Natural Resources Chapter 14 PDF

C hapter 14 NATURAL RESOURCES Our planet, Earth is the only one on which dioxide constitutes up to 95-97% of the d life, as we know it, exists. Life on Earth is...

C hapter 14 NATURAL RESOURCES Our planet, Earth is the only one on which dioxide constitutes up to 95-97% of the d life, as we know it, exists. Life on Earth is atmosphere on Venus and Mars. dependent on many factors. Most life-forms Eukaryotic cells and many prokaryotic he we know need an ambient temperature, cells, discussed in Chapter 5, need oxygen to water, and food. The resources available on break down glucose molecules and get energy the Earth and the energy from the Sun are for their activities. This results in the necessary to meet the basic requirements of production of carbon dioxide. Another process all life-forms on the Earth. which results in the consumption of oxygen is What are these resources on the Earth? and the concomitant production of carbon dioxide is combustion. This includes not just These are the land, the water and the air. bl human activities, which burn fuels to get The outer crust of the Earth is called the energy, but also forest fires. lithosphere. Water covers 75% of the Earth’s Despite this, the percentage of carbon surface. It is also found underground. These pu dioxide in our atmosphere is a mere fraction comprise the hydrosphere. The air that covers the whole of the Earth like a blanket, is called of a percent because carbon dioxide is ‘fixed’ the atmosphere. Living things are found in two ways: (i) Green plants convert carbon be T where these three exist. This life-supporting dioxide into glucose in the presence of Sunlight and (ii) many marine animals use re zone of the Earth where the atmosphere, the o R hydrosphere and the lithosphere interact and carbonates dissolved in sea-water to make make life possible, is known as the biosphere. their shells. Living things constitute the biotic 14.1.1 T HE ROLE OF THE tt E component of the biosphere. The air, the ATMOSPHERE IN water and the soil form the non-living or CLIMATE CONTROL abiotic component of the biosphere. Let us C study these abiotic components in detail in We have talked of the atmosphere covering order to understand their role in sustaining the Earth, like a blanket. We know that air is life on Earth. a bad conductor of heat. The atmosphere no N keeps the average temperature of the Earth 14.1 The Breath of Life: Air fairly steady during the day and even during the course of the whole year. The atmosphere We have already talked about the composition © prevents the sudden increase in temperature of air in the first chapter. It is a mixture of during the daylight hours. And during the many gases like nitrogen, oxygen, carbon dioxide and water vapour. It is interesting to night, it slows down the escape of heat into note that even the composition of air is the outer space. Think of the moon, which is result of life on Earth. In planets such as about the same distance from the Sun that Venus and Mars, where no life is known to the Earth is. Despite that, on the surface of exist, the major component of the atmosphere the moon, with no atmosphere, the is found to be carbon dioxide. In fact, carbon temperature ranges from –190º C to 110º C.  Activity _____________ 14.1 the heating of water bodies and the activities of living organisms. The atmosphere can be Measure the temperature of the heated from below by the radiation that is following : reflected back or re-radiated by the land or Take (i) a beaker full of water, (ii) a water bodies. On being heated, convection beaker full of soil/sand and (iii) a closed currents are set up in the air. In order to gain bottle containing a thermometer. Keep some understanding of the nature of them in bright Sunlight for three hours. Now measure the temperature of all 3 convection currents, let us perform the vessels. Also, take the temperature following activity: reading in shade at the same time. Activity _____________ 14.2 d Now answer Place a candle in a beaker or wide- mouthed bottle and light it. Light an he 1. Is the temperature reading more in incense stick and take it to the mouth activity (i) or (ii)? of the above bottle (Figure 14.1). 2. Based on the above finding, which Which way does the smoke flow when the incense stick is kept near the edge would become hot faster – the land or of the mouth? is the sea? Which way does the smoke flow when 3. Is the thermometer reading of the the incense stick is kept a little above temperature of air (in shade) the same the candle? bl as the temperature of sand or water? Which way does the smoke flow when What do you think is the reason for the incense stick is kept in other this? And why does the temperature regions? pu have to be measured in the shade? 4. Is the temperature of air in the closed glass vessel/bottle the same as the be T temperature taken in open air? (i) What do you think is the reason for this? re (ii) Do we ever come across this o R phenomenon in daily life? As we have seen above, sand and water tt E do not heat up at the same rate. What do you think will be their rates of cooling? Can we think of an experiment to test the prediction? C 14.1.2 THE MOVEMENT OF AIR: WINDS no N We have all felt the relief brought by cool Fig. 14.1: Air currents being caused by the uneven evening breezes after a hot day. And heating of air. sometimes, we are lucky enough to get rains after some days of really hot weather. What The patterns revealed by the smoke show © causes the movement of air, and what decides us the directions in which hot and cold air whether this movement will be in the form of move. In a similar manner, when air is heated a gentle breeze, a strong wind or a terrible by radiation from the heated land or water, it storm? What brings us the welcome rains? rises. But since land gets heated faster than All these phenomena are the result of water, the air over land would also be heated changes that take place in our atmosphere faster than the air over water bodies. due to the heating of air and the formation of So, if we look at the situation in coastal water vapour. Water vapour is formed due to regions during the day, the air above the land 190 SCIENCE  gets heated faster and starts rising. As this enter the bottle. Quickly close the bottle air rises, a region of low pressure is created once more. Make sure that the cap is and air over the sea moves into this area of fitting tightly. Press the bottle hard between your hands and crush it as low pressure. The movement of air from one much as possible. Wait for a few region to the other creates winds. During the seconds and release the bottle. Again day, the direction of the wind would be from press the bottle as hard as you can. the sea to the land. At night, both land and sea start to cool. Since water cools down slower than the land, Now answer the air above water would be warmer than 1. When did you observe that the air the air above land. inside seemed to become ‘foggy’? d On the basis of the above discussion, what 2. When does this fog disappear? can you say about: 3. When is the pressure inside the bottle he 1. the appearance of areas of low and higher? high pressure in coastal areas at night? 2. the direction in which air would flow 4. Is the ‘fog’ observed when the pressure at night in coastal areas? in the bottle is high or when it is low? Similarly, all the movements of air 5. What is the need for smoke particles is resulting in diverse atmospheric phenomena inside the bottle for this experiment? are caused by the uneven heating of the 6. What might happen if you do the bl atmosphere in different regions of the Earth. experiment without the smoke from the But various other factors also influence these incense stick? Now try it and check if winds – the rotation of the Earth and the the prediction was correct. What might presence of mountain ranges in the paths of be happening in the above experiment pu the wind are a couple of these factors. We in the absence of smoke particles? will not go into these factors in detail in this The above experiment replicates, on a very be T chapter, but think about this: how do the small scale, what happens when air with a presence of the Himalayas change the flow of very high content of water vapour goes from re a wind blowing from Allahabad towards the o R a region of high pressure to a region of low north? pressure or vice versa. When water bodies are heated during the 14.1.3 RAIN tt E day, a large amount of water evaporates and Let us go back now to the question of how goes into the air. Some amount of water clouds are formed and bring us rain. We could vapour also get into the atmosphere because C start by doing a simple experiment which of various biological activities. This air also demonstrates some of the factors influencing gets heated. The hot air rises up carrying the these climatic changes. water vapour with it. As the air rises, it no N expands and cools. This cooling causes the Activity _____________ 14.3 water vapour in the air to condense in the form of tiny droplets. This condensation of Take an empty bottle of the sort in water is facilitated if some particles could act © which bottled water is sold. Pour about as the ‘nucleus’ for these drops to form 5-10 mL of water into it and close the around. Normally dust and other suspended bottle tightly. Shake it well or leave it particles in the air perform this function. out in the Sun for ten minutes. This causes the air in the bottle to be Once the water droplets are formed, they saturated with water vapour. grow bigger by the ‘condensation’ of these Now, take a lighted incense stick. Open water droplets. When the drops have grown the cap of the bottle and allow some of big and heavy, they fall down in the form of the smoke from the incense stick to rain. Sometimes, when the temperature of air NATURAL RESOURCES 191  is low enough, precipitation may occur in the 14.1.4 AIR POLLUTION form of snow, sleet or hail. Rainfall patterns are decided by the We keep hearing of the increasing levels of prevailing wind patterns. In large parts of oxides of nitrogen and sulphur in the news. India, rains are mostly brought by the south- People often bemoan the fact that the quality west or north-east monsoons. We have also of air has gone down since their childhood. heard weather reports that say ‘depressions’ How is the quality of air affected and how in the Bay of Bengal have caused rains in does this change in quality affect us and other some areas (Figure 14.2). life forms? The fossil fuels like coal and petroleum contain small amounts of nitrogen and d sulphur. When these fuels are burnt, nitrogen and sulphur too are burnt and this produces he different oxides of nitrogen and sulphur. Not only is the inhalation of these gases dangerous, they also dissolve in rain to give rise to acid rain. The combustion of fossil fuels also increases the amount of suspended is particles in air. These suspended particles could be unburnt carbon particles or substances called hydrocarbons. Presence of bl high levels of all these pollutants cause visibility to be lowered, especially in cold weather when water also condenses out of pu Fig. 14.2: Satellite picture showing clouds over India. air. This is known as smog and is a visible indication of air pollution. Studies have shown that regularly breathing air that be T Activity _____________ 14.4 contains any of these substances increases re Collect information from newspapers the incidence of allergies, cancer and heart o R or weather reports on television about diseases. An increase in the content of these rainfall patterns across the country. harmful substances in air is called air Also find out how to construct a rain- pollution. tt E gauge and make one. What precautions are necessary in order to get reliable data from this rain-gauge? Now answer C the following questions : In which month did your city/town/ village get the maximum rainfall? no N In which month did your state/union territory get the maximum rainfall? Is rain always accompanied by thunder and lightning? If not, in which season do you get more of thunder and © lightning with the rain? Activity _____________ 14.5 Find out more about monsoons and cyclones from the library. Try and find out the rainfall pattern of any other country. Is the monsoon responsible for rains the world over? Fig. 14.3: Lichen 192 SCIENCE  Activity _____________ 14.6 people are forced to spend considerable amounts of time in fetching water from far- Organisms called lichens are found to away sources. be very sensitive to the levels of contaminants like sulphur dioxide in Activity _____________ 14.7 the air. As discussed earlier in section 7.3.3, lichens can be commonly found Many municipal corporations are trying growing on the barks of trees as a thin water-harvesting techniques to greenish-white crust. See if you can improve the availability of water. find lichen growing on the trees in your Find out what these techniques are and locality. how they would increase the water that Compare the lichen on trees near busy is available for use. d roads and trees some distance away. On the trees near roads, compare the But why is water so necessary? And do all organisms require water? All cellular he incidence of lichen on the side facing the road and on the side away from the processes take place in a water medium. All road. the reactions that take place within our body What can you say about the levels of and within the cells occur between polluting substances near roads and away substances that are dissolved in water. is from roads on the basis of your findings Substances are also transported from one above? part of the body to the other in a dissolved Q form. Hence, organisms need to maintain the bl level of water within their bodies in order to uestions stay alive. Terrestrial life-forms require fresh 1. How is our atmosphere different water for this because their bodies cannot pu from the atmospheres on Venus tolerate or get rid of the high amounts of and Mars? dissolved salts in saline water. Thus, water 2. How does the atmosphere act as sources need to be easily accessible for be T a blanket? animals and plants to survive on land. 3. What causes winds? re Activity _____________ 14.8 o R 4. How are clouds formed? 5. List any three human activities Select a small area (say, 1 m 2) near a that you think would lead to air water-body, it may be a river, stream, tt E pollution. lake or pond. Count the number of different animals and plants in this 14.2 Water: A Wonder Liquid area. Also, check the number of C individuals of each type or species. Water occupies a very large area of the Earth’s Compare this with the number of surface and is also found underground. Some individuals (both animals and plants) no N found in an area of the same size in a amount of water exists in the form of water dry, rocky region. vapour in the atmosphere. Most of the water Is the variety of plant and animal life on Earth’s surface is found in seas and oceans the same in both these areas? and is saline. Fresh water is found frozen in © the ice-caps at the two poles and on snow- Activity _____________ 14.9 covered mountains. The underground water and the water in rivers, lakes and ponds is Select and mark out a small area (about 1 m2 ) in some unused land in or near also fresh. However, the availability of fresh your school. water varies from place to place. Practically As in the above activity, count the every summer, most places have to face a number of different animals and plants shortage of water. And in rural areas, where in this area and the number of water supply systems have not been installed, individuals of each species. NATURAL RESOURCES 193  Remember to do this in the same place the water in rivers can be affected is when water twice in a year, once during summer is released from dams. The water inside the or the dry season and once after it has deep reservoir would be colder than the water rained. at the surface which gets heated by the Sun. All this can affect the life-forms that are Now answer found in these water bodies in various ways. It can encourage the growth of some life-forms 1. Were the numbers similar both times? and harm some other life-forms. This affects 2. In which season did you find more the balance between various organisms which variety of plants and animals? had been established in that system. So we 3. In which season did you find more use the term water-pollution to cover the d number of individuals of each variety? following effects: 1. The addition of undesirable After compiling the results of the above he substances to water-bodies. These two activities, think if there is any relationship substances could be the fertilisers and between the amount of available water and pesticides used in farming or they the number and variety of plants and animals could be poisonous substances, like that can live in a given area. If there is a mercury salts which are used by is relationship, where do you think you would paper-industries. These could also be find a greater variety and abundance of life – disease-causing organisms, like the in a region that receives 5 cm of rainfall in a bacteria which cause cholera. bl year or a region that receives 200 cm of 2. The removal of desirable substances rainfall in a year? Find the map showing from water-bodies. Dissolved oxygen rainfall patterns in the atlas and predict is used by the animals and plants that pu which States in India would have the live in water. Any change that reduces maximum biodiversity and which would have the amount of this dissolved oxygen the least. Can we think of any way of checking be T would adversely affect these aquatic whether the prediction is correct? organisms. Other nutrients could also The availability of water decides not only re be depleted from the water bodies. o R the number of individuals of each species that 3. A change in temperature. Aquatic are able to survive in a particular area, but it organisms are used to a certain range also decides the diversity of life there. Of of temperature in the water-body tt E course, the availability of water is not the only where they live, and a sudden marked factor that decides the sustainability of life change in this temperature would be in a region. Other factors like the temperature dangerous for them or affect their C and nature of soil also matter. But water is breeding. The eggs and larvae of one of the major resources which determine various animals are particularly life on land. no N susceptible to temperature changes. Q 14.2.1 WATER POLLUTION uestions Water dissolves the fertilisers and pesticides 1. Why do organisms need water? © that we use on our farms. So some percentage 2. What is the major source of fresh of these substances are washed into the water water in the city/town/village bodies. Sewage from our towns and cities and where you live? the waste from factories are also dumped into 3. Do you know of any activity rivers or lakes. Specific industries also use which may be polluting this water water for cooling in various operations and source? later return this hot water to water-bodies. Another manner in which the temperature of 194 SCIENCE  14.3 Mineral Riches in the Soil carries sand from one place to the other like water does. Soil is an important resource that decides the Living organisms also influence the diversity of life in an area. But what is the formation of soil. The lichen that we soil and how is it formed? The outermost layer read about earlier, also grows on the of our Earth is called the crust and the surface of rocks. While growing, they minerals found in this layer supply a variety release certain substances that cause of nutrients to life-forms. But these minerals the rock surface to powder down and will not be available to the organisms if the form a thin layer of soil. Other small minerals are bound up in huge rocks. Over plants like moss, are able to grow on long periods of time, thousands and millions this surface now and they cause the d of years, the rocks at or near the surface of rock to break up further. The roots of the Earth are broken down by various big trees sometimes go into cracks in he physical, chemical and some biological the rocks and as the roots grow bigger, processes. The end product of this breaking the crack is forced bigger. down is the fine particles of soil. But what are the factors or processes that make soil? Activity ____________14.10 is The Sun: The Sun heats up rocks Take some soil and put it into a beaker during the day so that they expand. containing water. The water should be At night, these rocks cool down and at least five times the amount of soil bl contract. Since all parts of the rock taken. Stir the soil and water vigorously do not expand and contract at the and allow the soil to settle down. Observe after some time. same rate, this results in the Is the soil at the bottom of the beaker pu formation of cracks and ultimately the homogenous or have layers formed? huge rocks break up into smaller If layers have formed, how is one layer pieces. different from another? be T Water: Water helps in the formation Is there anything floating on the of soil in two ways. One, water could surface of the water? re get into the cracks in the rocks formed Do you think some substances would o R due to uneven heating by the Sun. If have dissolved in the water? How would this water later freezes, it would cause you check? tt E the cracks to widen. Can you think As you have seen, soil is a mixture. It why this should be so? Two, flowing contains small particles of rock (of different water wears away even hard rock over sizes). It also contains bits of decayed living C long periods of time. Fast flowing water organisms which is called humus. In addition, often carries big and small particles soil also contains various forms of of rock downstream. These rocks rub microscopic life. The type of soil is decided no N against other rocks and the resultant by the average size of particles found in it abrasion causes the rocks to wear and the quality of the soil is decided by the down into smaller and smaller amount of humus and the microscopic particles. The water then takes these organisms found in it. Humus is a major © particles along with it and deposits it factor in deciding the soil structure because further down its path. Soil is thus it causes the soil to become more porous and found in places far away from its allows water and air to penetrate deep parent-rock. underground. The mineral nutrients that are Wind: In a process similar to the way found in a particular soil depends on the in which water rubs against rocks and rocks it was formed from. The nutrient wears them down, strong winds also content of a soil, the amount of humus erode rocks down. The wind also present in it and the depth of the soil are NATURAL RESOURCES 195  some of the factors that decide which plants Study the amount of soil that is will thrive on that soil. Thus, the topmost carried out of the trays now. Is the layer of the soil that contains humus and amount the same in both the trays? living organisms in addition to the soil Is the amount of soil that is carried out more or less or equal to the amount particles is called the topsoil. The quality of washed out earlier? the topsoil is an important factor that decides biodiversity in that area. Modern farming practices involve the use of large amounts of fertilizers and pesticides. Use of these substances over long periods of time can destroy the soil structure by killing d the soil micro-organisms that recycle nutrients in the soil. It also kills the he Earthworms which are instrumental in making the rich humus. Fertile soil can quickly be turned barren if sustainable practices are not followed. Removal of useful components from the soil and addition of Fig. 14.4: Effect of flowing water on the top-soil is other substances, which adversely affect the fertility of the soil and kill the diversity of The roots of plants have an important role organisms that live in it, is called soil pollution. in preventing soil erosion. The large-scale bl The soil that we see today in one place deforestation that is happening all over the has been created over a very long period of world not only destroys biodiversity, it also time. However, some of the factors that leads to soil erosion. Topsoil that is bare of pu created the soil in the first place and brought vegetation, is likely to be removed very the soil to that place may be responsible for quickly. And this is accelerated in hilly or the removal of the soil too. The fine particles mountainous regions. This process of soil be T of soil may be carried away by flowing water erosion is very difficult to reverse. Vegetative or wind. If all the soil gets washed away and cover on the ground has a role to play in the re the rocks underneath are exposed, we have o R percolation of water into the deeper lost a valuable resource because very little layers too. Q will grow on the rock. tt E Activity ____________14.11 uestions Take two identical trays and fill them 1. How is soil formed? C with soil. Plant mustard or green gram 2. What is soil erosion? or paddy in one of the trays and water 3. What are the methods of both the trays regularly for a few days, preventing or reducing soil no N till the first tray is covered by plant growth. Now, tilt both the trays and fix erosion? them in that position. Make sure that both the trays are tilted at the same 14.4 Biogeochemical Cycles angle. Pour equal amount of water © gently on both trays such that the water A constant interaction between the biotic and flows out of the trays (Fig. 14.4). abiotic components of the biosphere makes Study the amount of soil that is carried it a dynamic, but stable system. These out of the trays. Is the amount the interactions consist of a transfer of matter same in both the trays? Now pour equal amounts of water on and energy between the different components both the trays from a height. Pour three of the biosphere. Let us look at some or four times the amount that you processes involved in the maintenance of the poured earlier. above balance. 196 SCIENCE  14.4.1 T HE WATER- CYCLE water. Thus rivers carry many nutrients from the land to the sea, and these are used by You have seen how the water evaporates from the marine organisms. the water bodies and subsequent condensation of this water vapour leads to 14.4.2 T HE NITROGEN- CYCLE rain. But we don’t see the seas and oceans drying up. So, how is the water returning to Nitrogen gas makes up 78% of our these water bodies? The whole process in atmosphere and nitrogen is also a part of which water evaporates and falls on the land many molecules essential to life like proteins, as rain and later flows back into the sea via nucleic acids (DNA and RNA) and some rivers is known as the water-cycle. This cycle vitamins. Nitrogen is found in other d is not as straight-forward and simple as this biologically important compounds such as statement seems to imply. All of the water alkaloids and urea too. Nitrogen is thus an he that falls on the land does not immediately essential nutrient for all life-forms and life flow back into the sea. Some of it seeps into would be simple if all these life-forms could the soil and becomes part of the underground use the atmospheric nitrogen directly. reservoir of fresh-water. Some of this However, other than a few forms of bacteria, underground water finds its way to the life-forms are not able to convert the is surface through springs. Or we bring it to comparatively inert nitrogen molecule into the surface for our use through wells or tube- forms like nitrates and nitrites which can be wells. Water is also used by terrestrial animals taken up and used to make the required bl and plants for various life-processes molecules. These ‘nitrogen-fixing’ bacteria (Fig. 14.5). may be free-living or be associated with some species of dicot plants. Most commonly, the pu nitrogen-fixing bacteria are found in the roots of legumes (generally the plants which give us pulses) in special structures called root- be T nodules. Other than these bacteria, the only re other manner in which the nitrogen molecule o R is converted to nitrates and nitrites is by a physical process. During lightning, the high temperatures and pressures created in the tt E air convert nitrogen into oxides of nitrogen. These oxides dissolve in water to give nitric and nitrous acids and fall on land along with C rain. These are then utilised by various life- forms. What happens to the nitrogen once it is no N converted into forms that can be taken up and used to make nitrogen-containing molecules? Plants generally take up nitrates and nitrites and convert them into amino © Fig. 14.5: Water-cycle in nature acids which are used to make proteins. Some other biochemical pathways are used to make Let us look at another aspect of what the other complex compounds containing happens to water during the water-cycle. As nitrogen. These proteins and other complex you know, water is capable of dissolving a compounds are subsequently consumed by large number of substances. As water flows animals. Once the animal or the plant dies, through or over rocks containing soluble other bacteria in the soil convert the various minerals, some of them get dissolved in the compounds of nitrogen back into nitrates and NATURAL RESOURCES 197  d he is bl pu be T Fig.14.6: Nitrogen-cycle in nature re o R nitrites. A different type of bacteria converts nucleic acids and vitamins. The endoskeletons the nitrates and nitrites into elemental and exoskeletons of various animals are also tt E nitrogen. Thus, there is a nitrogen-cycle in formed from carbonate salts. Carbon is nature in which nitrogen passes from its incorporated into life-forms through the basic elemental form in the atmosphere into simple process of photosynthesis which is performed C molecules in the soil and water, which get in the presence of Sunlight by all life-forms that converted to more complex molecules in living contain chlorophyll. This process converts beings and back again to the simple nitrogen carbon dioxide from the atmosphere or no N molecule in the atmosphere. dissolved in water into glucose molecules. These glucose molecules are either converted 14.4.3 THE CARBON- CYCLE into other substances or used to provide © Carbon is found in various forms on the energy for the synthesis of other biologically Earth. It occurs in the elemental form as important molecules (Fig. 14.7). diamonds and graphite. In the combined The utilisation of glucose to provide energy state, it is found as carbon dioxide in the to living things involves the process of atmosphere, as carbonate and hydrogen- respiration in which oxygen may or may not carbonate salts in various minerals, while all be used to convert glucose back into carbon life-forms are based on carbon-containing dioxide. This carbon dioxide then goes back molecules like proteins, carbohydrates, fats, into the atmosphere. Another process that 198 SCIENCE  d he is bl pu be T Fig. 14.7: Carbon-cycle in nature re o R adds to the carbon dioxide in the atmosphere could be kept warm during the winters in is the process of combustion where fuels are colder climates. Such enclosures are called burnt to provide energy for various needs like greenhouses. Greenhouses have also lent tt E heating, cooking, transportation and their name to an atmospheric phenomenon. industrial processes. In fact, the percentage Some gases prevent the escape of heat from of carbon dioxide in the atmosphere is said the Earth. An increase in the percentage of C to have doubled since the industrial such gases in the atmosphere would cause revolution when human beings started the average temperatures to increase world- burning fossil fuels on a very large scale. wide and this is called the greenhouse effect. no N Carbon, like water, is thus cycled repeatedly Carbon dioxide is one of the greenhouse through different forms by the various gases. An increase in the carbon dioxide physical and biological activities. content in the atmosphere would cause more heat to be retained by the atmosphere and © lead to global warming. 14.4.3 ((ii ) T HE GREENHOUSE EFFECT Recall the reading taken by you under (iii) in Activity ____________14.12 Activity 14.1. Heat is trapped by glass, and Find out what the consequences of hence the temperature inside a glass global warming would be. enclosure will be much higher than the Also, find out the names of some other surroundings. This phenomenon was used greenhouse gases. to create an enclosure where tropical plants NATURAL RESOURCES 199  14.4.4 T HE OXYGEN- CYCLE bacteria, are poisoned by elemental oxygen. In fact, even the process of nitrogen-fixing by Oxygen is a very abundant element on our bacteria does not take place in the presence Earth. It is found in the elemental form in of oxygen. the atmosphere to the extent of 21%. It also occurs extensively in the combined form in the Earth’s crust as well as also in the air in 14.5 Ozone Layer the form of carbon dioxide. In the crust, it is Elemental oxygen is normally found in the found as the oxides of most metals and form of a diatomic molecule. However, in the silicon, and also as carbonate, sulphate, upper reaches of the atmosphere, a molecule nitrate and other minerals. It is also an containing three atoms of oxygen is found. d essential component of most biological This would mean a formula of O3 and this is molecules like carbohydrates, proteins, called ozone. Unlike the normal diatomic he nucleic acids and fats (or lipids). molecule of oxygen, ozone is poisonous and But when we talk of the oxygen-cycle, we we are lucky that it is not stable nearer to are mainly referring to the cycle that the Earth’s surface. But it performs an maintains the levels of oxygen in the essential function where it is found. It absorbs atmosphere. Oxygen from the atmosphere is harmful radiations from the Sun. This is used up in three processes, namely prevents those harmful radiations from combustion, respiration and in the formation reaching the surface of the Earth where they of oxides of nitrogen. Oxygen is returned to may damage many forms of life. bl the atmosphere in only one major process, Recently it was discovered that this ozone that is, photosynthesis. And this forms the layer was getting depleted. Various man-made broad outline of the oxygen-cycle in nature compounds like CFCs (carbon compounds pu (Fig. 14.8). having both fluorine and chlorine which are very stable and not degraded by any biological process) were found to persist in the be T atmosphere. Once they reached the ozone re layer, they would react with the ozone o R molecules. This resulted in a reduction of the ozone layer and recently they have discovered a hole in the ozone layer above the Antartica. tt E It is difficult to imagine the consequences for life on Earth if the ozone layer dwindles further, but many people think that it would C be better not to take chances. These people advocate working towards stopping all further damage to the ozone layer. no N © Fig. 14.8: Oxygen-cycle in nature October October October Though we usually think of oxygen as 1980 1985 1990 being necessary to life in the process of respiration, it might be of interest to you to Fig. 14.9: Satellite picture showing the hole (magenta learn that some forms of life, especially colour) in the ozone layer over Antartica 200 SCIENCE  Q Activity ____________14.13 uestions Find out which other molecules are 1. What are the different states in thought to damage the ozone layer. which water is found during the Newspaper reports often talk about the water cycle? hole in the ozone layer. 2. Name two biologically important Find out whether the size of this hole compounds that contain both is changing and in what manner scientists think this would affect life oxygen and nitrogen. on Earth (Fig. 14.9). 3. List any three human activities which would lead to an increase in the carbon dioxide content of air. d 4. What is the greenhouse effect? 5. What are the two forms of he oxygen found in the atmosphere? What is you have learnt bl Life on Earth depends on resources like soil, water and air, and energy from the Sun. pu Uneven heating of air over land and water-bodies causes winds. Evaporation of water from water-bodies and subsequent be T condensation give us rain. Rainfall patterns depend on the prevailing wind patterns in an re area. o R Various nutrients are used again and again in a cyclic fashion. This leads to a certain balance between the various components tt E of the biosphere. Pollution of air, water and soil affect the quality of life and C harm the biodiversity. We need to conserve our natural resources and use them in a sustainable manner. no N Exercises © 1. Why is the atmosphere essential for life? 2. Why is water essential for life? 3. How are living organisms dependent on the soil? Are organisms that 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? NATURAL RESOURCES 201  5. We know that many human activities lead to increasing levels of pollution of the air, water-bodies and soil. Do you think that isolating these activities to specific and limited areas would help in reducing pollution? 6. Write a note on how forests influence the quality of our air, soil and water resources. d he is bl pu be T re o R tt E C no N © 202 SCIENCE