Biology Notes for Class 12

Chapter 16: Environment

Chapter Summary

           Major issues relating to environmental pollution and depletion of valuable natural resources vary in dimension from local, regional to global levels. Air pollution primarily results from burning of fossil fuel, e.g., coal and petroleum, in industries and in automobiles. They are harmful to humans, animals and plants, and therefore must be removed to keep our air clean. Domestic sewage, the most common source of pollution of water bodies, reduces dissolved oxygen but increases biochemical oxygen demand of receiving water. Domestic sewage is rich in nutrients, especially, nitrogen and phosphorus, which cause eutrophication and nuisance creating algal blooms. Industrial waste waters are often rich in toxic chemicals, especially heavy metals and organic compounds. Industrial waste waters harm living organisms. Municipal solid wastes also create problems and must be disposed off in landfills. Disposal of hazardous wastes like defunct ships, radioactive wastes and e-wastes requires additional efforts. Soil pollution primarily results from agricultural chemicals (e.g., pesticides) and leachates from solid wastes deposited over it.

              Two major environmental issues of global nature are increasing greenhouse effect, which is warming Earth, and depletion of ozone in the stratosphere. Enhanced greenhouse effect is mainly due to increased emission of carbon dioxide, methane, nitrous oxide and CFCs., and also due to deforestation. It may drastically change rainfall pattern, global temperature, besides deleteriously affecting living organisms. Ozone in the stratosphere, which protects us from harmful effects of ultraviolet radiation, is depleting fast due to emission of CFCs thus increasing the risks of skin cancer, mutation and other disorders.


Ø  Pollution: is any undesirable changed in physical chemical or biological characteristics of air, land, water or soil.

Ø  Pollutant: Any solid, liquid or gas released into the environment in such a huge quantities that make our environment unhealthy is called pollutant.

Ø  Environment (protection) Act, 1986 to protect and improve the quality of our environment (air, water and soil).


Effect of air pollution:

Ø  Cause injury to all living organisms.

Ø  Reduce growth and yield of crops.

Ø  Cause premature death of plants.

Ø  Affects the respiratory system of human being.

Ø  Particulate size 2.5 micrometers or less are responsible for breathing and respiratory symptoms like irritation, inflammations and damage to the lungs and premature death.

Pollution caused by thermal power plant:

Ø  Sources of particulate matter: thermal power plant, smelters

Ø  These plants release particulate matter and gaseous air pollutant.

Ø  A harmless gas released by these plants is Nitrogen and Oxygen.

Prevention of air pollution: ways to remove particulate matter:

* Electrostatic precipitator

  • Widely used to remove particulate matter in the exhaust from a thermal power plant.
  • Electrode wires that are maintained at several thousand volts, which produce a corona that release electrons.
  • Electron binds with particulate matter giving them a net negative charge.
  • Positively charged collecting plates attract the charged dust particle.

* Scrubber:

  • Removes gases like sulphur dioxide.
  • The exhaust is passed through a spray of water or lime.

Methods to reduce vehicular pollution:

Ø  Use of lead free petrol or diesel can reduce vehicular pollution.

Ø  Catalytic converter:

  • Having expensive metals namely platinum, palladium and rhodium as the catalyst.
  • These metals reduce emission of poisonous gases.
  • The unburnt hydrocarbons are converted into CO2 and H2O.
  • Carbon         monoxide and nitric         oxide are         changed        to carbon dioxide and nitrogen gas respectively.
  • Motor vehicle equipped with catalytic converter should use unleaded petrol because lead in the petrol inactivates the catalyst.

16.1.1 Controlling Vehicular pollution: A case study of Delhi:

• Use of CNG (compressed natural gas):

Advantages of CNG

Ø  CNG burns most efficiently.

Ø  Very little remain unburnt.

Ø  Cannot be siphoned

Ø  Cannot be adulterated like petrol or diesel.

Ø  CNG is cheaper than petrol and diesel.

Problem of use of CNG:

Ø  Difficulty in laying down pipelines to deliver CNG

Ø  Non-assurance of uninterrupted supply.

Other parallel steps taken in Delhi:

Ø  Phasing out old vehicles.

Ø  Use of unleaded petrol.

Ø  Use of low-sulphur petrol and diesel.

Ø  Use of catalytic converter in vehicle.

Ø  Application of strict pollution level norms for vehicle.

New auto fuel policy to cut down vehicular pollution.

Ø  Steadily reducing the sulphur and aromatic content in petrol and diesel fuels.

Ø  Euro-II norms:

  • Sulphur reduced to 350 ppm in diesel.
  • Sulphur reduced to 150 ppm in petrol
  • Aromatic hydrocarbon to be reduced to 42 %.
  • Up gradation of vehicle engines.

Ø  Due to above steps taken by Delhi Govt. there is substantial fall in CO2 and SO2 level between 1997 and 2005.


Ø  Undesirable high level of sound is called noise pollution.

Harm full effect of noise pollution:

Ø  Psychological and physiological disorder in humans.

Ø  High sound level, 150dB or more may damage ear drums.

Ø  Noise causes sleeplessness

Ø  Increased heart rate.

Ø  Altered breathing pattern.

Prevention of Noise Pollution:

Ø  Use of sound absorbent materials or by muffling noise in industries

Ø  Demarcation of horn free zones around hospitals and schools.

Ø  Permissible sound levels of crackers,

Ø  Timings after which Loudspeakers cannot be played


Domestic sewage and industrial effluents:

Ø  A mere 0.1 percent impurities make domestic sewage unfit for human use.

Ø  Sewage contains dissolve salts like nitrates, phosphates, and other nutrients, and toxic metal ions and organic compounds.

Ø  The amount of organic matter in water is estimated by BOD.

Ø  Biochemical oxygen demand: the amount of Oxygen required oxidizing all organic matter present in one liter of water.

Ø  Changes take place on discharge of sewage into the river.

  • Micro-organism involved in biodegradation of organic matter in the receiving water body consume a lot of oxygen, hence there is sharp decline in dissolved oxygen downstream from the point of discharge.
  • Due to low DO there is mortality of fish and other aquatic animals.

Ø  Presence of large amount of nutrients in water also causes excessive growth of Planktonic (free floating) algae, called algal bloom.

  • Algal bloom imparts distinct color to water bodies.
  • Deterioration of water quality and fish mortality.
  • Some bloom-forming algae are extremely toxic to human and animals.

Ø  The world’s most problematic aquatic weed is water hyacinth (Eichhornia crassipes) called ‘Terror of Bengal’.

  • Introduced to India for their lovely flowers.
  • Excessive growth causes blocks in waterways.
  • They grow abundantly in eutrophic water bodies.
  • Causes imbalance in ecosystem and dynamics of water body.

Sewage associated with diseases:

Ø  Sewage from home and hospital contain pathogenic microbes.

Ø Discharge of such sewage without proper treatment causes diseases like dysentery, typhoid, jaundice, cholera etc.

Toxic heavy metals (defined as elements with density > 5g/cm3), released from:-

Ø  Petroleum industry.

Ø  Paper manufacturing.

Ø  Metal extraction and processing.

Ø  Chemical manufacturing industries.


Ø  Biomagnifications increase in concentration of the toxicant at successive trophic level is called biological magnification or biomagnifications.

  • Toxic substance accumulated by an organism cannot be metabolized or excreted.
  • The accumulated toxic passed to the next trophic level.
  • This phenomenon is well known for mercury and DDT.

Bio magnification of DDT in Aquatic food chain.


Ø  The process of nutrient enrichment of water and consequent loss of species diversity is referred to as Eutrophication.

* Natural Eutrophication:

  • Streams draining into the lake increase nutrients like nitrogen and phosphorus.
  • Increase in nutrient encourages growth of aquatic organisms.
  • Over centuries, as silt and organic debris pileup the lake grows shallower and warmer.
  • Warm-water organisms dominate over 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.

* Cultural or Accelerated Eutrophication:

  • Pollutants from man’s activities like effluents from the industries and homes can radically accelerate the aging process. This phenomenon is called Cultural or Accelerated Eutrophication.


Ø  Sewage and agricultural and industrial wastes.

Ø  Prime contaminants are nitrates and phosphates.


Ø  Unsightly scum and unpleasant odors.

Ø  Robbing the dissolved oxygen form water.

Ø  Pollutant inflow kills the fish.

Ø  Decomposition of dead fish causes further depletion of DO.

Ø  Finally a lake can literally choke to death.

Thermal pollution:


Ø  Heated (thermal) waste waters flowing out of electricity-generating units. E.g. thermal power plants.


Ø  Thermal wastewater eliminates or reduces the number of organism sensitive to high temperature.

Ø  Enhance the growth of plants and fish in extremely cold areas but only after causing damage to the indigenous flora and fauna.

A case study of integrated waste water treatment:

Ø  Wastewater including sewage can be treated in an integrated manner, by utilizing a mix of artificial and natural process.

Ø  It has been done in town of Arcata, in the northern coast of California.

Ø  The treatment is done in two stages:-

  • The conventional sedimentation, filtering and chorine treatment are given.
  • The biologist developed a series of six connected marshes over 60 hectares of marshland.
  • Appropriate plants, algae, fungi and bacteria were seeded into this area, which neutralize, absorb and assimilate the pollutant
  • The water flows through the marshes, it get purified naturally.

Ø  The marshes also constitute a sanctuary, with high level of biodiversity in the form of fishes, animals and birds that now reside there.

Ø  A citizens group called Friends of the Arcata Marsh (FOAM) is responsible for the upkeep and safeguarding of this project.

Ecological sanitation:

Ø  Ecological sanitation is a sustainable system for handling human excreta, using dry composting toilets.

Ø  This is a practical, hygienic, efficient and cost-effective solution to human waste disposal.

Ø  With this composting method human excreta can be recycled into are source (as natural fertilizer).

Ø  ‘EcoSan’ toilets are being used in Kerala and Srilanka.


Ø  Solid wastes refer to everything that goes out in trash.

Ø  Municipal solid wastes are wastes from homes, offices, stores, schools, hospitals etc. It comprises paper, food wastes, plastics, glass, metals rubber, leathers, textiles etc.

Ø  Open damp of these wastes serve as the breeding ground for rats and flies.

Ø  Sanitary landfills were adopted as substitute for open-burning dumps.

Sanitary landfills:

Ø  Wastes are dumped in a depression or trench after compaction and covered with dirt everyday.


Ø  Shortage of space for huge garbage’s.

Ø  Danger of seepage of chemicals, polluting the ground water resources.

Solution to solid wastes:

Ø  All solid wastes are categorized into thee types:

  • Bio-degradable.
  • Recyclable.
  • Non-biodegradable.

Ø  All the garbage generated is sorted first.

Ø  Recyclable material to be separated and send for recycles.

Ø  Biodegradable wastes can be put into deep pits in the ground and be left for natural breakdown.

Ø  Only Non-biodegradable wastes are left and required to be disposed.



Ø  The need to reduce our garbage generation should be a prime goal.

Ø  Reduction in use of plastics and use of eco-friendly packaging.

Ø  Carrying cloth or other natural fiber carry bags when we go shopping.

Ø  Refusing polythene bags.

Case study of Remedy for Plastic wastes: (Ahmed Khan)

Ø  Polyblend, a fine powder of recycled modified plastic, was developed by his company.

Ø  Polyblend is mixed with bitumen that is used to lay roads.

Ø  It increases the water repelling property of bitumen, and helped to increase road life by a factor of three.

Ø  The raw material used for polyblend is plastic film waste.

Hospital wastes:

Ø  Hospitals generate hazardous wastes that contain disinfectants and other harmful chemicals, and also pathogenic organisms.

Ø  The use of incinerators is crucial to disposal of hospital wastes.

Electronic wastes (e-wastes):

Ø  Irreparable computers and other electronic goods are known as electronic wastes (e-wastes).

Ø  E- Wastes are buried in landfills or incinerated.

Ø  Metals like copper, iron, silicon, nickel and gold are recovered during recycling process of e-wastes.

Ø  Manual recycling process exposes workers to toxic substances present in e-wastes.

Ø Recycling is the only solution for the treatment of e-wastes.


Ø  Use of inorganic fertilizers and pesticides has increased manifold for enhancing crop production.

Ø  Pesticides, herbicides, fungicides etc, are being increasingly used.

Ø  These are toxic to non-target organisms that are important components of the soil ecosystem?

Ø  Increasing amounts of artificial fertilizers causes eutrophication.

Case study of organic farming: (Ramesh Chandra Dagar of Sonipat)

Ø  Integrated organic farming is a cyclical, zero waste procedure, where waste products from one process are cycled in as nutrients for other processes.

Ø  Maximum utilization of resource and increase the efficiency of production.

Ø  He includes bee-keeping, diary management, water harvesting, composting and agriculture in a chain of processes, which support each other and allow an extremely economical and sustainable venture.


Ø  There is no need of use of chemical fertilizers for crops

Ø  Cattle excreta are used as manure.

Ø  Crop waste used to create compost, which can be used as a natural fertilizer or can be used to generate natural gas for energy need.


Ø  Nuclear energy was hailed as a non-polluting way for generating electricity.

Ø  Later on it was realized that it has two very serious inherent problem:

  • Accidental leakage, as occurred in Three Mile Island and Chernobyl.
  • Safe disposal of radioactive wastes.

Ø  Radiation from radioactive waste causes mutation at very high rate.

Ø  High dose of nuclear radiation is lethal, but lower doses create genetic disorders and also cause cancer.


Ø  The term “Greenhouse effect” has been derived from a phenomenon that occurs in greenhouse.

Ø  In a greenhouse the glass panel lets the light in, but does not allow heat to escape. Therefore the greenhouse warms up, very much like inside a car that has been parked in the sun for a few hours.

Ø  The greenhouse effect is a naturally occurring phenomenon that is responsible for heating of Earth’s surface

Ø  and atmosphere.

Ø  Without greenhouse effect the average temperature at surface of earth would have been a chilly -18° C rather than the present average of 15° C.

Ø  Clouds and gases reflect about one-fourth of the incoming solar radiation and absorb some of it but half of

Ø  incoming solar radiation falls on Earth’s surface heating it, while a small portion is reflected back.

Ø  Earth’s surface re-emits heat in the form of infrared radiation but some part of this does not escape into space because of atmospheric gases (e.g. carbon dioxide, methane etc).

Ø  The molecule of these gases radiate heat energy and a major part of which again comes to Earth’s surface,

Ø  thus heating it up once again.

Ø  Carbon dioxide and methane – are commonly called as greenhouse gases because they are responsible for greenhouse effect.

Ø  Increase in the level of greenhouse gases has led to considerable heating of Earth leading to global

Ø  warmingor enhanced green house effect.
Ø  During the past century, the temperature of Earth has increased by 0.6° C.

Effect of global warming:

Ø  Deleterious changes in the environment and resulting in odd climatic changes (e.g. El Nino effect).

Ø  Increased melting of polar ice caps as well as of other places like the Himalayan snow caps.

Ø  Rise in sea level that can submerge many coastal areas.

Control of global warming:

Ø  Reduce use of fossil fuel.

Ø  Improving efficiency of energy usage.

Ø  Reducing deforestation.

Ø  Promoting aforestation programme.

Ø  Slowing down growth of human population.

Ø  International initiative to be taken to reduce emission of green house gases.


Ø  ‘Bad’ ozone formed in the lower atmosphere (troposphere) that harms plants and animals.

Ø  There is ‘good’ ozone also; this ozone is found in the upper part of the atmosphere called stratosphere, and it acts as a shield absorbing ultraviolet radiation from the sun.

Ø  The thickness of ozone layer is measured in terms of Dobson units (DU)

Ø  Ozone (O3) gas is continuously formed by the action of UV rays on molecular oxygen, and also degraded into molecular oxygen in the stratosphere.

Ø  There should be proper balance of formation and degradation of ozone.

Ozone depletion:

Ø  Balance of ozone in stratosphere is disrupted due to enhancement of ozone degradation by chlorofluorocarbons (CFCs).

Ø  CFCs find wide use as refrigerants.

Ø  CFCs discharged in the lower part of atmosphere move upward and reach stratosphere.

Ø  In stratosphere, UV rays acts on CFCs and release active Cl atoms.

Ø  Cl degrades ozone releasing molecular oxygen.

Ø  Cl acts as catalysts and not consumed during reaction.

Ø  Whatever CFCs are added to the stratosphere, they have permanent and continuing affects one Ozone levels.

Ø  The depletion is marked particularly over the Antarctic region. This has resulted in formation of a large area of thinned ozone layer, commonly called as the ozone hole.

Effects of UV rays:

Ø  UV radiations shorter than UV-B are almost completely absorbed by Earth’s atmosphere, if the ozone layer is

Ø  intact.

Ø  DNA and proteins of living organisms are damaged by UV rays as they potentially absorb it.

Ø  The high energy of UV rays breaks the chemical bond in these molecules.

Ø  UV – B damages DNA and mutation may occur.

Ø  It causes ageing of skin.

Ø  Damage skin cells and causes skin cancers.

Ø  In human eye cornea absorb UV – B radiation and high dose of UV – B causes inflammation of cornea calledsnow-blindness, cataract etc.

Ø  Such exposes may damage cornea.


Ø  Montreal Protocol was signed at Montreal (Canada) in 1987 to control emission of ozone depleting substances.

Ø  Many efforts are being made to reduce emission of ozone depleting substances.


Soil erosion

Ø  The removal of top fertile layer due to human activities

Reasons: -

Ø  Over cultivation

Ø  Unrestricted grazing

Ø  Deforestation

Ø  Poor irrigation practices

Water logging and soil salinity:

Ø  Irrigation with proper drainage, leads to water lodging in the soil.

Ø  Draws salt to the surface of the soil.

Ø  The salt starts collecting at the roots of the plants.

Ø  The salt damages the roots and crop productions.

16.10 Deforestation:

Ø  Conversion of forested areas to non-forested one.

How deforestation does occurs:

Ø  Slash and burn agriculture/jhum cultivation

Ø  Farmers cut down the trees of the forest and burn the plant remains.

Ø  Ash is used as fertilizer and land is used for farming or cattle grazing

Ø  Later, Land is left uncultivated for several years for replenishment of minerals

Effects of deforestation

Ø  Leads to global warming due to excess carbon-dioxide

Ø  Loss of biodiversity

Ø  Damage to hydrological cycle

Ø  Leads to soil erosion

Ø  Desertification of land


Ø  Restoring forest that was existing earlier E.g. Observing Van-Mahotsavas

Ø  It also occurs naturally

Ø  Aforestation Developing a forest in a new area where no such forest existed in that area.

A case study of people‘s participation in forest conservation

Ø  A king of Jodhpur wanted to arrange wood for his new palace in 1731.

Ø  Few Bishnois hugged the trees and asked to cut them first rather than cutting trees.

Ø  365 persons lost their lives in this act

Ø  A small temple is now present there in remembrance of this act

Ø  Amrita Devi Bishnois Wild Life Protection Award is instituted for individuals of rural areas who take keen interest in protecting wild life.

Ø  Chipko movement

Ø  It was started by local women of Garhwali; they hugged the trees to protect them from the axes of contractors.

Ø  Joint forest management (jfm)

Ø  Strategy Government of India in 1980

Ø  Local communities worked with the government to save the forest.

Ø  Communities get forest products for encouragement.


Ø  CPCB: Central Pollution Control Board

Ø  BOD: Biological Oxygen demand

Ø  CNG: Compressed Natural gas

Ø  FOAM: Friends of Arcata Marsh

Ø  JFM: Joint Forest Management

Disclaimer: All contents are originally prepared by Shri K C Meena Ji, Principal, KVS. 

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