A constant interaction between the biotic and abiotic components of the biosphere makes it a dynamic, but stable system and because of these interactions there occurs transfer of matter and energy between the different components of the biosphere. Let us look at some processes involved in the maintenance of the above balance.

`color{green}(★)` Evaporation of water takes place from the water bodies and subsequent condensation of this water vapour leads to rain.

`color{green}(★)` The whole process in which water evaporates and falls on the land as rain and later flows back into the sea via rivers is known as the water-cycle.

`color{green}(★)` All of the water that falls on the land does not immediately flow back into the sea.

`color{green}(★)` Some of it seeps into the soil and becomes part of the underground reservoir of fresh-water.

`color{green}(★)` Some of this underground water finds its way to the surface through springs.

`color{green}(★)` Or we bring it to the surface for our use through wells or tubewells. Water is also used by terrestrial animals and plants for various life-processes (Fig. 14.5).


`color{green}(★)` As water is capable of dissolving a large number of substances so as water flows through or over rocks containing soluble minerals, some of them get dissolved in the water.

`color{green}(★)` Thus rivers carry many nutrients from the land to the sea, and these are used by the marine organisms.

`color{green}(★)` Nitrogen gas makes up `color{red}(78%)` of our atmosphere and nitrogen is also a part of many molecules essential to life like proteins, nucleic acids (DNA and RNA) and some vitamins.

`color{green}(★)` Nitrogen is found in other biologically important compounds such as alkaloids and urea too.

`color{green}(★)` Nitrogen is thus an essential nutrient for all life-forms and life would be simple if all these life-forms could use the atmospheric nitrogen directly.

`color{green}(★)` However, other than a few forms of bacteria, life-forms are not able to convert the comparatively inert nitrogen molecule into forms like nitrates and nitrites which can be taken up and used to make the required molecules.

`color{green}(★)` These ‘nitrogen-fixing’ bacteria may be free-living or be associated with some species of dicot plants.

`color{green}(★)` Most commonly, the nitrogen-fixing bacteria are found in the roots of legumes (generally the plants which give us pulses) in special structures called rootnodules.

`color{green}(★)` Other than these bacteria, the only other manner in which the nitrogen molecule is converted to nitrates and nitrites is by a physical process.

`color{green}(★)` During lightning, the high temperatures and pressures created in the air convert nitrogen into oxides of nitrogen. These oxides dissolve in water to give nitric and nitrous acids and fall on land along with rain which are then utilised by various lifeforms.


`color{green}(★)` Plants generally take up nitrates and nitrites and convert them into amino acids which are used to make proteins.

`color{green}(★)` These proteins and other complex compounds are subsequently consumed by animals.

`color{green}(★)` Once the animal or the plant dies, other bacteria in the soil convert the various compounds of nitrogen back into nitrates and nitrites.

`color{green}(★)` A different type of bacteria converts the nitrates and nitrites into elemental nitrogen.

Thus, there is a nitrogen-cycle in nature in which nitrogen passes from its elemental form in the atmosphere into simple molecules in the soil and water, which get converted to more complex molecules in living beings and back again to the simple nitrogen molecule in the atmosphere.

`color{green}(★)` Carbon is found in various forms on the Earth.

`color{green}(★)` It occurs in the elemental form as diamonds and graphite.

`color{green}(★)` In the combined state, it is found as carbon dioxide in the atmosphere, as carbonate and hydrogen carbonate salts in various minerals, while all life-forms are based on carbon-containing molecules like proteins, carbohydrates, fats, nucleic acids and vitamins.

`color{green}(★)` The endoskeletons and exoskeletons of various animals are also formed from carbonate salts.

`color{green}(★)` Carbon is incorporated into life-forms through the basic process of photosynthesis which is performed in the presence of Sunlight by all life-forms that contain chlorophyll.

`color{green}(★)` This process converts carbon dioxide from the atmosphere or dissolved in water into glucose molecules.

`color{green}(★)` These glucose molecules are either converted into other substances or used to provide energy for the synthesis of other biologically important molecules (Fig. 14.7).


`color{green}(★)` The utilisation of glucose to provide energy to living things involves the process of respiration in which oxygen may or may not be used to convert glucose back into carbon dioxide.

`color{green}(★)` This carbon dioxide then goes back into the atmosphere.

`color{green}(★)` Another process that adds to the carbon dioxide in the atmosphere is the process of combustion where fuels are burnt to provide energy for various needs like heating, cooking, transportation and industrial processes.

`color{green}(★)` In fact, the percentage of carbon dioxide in the atmosphere is said to have doubled since the industrial revolution when human beings started burning fossil fuels on a very large scale.

Carbon, like water, is thus cycled repeatedly through different forms by the various physical and biological activities.

`color{green}(★)` Heat is trapped by glass, and hence the temperature inside a glass enclosure will be much higher than the surroundings.

`color{green}(★)` This phenomenon was used to create an enclosure where tropical plants could be kept warm during the winters in colder climates. Such enclosures are called greenhouses.

`color{green}(★)` Some gases prevent the escape of heat from the Earth.

`color{green}(★)` An increase in the percentage of such gases in the atmosphere would cause the average temperatures to increase worldwide and this is called the greenhouse effect.

`color{green}(★)` Carbon dioxide is one of the greenhouse gases.

`color{green}(★)` An increase in the carbon dioxide content in the atmosphere would cause more heat to be retained by the atmosphere and lead to global warming.

`color{green}(★)` Oxygen is a very abundant element on our Earth.

`color{green}(★)` It is found in the elemental form in the atmosphere to the extent of `color{red}(21%)`.

`color{green}(★)` It also occurs extensively in the combined form in the Earth’s crust as well as also in the air in the form of carbon dioxide.

`color{green}(★)` In the crust, it is found as the oxides of most metals and silicon, and also as carbonate, sulphate, nitrate and other minerals.

`color{green}(★)` It is also an essential component of most biological molecules like carbohydrates, proteins, nucleic acids and fats (or lipids).

`color{green}(★)` Oxygen from the atmosphere is used up in three processes, namely combustion, respiration and in the formation of oxides of nitrogen.

`color{green}(★)` Oxygen is returned to the atmosphere in only one major process, that is, photosynthesis.

`color{green}(★)` And this forms the broad outline of the oxygen-cycle in nature (Fig. 14.8).


`color{green}(★)` Some forms of life, especially bacteria, are poisoned by elemental oxygen.

`color{green}(★)` In fact, even the process of nitrogen-fixing by bacteria does not take place in the presence of oxygen.

`color{green}(★)` Elemental oxygen is normally found in the form of a diatomic molecule.

`color{green}(★)` However, in the upper reaches of the atmosphere, a molecule containing three atoms of oxygen is found. This would mean a formula of `color{red}(O_3)` and this is called ozone.

`color{green}(★)` Ozone is poisonous and it is not stable nearer to the Earth’s surface.

`color{green}(★)` It absorbs harmful radiations from the Sun.

`color{green}(★)` This prevents those harmful radiations from reaching the surface of the Earth where they may damage many forms of life.


`color{green}(★)` Various man-made compounds like CFCs (carbon compounds having both fluorine and chlorine which are very stable and not degraded by any biological process) were found to persist in the atmosphere.

`color{green}(★)` Once they reached the ozone layer, they would react with the ozone molecules.

`color{green}(★)` This resulted in a reduction of the ozone layer and recently they have discovered a hole in the ozone layer above the Antartica.

`color{green}(★)` It is difficult to imagine the consequences for life on Earth if the ozone layer dwindles further, but many people think that it would be better not to take chances.