CBSE Class 12 Biology Environmental Issues Worksheet Set A

Very Short Answer

Question.  Who are responsible for the upkeep and safeguarding of the integrated waste water treatment project?
Answer. The species that invade a bare area is known as pioneer species.

Question.  Define pollution?
Answer. Humification leads to accumulation of a dark coloured amorphous substance is called humus.

Question.  How can we measure the thickness of the ozone in a column of air?
Answer. Humification and mineralisation occur during decomposition in the soil.

Question.  What are pollutants?
Answer. The humus is degraded by some microbes and release of inorganic nutrients occur by the process is called mineralisation.

Question. What is the use of electrostatic precipitator?
Answer. Vertical distribution of the different species occupying different levels is called stratification.

             

Short Answer

Question. What do you understand by the e-wastes?
Answer. A nutrient cycle refers to the movement and exchange of organic and inorganic matter back into the production of living matter. The process is regulated by the food web pathways previously presented, which decompose organic matter into inorganic nutrients. Nutrient cycles occur within ecosystems.

Question. What do you mean by algal bloom?
Answer. An ecosystem is a large community of living organisms (plants, animals and microbes) in a particular area. The living and physical components are linked together through nutrient cycles and energy flows. Ecosystems are of any size, but usually they are in particular places.

Question. What are the effects of the agro-chemicals?
Answer. 1. Primary productivity is the rate of synthesis of the biomass or energy fixation by the plants whereas secondary productivity is the rate of the synthesis of biomass by consumers i.e. herbivores and carnivores.
2. Primary productivity is comparatively quite high whereas secondary productivity is small and decrease with rise of the tropic level.

Question. What do you understand by the greenhouse effect?
Answer. Ecosystem services are the many and varied benefits to humans gifted by the natural environment and from healthy ecosystems. Such ecosystems include, for example, agro ecosystems, forest ecosystems, grassland ecosystems and aquatic ecosystems. These ecosystems, functioning in healthy relationship, offer such things like natural pollination of crops, clean air, and extreme weather mitigation, human mental and physical wellbeing.

Question.  Define radioactive wastes?
Answer. 1. Litter contains all kinds of wastes generated above the ground whereas detritus is composed of the remains of dead plants and animals.
2. Litter contains both biodegradable and non-biodegradable matter whereas detritus contains only biodegradable matter.

             

Long Answer

Question. Explain water pollution and how can its control?
Answer. 1. The series of processes by which compounds of carbon are interconverted in the ecosystem are referred to as the carbon cycle whereas the processes by which the compounds of phosphorous are interconverted in the ecosystem are referred as the phosphorous cycle.
2. Carbon cycle has a rapid process whereas phosphorous cycle has a slow process.
3. Carbon cycle interacts with atmosphere whereas phosphorous cycle does not interact with atmosphere.
4. Carbon cycle is a type of gaseous cycling whereas phosphorous cycle is a type of sedimentary cycle.
5. Carbon cycle releases carbon di oxide to the atmosphere by respiration whereas in phosphorous cycle there is no release of gaseous components to the atmosphere.

Question. State the difference between greenhouse effect and global warming?
Answer. An ecological pyramid is a graphical representation of the relationship between different organisms in an ecosystem. Each of the bars that make up the pyramid represents a different trophic level, and their order, which is based on who eats whom, represents the flow of energy. The three types of ecological pyramids include: pyramid of numbers, pyramid of biomass and pyramid of energy. Energy pyramid, sometimes called trophic pyramid or ecological pyramid, is useful in quantifying the energy transfer from one organism to another along the food chain. Energy decreases as one move through the trophic levels from the bottom to the top of the pyramid. Thus, the energy pyramid is always upward. The ecological pyramids do not take into account the same species belonging to more than one trophic level. It assumes a simple food chain that almost never exists in nature. It does not explain food webs. The bottom of an ecological pyramid is the broadest and is occupied the producers, which form the first trophic level. Producers are at the lowest level. Just as in a food chain, the producers are consumed by the primary consumers, in an ecological pyramid; the next level is occupied by the primary consumers. The next level of the pyramid is occupied by the secondary consumers and the last, by the tertiary consumers. An ecological pyramid not only shows us the feeding patterns of organisms in different ecosystems, but can also give us an insight into how inefficient energy transfer is, and show the influence that a change in numbers at one trophic level can have on the trophic levels above and below it .

Question. Write short not on ozone depletion?
Answer. 1. Food chain refers to a feeding hierarchy in which organisms in an ecosystem are grouped into trophic levels and are shown in a succession to represent a linear flow of food energy and the feeding relationships between them whereas food web refers to a graphical model showing the interconnecting food chains in an ecological community.
2. A food chain is a single unit whereas a food web is a collection of several interconnected food chains.
3. Food chain may consist of 4-6 trophic levels whereas food web consists of numerous trophic levels.
4. A food chain is a single, linear pathway of energy flow whereas food web consists of a number of interconnected pathways through which the energy flows within an ecosystem.
5. The food chain increases the instability of an ecosystem whereas a food web increases the stability of an ecosystem.

Question. Explain about air pollution and how it can be controlled?
Answer. Energy flow, also known as calorific flow, it refers to the flow of energy through a food chain, and is the focus of study in ecological energetics. In an ecosystem ecologists seek to the quantify the relative importance of different component species and feeding relationships. The flow of energy in the ecosystem is unidirectional because the energy lost as heat from the living organisms of a food chain cannot be reused by plants in photosynthesis. During the transfer of energy through successive trophic levels in an ecosystem, there is a loss of energy all along the path. Ecosystems maintain themselves by cycling energy and nutrients obtained from external sources. The low rate of energy transfer between trophic levels makes decomposers generally more important than producers in terms of energy flow. Energy is needed at all levels of the food chain, as are nutrients. However, when the energy passes on to organism after organism from the initial plants, it is also used up and exhausted, and ultimately, nothing remains that can be recycled to form more energy. Sun is the biggest and ultimate source of energy to Earth. Sunlight energy is trapped autotrophs, which use it to produce food for them. The trapped energy is passed to herbivores animals, which eat the producers. They in turn are eaten by carnivores’ animals, which are themselves eaten by tertiary consumers. This pathway is called a food chain.

Question. Write short note on solid wastes?
Answer. Decomposition is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water and mineral salt. The process is a part of the nutrient cycle and is essential for recycling the finite matter that occupies physical space in the biosphere. Decomposition of organic matter in soils is an important process in any ecosystem. Decomposer microorganisms feed on the organic matter and break it down into its simplest components, i.e. any excess nutrients are released and are available for plants to use to grow. During the process of the decomposition, the decomposers provide food for themselves by extracting chemicals from the dead bodies or organic wastes; using these to produce energy. The decomposers will then produce waste of their own. Decomposition helps us solve complex problems and manage large projects. This approach has many advantages. It makes the process a manageable and achievable one large problem is daunting, but a set of smaller, related tasks are much easier to take on. Oxygen is needed for many decomposers to respire, to enable them to grow and multiply. This is why we often seal food in bags or cling film before putting it in the fridge. As the volume of available oxygen increases, the rate of decomposition also increases. Some decomposers can survive without oxygen.

 

 

Very Short Answer Questions

Question. Why is Eichhornia crassipes nicknamed as “Terror of Bengal”? 
Answer. Eichhornia crassipes is an aquatic weed that grows abundantly and very fast in eutrophic water bodies and causes imbalances in water ecosystem. It causes oxygen depletion leading to death of aquatic life (eutrophication).

Question. Name an industry which can cause air pollution, thermal pollution and eutrophication.
Answer. Fertiliser factory.

Question. State the cause of accelerated eutrophication. 
Answer. Pollutants from human activities, effluents from industries, effluents from home, sewage and agricultural wastes (chemical wastes) radically accelerate the aging process.

Question. In spite of being non-polluting, why are there great apprehensions in using nuclear energy for generating electricity? 
Answer. There are great apprehensions in using nuclear energy because of accidental leakages and finding ways for safe disposal of radioactive waste.

Question. How do algal blooms affect the life in water bodies? 
Answer. (i) Algal bloom pollutes water and deteriorates the water quality by depleting the oxygen content. This turns the water toxic and increases its BOD.
(ii) Algal blooms lead to death of aquatic organisms due to oxygen depletion.

Question. How do plants control pollution?
Answer. Plants absorb and assimilate the pollutants like CO2 and provide oxygen by the process of photosynthesis. This reduces the pollution.

Question. What is reforestation? 
Answer. Reforestation is the process of restoring a forest that once existed but was removed at some point of time in the past.

Question State the effect of UV-B on human eye.
Answer. UV-B is absorbed by human eye and at high dose, it causes inflammation of cornea. This is called snow-blindness cataract.

Question. Write the name of the organism that is referred to as the ‘Terror of Bengal’.
Answer. Water hyacinth (Eichhornia crassipes).

Short Answer Questions

Question. Explain the relationship between green house gases and global warming. 
Answer. Greenhouse gases (CO2, CH4, N2O, CFCs) allow the solar radiations to enter but prevent the escape of heat radiations of longer wavelength. The absorbed radiations again come to earth’s surface and heat it up. Increase in the level of these greenhouse gases allow the heat waves to reach earth but prevent their escape and thus the earth becomes warm. There is gradual continuous increase in average temperature of earths’ surface leading to global warming.

Question. Explain the relationship between CFCs and ozone in the stratosphere. 
Answer. UV rays act on CFCs and release chlorine. These chlorine atoms act on ozone to release O2,resulting in ozone layer depletion.

Question. List four benefits to human life by eliminating the use of CFCs. 
Answer. (i) Delay in aging of skin (ii) Prevent damage to skin cells
(iii) Prevent skin cancer (iv) Prevent snow blindness or inflammation of cornea
(v) Prevent cataract (vi) Prevents ozone depletion
(vii) Prevents global warming (viii) Reduces greenhouse effect
(ix) Reduces odd climatic changes or El Nino effect. (Any four)

Question. Chlorofluorocarbons (CFCs) are widely used as refrigerants. Then why is it suggested to reduce its emission as far as possible? Explain. 
OR
Refrigerants are considered to be a necessity in modern living, but are said to be responsible for ozone holes detected in Antarctica. Justify. 
Answer. CFCs find wide use as refrigerants. CFCs discharged in the lower part of atmosphere move upward and reach stratosphere. In stratosphere, UV rays act on them releasing chloride atoms. Chloride atoms degrade ozone, releasing molecular oxygen. Whatever CFCs are added to the stratosphere have permanent and continuing effects on ozone. These have resulted in ozone hole.

Question5. Why are there regular reminders to reduce the use of CFCs in the production of industrial and household appliances? Explain.
Answer. CFCs discharged in the lower part of atmosphere move upward and reach stratosphere. In stratosphere, UV rays act on them releasing Cl atoms, which in turn degrade ozone. Ozone depletion will result in entry of harmful UV-B radiations into the earth’s atmosphere resulting in deleterious effects on all living organisms.

Question. Is it true that if the dissolved oxygen level drops to zero the water will become septic? Given example which could lower the dissolved oxygen content of an aquatic body.
Answer. Yes, the water will become septic if the dissolved oxygen drops to zero. Organic pollution (biodegradable) is an example.

Question. (a) State the cause of depletion of ozone layer.
(b) Specify any two ill-effects that it can cause in the human body.
Answer. (a) UV radiations act upon CFCs (chlorofluorocarbons) releasing Cl atoms in the stratosphere. These Cl atoms combine with O3 and degrade it.
(b) Ill-effects of ozone depletion:
(i) UV-B damages DNA and proteins of living organisms causing mutation.
(ii) It causes skin aging, skin cell damage and skin cancers.
(iii) UV-B is absorbed by human eye and at high dose it causes inflammation of cornea. This is called snow-blindness cataract.

Question. Match the items given in column A and B.
Column A Column 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. (a) Rearrange the following greenhouse gases in increasing order of their relative contribution to the total global warming:
N2O; CFC; CO2; C2H4
(b) What is the effect of global warming on polar ice-caps? Comment on its possible ecological impact. 
Answer. (a) C2H4 < N2O < CFC < CO2
(b) Global warming results in rise of atmospheric temperature. This leads to the increased melting of polar ice caps which will cause the rise in sea level and many coastal areas will be submerged.

Question. Justify the need for signing of Montreal Protocol by the participating nations.
Answer. The Montreal Protocol was signed to control the emission of ozone depleting substances. Excessive use of CFCs and other ozone depleting chemicals has resulted in thinning of ozone layer. Further thinning would allow harmful UV-B radiations to enter Earth’s atmosphere and have deleterious effects on living organisms.

Question. It is a common practice to plant tree and shrubs near the boundary walls of building. What purpose do they serve? 
Answer. The plant growing near the boundary wall act as barrier for sound pollution and act as dust catchers.

Long Answer Questions

Question. Why is the concentration of toxins found to be more in the organisms occupying the highest trophic level in the food chain in a polluted water body? Explain with the help of a suitable example. 
Answer. The concentration of toxic materials like heavy metals and pesticides increase at each trophic level of a food chain and is more in organisms of highest trophic level due to their accumulation at each trophic level. For example, when DDT was used to control mosquitoes in a lake of USA, 800 times more DDT was found in the phytoplanktons than in the water of the lake. Zooplanktons had about 13 times more DDT than phytoplanktons. It was also observed that the fishes population had 9–40 times more DDT than zooplanktons and fish eating birds had 25 times more DDT than fish.

Question. Mention the six harmful effects of noise on human health.
Answer. Six harmful effects of noise on human population are:
(ii) Harmful effects (Psychological and Physiological disorders)
(a) Sleeplessness
(b) Stress
(c) Increased rate of heartbeat and hypertension
(d) Breathing problems
(e) Damage of ear drums impairing hearing ability permanently (by the sound of jet plane/rocket or chronic exposure to sound).
(f) Gastric problems—nausea
(g) Emotional disturbance

Question. Write critical notes on the following:
(a) Eutrophication (b) Biological magnification
(c) Groundwater depletion and ways for its replenishment.
Answer. (a) Eutrophication:
• Marsh plants take root in the shallows and begin to fill in original lake basin.
• Eventually, floating plants develop in the lake, finally converting it into land.
• According to climate, size of the lake and other factors, natural ageing of lake may span up to thousands of years.
• The accelerated aging of lakes due to sewage, agricultural and industrial wastes is called cultural or accelerated eutrophication.
(b) Biological magnification: Unknowingly some harmful chemicals enter our bodies through the food chain. We use several pesticides and other chemicals to protect our crops from diseases and pests. These chemicals are either washed down into the soil or into the water bodies. From the soil, these are absorbed by the plants along with water and minerals, and from the water bodies these are taken up by aquatic plants and animals. This is one of the ways in which they enter the food chain. As these chemicals are not degradable, these get accumulated progressively at each trophic level. As human beings occupy the topmost level in any food chain, the maximum concentration of these chemicals get accumulated in our bodies. This phenomenon is known as biological magnification.
(c) Groundwater depletion and ways for its replenishment: Groundwater is depleting due to various reasons like deforestation, agricultural use, daily use at home, etc. Groundwater level is going down in several states due to its unjudicious use. Replenishment of groundwater can be done by rainwater harvesting and reforestation. Rainwater can be collected on the roof top of the buildings and can be stored underground for later use.

Question. (a) Expand BOD.
(b) At a particular segment of a river near a sugar factory, the BOD is much higher than the normal level. What is it indicative of? What will happen to the living organisms in this
part of the river?
(c) Under what conditions will the BOD be lowered in the river? How will it affect the aquatic life? 
Answer. (a) BOD—Biochemical Oxygen Demand
(b) It indicates the addition of lot of organic matter; microorganisms involved in the biodegradation of organic matter in the water body consume a lot of oxygen and as a result, there is a sharp decline in the dissolved oxygen content, downstream from the point of addition of effluent from the factory. This causes mortality of fish and other aquatic organisms.
(c) When the amount of organic matter decreases and the microbes do not need oxygen for decomposition, the BOD decreases. Thus, aquatic organisms will start flourishing.

Question. What is global warming? List four strategies for reducing global warming.
Answer. Increase in the level of greenhouse gases in the atmosphere causes rise in global mean temperature called global warming.
Four strategies for reducing global warming are: Control of global warming
• Global warming can be controlled by:
(a) Reducing deforestation
(b) Planting trees (afforestation)
(c) Slowing down the growth of human population
(d) Reduction of emission of greenhouse gases into the atmosphere
(e) Cutting down use of fossil fuels
(f) Improving efficiency of energy usage.

Question. Explain the causes of global warming. Why is it a warning to mankind?
Answer. Causes of global warming:
(i) Deforestation
(ii) Rise in the concentration of greenhouse gases (CO2, CH4, CFCs, N2O).
(iii) Burning of fossil fuels
(iv) Rise in industrial wastes and pollutants.
Global warming is a warning to mankind because:
(i) Rise in temperature is leading to increased melting of polar ice-caps as well as of other places like the Himalayan snow caps. This will result in a rise in sea level that can submerge many coastal areas.
(ii) Deleterious changes in the environment results in odd weather and climate changes, e.g.,El Nino effect.

Question. A plastic sack manufacturer in Bengaluru, Ahmed Khan has managed to find an ideal solution to the problem of plastic waste. Explain in five steps the efforts of Ahmed Khan to meet the challenges of solid waste management. 
Answer. (i) Ahmed Khan developed polyblend, which is a fine powder of recycled modified plastic.
(ii) Polyblend is mixed with bitumen and used to lay roads. This work is done in collaboration with R V engineering college and Bangalore city corporation.
(iii) Polyblend enhanced water-repellant property of bitumen and increase the road life.
(iv) Khan offered a price to rag pickers which enhanced their income and improved their livelihood.
(v) Thus solid waste management was achieved by removal and proper disposal of plastic waste from the city.

Question. Mention the information that the health workers derive by measuring BOD of a water body. 
Answer. BOD (biochemical oxygen demand) is the amount of the oxygen required by microorganisms to decompose the organic matter present in one litre of water during a particular time period. BOD test measures degree of impurity of water. As higher the BOD, more will be the pollution. Thus, indirectly BOD is a measure of the organic matter present in the water.

Question. Explain how does the inflow of large amount of nutrients like phosphates and nitrates into the water body drastically affects the aquatic life there. Name the phenomenon responsible.
Answer. Passage of sewage and run off from fertilised fields into water bodies causes nutrient enrichment particularly with nitrogen and phosphorus. Nutrients present in sewage and fertilisers cause dense growth of plants and planktonic algae. The algae use oxygen at night and may deoxygenate the water enough to kill the fish and other animals. The excess growth of planktonic algae that imparts a distinct colour to water bodies is called algal bloom. They are toxic to animals and humans. Algal blooms and floating plants cut off light from submerged plants and the latter die. There is drastic decrease in oxygen replenishment inside water. It causes organic loading of water and death of fish and other animals. The phenomenon responsible for this is eutrophication.

Question. Write the harmful effects of the presence of large amount of nutrients in a water body.
Answer. Excessive nutrients in a fresh water body cause fish mortality because of the following two reasons:
(i) Excessive nutrients cause dense growth of floating plants and planktonic algae (algal bloom) which cut off light from submerged plants and the latter die.
(ii) Increase in nutrients reduces dissolved oxygen and increases biochemical oxygen demand (BOD) of water body, leaving nothing for respiration of fish and therefore, they get killed. Algal blooms formed by some blue green algae are toxic to aquatic animals and humans.
Influx of nutrients would increase the BOD level of water body, because more oxygen would be required by microorganisms to metabolise organic wastes.

Question. How is it that the mortality rate of fish increases downstream from the point of sewage discharge into a water body? Explain 
Answer. Passage of sewage and run off from fertilised fields into water bodies causes nutrient enrichment particularly with nitrogen and phosphorus. Nutrients present in sewage and fertilisers cause dense growth of plants and planktonic algae. The algae use oxygen at night and may deoxygenate the water enough to kill the fish and other animals. The excess growth of planktonic algae that imparts a distinct colour to water bodies is called algal bloom. They are toxic to animals and humans. Algal blooms and floating plants cut off light from submerged plants and the latter die. There is drastic decrease in oxygen replenishment inside water. It causes organic loading of water and death of fish and other animals. The phenomenon responsible for this is eutrophication.

Question. Why is it dificult to get rid of ‘water-hyacinth’ from a water body? Name one abiotic component and one biotic component of the ecosystem that get affected by its spread in the water body.
Answer. It is difficult to get rid of ‘water-hyacinth’ from water body because eutrophic water bodies support excessive growth of water hyacinth, a floating plant, also called ‘Terror of Bengal’. Abiotic and biotic components affected by spread of water hyacinth are light and submerged plants respectively, as its growth cuts off light for submerged plants.

Question. A crane had DDT level as 5 ppm in its body.What would happen to the population of such bird? Explain giving reasons.
Answer. The population of birds with high DDT level will decrease. High concentration of DDT disturbs calcium metabolism in birds which causes thinning of egg shell and their premature breaking. This will eventually lead to decline in bird population.

Question. Explain the cause of algal bloom in a water body. How does it affect an ecosystem?
Answer. Algal bloom occurs due to presence of excess nutrients especially phosphates and nitrates in the water body. The growth of algal blooms will result in depletion of oxygen level in the water body, killing fishes and other aquatic animals and plants.

Question. DDT content in the water of a lake that supplies drinking water to the nearby villages, is found to be 0.003 ppm. The kingfishers of that area reported to have 2 ppm of DDT. Why has the concentration increased in these birds? What harm will this cause to the bird population? Name the phenomenon. 
Answer. The concentration of DDT in birds has increased because it is a toxic, non-biodegradable substance and it cannot be excreted or metabolised and is passed on to the next higher trophic level. During the transfer along food chain, its concentration in tissues increases. This is why its highest concentration is found in fish eating bird kingfisher which occupies the highest tropic level. This will result in decline in bird population. The phenomenon is called biological magnification.

Question. Drinking water problem in our urban areas is caused mainly because we fail to protect our water bodies. Explain how accelerated eutrophication chokes our water bodies to death.
Answer. Accelerated eutrophication is nutrient enrich- ment of water bodies due to human activities like passage of sewage, industrial effiuents and run off from fertilised fields rich in nitrates and phosphates. Nutrients present in sewage, agricultural wastes and fertilisers cause dense growth of plants and planktonic algae. They support a good number of animals. The excess growth of planktonic algae that causes colouration of water is called algal bloom. They are toxic to animals and humans. In some cases eutrophic water bodies support excessive growth of floating plants e.g., water hyacinth (Eichhornia crassipes). Algal blooms and floating plants cut off light for submerged plants. The latter die. There is drastic decrease in oxygen replenishment inside water. It causes organic loading of water. Decreased oxygen level also kills aquatic animals, further adding to organic loading. Decomposition is replaced by putrefaction which is anaerobic. It produces secondary pollutants that kill the bloom forming plants as well and thus choking water bodies to death.

Question. How does a water body age naturally? Explain.State how this phenomenon of ageing of a water body gets accelerated.
Answer. Eutrophication is the natural ageing of a water body by nutrient enrichment of its water, for example in a young lake the water is cold and clear, supporting little life. With time, streams draining into the lake introduce nutrients such as nitrogen and phosphorus, which encourage the growth of aquatic organisms. As the lake’s fertility increases, plant and animal life burgeons and organic remains begin to be deposited at the lake bottom. Over the centuries, as silt and organic debris pile up, the lake grows shallower and warmer, with warm-water organisms supplanting those that thrive in a cold environment. Marsh plants take root in the shallows and begin to fill in the original lake basin. Eventually, the lake gives way to large masses of floating plants (bog), finally converting into land. Depending on climate, size of the lake and other factors, the natural ageing of a lake may span thousands of years. However, pollutants from human activities like effiuents from the industries and homes can radically accelerate the ageing process. This phenomenon has been called cultural or accelerated eutrophication. During the past century, lakes in many parts of the earth have been severely eutrophied by sewage and agricultural and industrial wastes. The prime contaminants are nitrates and phosphates, which act as plant nutrients. They overstimulate the growth of algae, causing unsightly scum and unpleasant odours, and robbing the water of dissolved oxygen vital to other aquatic life.

Question. How does an algal bloom cause eutrophication of a water body? Name the weed that can grow in such a eutrophic lake. 
Answer. Excessive nutrients cause dense growth of floating plants and planktonic algae (algal bloom) which cut off light for submerged plants. The latter die thereby causing organic loading which increases biochemical oxygen demand (BOD) of water and in turn reduces dissolved oxygen content of water. Decreased oxygen level kills many aquatic animals including fish. Algal bloom formed by some blue green algae are toxic to aquatic animals like fish etc. Eichhornia crassipes is the weed which grows in eutrophic lake.

Question. Explain biomagnification of DDT in an aquatic food chain. How does it affect the bird population? 
Answer. Biomagnification is continuous increase in concentration of insecticide and pesticide in successive trophic levels in a food chain. Regular DDT spray for a few years, resulted in drastic decline in the population of fish eating birds.
There was 1000 times increase in concentration of DDT in phytoplankton as compared to water, in zooplankton as compared to phytoplankton, in different fish as compared to zooplankton and more DDT in fish eating birds as compared to fish. Higher amounts of pesticide disturb calcium metabolism of birds resulting in thinning of egg shells and their premature breaking that kills the embryos.

Question. A factory drains its waste water into the nearby lake. It has caused algal bloom.
(a) How was the algal bloom caused?
(b) What would be the consequences?
(c) Name the phenomenon that caused it.
Answer. (a) As the factory drains waste water into the lake, nutrient level (nitrogen and phosphorous) of the lake increases. Presence of large amount of nutrients in lake causes excessive growth of planktonic algae, called algal bloom.
(b) Algal bloom cut off light for submerged plants, thus leading to their death. This causes decrease in oxygen level in water which leads to organic loading of water. Decreased oxygen level also kills aquatic animals, further adding to organic loading.
(c) Eutrophication

Question. What is biological magnification? Explain how DDT as a water pollutant undergoes biological magnification?
Answer. DDT keeps on moving from water to different living components of the ecosystem and DDT concentration continuously increases in successive trophic levels in a food chain. This phenomenon is known as biological magnification or biological amplification. 

Question. Name any two sources of e-wastes and write two different ways for the disposal.
Answer. E-wastes or electronic wastes are generated from electronic appliances. It includes broken mobile phones, televisions, irreparable computers, etc. Sources of e-wastes come from a vast area of our daily life as each and every field is dependent on electronic appliances.
Two ways of disposal of e-wastes include – (i) incineration
(ii) landfilling.

Question. How does Ahmed Khan, plastic sacks manufacturer from Bangalore, solve the ever increasing problem of accumulating plastic waste? 
Answer. Ahmed Khan of Bangalore came up with a remedy for efficient use of plastic waste. He developed polyblend, a fine powder of recycled modified plastic. This mixture is mixed with bitumen that is used to lay roads. In collaboration with R.V. College of Engineering and the Bangalore City Corporation, Ahmed Khan proved that blends of polyblend and bitumen, when used to lay roads, enhanced the bitumen’s water repellant properties, and helped to increase road life by a factor of three. The raw material for creating polyblend is any plastic film waste. Using Khan’s technique, by the year 2002, more than 40 kms of road in Bangalore has already been laid. At this rate, Khan will soon be running short of plastic waste in Bangalore, to produce polyblend.

Question. What is polyblend? Why did the plastic manufactures think of producing it? Write its usefulness. 
Answer. Ahmed Khan of Bangalore came up with a remedy for efficient use of plastic waste. He developed polyblend, a fine powder of recycled modified plastic. This mixture is mixed with bitumen that is used to lay roads. In collaboration with R.V. College of Engineering and the Bangalore City Corporation, Ahmed Khan proved that blends of polyblend and bitumen, when used to lay roads, enhanced the bitumen’s water repellant properties, and helped to increase road life by a factor of three. The raw material for creating polyblend is any plastic film waste. Using Khan’s technique, by the year 2002, more than 40 kms of road in Bangalore has already been laid. At this rate, Khan will soon be running short of plastic waste in Bangalore, to produce polyblend.

Question. Mention how e-waste is produced and disposed off. Write the solution for its treatment.
Answer. E-wastes are irreparable computers, mobiles and other electronic goods. Electronic wastes are generally sent to developing countries like India, China and Pakistan for extraction of metals through recycling. E-waste should be disposed off in landfills or should be incinerated. Recycling is the only solution for the treatment of e-waste, provided it is carried out in an environment-friendly manner.

Question. Why should the spraying of DDT as an insecticide on vegetable crops be banned?
Explain. 
Answer. Spraying of DDT on vegetable crops should be banned because DDT is heavy metal and persistent pesticides passes into food chain and increases in amount per unit weight of organisms with the rise in trophic level due to their accumulation in fat. With agricultural run off, it enters into water body and harms aquatic life. Regular DDT spray for a few years, resulted in drastic decline in the population of fish eating birds. There was 1000 times increase in concentration of DDT in phytoplankton as compared to water, in zooplankton as compared to phytoplankton, in different fish as compared to zooplankton and more DDT in fish eating birds as compared to fish. Higher amounts of pesticide disturb calcium metabolism of birds resulting in thinning of egg shells and their premature breaking that kills the embryos. Other disorders due to DDT can be cerebral haemorrhage, softening of brain, liver cirrhosis, hypertension and malfunctioning of sex hormones.

Question. Why is there a decline in the population of the fish-eating birds when the water body is amidst agricultural fields? 
Answer. Spraying of DDT on vegetable crops should be banned because DDT is heavy metal and persistent pesticides passes into food chain and increases in amount per unit weight of organisms with the rise in trophic level due to their accumulation in fat. With agricultural run off, it enters into water body and harms aquatic life. Regular DDT spray for a few years, resulted in drastic decline in the population of fish eating birds. There was 1000 times increase in concentration of DDT in phytoplankton as compared to water, in zooplankton as compared to phytoplankton, in different fish as compared to zooplankton and more DDT in fish eating birds as compared to fish. Higher amounts of pesticide disturb calcium metabolism of birds resulting in thinning of egg shells and their premature breaking that kills the embryos. Other disorders due to DDT can be cerebral haemorrhage, softening of brain, liver cirrhosis, hypertension and malfunctioning of sex hormones.

Question. In spite of being non-polluting, why are there great apprehension in using nuclear energy for generating electricity?
Answer. Nuclear energy is not used for generating electricity because various radioactive pollutants damage white blood corpuscles, bone marrow, spleen, lymph nodes and may cause tumours, skin cancer, sterility etc. Even dairy animals may get highly contaminated by radioactive pollutants.

Question. Name the greenhouse gases that contribute to global warming. 
Answer. Carbon dioxide, methane, chlorofluorocarbons and nitrous oxide.

Question. Name the two gases contributing maximum to the greenhouse effect. 
Answer. CO₂ - 60% warming effect
CH₄ - 20% warming effect

Question. How is snow-blindness caused in humans?
Answer. Snow-blindness is inflammation of cornea due to absorption of UV-B radiations. It occurs due to depletion of ozone layer which increases amount of UV-B radiations reaching the earth.

Question. Chlorofluorocarbons (CFCs) are widely used as refrigerants. Then why it is suggested to reduce its emission as far as possible? Explain.
Answer. Chlorofluorocarbons (CFCs) are greenhouse gases which contribute to 14% of global warming. CFCs cause ozone hole. In the stratosphere, chlorofluorocarbon is degraded in the presence of UV- radiation, releasing Cl atoms. Cl acts as catalyst in the degradation of ozone releasing molecular oxygen.

Question. How does global warming pose a threat to the coastal areas of the Earth? Explain. 
Answer. The global warming will raise the sea level due to melting of glaciers. A rise of even half a meter in sea level will be disastrous as thousands of islands and several important cities of the world and coastal areas will be submerged.

Question. Explain the causes of global warming. Why is it a warning to mankind?
Answer. The main causes of global warming are : (i) Deforestation
(ii) Increase in the use of CFCs. (iii) Burning of fossil fuels
(iv) Increase in the use of nitrogen fertilisers. Global warming is a warning to mankind because:
(i) Rise in temperature is leading to increased melting of polar ice-caps as well as of other places like the Himalayan snow caps. This will result in a rise in sea level that can submerge many coastal areas.
(ii) Changes in the environment results in odd weather and climate changes, e.g., El Nino effect.
(iii) Many tree species and others which are sensitive to temperature will die out resulting in conversion of forests into scrub vegetation.
(iv) Small temperature rise may increase crop productivity in temperate areas but higher temperature rise will be detrimental.

Question. It has been recorded that the temperature of the earth’s atmosphere has increased by 0.6 °C.
(a) What has caused this increase?
(b) Explain its consequences.
Answer. (a) The temperature has increased due to increase in concentration of greenhouse gases (carbon dioxide, methane, chlorofluorocarbons, nitrous oxide) in the atmosphere.
(b) The main causes of global warming are : (i) Deforestation
(ii) Increase in the use of CFCs. (iii) Burning of fossil fuels
(iv) Increase in the use of nitrogen fertilisers. Global warming is a warning to mankind because:
(i) Rise in temperature is leading to increased
melting of polar ice-caps as well as of other places like the Himalayan snow caps. This will result in a rise in sea level that can submerge many coastal areas.
(ii) Changes in the environment results in odd weather and climate changes, e.g., El Nino effect.
(iii) Many tree species and others which are sensitive to temperature will die out resulting in conversion of forests into scrub vegetation.
(iv) Small temperature rise may increase crop productivity in temperate areas but higher temperature rise will be detrimental.

Question. Describe greenhouse effect and list two greenhouse gases.
Answer. Greenhouse effect is the progressive, gradual warming of the earth’s atmosphere caused by the insulating effect of carbon dioxide and other green- house gases that have proportionately increased in the atmosphere. The greenhouse effect disturbs the way the earth’s climate maintains the balance between incoming and outgoing energy by allowing short-wave radiations from the sun to penetrate through to warm the earth, but preventing the resulting long-wave infrared radiations from escaping back into the atmosphere. There is concern that increasing concentration of greenhouse gases, including carbon dioxide, methane, and man made chlorofluorocarbons, may enhance the greenhouse effect and cause global warming.

Question. (a) What is El Nino effect? Explain how it accounts for biodiversity loss.
(b) Explain any three measures that you as an individual would take, to reduce environmental pollution.
Answer. (a) EL Nino effect can be described as the warming of sea surface temperatures in the Pacific Ocean along the equator at irregular intervals. It influences weather patterns across the globe. It results in increasing the atmospheric temperature which results in global warming. This leads to rise in sea level and thus causing the biodiversity loss due to floods and also drought at some places due to very high temperature.
(b) Three measures to reduce environmental pollution are:
(i) By using unleaded petrol and diesel with reduced sulphur content.
(ii) By planting more trees.
(iii) By using television and loudspeakers at low volume.

Question. State the purpose of signing the Montreal Protocol.
Answer. Industrialised countries signed Montreal Protocol to limit the production of chlorofluorocarbons to half to reduce depletion of ozone layer.

Question. Where is good ozone present? Why is it so called?
Answer. Good ozone is present in stratosphere. It is so called because it functions as shield for living organisms against strong harmful UV radiations from the sun.

Question. Mention two harmful effects of UV-B exposure on human eye.
Answer. UV-B exposure on human eye causes inflammation of cornea and cataract.

Question. Write the unit used for measuring ozone thickness.
Answer. Dobson Unit (D.U).

Question. Mention the causes of thinning of ozone layer. 
Answer. Thinning of ozone layer is due to various ozone depleting substances such as:
(i) CFCs used in air conditioners and refrigerators. (ii) Nitrogen oxides, sulphur dioxide, carbon tetrachloride etc., released by jets flying in stratosphere depletes ozone.

Question. (a) State the cause of depletion of ozone layer.
(b) Specify any two ill-effects that it can cause in the human body.
Answer. (a) Depletion of ozone layer is due to reaction of ozone depleting substances (ODS) with ozone present in the stratosphere. The major ODS are chlorofluorocarbons (14% of total depletion), nitrogen oxides (3.5% depletion), sulphur dioxide, halon, carbon tetrachloride, methyl chloroform, chlorine, etc. Many of these are being released by jets flying in the stratosphere and rockets being fired into space. Maximum ozone depleting potential or ODP is of chlorofluorocarbon (used in ACs and refrigerators) due to release of active chlorine (Cl, ClO) by it. Active chlorine gets perched over atmospheric ice crystals and remains functional for a long time. A single chlorine atom converts 1 lakh molecules of ozone into oxygen.
(b)  It severely affects human health. Cornea absorbs UV-B radiations and becomes inflamed called “snow blindness” cataract.
UV-B radiations can also damage skin cells, cause ageing of skin and skin cancer, increased incidence of herpes and deficient functioning of immune system.

Question. Explain greenhouse effect and depletion of ozone layer with reference to global warming.
Answer. Increase in concentration of greenhouse gases enhance greenhouse effect and leads to global warming which has resulted in rise of atmospheric temperature and due to this there will be melting of polar ice caps and mountain snow caps (e.g., Himalayan snow caps) which will raise sea level. The other effects of global warming include warming of troposphere accompanied by cooling of the upper Ozone depletion contributes in the thinning of ozone layer which increases the UV radiations reaching the earth. UV radiations inhibit photosynthesis and decreased photosynthetic activity will increase CO2 concentration of the atmosphere resulting in global warming.

Question. How is ozone formed in the stratosphere? Why is it called ‘good ozone’? CFCs contribute to ozone-hole formation. Explain. 
Answer. Ozone in stratosphere is formed by action of UV
radiations on molecular oxygen.
         UV
O3     ⇔   O2 +[O]
It is called good ozone as ozonosphere acts as shield against strong UV radiations. CFCs discharged in the lower part of the atmosphere move upward and reach stratosphere. In the stratosphere, UV radiations act on CFCs and release Cl atoms. Cl atoms act as catalyst and degrade the ozone into molecular oxygen. Since Cl atoms are not consumed, whatever CFCs are added to the stratosphere, they have a permanent and continuing effect on ozone levels which may result in ozone hole formation.

Question. What is the ozone shield and why is it important? Name the gases that cause stratospheric ozone depletion. 
Answer. Ozone shield is the region of stratosphere, where ozone is present in high concentration. It is called ozone shield because it protects the living beings from harmful effects of ultraviolet radiations. Damage to ozone shield results in impairment of photosynthetic machinery in plants which increase CO2 concentration and results in global warming and skin cancer, cataract in humans.
The two gases that can cause damage to ozone shield are chlorofluorocarbons (CFCs) and methane (CH₄).

Question. (a) Why is the ozone layer required in the stratosphere? How does it get degraded?
Explain.
(b) Why is the ozone depletion a threat to mankind?
Answer. Ozone depletion is a threat to mankind as it leads to thinning of ozone layer which results in the harmful UV radiations reaching the earth. The various effects of ozone depletion are :
– UV radiations inhibit photosynthesis which results in high CO₂ concentration of the atmosphere resulting in global warming. Due to this there will be rise in temperature resulting in melting of glaciers which will raise sea level due to thermal expansion of sea water leading to floods etc.
– Cornea absorbs UV-B radiations and becomes inflamed called “snow blindness” cataract. It leads to diminishing of eye sight, photoburning and later permanent damage to cornea that results in actual cataract.
– UV-B radiations damage skin cells, cause ageing of skin and skin cancer.
– There is increased incidence of herpes and deficient functioning of immune system.

Question. How have human activities caused desertification?
Explain.
Answer. Loss of soil productivity by erosion of top soil results in the desert formation. Deserts are spreading in all continents, destroying the fertile land. Various human activities that cause desertification are:
(i) Human establishment : Forest areas have been cleared for building more residential complexes and industrial townships for ever increasing human population.
(ii) Overgrazing : The livestock graze in forest transpiring seedlings and cause soil compaction. This reduces water storing capacity and increases run off. (iii) Requirement of wood for timber and paper industry resulted in clearing of forest land which leads to soil erosion resulting in the desert formation

Question. How does Jhum cultivation promote deforestation? 
Answer. In Jhum cultivation, or slash and burn agricul- ture, the farmers cut down the trees of the forest and burn the plant remains. Ash is mixed with soil and crops are sown in the cleared land. The land is used for two-three years without manuring, which results in nutrient depletion, reduced moisture retention and increased soil erosion resulting in deforestation.

Question. What is joint forest management? How can it help in conservation of forests? 
Answer. Realising the importance of participation by local communities, the Government of India in 1980s has introduced concept of Joint Forest Management (JFM) so as to work in close association with the local communities for protecting and managing forests. For such cooperation, the communities in return get benefits of various forest products (e.g., fruits, gum, rubber, medicine etc.). In this way, forests can be conserved in a sustainable manner.

Question. Write what was the percentage of forest cover of India at the beginning and at the end of the twentieth century. How different is it from the one recommended by the National Forest Policy of our country?
Answer. In India, 30% of the land was under forest cover at the beginning of the twentieth century. However, it has been reduced to 19.4% by the year
1999. National Forest Policy (1988) recommends an optimum 33% forest area for the plains and at least
67% for the hills. However, at present, it is much less than the recommended optimum figures.

Question. Enumerate the consequences of deforestation.
Answer. The consequences of deforestation are as follows: (i) Change in climate – Deforestation results in reduced rainfall, increased drought, hotter summers
and colder winters.
(ii) Soil erosion – Soil is exposed to insolation, dries up and gets eroded by wind and water. It is estimated that 6000 million tons of top soil is lost annually in India due to water erosion in the absence of forest cover.
(iii) Drought – Deforestation negatively affects water cycle causing reduced precipitation. It increases the occurrence of drought.
(iv) Loss of biodiversity and germplasm.
(v) Global warming – Deforestation increases atmospheric CO2 content by releasing carbon stored in organic matter and reduced primary productivity.

Question. What is agroforestry? How do shifting cultivation and taungya system of cultivation contribute to it? What is the difference between these two systems?
Answer. Agroforestry is plantation of multipurpose trees/shrubs/horticulture plants/grasses alongwith crops over agricultural land for stabilising soil, meeting the needs of fodder, fruit and timber of the community. Woody plants cover the land partially as they are grown in combination of herbaceous crops either at the same time or in time sequences. It adds to the income of farmers, stabilises soil and acts as wind break. In taungya system, agricultural crops are grown in between rows of planted trees like Sal and Teak. Jhum or shifting cultivation is a traditional system of agroforestry which allows regrowth of forests after clearing and cultivation in an area for a few years. The major difference between these two systems can be that in jhum cultivation system a part of forest is cleared for further cultivation while in taungya system, agricultural crops can be planted in between the rows of trees e.g., teak, sal, etc. without cutting the trees.

Please click on below link to download CBSE Class 12 Biology Environmental Issues Worksheet Set A