1. The Biosphere and its Components

Life on Earth is possible because of the presence of essential resources like land, water, and air, along with energy from the Sun.

• Lithosphere: The solid outer crust of the Earth.
• Hydrosphere: The water that covers 75% of the Earth's surface (oceans, rivers, lakes, underground water).
• Atmosphere: The blanket of air that covers the whole Earth.
• Biosphere: The life-supporting zone of the Earth where the atmosphere, the hydrosphere, and the lithosphere interact and make life possible.

The biosphere consists of biotic components (living things like plants and animals) and abiotic components (non-living things like air, water, and soil).

2. The Breath of Life: Air

Air is a mixture of many gases like nitrogen, oxygen, carbon dioxide, and water vapour. On Earth, life is sustained because of this specific composition (unlike Venus and Mars, where the atmosphere is mainly CO₂, resulting in no life).

2.1 The Role of the Atmosphere in Climate Control

The atmosphere acts like a blanket covering the Earth.

• Air is a bad conductor of heat.
• It keeps the average temperature of the Earth fairly steady during the day and even during the course of the whole year.
• It prevents the sudden increase in temperature during daylight hours and slows down the escape of heat into outer space during the night. (Unlike the Moon, which lacks an atmosphere and experiences temperature variations from -190°C to 110°C).

2.2 The Movement of Air: Winds

The movement of air is caused by the uneven heating of the atmosphere in different regions of the Earth. As air gets heated, it becomes lighter and rises. As it rises, a region of low pressure is created, and cooler air from high-pressure areas rushes in to fill the gap. This movement of air causes winds.

• Coastal Breezes: During the day, land heats up faster than the sea. The air over the land rises, and cooler air from the sea moves in (Sea Breeze). At night, land cools faster than the sea. The air over the sea is warmer, so it rises, and cooler air from the land moves towards the sea (Land Breeze).

2.3 Rain

When water bodies heat up during the day, a large amount of water evaporates and goes into the air. This warm air rises carrying the water vapour. As the air rises, it expands and cools. This cooling causes the water vapour to condense in the form of tiny droplets. Dust and other suspended particles act as the 'nucleus' for these drops to form around. Once the drops grow big and heavy, they fall down in the form of rain.

3. Water: A Wonder Liquid

Water occupies a very large area of the Earth's surface. A major part is in oceans (saline). Freshwater is found in ice caps (poles, snow-covered mountains), underground, and in rivers, lakes, and ponds.

• Importance of Water: All cellular processes take place in a water medium. Substances are transported from one part of the body to another in a dissolved form. Organisms need to maintain the level of water within their bodies to stay alive. It also determines the biodiversity of an area.

3.1 Water Pollution

Water pollution is the addition of undesirable substances to water bodies or the removal of desirable substances from them. Causes include:

1. Addition of undesirable substances: Fertilizers and pesticides from farms, poisonous mercury salts from paper industries, or disease-causing organisms (like bacteria causing cholera).
2. Removal of desirable substances: Dissolved oxygen is essential for aquatic life. Degrading pollution consumes this oxygen, suffocating fish and plants.
3. Change in temperature (Thermal Pollution): Water released from cooling towers of power plants is very hot, which suddenly changes the temperature of the water body, destroying aquatic ecosystems and affecting breeding.

4. Mineral Riches in the Soil

Soil is an important resource that decides the diversity of life in an area. It is formed over millions of years by the breakdown of rocks.

4.1 Factors that Make Soil

• The Sun: Heats up rocks during the day causing them to expand. At night, they cool and contract. Uneven expansion and contraction form cracks, eventually breaking the rocks into smaller pieces.
• Water: It enters cracks in the rocks. If it freezes, it expands and widens the cracks. Fast-flowing water also carries hard rock particles, rubbing against other rocks, turning them into smaller particles.
• Wind: Strong winds erode rocks down, similar to water, and carry sand from one place to another.
• Living Organisms: Lichens growing on rocks release substances that powder the rock surface to form a thin layer of soil. Plants like mosses grow on this and further break the rock. Roots of big trees go into cracks and force them wider.

4.2 Soil Erosion

The removal of the topsoil by wind or flowing water is called soil erosion. Deforestation is a major cause. The roots of plants bind the soil firmly; without plants, the topsoil is easily blown or washed away, turning fertile land into barren deserts.

5. Biogeochemical Cycles

The constant interaction between the biotic and abiotic components of the biosphere makes it a dynamic but stable system. These interactions consist of a transfer of matter and energy between different components. These cyclical pathways are known as biogeochemical cycles.

5.1 The Water-Cycle

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.

• It involves evaporation, condensation, and precipitation.
• Water is not just flowing; it dissolves many minerals from rocks as it flows over them and carries them to the sea, where marine organisms use them.

5.2 The Nitrogen-Cycle

Nitrogen gas makes up 78% of our atmosphere. It is a part of many molecules essential to life like proteins, nucleic acids (DNA/RNA), and some vitamins.

1. Nitrogen Fixation: Most life forms cannot use atmospheric nitrogen directly. It must be converted into nitrates and nitrites. This is done by nitrogen-fixing bacteria (like Rhizobium found in root nodules of leguminous plants) or by physical processes like lightning (which converts nitrogen and oxygen into nitrogen oxides that dissolve in rain to form nitric and nitrous acids).
2. Assimilation: Plants take up nitrates/nitrites and convert them into amino acids which are used to make proteins. Animals consume plants and get these proteins.
3. Ammonification & Nitrification: When plants and animals die, bacteria in the soil turn the nitrogen from their proteins back into ammonia, and other bacteria convert ammonia into nitrites and nitrates.
4. Denitrification: A different type of bacteria converts the nitrates back into elemental nitrogen gas, which escapes into the atmosphere.

5.3 The Carbon-Cycle

Carbon is found in various forms on Earth: elemental forms like diamond and graphite, and combined forms like carbon dioxide in the atmosphere, carbonates in rocks, and in all organic molecules (carbohydrates, proteins, fats) of life forms.

1. Photosynthesis: Plants use sunlight to convert atmospheric CO₂ into glucose molecules.
2. Respiration: All living organisms break down glucose for energy, releasing CO₂ back into the atmosphere.
3. Decomposition and Combustion: Dead plants and animals decompose, releasing CO₂. The burning of fossil fuels (coal, petroleum) for cooking, heating, and transport adds massive amounts of CO₂ to the atmosphere.

5.4 The Oxygen-Cycle

Oxygen constitutes 21% of our atmosphere. It is an essential component of most biological molecules (carbohydrates, proteins, nucleic acids, fats).

• Usage: Oxygen from the atmosphere is used up in three main processes: combustion, respiration, and the formation of oxides of nitrogen.
• Return: Oxygen is returned to the atmosphere by only one major process: Photosynthesis.

6. Ozone Layer

Elemental oxygen is normally found in the form of a diatomic molecule (O₂). However, in the upper reaches of the atmosphere, a molecule containing three atoms of oxygen is found. This is called ozone (O₃).

• Unlike normal diatomic oxygen, ozone is poisonous. But it is vital at a distance: it absorbs harmful ultraviolet (UV) radiations from the Sun, preventing them from reaching the surface of the Earth where they could damage living forms.
• Ozone Depletion: In the 1980s, a hole in the ozone layer was discovered over Antarctica. This depletion is caused by man-made carbon compounds containing fluorine and chlorine, called Chlorofluorocarbons (CFCs). CFCs were widely used in refrigerants and aerosols. They reach the ozone layer and react with ozone molecules, breaking them down and causing the thinning of this protective shield.