NCERT Solutions Class 12 Biology Chapter 16

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

Ans-

Domestic sewage is constituted of waste that originates from kitchen, toilets, bathrooms, laundry and other cleaning activities that flow into the drains. Following are the most important constituents of domestic sewage:

• Suspended solids like sand, silt and clay.
• Colloidal material like faecal matter, bacteria, cloth, paper.
• Dissolved materials like nitrate, phosphate, calcium, ammonia, house-hold chemicals like pesticides, disinfectants and soaps (e.g. sodium dodecyl sulphate).

Effect of sewage discharge on a river: Sewage discharge into rivers not only hazardous to human health but can also disrupt the river ecosystem. The following points illustrate the ill-effects of sewage discharge in the river:

1. Micro-organisms consume a lot of dissolved oxygen during the degradation of organic waste. This leads to a sharp decline in dissolved oxygen (or increase in Biological Oxygen Demand also called BOD) causing mortality of fish and other aquatic creatures.
2. Nutrient availability in water increases causing an overgrowth of planktonic algae which leads to the phenomenon called ‘Algal Bloom’. Algal bloom affects water quality and causes fish mortality. Some algae can be toxic to humans too.
3. Sewage can increase the turbidity of water adversely affecting aquatic plant growth due to reduced light penetration in water.
4. Sewage can also introduce heavy metals and other toxic substances like chemicals and pesticides. It can accumulate in the flora and fauna which can cause their mortality or have long term toxic effects.
5. Microbial pathogens can get introduced into the rivers and cause disease outbreaks and other health problems.

Q.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?

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All these wastes can be categorised into recyclable, biodegradable and non-recyclable waste. We should segregate and dispose of our waste responsibly whenever possible according to these categories.

Waste generated can be minimised by judicious use of the objects listed in the table.

1. Plastic bags can be replaced with reusable materials like jute and cloth bags.
2. Paper can be conserved by avoiding unnecessary printing, optimal usage, using recycled paper.
3. Domestic sewage can be reduced by optimising the use of water while bathing, cooking, cleaning and avoiding any leaks at home.

Some of the waste which would be difficult or impossible for us to reduce would include waste generated by large manufacturing units, factories, hospital waste etc because it requires special waste management techniques to manage wastes generated from these facilities.

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

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In the recent times, the levels of greenhouse gases such as CO₂, methane, tropospheric ozone, CFCs and nitrous oxide have increased due to which the earth’s temperature is on the rise leading to global warming.

Causes of global warming are:

• Increased human activity since the industrial revolution has led to an increase in the number of greenhouse gases released in the atmosphere. According to a study in 2007, the concentrations of CO₂ and methane have increased by 36% and 148% respectively since 1750.
• Burning of fossil fuels has produced about three-quarters of the increase in CO₂ over the past decades.
• Changes in land-use, particularly deforestation has also contributed to global warming.

Effects of global warming are:

As compared to the last century the earth’s temperature has increased by 0.6 °C.

Increased temperature has caused the melting of Himalayan snowcaps and Polar ice caps.

Increased melting of snow has resulted in increased sea levels inundating many coastal areas.

Rainfall patterns have become erratic.

Measures to control global warming:

1. Reduce the use of fossil fuels.
2. Improve the efficiency of energy usage.
3. Reduce deforestation.
4. Plant more trees.
5. Curb the rate of growth of the human population.
6. Reduce the emission of greenhouse gases by checking air pollution.

Q.4 Match the items given in column A and B:

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Q.5 Write critical notes on the following:

1. Eutrophication
2. Biological magnification
3. Ground water depletion and ways for its replenishment

Ans- 

a. Eutrophication

Eutrophication is the enrichment of an ecosystem with chemical nutrients, typically compounds containing nitrogen, phosphorus, or both. Eutrophication can be a natural process in lakes, occurring as they age through geological time. Natural eutrophication encourages the growth of various types of flora in the lake which leads to deposition of organic matter and silt on the lake bottom finally converting the lake into the land. This type of natural ageing may span thousands of years.

Human activities can accelerate the rate at which nutrients enter ecosystems, in turn, accelerating the ageing process of lakes. This phenomenon is called ‘Cultural or Accelerated Eutrophication’. Agriculture, industrial and sewage waste and other human-related activities increase the flux of both inorganic nutrients and organic substances into terrestrial, aquatic, and coastal marine ecosystems. Contaminants like nitrates and phosphates overstimulate the growth of algae causing algal bloom and reducing the dissolved oxygen content of the water, which in turn causes other life forms to perish.

b. Biological magnification

Biological magnification (also called biomagnification) is a process where the concentration of a contaminant increases as one moves up the trophic levels in a food chain. Biomagnification happens when pesticides or heavy metals find their way into rivers and lakes as pollutants, they get accumulated in planktons which in turn are eaten by aquatic animals like fish which are eaten by higher-order predators like birds, animals and humans. The substances become concentrated in tissues or internal organs as they move up the chain because they cannot be excreted/metabolised.

Example of biomagnification of DDT:

(C) Groundwater depletion and ways for its replacement

Groundwater is the largest source of usable, fresh water in the world. In many parts of the world, including India, domestic, agricultural, and industrial water needs are met by using the groundwater. Groundwater depletion is primarily caused by sustained groundwater pumping, overuse of surface water from ponds and lakes (which is the source for replenishment of groundwater), deforestation due to which rainfall and water seepage are affected. Some of the negative effects of groundwater depletion are as follows:

• Lowering of the water table.
• The increased cost of pumping water because of the lowered water table.
• Reduced surface water supplies because groundwater and surface water are connected. When groundwater is overused, the lakes, streams, and rivers connected to groundwater also have diminished supply.
• Land subsidence caused by soil collapse when there is a loss of support below the ground.
• Water quality degrades. Excessive pumping in coastal areas can cause saltwater to move inland and upward, resulting in saltwater contamination of the water supply.

Ways to replace groundwater:

• Judicial use of groundwater and surface water
• Rainwater harvesting
• Afforestation

Q.6 Why does ozone hole form over Antarctica? How will enhanced ultraviolet radiation affect us?

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The ozone hole is more prominent in the region of Antarctica due to the presence of more concentration of chlorine in the atmosphere. One of the main sources of chlorine is chlorofluorocarbons (CFCs) which were widely used in the refrigerators as refrigerants. These CFCs can move from the troposphere to stratosphere. In the stratosphere, by the action of UV rays, CFCs release chlorine ions. These chlorine ions convert ozone molecules into oxygen gas. Each chlorine ion has the potential of destroying almost 10,000 ozone molecules and thus, causes ozone depletion. Ozone hole formation results in an increased amount of UV-B rays to the earth’s surface. UV- B rays can damage DNA and results in the ageing of the skin. It also causes skin cancer and also darkens the skin. The increased level of UV-B rays can cause corneal cataract in human beings.

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

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Co-existence with nature and respect for the environment is engrained in our cultural roots. Though the need for urbanisation and new technologies has caused much destruction, many communities have relentlessly worked in conserving their habitats.

Case 1: As long back as 1731, the Bishnoi community had protested cutting down of trees ordered by the king of Jodhpur. Amrita Devi, her three daughters and hundreds of Bishnoi women lost their lives because they embraced the trees and refused to let go. The widespread protest of the people led the king to give up the idea of cutting the trees for the construction of his palace.

Case 2: Chipko movement was started in the Garwhal region in 1974. Here too women hugged the trees preventing them from being cut by the timber contractors.

Q.8 What measures, as an individual, would you take to reduce environmental pollution?

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Some of the measures for reducing air pollution are:

1. Reduce the consumption of fossil fuels by using public transport and by sharing vehicles
2. Use catalytic converters in vehicles
3. Regular pollution check of automobiles
4. Use eco-friendly fuel like CNG and biofuel
5. Plant trees to increase the green cover
6. Light Diwali crackers sparingly

Some of the measures for reducing noise pollution are:

1. Limit the use of horns on roads
2. Light Diwali crackers sparingly
3. Avoid misuse of loudspeakers

Some of the measures for reducing water pollution are:

1. Do not throw oil, chemicals, medicines and pesticides in the drain.
2. Do not spill diesel, petrol and other mineral oils.
3. Use soap for cleaning and bathing judiciously.
4. Encourage the use of recycled water for gardening, car wash, and in the toilet.

Some of the measures for reducing solid waste are:

1. Composting of biodegradable kitchen waste
2. Recycling and reuse of plastic and paper
3. Responsible disposal of e-waste

Q.9 Discuss briefly the following:
(a) Radioactive wastes
(b) Defunct ships and e-wastes
(c) Municipal solid wastes

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(a) Radioactive wastes: Any application of nuclear technology (such as energy generation, medicine, and research) generates radioactive wastes. These wastes remain radioactive for a long period of time and radiate gamma rays. These gamma rays are extremely harmful to flora, fauna, and the environment. Genetic mutations (causing limb deformities and other developmental disorders) are a common repercussion of exposure to radiations, while prolonged exposure could also be lethal. Radioactive waste is regulated by government agencies around the world. The radioactive waste needs to be isolated and must be kept confined for a sufficiently long period of time for the radiation intensity to come down to below hazard limit.

(b) Defunct ships and e-wastes: Ships, like any other product, have a finite life. At the end of their life, they need to be dismantled as it would be dangerous to keep on using them for the transport of people or goods. Apart from steel, that could be recycled, these defunct ships also contain hazardous materials such as asbestos, electronic wastes and waste oils. These materials cannot be recycled and if not disposed of safely. These materials pose a serious risk of contaminating seawater and endangering marine life. Dealing with materials such as, asbestos and waste oil also pose health hazards (lung and skin diseases) for the workers who are involved in the dismantling of these defunct ships. E‑wastes or e-scrap or electronic waste is a term that is used for discarded electrical or electronic devices. Improper processing of e-waste poses serious health and pollution risks. E-waste may contain contaminants such as lead, cadmium, beryllium, or brominated flame retardants. Recycling and disposal of e-waste are done either by landfilling or incarcerating. This process may involve a significant risk of exposing workers to hazardous chemicals along with leaking heavy metals.

(c) Municipal solid wastes: Municipal solid wastes are the wastes that humans generate in their daily lives. These are the things that the human population discards in the trash of the house or community. The composition of the solid waste is country-specific. For example, in India paper (or newspaper), glass bottles, and metal items are generally sold while in developed countries, one is required to put the waste in specified community bins. In general, solid waste could be categorised as follows –

• Biodegradable: food and kitchen waste, green waste, paper (can be recycled)
• Recyclable: paper, glass, bottles, cans, metals, certain plastics, fabrics, clothes, batteries etc.
• Electrical and electronic waste: electrical appliances, TVs, computers etc.
• Composite wastes: waste clothing, tetra packs, waste plastics such as toys
• Inert waste: construction and demolition waste, dirt, rocks, debris
• Hazardous waste including most paints, chemicals, light bulbs, fluorescent tubes, spray cans, fertilizer and containers
• Toxic waste including pesticide, herbicides, fungicides
• Medical waste

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

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In 1990, Delhi ranked as the 4^th most polluted city in the world. Govt. of India decided to curb and control air pollution by taking the following measures:

1. Buses were converted to CNG. Government made it mandatory for new light commercial vehicles to run on CNG.
2. Restricted commercial vehicles from entering the city during peak hours.
3. Enforced phasing out of old vehicles.
4. Enforced use of unleaded petrol.
5. Enforced use of low-sulphur petrol and diesel.
6. Use of catalytic converters in vehicles.
7. Enforced application of stringent pollution level norms for vehicles.

The above measures did improve the air quality of Delhi and it was shown that by 2005 levels of noxious gases like CO₂ and SO₂ had reduced. Today, Delhi has more than four million registered vehicles, the highest vehicular population in the country. Currently, the city adds over 1,000 new personal vehicles each day on its roads. The latest estimates show that the respirable suspended particulate matter (RSPM) since 2005 has increased and stands at 3 times more than the permissible limits. Persistent efforts, new methods and regulations are the need of the hour.

Q.11 Discuss briefly the following:
(a) Greenhouse gases
(b) Catalytic converter
(c) Ultraviolet B

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(a) Greenhouse gases: The major greenhouse gases are: water vapour, which causes about 36–70%, carbon dioxide (CO₂) which causes 9–26%, methane (CH₄) which causes 4–9%, and ozone (O₃) which causes 3–7% of the greenhouse effect. Clouds also affect the radiation balance in a way similar way to greenhouse gases. The greenhouse effect is a naturally occurring phenomenon which keeps the earth surface and its atmosphere warm. Without the greenhouse effect, the average temperature of the earth’s surface would be -18°C as opposed to the habitable average of 15°C. This greenhouse effect is caused by a blanket of these greenhouse gases that form the earth’s atmosphere. The solar radiation that falls on the earth’s surface causes it to heat up. The earth’s surface re-emits the heat in the form of infrared radiations which are absorbed by the greenhouse gases and radiate the heat back on the earth’s surface.

(b) Catalytic converter: Catalytic converter is a device that is fitted into the automobiles for reducing the emission of toxic gases caused due to burning of fossil fuels. In the catalytic converters, metals like rhodium and platinum-palladium act as a catalyst to convert the poisonous gases to harmless ones. For example, unburnt hydrocarbons are converted into carbon dioxide and water, carbon monoxide is converted into carbon dioxide, and nitric oxide is converted into nitrogen gas.

(c) Ultraviolet B: Ultraviolet B or UV-B is electromagnetic radiation which is part of the invisible spectrum of light. UV-B is harmful radiation that can cause the following damages: