Question. How are flocs produced in the secondary treatment plant of the sewage? Explain their role. 
Answer.During secondary treatment, the primary effluent is taken to aeration tanks. A large number of aerobic heterotrophic microbes grow in the aeration tank. They form flocs which are masses of bacteria held together by slime and fungal filaments to form mesh like structures. The microbes digest a lot of organic matter, converting it into microbial biomass and releasing a lot of minerals. As a result the BOD of the waste matter is reduced to 10-15% of raw sewage, it is passed into settling tank.

Question. Name the two different categories of microbes naturally occurring in sewage water. Explain their role in cleaning sewage water into usable water. 
Answer. 
Aerobic heterotrophs like bacteria and fungi occur in sewage water. They are natural decomposers and digiest a lot of organic matter present in the polluted water thereby releasing minerals and reducing organic waste. Hence, they play an important role in cleaning water and making it fit for various domestic uses

Question. (a) How is activated sludge formed during sewage treatment?
(b) This sludge can be used as an inoculum or as a source of biogas. Explain.
Answer. (a)Sewage water can be purified by passing it through sewage treatment plants with the action of microorganisms. A sewage treatment plant separates solids from liquids by physical processes and purifies the liquid by biological processes. There are three stages of this treatment; primary, secondary and tertiary. Primary treatment is physical, secondary biological and tertiary chemical.
Primary treatment phase of sewage treatment removes floating and suspended solids from sewage through two processes of filtration and sedimentation. First floating matter is removed through sequential filtration. The filtrate is kept in large open settling tanks where grit settles down. Aluminium or iron sulphate is added in certain places to flocculation and settling down of solids. The sediment is called primary sludge while the supernatant is called effuent. The primary sludge traps a lot of microbes and debris. It is subjected to composting or land fill where anaerobic digestion removes the organic matter.
During secondary treatment, the primary effuent is taken to aeration tanks. A large number of aerobic heterotrophic microbes grow in the aeration tank. They form flocs are masses of bacteria held together by slime and fungal filaments to form mesh like structures. The microbes digest a lot of organic matter, converting it into microbial biomass and releasing a lot of minerals. As a result the BOD of the waste matter is reduced to 10-15% of raw sewage, it is passed into settling tank. In settling tank, the bacterial flocs are allowed to undergo sedimentation. The effuent or spernatant is generally passed into natural water bodies and sediment of settling tank is called activated sludge.
(b) A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the activated sludge is pumped into large tanks called anaerobic sludge digesters. The anaerobic bacteria digest the bacteria and fungi in a sludge and produce mixture of gases like methane, hydrogen sulphide and CO2 which constitute biogas.

Question. Explain the different steps involved in sewage treatment before it can be released into natural water bodies. 
Answer. Sewage water can be purified by passing it through sewage treatment plants with the action of microorganisms. A sewage treatment plant separates solids from liquids by physical processes and purifies the liquid by biological processes. There are three stages of this treatment; primary, secondary and tertiary. Primary treatment is physical, secondary biological and tertiary chemical.
Primary treatment phase of sewage treatment removes floating and suspended solids from sewage through two processes of filtration and sedimentation. First floating matter is removed through sequential filtration. The filtrate is kept in large open settling tanks where grit settles down. Aluminium or iron sulphate is added in certain places to flocculation and settling down of solids. The sediment is called primary sludge while the supernatant is called effuent. The primary sludge traps a lot of microbes and debris. It is subjected to composting or land fill where anaerobic digestion removes the organic matter.
During secondary treatment, the primary effuent is taken to aeration tanks. A large number of aerobic heterotrophic microbes grow in the aeration tank. They form flocs are masses of bacteria held together by slime and fungal filaments to form mesh like structures. The microbes digest a lot of organic matter, converting it into microbial biomass and releasing a lot of minerals. As a result the BOD of the waste matter is reduced to 10-15% of raw sewage, it is passed into settling tank. In settling tank, the bacterial flocs are allowed to undergo sedimentation. The effuent or spernatant is generally passed into natural water bodies and sediment of settling tank is called activated sludge.

Microbes in Production of Biogas

Very Short Answer Type Questions

Question. Name the group of organisms and the substrate they act on to produce biogas. 
Answer. Methanogenic bacteria or methanogens are the group of anaerobic microbes which digest organic mass as well as aerobic microbes of the sludge to produce a mixture of gases containing methane, H2S and CO2 called biogas.

Short Answer Type Questions

Question. (a) What are methanogens?
(b) Name the animals in which methanogens occur and the role they play there.
Answer. (a) Methanogenic bacteria or methanogens are the group of anaerobic microbes which digest organic mass as well as aerobic microbes of the sludge to produce a mixture of gases containing methane, H2S and CO2 called biogas.
(b) Methanogens are present in the rumen (a part of stomach of cattle) such as cow, buffalo. In rumen, these bacteria help in the breakdown of glucose and play an important role in the nutrition of cattle. The excreta of cattle is rich in these bacteria (methanogens) and can be used for generation of biogas.

Question. How can sewage be used to generate biogas?
Explain. 
Answer. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the activated sludge is pumped into large tanks called anaerobic sludge digesters. The anaerobic bacteria digest the bacteria and fungi in a sludge and produce mixture of gases like methane, hydrogen sulphide and CO2 which constitute biogas.

Question. What are methanogens? How do they help to generate biogas?
Answer.  Methanogens are a group of anaerobic bacteria which obtain their energy by reducing carbondioxide and oxidising hydrogen with the production of methane. They are found in oxygen deficient environment such as marshes, swamps, sludge and digestive systems of ruminant animals like cow, buffalo etc. These microorganims are present in anaerobic sludge digester where they digest organic mass as well as aerobic microbes of the sludge to produce mixture of gases containing methane, H 2 S and CO2 called biogas.

Question. Describe how biogas is generated from activated sludge. List the components of biogas.
Answer. Methanogens are present in the rumen (a part of stomach of cattle) such as cow, buffalo. In rumen, these bacteria help in the breakdown of glucose and play an important role in the nutrition of cattle. The excreta of cattle is rich in these bacteria (methanogens) and can be used for generation of biogas.

Question.

VBQs Class 12 Biology Microbes in Human Welfare

The diagram above is that of a typical biogas plant. Explain the sequence of events occurring in a biogas plant. Identify a, b and c.
Answer. In the given diagram, a is sludge, b is gas holder and c is dung and water. The biogas production is an anaerobic process and is carried out in an air tight, closed cylindrical concrete tank called a digester. The tank has a concrete inlet basin on one side for feeding fresh cattle dung. There is a concrete outlet on the outer side for removing the digested sludge. The top of the tank serves as the gas tank. It has an outlet pipe for the biogas. Biogas generation is a three-stage anaerobic digestion of animal and other organic wastes. In the first stage of anaerobic digestion, facultative anaerobic decomposer microbes bring about enzymatic breakdown of complex organic compounds into simpler and soluble compounds often called monomers. For this, the decomposer microbes secrete cellulases, proteases and lipases (cellulolytic, proteolytic and lipolytic enzymes). In the second stage, the simple soluble compounds of microbial digestion or monomers are acted upon by fermentation causing microbes.The latter change the monomers into organic acids. Organic acids, especially acetic acid, are acted upon by methanogenic bacteria in the third or final stage. The methane bacteria convert organic acids as well as carbon dioxide into methane.The biogas thus formed is conducted through the outlet pipe and used for domestic purposes. The digested sludge is removed from tank and is used as fertiliser.

Microbes as Biocontrol Agents

Very Short Answer Type Questions

Question. What makes the Nucleopolyhedrovirus a desirable biological control agent?
Answer.  Nucleopolyhedrovirus a genus of baculoriviruses are useful in controlling many insects and other arthropods. They are species specific narrow spectrum bioinsecticides with no side effects on plants, mammals, birds, fish and non-target insects. Therefore, they serve as an important component of integrated pest management programme in dealing with ecological sensitive areas. These properties are useful in organic farming.

Short Answer Type Questions

Question. (a) How do organic farmers control pests?
Give two examples.
(b) State the difference in their approach from that of conventional pest control methods.
Answer. 
(a) Organic farming involves the use of biopesticides, i.e., biocontrol agents to control weeds, insects and pathogen. These biocontrol agents include viruses, bacteria, protozoa, fungi and mites. Example : Baculovirus heliothis can control cotton bollworm (Helicoverpa zea) whereas Bacillus thuringeinsis is effective against cabbage looper (Trichoplusia ni).
(b) Conventional pest control involves the use of chemical pesticides to control pests in crop-field. Pesticides are toxic and harmful to crop plants. Other crop field animals and humans. On the other hand organic farmers use biopesticides to control field pests. This approach is environment friendly, exercises specific control over pests and is harmless to humans.

Question. How are baculoviruses and Bacillus thuringiensis used as bio-control agents? Why are they preferred over readily available chemical pesticides? 
Answer.  Nucleopolyhedrovirus a genus of baculoriviruses are useful in controlling many insects and other arthropods.They are species specific narrow spectrum bioinsecticides with no side effects on plants, mammals, birds, fish and non-target insects. Bacillus thuringiensis is a soil bacterium. The spores of this bacterium possess insecticidal cry protein, hence, kill larvae of certain insects. This bacterium was first biopesticide to be used on commercial scale. The commercial preparations of B. thuringiensis contain a mixture of spores, cry protein and inert carrier.
Chemical pesticides used in agricultural fields are toxic and biocides. They kill even useful organisms along with harmful ones, harm human beings and animals, pollute soil and water and crop plants. It is estimated that despite the use of chemical pesticides 30% of the agricultural produces is lost to pathogens and pests because these continue to develop resistance against various pesticides. On the other hand biopesticides are the biological agents that control the growth of weeds, insects and pathogens in an agricultural field. They have targeted actions and are harmless to the crop plants, other beneficial field animals and humans. Therefore, nowadays biopesticides are preferred over chemical pesticides.

Question. Given below is a list of six microorganisms. State their usefulness to humans.
(a) Nucleopolyhedrovirus
(b) Saccharomyces cerevisiae
(c) Monascus purpureus
(d) Trichoderma polysporum
(e) Penicillium notatum
(f) Propionibacterium sharmanii 
Answer. 
(a) Nucleopolyhedrovirus :
 It is a baculovirus which is an effective biocontrol agent as it is useful in controlling many insects and other arthropods. It serves as species specific narrow spectrum bioinsecticide.
(b) Saccharomyces cerevisiae : It is used to ferment dough, in order to make bread. It is also used for the preparation of ethanol.
(c) Monascus purpureus : Its fermentation activity produces statins which inhibits cholesterol synthesis and is therefore used in lowering blood cholesterol.
(d) Trichoderma polysporum : It is a fungus that produces cyclosporin–A, which is used as immunosuppressive agent in organ-transplantation.
(e) Pencillium notatum : It secretes the antibiotic, penicillin.
(f) Propionibacterium sharmanii : It is used in processing of ‘Swiss cheese’. The large holes in Swiss cheese are due to production of a large amount of CO2 by this bacterium.

Question. What are biopesticides? Give the scientific name and the use of the first commercially used biopesticide in the world.
Answer.Biopesticides are those biological agents that are used for control of weeds, insects and pathogens. The microorganisms used as biopesticides are viruses, bacteria, protozoa, fungi etc.
Nucleopolyhedrovirus a genus of baculoriviruses are useful in controlling many insects and other arthropods.They are species specific narrow spectrum bioinsecticides with no side effects on plants, mammals, birds, fish and non-target insects. Bacillus thuringiensis is a soil bacterium. The spores of this bacterium possess insecticidal cry protein, hence, kill larvae of certain insects. This bacterium was first biopesticide to be used on commercial scale. The commercial preparations of B. thuringiensis contain a mixture of spores, cry protein and inert carrier.
Chemical pesticides used in agricultural fields are toxic and biocides. They kill even useful organisms along with harmful ones, harm human beings and animals, pollute soil and water and crop plants. It is estimated that despite the use of chemical pesticides 30% of the agricultural produces is lost to pathogens and pests because these continue to develop resistance against various pesticides. On the other hand biopesticides are the biological agents that control the growth of weeds, insects and pathogens in an agricultural field. They have targeted actions and are harmless to the crop plants, other beneficial field animals and humans. Therefore, nowadays biopesticides are preferred over chemical pesticides.

Microbes as Biofertilisers

Very Short Answer Type Questions

Question. Which of the following is cynobacterium that can fix atmospheric nitrogen?
Azospirillum, Oscillatoria, Spirulina.
Answer.Oscillatoria is a nitrogen fixing cyanobacterium.

Question. List two advantages that a mycorrhizal association provides to the plant.
Answer.Mycorrhiza is a mutually beneficial or symbiotic association of a fungus with root of a higher plant.The two advantages that a mycorrhizal association provides to the plant is as follows:
(i) This association provides phosphorous to the plant. The fungal symbiont of mycorrlizal association absorbs phosphorus from soil and passes it to the plant.
(ii) This association also provides other benefits such as resistance to root borne pathogens, tolerance to salinity and drought and an overall increase in plant growth and development.

Question. Name any one symbiont which serves as a biofertiliser. Mention its specific role.
Answer.Rhizobium is the symbiotic nitrogen fixing bacteria which forms nodules on the roots of legume plants. Hence, it help the plants in obtaining their nitrogen nutrition.

Question. Which of the following is a free living bacteria that can fix nitrogen in the soil? Spirulina, Azospirillum, Sonalika 
Answer.Azospirillum is a free living nitrogen fixing bacteria.

Question. State one reason for adding blue-green algae to the agricultural soil.
Answer. A number of free living and symbiotic blue green algae or cyanobacteria have the property of nitrogen fixation and are photosynthetic. Therefore, they add organic matter as well as extra nitrogen to the soil. Hence, blue green algae serve as biofertilisers and are added to agricultural fields such as cotton, maize, jowar, rice, etc.

Question. How is presence of cyanobacteria in the paddy field beneficial to rice crop? 
Answer.A number of free living and symbiotic blue green algae or cyanobacteria have the property of nitrogen fixation and are photosynthetic. Therefore, they add organic matter as well as extra nitrogen to the soil. Hence, blue green algae serve as biofertilisers and are added to agricultural fields such as cotton, maize, jowar, rice, etc.

Short Answer Type Questions

Question. How do mycorrhizae act as biofertilisers?
Explain. Name a genus of fungi that forms a mycorrhizal association with plants.
Answer. 
Mycorrhiza is a mutually beneficial or symbiotic association of a fungus with the root of a higher plant. The most common fungal partners of mycorrhiza are Glomus species. Mycorrhizal roots show a sparse or dense wooly growth of fungal hyphae on their surface.
They perform several functions for the plant— (i) Absorption of water, (ii) Solubilisation of organic matter of the soil humus, release of inorganic nutrients, absorption and their transfer to root. (iii) Direct absorption of minerals from the soil over a large area and handing over the same to the root. Plants with ectomycorrhiza are known to absorb 2–3 times more of nitrogen, phosphorus, potassium and calcium. (iv)The fungus secretes antimicrobial substances which protect the young roots from attack of pathogens.

Question. Why is Rhizobium categorised as a ‘symbiotic bacterium’? How does it act as a biofertiliser?
Answer. Rhizobium is a soil bacterium which either lives freely in soil or lives in symbiotic association with roots of leguminous plants. The bacterium forms nodules or roots of leguminous plants where it lie in groups. When it occurs freely in soil, it cannot fix nitrogen. Nitrogen fixing ability develops only when it is present inside root nodules. Hence, Rhizobium is categorised as symbiotic bacterium. It acts as a biofertiliser as it helps plants in obtaining their nitrogen nutrition.

Question. What is mycorrhiza? How does it help as biofertiliser? 
Answer.Rhizobium is a soil bacterium which either lives freely in soil or lives in symbiotic association with roots of leguminous plants. The bacterium forms nodules or roots of leguminous plants where it lie in groups. When it occurs freely in soil, it cannot fix nitrogen. Nitrogen fixing ability develops only when it is present inside root nodules. Hence, Rhizobium is categorised as symbiotic bacterium. It acts as a biofertiliser as it helps plants in obtaining their nitrogen nutrition.

Short Answer Type Questions

Question. (a) Why do farmers prefer biofertilisers to chemical fertilisers these days? Explain.
(b) How do Anabaena and mycorrhiza act as biofertilisers? 
A
nswer. 
(a) Chemical fertilisers cause pollution of water bodies as well as ground water, besides getting stored in crop plants. Therefore, farmers are pressing for switch over to organic farming which includes the use of manures biofertilisers, biopesticides. Biofertilisers are microorganisms which bring about nutrient enrichment of soil by enhancing the availability of nutrients to crops. The microorganisms which act as biofertilisers are bacteria, cyanobacteria (blue green algae) and mycorrhizal fungi. Bacteria and cynobacteria have the property of nitrogen fixation while mycorrhizal fungi preferentially withdraw minerals from organic matter for the plant with which they are associated. Phosphate is also solubilised by some bacteria and by some fungi that form association with plant roots.
(b) Anabaena is a free living nitrogen fixing cyanobacterium. Therefore, it adds organic matter as well as extra nitrogen to agricultural field.Cyanobacteria are an extremely low cost biofertilisers.
Rhizobium is a soil bacterium which either lives freely in soil or lives in symbiotic association with roots of leguminous plants. The bacterium forms nodules or roots of leguminous plants where it lie in groups. When it occurs freely in soil, it cannot fix nitrogen. Nitrogen fixing ability develops only when it is present inside root nodules. Hence, Rhizobium is categorised as symbiotic bacterium. It acts as a biofertiliser as it helps plants in obtaining their nitrogen nutrition.

Question. Choose any three microbes, from the following which are suited for organic farming which is in great demand these days for various reasons.
Mention one application of each one chosen.
Mycorrhiza; Monascus; Anabaena; Rhizobium; Methanobacterium; Trichoderma.
Answer. Among the given microbes, the ones which are in great demand for organic farming are: Mycorrhiza,Anabaena and Rhizobium.
Mycorrhiza is a mutually beneficial or symbiotic association of a fungus with the roots of a higher plant. Mycorrhizal roots show a sparse or dense wooly growth of fungal hyphae on their surface. Plants having mycorrhizal associations show resistance to root-borne pathogens.
Anabaena is free living and symbiotic nitrogen fixing cyanobacteria. Cyanobacteria are photosynthetic and have the property of nitrogen fixation. They add organic matter as well as extra nitrogen to the soil. Cyanobacteria are an extremely low cost biofertilisers.
Rhizobium is symbiotic nitrogen fixing bacteria. They form a mutually beneficial association with the plants. The bacteria obtain food and shelter from plants. In return, they give a part of their fixed nitrogen to the plants, thus enhancing the availability of nutrient to crops. It forms nodules on the roots of legume plants. They develop the ability to fix nitrogen only when they are present inside the root nodules.

Long Answer Type Questions

Question. What are biofertilisers? Describe their role in agriculture. Why are they preferred to chemical fertilisers?
Answer.  Biofertilisers are microorganisms which bring about nutrient enrichment of soil by enhancing the availability of nutrients like nitrogen (N) and phosphorus (P) to crops. Biofertilisers includes– nitrogen fixing bacteria, nitrogen fixing cyanobacteria and mycorrhiza. Azotobacter occurring in fields of cotton, maize, jowar and rice, not only increases yield but also saves nitrogen fertiliser upto 10–25 kg/ha. A number of free living cyanobacteria or blue-green algae have the property of nitrogen fixation, e.g., Anabaena, Nostoc, Oscillatoria, Aulosira, Tolypothrix, Cylindrospermum, Stigonema. Cyanobacteria are extremely low cost biofertilizers.
The most important of the symbiotic nitrogen fixing bacteria is Rhizobium. It forms nodules on the roots of legume plants.There are about a dozen species of Rhizobium which form association with different legume roots, e.g., R. leguminosarum, R. lupini, R. trifolii, R. meliloti, R. phaseoli.
Nitrogen fixing cyanobacteria (blue–green algae) form symbiotic association with several plants, e.g., cycad roots, lichens, liverworts, Azolla (fern). Azolla–Anabaena association is of great importance to agriculture. Azolla pinnata is a free floating fresh water fern which multiplies rapidly, doubling every 5–7 days. The fern can coexist with rice plants because it does not interfere with their growth. In some South-East Asian countries, especially China, the rice fields are regularly provided with Azolla.
Chemical fertilisers cause pollution of water bodies as well as ground water, besides getting stored in crop plants. Therefore, farmers are pressing for switch over to organic farming which includes the use of manures biofertilisers, biopesticides. Biofertilisers are microorganisms which bring about nutrient enrichment of soil by enhancing the availability of nutrients to crops. The microorganisms which act as biofertilisers are bacteria, cyanobacteria (blue green algae) and mycorrhizal fungi. Bacteria and cynobacteria have the property of nitrogen fixation while mycorrhizal fungi preferentially withdraw minerals from organic matter for the plant with which they are associated. Phosphate is also solubilised by some bacteria and by some fungi that form association with plant roots.