CBSE NOTES CLASS 9 SCIENCE CHAPTER 14
The different resources available on the earth are land, water and air.
All of these three things are required for the existence of life.
Lithosphere: The outer crust of the Earth.
Hydrosphere: 75% of the earth’s surface is covered with water. This water along with underground water comprises the hydrosphere.
Atmosphere: The air covering of earth is called atmosphere.
Biosphere: The life-supporting zone of the earth where the atmosphere, hydrosphere and the lithosphere interact and make life possible is known as the biosphere.
Biotic components: The living things constitute the biotic components of the biosphere.
Abiotic components: The non-living things air, water and soil, rain etc. form the abiotic components of the biosphere.
Air - The Breath of Life
Air is a mixture of many gases like nitrogen, oxygen, carbon dioxide and water vapours. On Venus and Mars carbon dioxide constitutes 95-97% of the atmosphere and there is no life.
Oxygen is consumed and carbon dioxide is produced in the atmosphere by following activities:
(i) Breakdown of glucose in presence of oxygen by organisms during respiration.
(ii) Combustion of fuels, including household activities, forest fires, industrial activities etc.
Carbon dioxide is fixed in two ways
Despite large production, the percentage of carbon dioxide in our atmosphere is a mere fraction of a percent because carbon dioxide is ‘fixed’ in two ways:
(i) Green plants convert carbon dioxide into glucose in the presence of Sunlight and
(ii) Many marine animals use carbonates dissolved in sea-water to make their shells.
The Role of Atmosphere in Climate Control
(i) Atmosphere keeps the average temperature of the earth steady during the day and whole year.
(ii) Atmosphere prevents sudden increase in temperature during daytime and sudden fall of temperature during night.
Moon has no atmosphere, its temperature ranges from -190°C to 110°C.
Movement of Air – Winds
The movement of air from one region to the other creates winds.
Reason: Sand and water do not heat up evenly. Also different parts of the earth do not receive sunlight equally. Because of this uneven heating there is movement of air and water which cause various climatic conditions like rain, winds, thunder etc.
When air is heated by radiation from the heated land or water, it rises. But since land gets heated faster than water, the air over land is also heated faster than the air over water bodies.
In coastal regions during the day, the air above the land gets heated faster and starts rising. A region of low pressure is created and air over the sea moves into this area of low pressure.
During the day, the direction of the wind would be from the sea to the land. This wind from sea to land is called sea breeze.
At night, both land and sea start to cool. Since water cools down slower than the land, the air above water would be warmer than the air above land. This results in wind from land to sea, which is called land breeze.
Uneven heating – region near equator receives more heat than poles. As a result the air in equatorial regions rises and creates a low pressure. The cold air from polar regions rushes to take its place creating a wind. These winds are called trade winds.
But various other factors also influence these winds – the rotation of the Earth and the presence of mountain ranges in the paths of the wind.
Cloud Formation and Rains
When water bodies are heated during the day, a large amount of water evaporates and goes into the air. Some amount of water vapour also gets into the atmosphere because of various biological activities like respiration, transpiration etc.
The hot air rises up carrying the water vapour with it. As the air rises, it expands and cools. This cooling causes the water vapour in the air to condense in the form of tiny droplets around dust and other suspended particles in the air, which act as nucleuses. This is cloud.
When the drops have grown big and heavy, they fall down in the form of rain.
When the temperature of air is low enough, precipitation may occur in the form of snow, sleet or hail.
Rainfall Distribution in India
Rainfall patterns are decided by the prevailing wind patterns.
In large parts of India, rains are mostly brought by the southwest or northeast monsoons or sometimes due depressions in pressure.
All parts of India do not receive the same amount of rainfall. This affects the lives of people and animals as well as plants and crops.
The addition of substances which have harmful or poisonous effects or removal of desirable substances from the environment is called pollution.
Causes of Air Pollution
- Burning of Fossil Fuels by Vehicles: Burning of fossil fuels by vehicles produces harmful gases like Sulfur dioxide, Carbon Monoxide, Nitrogen Oxides, Carbon dioxide etc. Particulate matter and suspended particles are also contributed due to the soot or smoke.
- Agricultural activities: Ammonia is released during agriculture related activities and is one of the most hazardous gases in the atmosphere. Use of insecticides, pesticides and fertilizers in agricultural activities has grown quite a lot. They emit harmful chemicals into the air.
- Exhaust from factories and industries: Industries release large amount of carbon monoxide, hydrocarbons, organic compounds, and chemicals into the air thereby depleting the quality of air.
- Mining operations: Mining is a process wherein minerals below the earth are extracted using large equipments. During the process dust and chemicals are released in the air causing massive air pollution. This is one of the reasons responsible for the deteriorating health conditions of workers and nearby residents.
- Construction activities: Sand and other particulate matter due to construction activity get into air and cause air pollution.
- Indoor air pollution: Household cleaning products, painting supplies emit toxic chemicals in the air and cause air pollution.
Effects of Air pollution
- Respiratory and heart problems: They are known to create several respiratory problems like pneumonia and asthma, heart conditions and cancer.
- Greenhouse Effect & Global warming:
Some gases like carbon dioxide, methane, nitro us oxide prevent the escape of heat from the Earth’s surface by trapping it. This increases the average temperature of the Earth. This is called the greenhouse effect. An increase in the content of such gases would lead to a situation of global warming.
Global Warming is the increase of Earth's average surface temperature due to effect of greenhouse gases. Green house gases are – carbon dioxide, methane and CFCs.
Increase in the carbon dioxide or other greenhouse gases in the atmosphere causes more heat to be retained by the atmosphere and leads to global warming.
Effects of global warming:
- Increased temperatures worldwide,
- Increase in sea levels and submerging of land into sea.
- Melting of ice from colder regions and icebergs,
- Displacement and loss of habitat for flora and fauna, including human beings.
- Acid Rain: Gases like nitrogen oxides and sulphur oxides released into the atmosphere during the burning of fossil fuels combine with rain water produce acid rain. Acid rain can cause great damage to human, animals and crops.
- Depletion of Ozone Layer: Ozone (O3) exists in earth’s stratosphere and is responsible for protecting humans from harmful ultraviolet (UV) rays. Earth’s ozone layer is depleting due to the presence of chlorofluorocarbons (CFC) in the atmosphere. These are released by ACs, refrigerators etc. As ozone layer becomes thinner, it will no longer provide protection from UV rays.
UV rays can cause skin and eye related problems, including cancer.
Importance of water
Distribution of water is not uniform throughout the world, due to uneven rains and climatic conditions.
- Water is very important in sustaining the plant and animal life on earth.
- Water is used for making food during photosynthesis in plants.
- It also is a moderator of temperature on earth, does not allow sudden rise or fall of temperature.
- The areas in which plenty of water is available are able to support larger number of individuals of each species.
- It also decides the diversity of life, meaning the number of species in a particular area.
Other factors like the temperature and nature of soil also matter in deciding the sustainability of life in a particular region.
The main causes of water pollution are:
(i) The addition of undesirable substances to water-bodies. These substances could be
(a) The fertilisers and pesticides used in farming or
(b) They could be poisonous substances, like mercury salts which are used by paper-industries.
(c) These could also be disease-causing organisms, like the bacteria which cause cholera.
(ii) The removal of desirable substances from water-bodies. For example
(a) Dissolved oxygen is used by the animals and plants that live in water. Any change that reduces the amount of this dissolved oxygen would adversely affect these aquatic organisms.
(b) Other nutrients could also be depleted from the water bodies.
(iii) A change in temperature. Aquatic organisms are used to a certain range of temperature in the water-body where they live, and a sudden marked change in this temperature would be dangerous for them or affect their breeding. The eggs and larvae of various animals are particularly susceptible to temperature changes.
Eutrophication: Eutrophication is a condition in which high amount of fertilizers and chemicals in the water bodies leads to plants and algae on the surface of water bodies. It reduces the dissolved oxygen and nutrients from the water and adversely affects fish, plants and animal species.
Formation of soil
Soil is the most important natural resource which supplies nutrients to the life forms. Soil is formed by weathering of rocks in thousands of years, due to,
- Sun - The Sun heats up rocks during the day so that they expand. At night, these rocks cool down and contract. Since all parts of the rock do not expand and contract at the same rate, this results in the formation of cracks and ultimately the huge rocks break up into smaller pieces.
- Water - Water helps in the formation of soil in two ways.
- Water could get into the cracks in the rocks formed due to uneven heating by the Sun. If this water later freezes, it would cause the cracks to widen.
- Flowing water wears away even hard rock over long periods of time. Fast flowing water carries big and small particles of rock downstream. These rocks rub against other rocks and the resultant abrasion causes the rocks to wear down into smaller and smaller particles. The water then takes these particles along with it and deposits it further down its path. Soil is thus found in places far away from its parent rock.
- Wind - Strong winds erode rocks down. The wind also carries sand from one place to the other.
- Some organisms like lichen and mosses grow on the surface of rocks and they release certain substances that cause weathering of rocks and a thin layer of soil is formed. Plants now grow on this and break the rocks further to form soil.
Humus: A dark-brown or black organic substance made up of decayed plant or animal matter is called humus Humus provides nutrients for plants and increases the ability of soil to retain water. Humus makes the soil porous and allows water and air to penetrate deep underground.
Topsoil: The topmost layer of the soil that contains humus and living organisms in addition to the soil particles is called the topsoil. The quality of the topsoil is an important factor that decides biodiversity in that area.
Removal of useful components from the soil and addition of other substances, which adversely affect the fertility of the soil and kill micro-organisms living in it, is called soil pollution.
Use of large amounts of fertilizers and pesticides over long periods of time can destroy the soil structure by killing the soil micro-organisms that recycle nutrients in the soil. It also kills the earthworms which are instrumental in making the rich humus.
Removal of topsoil by wind, water or other activities is called soil erosion.
Roots of plants prevent soil erosion by firmly holding the soil particles. The large-scale deforestation destroys biodiversity and also leads to soil erosion.
Topsoil that is bare of vegetation, is likely to be removed very quickly by wind and water.
Soil erosion is accelerated in hilly or mountainous regions.
Harmful Effects of Soil Erosion
(i) The implications of soil erosion leads to removal of valuable topsoil. Crop emergence, growth and yield are directly affected by the loss of natural nutrients and applied fertilizers.
(ii) Seeds and plants can be disturbed or completely removed by the erosion.
(iii) Soil quality, structure, stability and texture can be affected by the loss of soil.
(iv) It affects the water-holding capacity of the soil, making it more susceptible to extreme conditions such as drought.
Methods of preventing soil erosion
(i) Forestation: Growing plant grass and shrubs
(ii) Build retaining walls or terraces for steep slopes
Various biogeochemical cycles exist in the nature due to interaction between biotic and abiotic components of the biosphere to make a stable system.
(A) Water cycle
The Water Cycle (also known as the hydrologic cycle) is the journey water takes as it circulates from the land to the sky and back again.
The Sun's heat provides energy to evaporate water from the Earth's surface (oceans, lakes, etc.).
Plants also lose water to the air through transpiration.
Animals lose water due to respiration and perspiration.
The water vapor eventually condenses, forming tiny droplets in clouds. When the clouds meet cool air over land, precipitation (rain, sleet, or snow) is triggered, and water returns to the land (or sea).
Some of the precipitation soaks into the ground. Some of the underground water is trapped between rocks or layers of clay which is called groundwater.
But most of the water flows downhill as runoff (above ground or underground), eventually returning to the seas as slightly salty water.
The water running off to sea dissolves lots of sand and chemicals, thereby making the sea water salty.
(B) Nitrogen cycle
The series of processes by which nitrogen and its compounds are inter-converted in the environment and in living organisms, including nitrogen fixation and decomposition, is called nitrogen cycle.
Nitrogen is found in biologically important compounds such as proteins, alkaloids and urea. Nitrogen is thus an essential nutrient for all life-forms and life.
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.
Nitrogen is used by life forms to carry out many of the functions of life.
It is converted or ‘fixed’ into compounds usable by living organism.
The process of converting nitrogen into its compounds is called nitrogen fixation.
Specialized bacteria fix the nitrogen, converting it to compounds, so that it can be used by plants.
It can also be done 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.
After nitrogen is fixed, it can be absorbed and used by plants, and subsequently by animals.
Animal and plant waste and dead bodies are decomposed and the nitrogen is returned to atmosphere in its gaseous form.
The process of nitrogen being fixed, used by plants and animals, and later returned to the atmosphere is referred to as the nitrogen cycle.
(C) Carbon cycle
The series of processes by which carbon compounds are interconverted in the environment, involving the incorporation of carbon dioxide into living tissue by photosynthesis and its return to the atmosphere through respiration, the decay of dead organisms, and the burning of fossil fuels.
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. This process converts carbon dioxide from the atmosphere or dissolved in water into glucose molecules.
These glucose molecules are either converted into other substances or used to provide energy for the synthesis of other molecules.
The carbon dioxide is produced during the process of respiration and goes into the atmosphere.
Carbon dioxide is also produced during burning of fuels.
Carbon dioxide is also found as carbonate and hydrogencarbonate salts in various minerals.
Carbon containing molecules like proteins, carbohydrates, fats, nucleic acids and vitamins are the basis of life.
The endoskeletons and exoskeletons of various animals are also formed from carbonate salts.
(D) Oxygen cycle
Oxygen cycle is a biogeochemical cycle that describes the movement of oxygen within the earth’s three main reservoirs – the lithosphere (land), the hydrosphere (water), and the atmosphere (air), which make up the earth’s biosphere.
Plants produce oxygen during photosynthesis.
All organisms use oxygen for respiration.
When plants and animals die, they decompose. This process uses up oxygen and releases carbon dioxide into the air.
Combustion also uses up oxygen and releases carbon dioxide into the atmosphere.
- Carbon cycle and oxygen cycle are related.