Class 12 Biology

Environmental Issues

NCERT Exercise

Question 1: What are the various constituents of domestic sewage? Discuss the effects of sewage discharge on a river.

Answer: Various constituents of domestic sewage are; leftover food, vegetable peels, rotten fruits and vegetables, detergents, oil and fat, soap, some chemicals, etc. A major portion of domestic sewage is composed of organic matter. Some bacteria and other microbes decompose this organic matter. While decomposing organic matter, these microbes consume of a lot of oxygen. This results in a sharp decline in dissolved oxygen downstream; from the point of sewage discharge. Reduced level of oxygen causes mortality of fish and other aquatic creatures.

Question 2: List all the wastes that you generate, at home, school or during your trips to other places, could you very easily reduce? Which would be difficult or rather impossible to reduce?

Answer: Some of the items are; plastic cups and plates, polythene bags, leftover food, paper, etc. We can easily reduce the amount of plastic in the waste. Waste from food is almost impossible to reduce.

Question 3: Discuss the causes and effects of global warming. What measures need to be taken to control global warming?

Answer: Increased percentage of greenhouse gases in the atmosphere is causing global warming. Global warming is causing massive changes in the environment and resulting in odd climatic changes. Following steps can be taken to control global warming:

Question 4: Match the items given in column A and B:

Column AColumn B
(a) Catalytic converter(i) Particulate matter
(b) Electrostatic precipitator(ii) Carbon monoxide and nitrogen oxides
(c) Earmuffs(iii) High noise level
(d) Landfills(iv) Solid wastes

Answer: (a) ii, (b) i, (c) iii, (d) iv

Question 5: Write critical notes on the following:

(a) Eutrophication

Answer: Presence of large amounts of nutrients in water body is called eutrophication. It causes excessive growth of planktonic algae. This phenomenon is called algal bloom. Eutrophication accelerates the aging process of a lake. Release of untreated sewage results in eutrophication of water bodies. It destroys the aquatic ecosystem and results in irreparable damage to the environment.

(b) Biological magnification

Answer: Wastewater from industries often contain toxic substances; notably; heavy metals. Some of the toxic substances can undergo biological magnification in the aquatic food chain. The increase in concentration of a toxicant at successive trophic levels is called biomagnification. This often proves harmful for animals which are at the top of the food pyramid.

(c) Groundwater depletion and ways for its replenishment

Answer: The natural process of recharge of groundwater has significantly reduced because of construction all around us. This has reduced the supply of water to underground reservoirs. Increase in human population has increased the demand for groundwater. Thus, there is a mismatch of demand and supply of water. This mismatch can be rectified by applying various techniques of rainwater harvesting. Traditional structures of rainwater harvesting should be revived. Rooftop rainwater harvesting should be practiced and encourages in urban areas.

Question 6: Why ozone hole forms over Antarctica? How will enhanced ultraviolet radiation affect us?

Answer: Extreme cold in Antarctica changes unreactive chlorine into reactive form. The polar vortex formed over Antarctica is very tight. Hence, the reaction occurring on cloud surface over Antarctica is entirely different than at any other place in the atmosphere. This contributes significantly to ozone hole formation over Antarctica. Ultraviolet radiation is highly harmful for skin and retina. It can cause faster ageing of skin and can cause skin cancer. It can result in inflammation of cornea. This condition is called snow-blind cataract.

Question 7: Discuss the role of women and communities in protection and conservation of forests.

Answer: Women have been at the forefront of protection and conservation of forests. The revolt against king’s men in Khejrali village of Rajasthan in 1731 was led by a woman named Amrita Devi. The Chipko Movement was started by women of Reni village of Garhwal district in the 1970s. Even in modern times, many women activists have played the role of torchbearers of conservation movement. Medha Patkar is one example. She fought against increasing the height of the Sardar Sarovar Dam on Narmada river. It is generally a woman who has to face the burnt of dwindling resources. Women have to trek for miles in search of drinking water and firewood. So, it is quite natural for a woman to take up the cudgels against forces which destroy the nature.

Question 8: What measures, as an individual, you would take to reduce environmental pollution?

Answer: I will take following steps to reduce environmental pollution:

Question 9: Discuss briefly the following:

(a) Radioactive wastes

Answer: Waste from nuclear power plants generate radiation which is extremely damaging to living beings because it causes mutations at a very high rate. Radiation at lower doses causes various disorders; including cancer. Nuclear waste should ideally be buried in shielded containers about 500 m deep below the earth’s surface. But this method is meeting with stiff resistance from public

(b) Defunct ships and e-wastes

Answer: Defunct ships are generally sent for scrapping so that iron from it can be recycled to make new items. But a defunct ship also produces waste which contains non-recyclable and non-biodegradable substances; like grease, harmful chemicals, etc. Oil spill in the coastal areas damages the coral reef and is detrimental to the coastal ecosystem.

e-waste: Irreparable computers and other electronic goods comprise e-waste. It can be buried in landfills or incinerated. Primitive methods of recycling of e-waste produce many toxic substances in the air. Workers who are involved in such methods of recycling are always at risk of exposure to harmful substances.

(c) Municipal solid wastes

Answer: Sanitary landfills were preferred over open dumps with an aim at better management of solid waste. But, generation of huge amounts of garbage in cities has turned many landfill sites into hills of garbage. Solid waste can be segregated into biodegradable, recyclable and non-biodegradable wastes. Proper segregation can help in better management of solid waste. Biodegradable waste should be converted into compost so that it can find better use. Biodegradation also helps in power generation. Recyclable waste should be sent for recycling; to make new products.

Question 10: What initiatives were taken for reducing vehicular air pollution in Delhi? Has air quality improved in Delhi?

Answer: Delhi has more cars than the states of Gujarat and West Bengal put together. In the 1990s, the Supreme Court of India directed the government to take suitable measures to tackle air pollution in Delhi. As a result of this directive, all public transport buses were fitted with CNG kits. CNG is a cleaner fuel than petrol or diesel. This helped a lot in controlling air pollution in cities.

Question 11: Discuss briefly the following :

(a) Greenhouse gases

Answer: Gases which trap solar radiation and produce greenhouse effect are called greenhouse gases, e.g. carbon dioxide, methane, water vapor, etc. Greenhouse effect results in global warming which detrimental to the environment.

(b) Catalytic converter

Answer: It is a device which is fitted in exhaust system of automobiles. It has expensive metals (platinum, palladium and rhodium) as catalysts. When the exhaust passes through the catalytic converter, unburnt hydrocarbons are converted into carbon dioxide and water, and carbon monoxide and nitric oxide are changed to carbon dioxide and nitrogen gas, respectively. Lead in petrol inactivates the catalyst. Hence, unleaded petrol should be used in vehicles fitted with catalytic converters.

(c) Ultraviolet B

Answer: UV B is a type of ultraviolet rays. It can penetrate the ozone layer to reach the earth’s surface. It damages the DNA and may cause mutation. It causes ageing of skin and can cause skin cancer. It causes inflammation of cornea. This condition is called snow-blindness cataract.