Our planet, Earth is the only one on whichlife, as we know it, exists. Life on Earth isdependent on many factors. Most life-formswe know need an ambient temperature,water, and food. The resources available onthe Earth and the energy from the Sun arenecessary to meet the basic requirements ofall life-forms on the Earth.

What are these resources on the Earth?

These are the land, the water and the air.The outer crust of the Earth is called thelithosphere. Water covers 75% of the Earth’ssurface. It is also found underground. Thesecomprise the hydrosphere. The air that coversthe whole of the Earth like a blanket, is calledthe atmosphere. Living things are foundwhere these three exist. This life-supportingzone of the Earth where the atmosphere, thehydrosphere and the lithosphere interact andmake life possible, is known as the biosphere.

Living things constitute the bioticcomponent of the biosphere. The air, thewater and the soil form the non-living orabiotic component of the biosphere. Let usstudy these abiotic components in detail inorder to understand their role in sustaininglife on Earth.

14.1 The Breath of Life: AirWe have already talked about the compositionof air in the first chapter. It is a mixture ofmany gases like nitrogen, oxygen, carbondioxide and water vapour. It is interesting tonote that even the composition of air is theresult of life on Earth. In planets such asVenus and Mars, where no life is known toexist, the major component of the atmosphereis found to be carbon dioxide. In fact, carbon

dioxide constitutes up to 95-97% of theatmosphere on Venus and Mars.

Eukaryotic cells and many prokaryoticcells, discussed in Chapter 5, need oxygen tobreak down glucose molecules and get energyfor their activities. This results in theproduction of carbon dioxide. Another processwhich results in the consumption of oxygenand the concomitant production of carbondioxide is combustion. This includes not justhuman activities, which burn fuels to getenergy, but also forest fires.

Despite this, the percentage of carbondioxide in our atmosphere is a mere fractionof a percent because carbon dioxide is ‘fixed’in two ways: (i) Green plants convert carbondioxide into glucose in the presence ofSunlight and (ii) many marine animals usecarbonates dissolved in sea-water to maketheir shells.

14.1.1 THE ROLE OF THE ATMOSPHERE IN

CLIMATE CONTROL

We have talked of the atmosphere coveringthe Earth, like a blanket. We know that air isa bad conductor of heat. The atmospherekeeps the average temperature of the Earthfairly steady during the day and even duringthe course of the whole year. The atmosphereprevents the sudden increase in temperatureduring the daylight hours. And during thenight, it slows down the escape of heat intoouter space. Think of the moon, which isabout the same distance from the Sun thatthe Earth is. Despite that, on the surface ofthe moon, with no atmosphere, thetemperature ranges from –190º C to 110º C.

1414141414NNNNNATURALATURALATURALATURALATURAL R R R R RESOURCESESOURCESESOURCESESOURCESESOURCES

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the heating of water bodies and the activitiesof living organisms. The atmosphere can beheated from below by the radiation that isreflected back or re-radiated by the land orwater bodies. On being heated, convectioncurrents are set up in the air. In order to gainsome understanding of the nature ofconvection currents, let us perform thefollowing activity:

Activity _____________14.2• Place a candle in a beaker or wide-

mouthed bottle and light it. Light anincense stick and take it to the mouthof the above bottle (Figure 14.1).

• Which way does the smoke flow whenthe incense stick is kept near the edgeof the mouth?

• Which way does the smoke flow whenthe incense stick is kept a little abovethe candle?

• Which way does the smoke flow whenthe incense stick is kept in otherregions?

Activity _____________14.1• Measure the temperature of the

following :Take (i) a beaker full of water, (ii) abeaker full of soil/sand and (iii) a closedbottle containing a thermometer. Keepthem in bright Sunlight for three hours.Now measure the temperature of all 3vessels. Also, take the temperaturereading in shade at the same time.

Now answer1. Is the temperature reading more in

activity (i) or (ii)?

2. Based on the above finding, whichwould become hot faster – the land orthe sea?

3. Is the thermometer reading of thetemperature of air (in shade) the sameas the temperature of sand or water?What do you think is the reason forthis? And why does the temperaturehave to be measured in the shade?

4. Is the temperature of air in the closedglass vessel/bottle the same as thetemperature taken in open air? (i) Whatdo you think is the reason for this?(ii) Do we ever come across thisphenomenon in daily life?

As we have seen above, sand and waterdo not heat up at the same rate. What do youthink will be their rates of cooling? Can wethink of an experiment to test the prediction?

14.1.2 THE MOVEMENT OF AIR: WINDS

We have all felt the relief brought by coolevening breezes after a hot day. Andsometimes, we are lucky enough to get rainsafter some days of really hot weather. Whatcauses the movement of air, and what decideswhether this movement will be in the form ofa gentle breeze, a strong wind or a terriblestorm? What brings us the welcome rains?

All these phenomena are the result ofchanges that take place in our atmospheredue to the heating of air and the formation ofwater vapour. Water vapour is formed due to

Fig. 14.1: Air currents being caused by the unevenheating of air.

The patterns revealed by the smoke showus the directions in which hot and cold airmove. In a similar manner, when air is heatedby radiation from the heated land or water, itrises. But since land gets heated faster thanwater, the air over land would also be heatedfaster than the air over water bodies.

So, if we look at the situation in coastalregions during the day, the air above the land

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gets heated faster and starts rising. As thisair rises, a region of low pressure is createdand air over the sea moves into this area oflow pressure. The movement of air from oneregion to the other creates winds. During theday, the direction of the wind would be fromthe sea to the land.

At night, both land and sea start to cool.Since water cools down slower than the land,the air above water would be warmer thanthe air above land.

On the basis of the above discussion, whatcan you say about:

1. the appearance of areas of low andhigh pressure in coastal areas at night?

2. the direction in which air would flowat night in coastal areas?

Similarly, all the movements of airresulting in diverse atmospheric phenomenaare caused by the uneven heating of theatmosphere in different regions of the Earth.But various other factors also influence thesewinds – the rotation of the Earth and thepresence of mountain ranges in the paths ofthe wind are a couple of these factors. Wewill not go into these factors in detail in thischapter, but think about this: how do thepresence of the Himalayas change the flow ofa wind blowing from Allahabad towards thenorth?

14.1.3 RAIN

Let us go back now to the question of howclouds are formed and bring us rain. We couldstart by doing a simple experiment whichdemonstrates some of the factors influencingthese climatic changes.

Activity _____________14.3• Take an empty bottle of the sort in

which bottled water is sold. Pour about5-10 mL of water into it and close thebottle tightly. Shake it well or leave itout in the Sun for ten minutes. Thiscauses the air in the bottle to besaturated with water vapour.

• Now, take a lighted incense stick. Openthe cap of the bottle and allow some ofthe smoke from the incense stick to

enter the bottle. Quickly close the bottleonce more. Make sure that the cap isfitting tightly. Press the bottle hardbetween your hands and crush it asmuch as possible. Wait for a fewseconds and release the bottle. Againpress the bottle as hard as you can.

Now answer1. When did you observe that the air

inside seemed to become ‘foggy’?

2. When does this fog disappear?

3. When is the pressure inside the bottlehigher?

4. Is the ‘fog’ observed when the pressurein the bottle is high or when it is low?

5. What is the need for smoke particlesinside the bottle for this experiment?

6. What might happen if you do theexperiment without the smoke from theincense stick? Now try it and check ifthe prediction was correct. What mightbe happening in the above experimentin the absence of smoke particles?

The above experiment replicates, on a verysmall scale, what happens when air with avery high content of water vapour goes froma region of high pressure to a region of lowpressure or vice versa.

When water bodies are heated during theday, a large amount of water evaporates andgoes into the air. Some amount of watervapour also get into the atmosphere becauseof various biological activities. This air alsogets heated. The hot air rises up carrying thewater vapour with it. As the air rises, itexpands and cools. This cooling causes thewater vapour in the air to condense in theform of tiny droplets. This condensation ofwater is facilitated if some particles could actas the ‘nucleus’ for these drops to formaround. Normally dust and other suspendedparticles in the air perform this function.

Once the water droplets are formed, theygrow bigger by the ‘condensation’ of thesewater droplets. When the drops have grownbig and heavy, they fall down in the form ofrain. Sometimes, when the temperature of air

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14.1.4 AIR POLLUTION

We keep hearing of the increasing levels ofoxides of nitrogen and sulphur in the news.People often bemoan the fact that the qualityof air has gone down since their childhood.How is the quality of air affected and howdoes this change in quality affect us and otherlife forms?

The fossil fuels like coal and petroleumcontain small amounts of nitrogen andsulphur. When these fuels are burnt, nitrogenand sulphur too are burnt and this producesdifferent oxides of nitrogen and sulphur. Notonly is the inhalation of these gasesdangerous, they also dissolve in rain to giverise to acid rain. The combustion of fossil fuelsalso increases the amount of suspendedparticles in air. These suspended particlescould be unburnt carbon particles orsubstances called hydrocarbons. Presence ofhigh levels of all these pollutants causevisibility to be lowered, especially in coldweather when water also condenses out ofair. This is known as smog and is a visibleindication of air pollution. Studies haveshown that regularly breathing air thatcontains any of these substances increasesthe incidence of allergies, cancer and heartdiseases. An increase in the content of theseharmful substances in air is called airpollution.

is low enough, precipitation may occur in theform of snow, sleet or hail.

Rainfall patterns are decided by theprevailing wind patterns. In large parts ofIndia, rains are mostly brought by the south-west or north-east monsoons. We have alsoheard weather reports that say ‘depressions’in the Bay of Bengal have caused rains insome areas (Figure 14.2).

Activity _____________14.4• Collect information from newspapers

or weather reports on television aboutrainfall patterns across the country.Also find out how to construct a rain-gauge and make one. What precautionsare necessary in order to get reliabledata from this rain-gauge? Now answerthe following questions :

• In which month did your city/town/village get the maximum rainfall?

• In which month did your state/unionterritory get the maximum rainfall?

• Is rain always accompanied by thunderand lightning? If not, in which seasondo you get more of thunder andlightning with the rain?

Activity _____________14.5• Find out more about monsoons and

cyclones from the library. Try and findout the rainfall pattern of any othercountry. Is the monsoon responsible forrains the world over?

Fig. 14.2: Satellite picture showing clouds over India.

Fig. 14.3: Lichen

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people are forced to spend considerableamounts of time in fetching water from far-away sources.

Activity _____________14.7• Many municipal corporations are trying

water -harvesting techniques toimprove the availability of water.

• Find out what these techniques are andhow they would increase the water thatis available for use.

But why is water so necessary? And doall organisms require water? All cellularprocesses take place in a water medium. Allthe reactions that take place within our bodyand within the cells occur betweensubstances that are dissolved in water.Substances are also transported from onepart of the body to the other in a dissolvedform. Hence, organisms need to maintain thelevel of water within their bodies in order tostay alive. Terrestrial life-forms require freshwater for this because their bodies cannottolerate or get rid of the high amounts ofdissolved salts in saline water. Thus, watersources need to be easily accessible foranimals and plants to survive on land.

Activity _____________14.8• Select a small area (say, 1 m2) near a

water-body, it may be a river, stream,lake or pond. Count the number ofdifferent animals and plants in thisarea. Also, check the number ofindividuals of each type or species.

• Compare this with the number ofindividuals (both animals and plants)found in an area of the same size in adry, rocky region.

• Is the variety of plant and animal lifethe same in both these areas?

Activity _____________14.9• Select and mark out a small area (about

1 m2) in some unused land in or nearyour school.

• As in the above activity, count thenumber of different animals and plantsin this area and the number ofindividuals of each species.

Activity _____________14.6• Organisms called lichens are found to

be very sensitive to the levels ofcontaminants like sulphur dioxide inthe air. As discussed earlier in section7.3.3, lichens can be commonly foundgrowing on the barks of trees as a thingreenish-white crust. See if you canfind lichen growing on the trees in yourlocality.

• Compare the lichen on trees near busyroads and trees some distance away.

• On the trees near roads, compare theincidence of lichen on the side facingthe road and on the side away from theroad.

What can you say about the levels ofpolluting substances near roads and awayfrom roads on the basis of your findingsabove?

uestions1. How is our atmosphere different

from the atmospheres on Venusand Mars?

2. How does the atmosphere act asa blanket?

3. What causes winds?4. How are clouds formed?5. List any three human activities

that you think would lead to airpollution.

14.2 Water: A Wonder LiquidWater occupies a very large area of the Earth’ssurface and is also found underground. Someamount of water exists in the form of watervapour in the atmosphere. Most of the wateron Earth’s surface is found in seas and oceansand is saline. Fresh water is found frozen inthe ice-caps at the two poles and on snow-covered mountains. The underground waterand the water in rivers, lakes and ponds isalso fresh. However, the availability of freshwater varies from place to place. Practicallyevery summer, most places have to face ashortage of water. And in rural areas, wherewater supply systems have not been installed,

Q

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• Remember to do this in the same placetwice in a year, once during summeror the dry season and once after it hasrained.

Now answer1. Were the numbers similar both times?

2. In which season did you find morevariety of plants and animals?

3. In which season did you find morenumber of individuals of each variety?

After compiling the results of the abovetwo activities, think if there is any relationshipbetween the amount of available water andthe number and variety of plants and animalsthat can live in a given area. If there is arelationship, where do you think you wouldfind a greater variety and abundance of life –in a region that receives 5 cm of rainfall in ayear or a region that receives 200 cm ofrainfall in a year? Find the map showingrainfall patterns in the atlas and predictwhich States in India would have themaximum biodiversity and which would havethe least. Can we think of any way of checkingwhether the prediction is correct?

The availability of water decides not onlythe number of individuals of each species thatare able to survive in a particular area, but italso decides the diversity of life there. Ofcourse, the availability of water is not the onlyfactor that decides the sustainability of lifein a region. Other factors like the temperatureand nature of soil also matter. But water isone of the major resources which determinelife on land.

14.2.1 WATER POLLUTION

Water dissolves the fertilisers and pesticidesthat we use on our farms. So some percentageof these substances are washed into the waterbodies. Sewage from our towns and cities andthe waste from factories are also dumped intorivers or lakes. Specific industries also usewater for cooling in various operations andlater return this hot water to water-bodies.Another manner in which the temperature of

the water in rivers can be affected is when wateris released from dams. The water inside thedeep reservoir would be colder than the waterat the surface which gets heated by the Sun.

All this can affect the life-forms that arefound in these water bodies in various ways.It can encourage the growth of some life-formsand harm some other life-forms. This affectsthe balance between various organisms whichhad been established in that system. So weuse the term water-pollution to cover thefollowing effects:

1. The addition of undesirablesubstances to water-bodies. Thesesubstances could be the fertilisers andpesticides used in farming or theycould be poisonous substances, likemercury salts which are used bypaper-industries. These could also bedisease-causing organisms, like thebacteria which cause cholera.

2. The removal of desirable substancesfrom water-bodies. Dissolved oxygenis used by the animals and plants thatlive in water. Any change that reducesthe amount of this dissolved oxygenwould adversely affect these aquaticorganisms. Other nutrients could alsobe depleted from the water bodies.

3. A change in temperature. Aquaticorganisms are used to a certain rangeof temperature in the water-bodywhere they live, and a sudden markedchange in this temperature would bedangerous for them or affect theirbreeding. The eggs and larvae ofvarious animals are particularlysusceptible to temperature changes.

uestions1. Why do organisms need water?2. What is the major source of fresh

water in the city/town/villagewhere you live?

3. Do you know of any activitywhich may be polluting this watersource?Q

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14.3 Mineral Riches in the SoilSoil is an important resource that decides thediversity of life in an area. But what is thesoil and how is it formed? The outermost layerof our Earth is called the crust and theminerals found in this layer supply a varietyof nutrients to life-forms. But these mineralswill not be available to the organisms if theminerals are bound up in huge rocks. Overlong periods of time, thousands and millionsof years, the rocks at or near the surface ofthe Earth are broken down by variousphysical, chemical and some biologicalprocesses. The end product of this breakingdown is the fine particles of soil. But whatare the factors or processes that make soil?

• The Sun: The Sun heats up rocksduring the day so that they expand.At night, these rocks cool down andcontract. Since all parts of the rockdo not expand and contract at thesame rate, this results in theformation of cracks and ultimately thehuge rocks break up into smallerpieces.

• Water: Water helps in the formationof soil in two ways. One, water couldget into the cracks in the rocks formeddue to uneven heating by the Sun. Ifthis water later freezes, it would causethe cracks to widen. Can you thinkwhy this should be so? Two, flowingwater wears away even hard rock overlong periods of time. Fast flowing wateroften carries big and small particlesof rock downstream. These rocks rubagainst other rocks and the resultantabrasion causes the rocks to weardown into smaller and smallerparticles. The water then takes theseparticles along with it and deposits itfurther down its path. Soil is thusfound in places far away from itsparent-rock.

• Wind: In a process similar to the wayin which water rubs against rocks andwears them down, strong winds alsoerode rocks down. The wind also

carries sand from one place to theother like water does.

• Living organisms also influence theformation of soil. The lichen that weread about earlier, also grows on thesurface of rocks. While growing, theyrelease certain substances that causethe rock surface to powder down andform a thin layer of soil. Other smallplants like moss, are able to grow onthis surface now and they cause therock to break up further. The roots ofbig trees sometimes go into cracks inthe rocks and as the roots grow bigger,the crack is forced bigger.

Activity ____________14.10• Take some soil and put it into a beaker

containing water. The water should beat least five times the amount of soiltaken. Stir the soil and water vigorouslyand allow the soil to settle down.Observe after some time.

• Is the soil at the bottom of the beakerhomogenous or have layers formed?

• If layers have formed, how is one layerdifferent from another?

• Is there anything floating on thesurface of the water?

• Do you think some substances wouldhave dissolved in the water? How wouldyou check?

As you have seen, soil is a mixture. Itcontains small particles of rock (of differentsizes). It also contains bits of decayed livingorganisms which is called humus. In addition,soil also contains various forms ofmicroscopic life. The type of soil is decidedby the average size of particles found in itand the quality of the soil is decided by theamount of humus and the microscopicorganisms found in it. Humus is a majorfactor in deciding the soil structure becauseit causes the soil to become more porous andallows water and air to penetrate deepunderground. The mineral nutrients that arefound in a particular soil depends on therocks it was formed from. The nutrientcontent of a soil, the amount of humuspresent in it and the depth of the soil are

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some of the factors that decide which plantswill thrive on that soil. Thus, the topmostlayer of the soil that contains humus andliving organisms in addition to the soilparticles is called the topsoil. The quality ofthe topsoil is an important factor that decidesbiodiversity in that area.

Modern farming practices involve the useof large amounts of fertilizers and pesticides.Use of these substances over long periods oftime can destroy the soil structure by killingthe soil micro-organisms that recyclenutrients in the soil. It also kills theEarthworms which are instrumental inmaking the rich humus. Fertile soil canquickly be turned barren if sustainablepractices are not followed. Removal of usefulcomponents from the soil and addition ofother substances, which adversely affect thefertility of the soil and kill the diversity oforganisms that live in it, is called soil pollution.

The soil that we see today in one placehas been created over a very long period oftime. However, some of the factors thatcreated the soil in the first place and broughtthe soil to that place may be responsible forthe removal of the soil too. The fine particlesof soil may be carried away by flowing wateror wind. If all the soil gets washed away andthe rocks underneath are exposed, we havelost a valuable resource because very littlewill grow on the rock.

Activity ____________14.11• Take two identical trays and fill them

with soil. Plant mustard or green gramor paddy in one of the trays and waterboth the trays regularly for a few days,till the first tray is covered by plantgrowth. Now, tilt both the trays and fixthem in that position. Make sure thatboth the trays are tilted at the sameangle. Pour equal amount of watergently on both trays such that the waterflows out of the trays (Fig. 14.4).

• Study the amount of soil that is carriedout of the trays. Is the amount thesame in both the trays?

• Now pour equal amounts of water onboth the trays from a height. Pour threeor four times the amount that youpoured earlier.

• Study the amount of soil that iscarried out of the trays now. Is theamount the same in both the trays?

• Is the amount of soil that is carried outmore or less or equal to the amountwashed out earlier?

Fig. 14.4: Effect of flowing water on the top-soil

The roots of plants have an important rolein preventing soil erosion. The large-scaledeforestation that is happening all over theworld not only destroys biodiversity, it alsoleads to soil erosion. Topsoil that is bare ofvegetation, is likely to be removed veryquickly. And this is accelerated in hilly ormountainous regions. This process of soilerosion is very difficult to reverse. Vegetativecover on the ground has a role to play in thepercolation of water into the deeperlayers too.

uestions1. How is soil formed?2. What is soil erosion?3. What are the methods of

preventing or reducing soilerosion?

14.4 Biogeochemical CyclesA constant interaction between the biotic andabiotic components of the biosphere makesit a dynamic, but stable system. Theseinteractions consist of a transfer of matterand energy between the different componentsof the biosphere. Let us look at someprocesses involved in the maintenance of theabove balance.

Q

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14.4.1 THE WATER-CYCLE

You have seen how the water evaporates fromthe water bodies and subsequentcondensation of this water vapour leads torain. But we don’t see the seas and oceansdrying up. So, how is the water returning tothese water bodies? The whole process inwhich water evaporates and falls on the landas rain and later flows back into the sea viarivers is known as the water-cycle. This cycleis not as straight-forward and simple as thisstatement seems to imply. All of the waterthat falls on the land does not immediatelyflow back into the sea. Some of it seeps intothe soil and becomes part of the undergroundreservoir of fresh-water. Some of thisunderground water finds its way to thesurface through springs. Or we bring it tothe surface for our use through wells or tube-wells. Water is also used by terrestrial animalsand plants for various life-processes(Fig. 14.5).

water. Thus rivers carry many nutrients fromthe land to the sea, and these are used bythe marine organisms.

14.4.2 THE NITROGEN-CYCLE

Nitrogen gas makes up 78% of ouratmosphere and nitrogen is also a part ofmany molecules essential to life like proteins,nucleic acids (DNA and RNA) and somevitamins. Nitrogen is found in otherbiologically important compounds such asalkaloids and urea too. Nitrogen is thus anessential nutrient for all life-forms and lifewould be simple if all these life-forms coulduse the atmospheric nitrogen directly.However, other than a few forms of bacteria,life-forms are not able to convert thecomparatively inert nitrogen molecule intoforms like nitrates and nitrites which can betaken up and used to make the requiredmolecules. These ‘nitrogen-fixing’ bacteriamay be free-living or be associated with somespecies of dicot plants. Most commonly, thenitrogen-fixing bacteria are found in the rootsof legumes (generally the plants which giveus pulses) in special structures called root-nodules. Other than these bacteria, the onlyother manner in which the nitrogen moleculeis converted to nitrates and nitrites is by aphysical process. During lightning, the hightemperatures and pressures created in theair convert nitrogen into oxides of nitrogen.These oxides dissolve in water to give nitricand nitrous acids and fall on land along withrain. These are then utilised by various life-forms.

What happens to the nitrogen once it isconverted into forms that can be taken upand used to make nitrogen-containingmolecules? Plants generally take up nitratesand nitrites and convert them into aminoacids which are used to make proteins. Someother biochemical pathways are used to makethe other complex compounds containingnitrogen. These proteins and other complexcompounds are subsequently consumed byanimals. Once the animal or the plant dies,other bacteria in the soil convert the variouscompounds of nitrogen back into nitrates and

Fig. 14.5: Water-cycle in nature

Let us look at another aspect of whathappens to water during the water-cycle. Asyou know, water is capable of dissolving alarge number of substances. As water flowsthrough or over rocks containing solubleminerals, some of them get dissolved in the

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nitrites. A different type of bacteria convertsthe nitrates and nitrites into elementalnitrogen. Thus, there is a nitrogen-cycle innature in which nitrogen passes from itselemental form in the atmosphere into simplemolecules in the soil and water, which getconverted to more complex molecules in livingbeings and back again to the simple nitrogenmolecule in the atmosphere.

14.4.3 THE CARBON-CYCLE

Carbon is found in various forms on theEarth. It occurs in the elemental form asdiamonds and graphite. In the combinedstate, it is found as carbon dioxide in theatmosphere, as carbonate and hydrogen-carbonate salts in various minerals, while alllife-forms are based on carbon-containingmolecules like proteins, carbohydrates, fats,

Fig.14.6: Nitrogen-cycle in nature

nucleic acids and vitamins. The endoskeletonsand exoskeletons of various animals are alsoformed from carbonate salts. Carbon isincorporated into life-forms through the basicprocess of photosynthesis which is performedin the presence of Sunlight by all life-forms thatcontain chlorophyll. This process convertscarbon dioxide from the atmosphere ordissolved in water into glucose molecules.These glucose molecules are either convertedinto other substances or used to provideenergy for the synthesis of other biologicallyimportant molecules (Fig. 14.7).

The utilisation of glucose to provide energyto living things involves the process ofrespiration in which oxygen may or may notbe used to convert glucose back into carbondioxide. This carbon dioxide then goes backinto the atmosphere. Another process that

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adds to the carbon dioxide in the atmosphereis the process of combustion where fuels areburnt to provide energy for various needs likeheating, cooking, transportation andindustrial processes. In fact, the percentageof carbon dioxide in the atmosphere is saidto have doubled since the industrialrevolution when human beings startedburning fossil fuels on a very large scale.Carbon, like water, is thus cycled repeatedlythrough different forms by the variousphysical and biological activities.

14.4.3 (i) THE GREENHOUSE EFFECT

Recall the reading taken by you under (iii) inActivity 14.1. Heat is trapped by glass, andhence the temperature inside a glassenclosure will be much higher than thesurroundings. This phenomenon was usedto create an enclosure where tropical plants

Fig. 14.7: Carbon-cycle in nature

could be kept warm during the winters incolder climates. Such enclosures are calledgreenhouses. Greenhouses have also lenttheir name to an atmospheric phenomenon.Some gases prevent the escape of heat fromthe Earth. An increase in the percentage ofsuch gases in the atmosphere would causethe average temperatures to increase world-wide and this is called the greenhouse effect.Carbon dioxide is one of the greenhousegases. An increase in the carbon dioxidecontent in the atmosphere would cause moreheat to be retained by the atmosphere andlead to global warming.

Activity ____________14.12• Find out what the consequences of

global warming would be.• Also, find out the names of some other

greenhouse gases.

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14.4.4 THE OXYGEN-CYCLE

Oxygen is a very abundant element on ourEarth. It is found in the elemental form inthe atmosphere to the extent of 21%. It alsooccurs extensively in the combined form inthe Earth’s crust as well as also in the air inthe form of carbon dioxide. In the crust, it isfound as the oxides of most metals andsilicon, and also as carbonate, sulphate,nitrate and other minerals. It is also anessential component of most biologicalmolecules like carbohydrates, proteins,nucleic acids and fats (or lipids).

But when we talk of the oxygen-cycle, weare mainly referring to the cycle thatmaintains the levels of oxygen in theatmosphere. Oxygen from the atmosphere isused up in three processes, namelycombustion, respiration and in the formationof oxides of nitrogen. Oxygen is returned tothe atmosphere in only one major process,that is, photosynthesis. And this forms thebroad outline of the oxygen-cycle in nature(Fig. 14.8).

bacteria, are poisoned by elemental oxygen.In fact, even the process of nitrogen-fixing bybacteria does not take place in the presenceof oxygen.

14.5 Ozone LayerElemental oxygen is normally found in theform of a diatomic molecule. However, in theupper reaches of the atmosphere, a moleculecontaining three atoms of oxygen is found.This would mean a formula of O3 and this iscalled ozone. Unlike the normal diatomicmolecule of oxygen, ozone is poisonous andwe are lucky that it is not stable nearer tothe Earth’s surface. But it performs anessential function where it is found. It absorbsharmful radiations from the Sun. Thisprevents those harmful radiations fromreaching the surface of the Earth where theymay damage many forms of life.

Recently it was discovered that this ozonelayer was getting depleted. Various man-madecompounds like CFCs (carbon compoundshaving both fluorine and chlorine which arevery stable and not degraded by any biologicalprocess) were found to persist in theatmosphere. Once they reached the ozonelayer, they would react with the ozonemolecules. This resulted in a reduction of theozone layer and recently they have discovereda hole in the ozone layer above the Antartica.It is difficult to imagine the consequences forlife on Earth if the ozone layer dwindlesfurther, but many people think that it wouldbe better not to take chances. These peopleadvocate working towards stopping all furtherdamage to the ozone layer.

Fig. 14.8: Oxygen-cycle in nature

Though we usually think of oxygen asbeing necessary to life in the process ofrespiration, it might be of interest to you tolearn that some forms of life, especially

Fig. 14.9: Satellite picture showing the hole (magentacolour) in the ozone layer over Antartica

October1980

October1985

October1990

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Activity ____________14.13• Find out which other molecules are

thought to damage the ozone layer.• Newspaper reports often talk about the

hole in the ozone layer.• Find out whether the size of this hole

is changing and in what mannerscientists think this would affect lifeon Earth (Fig. 14.9).

QWhatyou havelearnt• Life on Earth depends on resources like soil, water and air,

and energy from the Sun.

• Uneven heating of air over land and water-bodies causes winds.

• Evaporation of water from water-bodies and subsequentcondensation give us rain.

• Rainfall patterns depend on the prevailing wind patterns in anarea.

• Various nutrients are used again and again in a cyclic fashion.This leads to a certain balance between the various componentsof the biosphere.

• Pollution of air, water and soil affect the quality of life andharm the biodiversity.

• We need to conserve our natural resources and use them in asustainable manner.

Exercises1. Why is the atmosphere essential for life?

2. Why is water essential for life?

3. How are living organisms dependent on the soil? Are organismsthat live in water totally independent of soil as a resource?

4. You have seen weather reports on television and in newspapers.How do you think we are able to predict the weather?

uestions1. What are the different states in

which water is found during thewater cycle?

2. Name two biologically importantcompounds that contain bothoxygen and nitrogen.

3. List any three human activitieswhich would lead to an increasein the carbon dioxide content of air.

4. What is the greenhouse effect?5. What are the two forms of

oxygen found in the atmosphere?

SCIENCE202

5. We know that many human activities lead to increasinglevels of pollution of the air, water-bodies and soil. Do youthink that isolating these activities to specific and limitedareas would help in reducing pollution?

6. Write a note on how forests influence the quality of our air,soil and water resources.