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NCERT Solutions for Class 12 Biology
Page No 286
Exercises
Q1. What are the various constituents of domestic sewage? Discuss the effects of sewage discharge on a river.
Answer: Constituents of domestic sewage.
It mainly consists of biodegradable organic matter
(i) suspended solids e.g., sand, silt and clay.
(ii) colloidal materials eg. faecal matter, bacteria, disease causing microbes, cloth fibre, plastic and paper.
(iii)Dissolved materials, e.g. nutrients (nitrates, ammonia, phosphates, sodium and calcium).
Effects of Sewage discharge on a river :
(i) The micro-organisms in the sewage consume alot of oxygen and this pose a problem for other organisms in the river.
(ii) The Biochemical oxygen demand (BOD) of the river increases.
(iii) Presence of large amount nutrients causes excessive growth of planktonic algae or Algal bloom which imparts distinct color and degrades the quality of water and fish mortality. Some bloom-forming algae are extremely toxic to human beings and animals.
(iv) Various disease causing microbes may cause outbreak of serious diseases, such as, dysentery, typhoid, jaundice, cholera, etc
Q2. List all the wastes that you generate, at home, school or during your trips to other places. Could you very easily reduce the generation of these wastes? Which would be difficult or rather impossible to reduce?
Answer: Wastes generated at home include plastic bags, paper napkin, toiletries, kitchen wastes (such as peelings of vegetables and fruits, tea leaves), domestic sewage, glass, etc. Wastes generated at schools include waste paper, plastics, vegetable and fruit peels, food wrappings, sewage etc. Wastes generated at trips or picnics include plastic, paper, vegetable and fruit peels, disposable cups, plates, spoons etc. Yes, wastes can be easily reduced by the judicious use of the above materials.
Wastage of paper can be minimized by writing on both sides of the paper and by using recycled paper. Plastic and glass waste can also be reduced by recycling and re-using. Also, substituting plastics bags with biodegradable jute bags can reduce wastes generated at home, school, or during trips. Domestic sewage can be reduced by optimizing the use of water while bathing, cooking, and other household activities. Non- biodegradable wastes such as plastic, metal, broken glass, etc are difficult to decompose because micro-organisms do not have the ability to decompose them.
Q3. Discuss the causes and effects of global warming. What measures need to be taken to control global warming?
Answer:
- Effects: Heating of earth surface [mean temperature is increased] Climatic changes e.g.: El Nino effect.
- Increased melting of polar ice caps and Himalayan snowcaps. Increased sea levels and coastal areas will submerge.
- Measures – Decreased use of fossil fuels, improve the efficiency of energy usage, Reduce deforestation, plant trees Control of man-made sources of greenhouse gases like vehicles, aerosol sprays.
- Global warming is a rise in the mean temperature of the lower atmosphere and the earth’s surface. Causes – increase in the quantity of radioactively active greenhouse gases CO2, CH4, N2O, CFCs. They allow heat waves to reach the surface and prevent their escape.
- They are produced by combustion of fossil fuels, biomass [CO2]; burning of nitrogen-rich fuels [N2O]; paddy fields, fermentation in cattle and wetlands [CH4]; refrigerators, aerosols, drying, cleaning [CFCs].
Q4. Match the items given in column A and B:
Column A
(a) Catalytic converter
(b) Electrostatic precipitator
(c) Earmuffs
(d) Landfills
Column B
i) Particulate matter
ii) Carbon monoxide and nitrogen oxides
iii) High noise level
iv) Solid wastes
Answer:
Column B
(ii) Carbon monoxide and nitrogen oxides
(i) Particulate matter
(iii) High noise level
(iv) Solid wastes
Q5. Write critical notes on the following:
(a) Eutrophication
Answer: It is referred to the natural ageing process of a lake as a result of nutrient enrichment caused by the overflow of nutrients such as fertilizers, animal wastes and sewage from land leading to an increased fertility of the lake. Consequently, there is a huge increase in the primary productivity of the ecosystem bringing about an increased growth of algae, hence in algal blooms. After sometime, the decomposition of these algae reduces oxygen supply, resulting in the death of aquatic animal life.
(b) Biological magnification
Answer: To protect the crops from the several diseases and pests, a large number of pesticides are used. These pesticides reach the soil and are absorbed by plants with water and minerals from the soil. Due to rain, these chemicals can also enter water sources and into the body of aquatic plants and animals. As a result, chemicals enter the food chain. Since these chemicals cannot be decomposed, they keep on accumulating at each trophic level. The maximum concentration is accumulated at the top carnivore’s level.
This increase in the concentration of pollutants or harmful chemicals with an increase in the trophic level is called biological magnification. For example, high DDT concentrations were found in a pond. The producers (phytoplankton) were found to have 0.04 ppm concentration of DDT. Since many types of phytoplankton were eaten by zooplankton (consumers), the concentration of DDT in the bodies of zooplankton was found to be 0.23 ppm. Small fish that feed on zooplankton accumulate more DDT in their body. Thus, large fish (top carnivore) that feed on several small fish have the highest concentration of DDT.
(c) Groundwater depletion and ways for its replenishment
Answer: The level of ground water has decreased in the recent years. The source of water supply is rapidly diminishing each year because of an increase in the population and water pollution. To meet the demand of water, water is withdrawn from water bodies such as ponds, rivers etc. As a result, the source of ground water is depleting. This is because the amount of groundwater being drawn for human use is more than the amount replaced by rainfall. Lack of vegetation cover also results in very small amounts of water seeping through the ground. An increase in water pollution is another factor that has reduced the availability of ground water.
Measures for replenishing ground water:-
(i) Preventing over-exploitation of ground water
(ii) Optimizing water use and reducing water demand
(iii) Rain water harvesting
(iv) Preventing deforestation and plantation of more trees
Q6. Why does ozone hole form over Antarctica? How will enhanced ultraviolet radiation affect us?
Answer: Ozone holes are more apparent in the region of Antarctica. They are formed as a result of an increased concentration of chlorine in the atmosphere. The release of chlorine is mainly from Chlorofluorocarbons (CFCs) which is extensively used as a refrigerant. The CFCs migrate from different layers of the atmosphere – troposphere to the stratosphere, where chlorine atoms are released by the action of UV radiations on them.
Liberation of chlorine atoms leads to the conversion of ozone into molecular oxygen. One chlorine atom can destruct 10,000 ozone molecules, causing ozone depletion. Ozone hole formation will result in an increased concentration of UV-B radiations on the surface of the earth. The UV – B is known to damage DNA, activating the process of skin ageing. Also, it causes darkening of skin and skin cancer. In humans, high levels of UV-B causes corneal cataract.
Q7. Discuss the role of women and communities in protection and conservation of forests.
Answer: Communities have played an important role in protecting the trees and in conserving the forests.
(i) Bishnoi community women and many others had sacrificed their lives in an effort to save the trees. The Bishnoi community in Rajasthan strictly believes in the concept of living peacefully with nature. In 1731, the king of Jodhpur ordered his ministers to arrange wood for the construction of his new palace. For this purpose, the minister and the workers went to Bishnoi village. There, a Bishnoi woman called Amrita Devi along with her daughter and hundreds of other Bishnois showed the courage to step forward and stop them from cutting trees. They embraced the trees and lost their lives at the hands of soldiers of the king. This resistance by the people of the village forced the king to give up the idea of cutting trees.
(ii) Chipko movement: The Chipko movement was started in 1974 in the Garhwal region of the Himalayas. In this movement, the women from the village stopped the contractors from cutting forest trees by embracing them. Gaura Devi of village Gopeshwar in Chamoli district had led a movement, where in village people hugged the trees to save them from being cut. Many other people in different villages under the leadership of Chandi Prasad and Sunder Lal Bahuguna participated in similar protests.
Later such movements spread to South of India as well.
Now, village and tribal communities are being involved in development and protection of damaged forest area to resurrect them and share profits with the government. Such activities are conducted under the Joint forest management schemes.
Q8. What measures, as an individual, would you take to reduce environmental pollution?
Answer: To reduce environmental pollution, we should change our habits and lifestyle so as to reduce the use of disposable materials. We should use preferably those items which can easily be recycled and also minimise the use of fossil fuels. We should also take measures to improve the quality of air by using CNG gases wherever possible instead of using diesel or petrol. We should also use the catalytic converter in our vehicles.
Q9. Discuss briefly the following:
(a) Radioactive wastes
Answer: These are the wastes produced when nuclear energy is generated from radioactive materials. This nuclear waste is richly supplied with radioactive material which generate large quantities of ionizing radiations like the gamma rays causing mutation in entities resulting in ailments such as skin cancer. These rays can be lethal at high dosages. Harmless disposal of radioactive wastes is a huge challenge. Ideally, nuclear wastes should be stored after pre-treatment in appropriate shielded containers and later buried in rocks.
(b) Defunct ships and e-wastes
Answer: Irrepairble electronic goods and computers are called electronic wastes (e-waste).
Ships that are no longer in use or that are to be dismantled are called defunct ships. Asbestos, Polychlorinated Biphenyl (PCB) produced during dismantling defunct ship cause serious health hazards especially cancer.
(c) Municipal solid wastes
Answer: Municipal solid wastes are generated from schools, offices, homes, and stores. It is generally rich in glass, metal, paper waste, food, rubber, leather, and textiles. The open dumps of municipal wastes serve as a breeding ground for flies, mosquitoes, and other disease-causing microbes. Hence, it is necessary to dispose municipal solid waste properly to prevent the spreading of diseases. Sanitary landfills and incineration are the methods for the safe disposal of solid wastes.
Q10. What initiatives were taken for reducing vehicular air pollution in Delhi? Has air quality improved in Delhi?
Answer: The several initiatives are:-
Introduction of CNG
Enforcement of pollution control law
Introduction of green zones
Use of unleaded fuels
Old vehicles can be replaced with a new ones.
Use of catalytic converters in vehicles.
For vehicles, the application of Euro II norms is adopted.
Q11. Discuss briefly the following:
(a) Greenhouse gases
Answer: Gases that trap the heat of the sun in the earth’s atmosphere increasing atmospheric temperature effect are called greenhouses gases. CO2, CH4, N2O and CFC, cause greenhouse. In the absence of greenhouse gases, the temperature of earth would go down to -18°C. The net effect of higher GHGs will be disastrous, (i) Melting of polar ice caps and mountain snow caps resulting in rising of sea level threatening submergence of many islands and coastal areas. Odd climate changes like El Nino. Increased floods and drought.
(b) Catalytic converter
Answer: These are devices fixed in automobiles to lessen the vehicular pollution and contain expensive metals such as palladium, platinum and rhodium that serve as catalysts. When the vehicular discharge passes through the catalytic converter, the unburnt hydrocarbons found in it is converted into water and carbon dioxide. Nitric oxide and carbon monoxide liberated by catalytic converters are converted into nitrogen gas and carbon dioxide respectively.
(c) Ultraviolet B
Answer: Ultraviolet-B is a part of ultraviolet radiation having a wavelength of 280-320 nm. It damages DNA by causing mutations that can also result in-
(i) Premature aging of skin.
(ii) Damage to skin cells and various types of skin cancers.
(iii) Inflammation of cornea, called snow-blindness cataract, etc.
Ozone layer absorbs 50% of ultraviolet-B and protects us from it
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