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NCERT Solutions for Class 12 Biology
Page No. 189
Exercises
Q1. Bacteria cannot be seen with the naked eyes, but these can be seen with the help of a microscope. If you have to carry a sample from your home to your biology laboratory to demonstrate the presence of microbes under a microscope, which sample would you carry and why?
Answer: The sample that can be easily carried is the curd. Curd contains the lactic acid bacteria (Lactobacillus) in large amount, so they can be easily observed. Lactic acid bacteria are also non-pathogenic, so there will be no risk of infection in case of mishandling.
Q2. Give examples to prove that microbes release gases during metabolism.
Answer: The examples of bacteria that release gases during metabolism are:
(a) Bacteria and fungi carry out the process of fermentation and during this process, they release carbon dioxide. Fermentation is the process of converting a complex organic substance into a simpler substance with the action of bacteria or yeast. Fermentation of sugar produces alcohol with the release of carbon dioxide and very little energy.
(b) The dough used for making idli and dosa gives a puffed appearance. This is because of the action of bacteria which releases carbon dioxide. This CO2 released from the dough gets trapped in the dough, thereby giving it a puffed appearance.
Q3. In which food would you find lactic acid bacteria? Mention some of their useful applications.
Answer: We can find lactic acid bacteria in curd. Some of the useful applications of these bacteria are:
1) It causes an increase in the Vitamin B12 level of curd, thus increasing the nutritional value of curd
2) It can be used to ferment or culture food
3) They check the health of the gut
Q4. Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products) which involve use of microbes.
Answer: Idli, Dhokla, Dosa. Several food items such as dosa, idli, jalebi and bread are prepared by fermentation process in which one or more kinds of microbes are used.
Q5. In which way have microbes played a major role in controlling diseases caused by harmful bacteria?
Answer: Several micro-organisms are used for preparing medicines. Antibiotics are medicines produced by certain micro-organisms to kill other disease-causing micro-organisms. These medicines are commonly obtained from bacteria and fungi. They either kill or stop the growth of disease-causing micro-organisms. Streptomycin, tetracycline, and penicillin are common antibiotics. Penicillium notatum produces chemical penicillin,
which checks the growth of staphylococci bacteria in the body. Antibiotics are designed to destroy bacteria by weakening their cell walls. As a result of this weakening, certain immune cells such as the white blood cells enter the bacterial cell and cause cell lysis. Cell lysis is the process of destroying cells such as blood cells and bacteria
Q6. Name any two species of fungus, which are used in the production of the antibiotics.
Answer: Antibiotics are medicines that are produced by certain micro-organisms to kill other disease-causing micro-organisms. These medicines are commonly obtained from bacteria and fungi.
The species of fungus used in the production of antibiotics are:
| Antibiotic | Fungus source |
| Penicillin Cephalosporin | Penicillium notatum Cephalosporium acremonium |
Q7. What is sewage? In which way can sewage be harmful to us?
Answer: The municipal waste matter is the sewage which is carried away in sewers and drains.
It consists of both liquid and solid wastes, which is rich in organic matter and microbes. Most of these microbes are pathogenic and may cause several water- borne diseases. Sewage water is a major source of polluting drinking water. Hence, it is important that sewage water should be properly collected, treated, then disposed of.
Q8. What is the key difference between primary and secondary sewage treatment?
Answer: There are three stages of sewage treatment : primary, secondary and tertiary. Primary treatment is a physical process while secondary is a biological process and tertiary is a chemical process.
Q9. Do you think microbes can also be used as source of energy? If yes, how?
Answer: Yes, microbes can certainly be a source of energy. Some bacteria such as the Methane bacterium can be used to generate biogas or gobar gas. In a biogas plant, under anaerobic conditions, biogas can be generated. The biogas plant consists of a concrete tank, with a depth of nearly 10-15 feet, comprising of adequate outlets and inlets. The gathered dung is mixed with water for the formation of slurry and flung into the tank. The tank consists of a digester, which is filled with plenty of anaerobic methane-producing bacteria. This produces biogas from the slurry. Biogas can be extracted through a pipe which can be used as a source of energy. The slurry that is used, is removed from the outlet and used as a fertilizer.
Q10. Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how this can be accomplished.
Answer: Microbes can be used as biofertilisers, organisms that enrich the nutrient quality of the soil.
The main sources of bio-fertilisers are bacteria, fungi and cyanobacteria.They help in increasing the fertility of the soil in many ways
(i) Rhizobium that forms nodules on the roots of leguminous plants(a symbiotic association) fixes atmospheric nitrogen into organic
forms, which is used by the plant as nutrient.
(ii) Azospirillum and Azotobacter fix atmospheric nitrogen, while living freely, and enriching the nitrogen content of the soil.
(iii) Many members of the genus Glomus (fungi) form symbiotic associations with plant known as mycorrhiza that
(a) Absorption of phosphorus from soil and pass it to the plant.
(b) Help the plants to develop resistance to root-borne pathogens.
(c) increase their tolerance to salinity and drought and thus, help inoverall increase in plant growth and development.(iv) Cyanobacteria autotrophic microbes, e.g., Anabaena, Nostoc,Oscillatoria can fix atmospheric nitrogen, in aquatic and terrestrial environment and also add organic matter to the soil and increase its fertility.
Q11. Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled A, B and C; but the laboratory attendant did not note which was which. The BOD values of the three samples A, B and C were recorded as 20mg/L, 8mg/L and 400mg/L, respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?
Answer: BOD or the biochemical oxygen demand refers to the amount of oxygen which would be consumed if all the organic matter in one liter of water were to be oxidized by bacteria. This BOD test measures the rate of uptake of oxygen by microbes in a sample of water, hence BOD is said to be a measure of the organic matter found in water. Therefore, greater the BOD of waste water, the greater is the polluting potential.
Sample A with BOD 20mg/L can be assigned as the secondary effluent discharged from a sewage treatment plant.
Sample B with BOD 8mg/L can be assigned as river water
Sample C with BOD 400mg/L can be assigned as untreated sewage water.
This assignment is on the basis that the highest BOD value is the sample carrying the most polluted water.
Q12. Find out the name of the microbes from which Cyclosporin A (an immunosuppressive drug) and Statins (blood cholesterol lowering agents) are obtained
Answer: (i) Cyclosporin-A is obtained from the fungus Trichoderma polysporum.
(ii) Statins is obtained from Monascus purpureus.
Q13. Find out the role of microbes in the following and discuss it with your teacher.
(a) Single cell protein (SCP)
Answer: Single Cell Protein (SCP) refers to harmless microbial cells that can be used as an alternate source of good protein.Just like mushrooms (a fungus) is eaten by many people and yeast isused by athletes as a protein source;similarly, other forms of microbial cells can also be used as food rich in protein, minerals, fats, carbohydrate and vitamins.Microbes like Spirulina and Methylophilus methylotrophus are being grown on an industrial scale on materials containing starch like wastewater from potato processing plants, straw,molasses, animal manure and even sewage. These single cell microbes can be used as source.
(b) Soil
Answer: Microbes play an important role in maintaining soil fertility. They help in the formation of nutrient-rich humus by the process of decomposition. Many species of bacteria and cyanobacteria have the ability to fix atmospheric nitrogen into usable form. Rhizobium is a symbiotic bacteria found in the root nodules of leguminous plants. Azospirillium and Azotobocter are free living nitrogen-fixing bacteria, whereas Anabena, Nostoc, and Oscillitoria are examples of nitrogen-fixing cyanobacteria.
Q14. Arrange the following in the decreasing order (most important first) of their importance, for the welfare of human society. Give reasons for your answer.
Biogas, Citric acid, Penicillin and Curd
Answer: Penicillin > Biogas > Curd > Citric acid
An antibiotic that aid kill pathogens that cause infections and diseases and
thus, it saves lives is Penicillin.
Biogas is a non-polluting decent fuel that is produced as a byproduct of sewage treatment. In the homes, it is used for cooking and lighting homes in rural areas. Curd has excellent nutrient value, providing vitamin-B12 and replaces unfavourable bacteria of the stomach with helpful ones.
Citric acid is employed as a preservative of food.
Q15. How do biofertilisers enrich the fertility of the soil?
Answer: Bio-fertilizers involve picking advantageous microbes that help in improving plant growth, by supplying nutrients. Bio-fertilizers are living entities that are known to increase soil fertility. These are introduced into roots, seeds and soil in order to make nutrients available, thus enriching the soil with organic nutrients. Many species of cyanobacteria and bacteria have the potential to fix the free atmospheric nitrogen. A symbiotic bacteria, Rhizobium, is found in the root nodules of leguminous plants. Some free living nitrogen-fixing bacteria are Azotobacter and Azospirillum, while Nostoc, Oscillitoria, Anabaena are examples of nitrogen-fixing cyanobacteria. These biofertilizers are eco-friendly and cost-effective.
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