CBSE Class 12 Biology Environmental Issues Assignment Set C

Very Short Answer Type Questions

Question. Mention the effect of global warming on the geographical distribution of stenothermals like amphibians. 
Answer: Due to global warming, stenothermals would either migrate or die due to change in the temperature.

Question. BOD of two samples of water A and B were 120 mg/L and 400 mg/L, respectively. Which sample is more polluted?
Answer: Sample B is more polluted than Sample A.

Question. Domestic sewage contains certain substances or salts which are difficult to remove. Name any four of them.
Answer: Nitrate, phosphate, metal ions and organic compound.

Question. Which of the following is not a greenhouse gas? CO₂, CH₄, O₂, CFCs.
Answer: O₂.

Question. Why is the use of unleaded petrol recommended for motor vehicles equipped with catalytic converters? 
                                                    OR
Why is it desirable to use unleaded petrol in vehicles fitted with catalytic converters?
Answer: Lead in petrol inactivates the catalysts (platinum, palladium and rhodium) which convert harmful pollutants (CO, unburnt hydrocarbons, nitric oxide) to lesser harmful pollutants (CO₂, H₂O, N₂).

Question. Why does ozone hole form in spring and not in winter? 
Answer: Sunlight is necessary for ozone degradation and it is available only during spring.

Question. Eutrophication is the natural aging of a lake; mention any other feature which defines this term. 
Answer: —Depletion of dissolved oxygen in water
—Nutrient enrichment (Any one)

Question. Mention the effect of UV rays on DNA and proteins in living organisms. 
Answer: The high energy of UV rays breaks the chemical bonds within DNA and protein molecules.

Question. Excessive nutrients in a fresh water body cause fish mortality. Give two reasons.
Answer: Excessive nutrients result in excessive algal growth or eutrophication in the water body. Thus, the water quality becomes poor as the oxygen level decreases.

Short Answer Type Questions

Question. (a) Name the green house gases that cause global warming.
(b) Which of them has caused ozone hole and how?
Answer: (a) CO₂, CH₄, N₂O, chlorofluorocarbons or CFCs
(b) Ozone degradation has increased due to chlorofluorocarbons (CFCs). CFCs are refrigerants which react with UV in stratosphere to release chloride atoms. Chloride atoms act as catalyst to degrade ozone and release molecular oxygen. CFCs have permanent and continued effect as chloride atoms are not consumed.

Question.  What measures, as an individual, you would take to reduce environmental pollution?
Answer: I will take following measures to reduce environmental pollution:
(i) Use of renewable energy resources.
(ii) Use of television and other instruments, at low pitch.
(iii) Minimum use of fossil fuel.
(iv) No tobacco smoking.
(v) Planting more trees.

Question. Ornithologists observed decline in the bird population in an area near a lake after the setting of an industrial unit in the same area. Explain the cause responsible for the decline observed.
Answer: Harmful wastes from the industrial unit must have entered the trophic levels of food chain causing biomagnification. It must have accumulated in the birds as it can neither be metabolised nor be excreted. High concentration of harmful chemical may disturb calcium metabolism in birds causing thinning of the egg shells and their premature breaking, ultimately causing decline in bird populations.

Question. Lower BOD of a water body helps reappearance of clean-water organisms. Explain.
Answer: Lowering of Biological Oxygen Demand (BOD) results in decreased biodegradable material in the water body. This results in reduced microbial decomposition. When there is no decomposition,oxygen utilisation is reduced and there is more Dissolved Oxygen (DO) available. Thus, clean water-organisms reappear.

Question. Suppose sewage water is disposed off in the river. What will be its effect on BOD and dissolved oxygen?
                                                    OR
Sewage discharge into clean water body leads to increased fish mortality. Explain.
Answer: Discharge of sewage water into a river will increase nutrients and thereby promotes algal growth. This will result in rise of BOD (Biochemical Oxygen Demand) as decomposers will consume more dissolved oxygen in river. If sewage quantity is large, the whole of the dissolved oxygen will be consumed leaving nothing for respiration of fishes and other aquatic organisms. This increases the mortality rate of aquatic creatures.

Question. Study the graph given below. Explain how is oxygen concentration affected in the river when sewage is discharged into it.

Answer: When sewage is discharged into the river, the oxygen concentration declines sharply because a large amount of oxygen is consumed by aerobic microorganisms in river to decompose the organic matter in river. When the amount of organic matter reduces, the amount of dissolved oxygen again increases.

Question. “Determination of Biological Oxygen Demand (BOD) can help in suggesting the quality of a water body.” Explain. 
Answer: High BOD of a water body indicates growth of more number of micro-organisms in water. This results in bad quality of water. Bad water quality will lead to death of aquatic creatures and hence more polluting potential. Whereas lower BOD of water body indicates less number of microorganisms in water. There is good quality of water in which aquatic life flourishes and there is less polluting potential.
The amount of oxygen required for microbial breakdown of biodegradable organic matter is called BOD.

Question. Explain accelerated eutrophication. Mention any two consequences of this phenomenon.
Answer: Accelerated eutrophication is nutrient enrichment of water bodies due to human activities like passage of sewage.
Consequences are:
(i) Large amount of nutrients in water causes excessive growth of planktonic algae (called algal bloom) which impart characteristic colour to water bodies.
(ii) Depletion of oxygen content of water leading to the death of the aquatic organisms.

Question. A crane had DDT level at 5 ppm in its body. What would happen to the population of such birds? Explain giving reasons.
                                                    ​​​​​​​                          OR
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 were 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: DDT being a toxic substance gets accumulated in the organism and passes on to the next higher trophic level because it cannot be metabolised or excreted. Thus, concentration of DDT has increased in the birds in the given case. The high concentration of DDT disturbs the calcium metabolism in birds, causes thinning of eggshells, their premature breaking and eventually causes a decline in the bird population. The phenomenon is called biomagnification.

Question. Explain the cause of algal bloom in a water body. How does it affect an ecosystem?
                          ​​​​​​​                          OR
How does algal bloom destroy the quality of a fresh water body? Explain.
Answer: Domestic sewage and industrial effluents contains nutrients like nitrogen and phosphorus which favour the excessive growth of planktonic (free-floating) algae.
Its harmful effects cause:
(i) sharp decline in dissolved oxygen content in the water.
(ii) deterioration of water quality and causes mortality of aquatic life forms.
(iii) distinct odour from the water bodies.

Question. How have human activities caused desertification? Explain. 
Answer: Human activities like over-cultivation, unrestricted grazing, deforestation and poor irrigation practices result in arid patches of land. The fertile top soil that may take centuries to develop is eroded due to these activities. When large barren patches extend and meet over time, a desert is created. Increased urbanisation is also one of the causes of desertification.

Question. Explain giving reasons the cause of appearance of peaks ‘a’ and ‘b’ in the graph shown below. (Img 570)
Answer: ‘a’–High BOD due to sewage discharge.
‘b’–Increase in dissolved oxygen due to sewage decomposition.
Micro-organisms involved in biodegradation of organic matter consume a lot of oxygen, therefore,there is a sharp decline in dissolved oxygen. When the sewage is completely degraded, oxygen concentration again increases.

Long Answer Type Questions

Question. With the help of a flow chart, show the phenomenon of biomagnification of DDT in an aquatic food chain.
Answer:

Question.  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: Wastes are used up, unwanted and discarded materials which have to be disposed off.
(i) Wastes generated at home include kitchen waste, food left-overs, old and torn clothes, paper,polythene bags, plastic bags, carry bags, broken glass, carton boxes, tissue papers, etc.
(ii) Wastes generated at school include pens, papers, chalk, plastic envelopes, polythene, food wrappers, waste food, etc.
(iii) Wastes generated during trips include food wrappers, waste papers, polythene, disposable plates, spoons, cups, discarded food, etc.
Yes, we can reduce most of the wastes by its judicious use. Biodegradable wastes include all articles of organic origin, i.e., food wastes, waste paper, wood, leather articles and disposable articles which will be broken down by micro-organisms. Non-biodegradable wastes can be recycled and often taken away by rag-pickers. It is difficult to reduce non-biodegradable wastes.

Question. With the help of a flow-chart exhibit the events of eutrophication. 
Answer:

Question. Eutrophication is the natural aging of a lake. Explain.
Answer: Eutrophication is the natural aging of a lake by biological enrichment of its water. In a young lake, the water being cold and clear, does not support much life. But 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 begins to develop and organic remains begin to be deposited on the lake’s bottom. Over the centuries, as silt and organic debris pile up, the lake grows shallower and warmer. Now, the warm water organisms replace those that live in a cold environment. Marsh plants take root in the shallows and begin to fill in the original lake basin. Eventually, the lake develops large masses of floating plants (bog), finally converting into land.

Question. What is eutrophication? How does a lake undergo accelerated eutrophication? 
Answer: Eutrophication
• It is defined as the natural aging of a lake by biological enrichment of its water.
• Water in a young lake is cold and clear to support life.
• With time, it is enriched with nutrients like nitrogen and phosphorus by streams draining into it.
• This encourages growth of aquatic life—plant and animal life.
• Organic remains deposit at the bottom of the lake and with time makes the water warmer and shallower. 
• 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.

Question. Discuss briefly the following:
(a) Greenhouse gases (b) Catalytic converter (c) Ultraviolet B
Answer: (a) Greenhouse gases: The gases which allow solar radiations to pass through them but retain and partially reflect back radiation of longer wavelength are called greenhouse gases, e.g., CO₂, NO₂, CH₄, etc. These gases are required to keep earth warm and hospitable. They prevent a substantial part of long wave radiations emitted by earth to escape into space.
Rather greenhouse gases radiate a part of this energy back to the earth.
(b) Catalytic converter: Refer to Basic Concepts Point 2 (iv)(c).
(c) Ultraviolet B: These are UV radiations with wavelength ranging between 280–320 nm. They are harmful radiations that penetrate through the ozone hole to reach the earth. Ozone shield is thinning over the equator. UV-B radiation causes cataract and skin cancer in human beings.
High levels of UV-B radiation damages DNA of an individual.