Earth functions as an interconnected system with four major spheres that constantly interact and exchange energy and matter. (1) Atmosphere: layer of gases surrounding Earth — mainly N₂ (78%), O₂ (21%), Ar (0.9%), CO₂ (0.04%). Extends up to ~10,000 km. Divided into troposphere, stratosphere, mesosphere, thermosphere, exosphere. Weather and climate occur in the troposphere. Ozone layer is in the stratosphere. (2) Hydrosphere: all water on Earth — oceans, rivers, lakes, glaciers, groundwater, ice caps. 97% is saline ocean water; only ~3% is freshwater. (3) Lithosphere: the solid outer layer — Earth's crust and upper mantle. Contains rocks, soil, minerals. (4) Biosphere: all living organisms and the environments they live in. Interacts with all other spheres.

Biogeochemical cycles describe how matter (elements and compounds) moves between living organisms and the environment in a continuous, cyclic manner. Water Cycle (Hydrological Cycle): Water continuously moves through evaporation (water bodies → water vapour, driven by solar energy), transpiration (water lost by plants as vapour), condensation (vapour cools → water droplets → clouds), precipitation (rain, snow, hail falls to Earth), runoff and infiltration (water flows to rivers, oceans or seeps into ground). Importance: distributes freshwater across the planet, regulates temperature, supports all life. Human impacts: deforestation reduces transpiration, reducing rainfall; pollution contaminates freshwater; overextraction of groundwater depletes aquifers.

Carbon is the basis of all organic molecules (carbohydrates, proteins, fats, nucleic acids). Carbon cycle: Photosynthesis: plants absorb CO₂ from air + H₂O → glucose + O₂ (carbon fixed into organic molecules). Respiration: all organisms break down glucose → CO₂ released back to atmosphere. Decomposition: dead organisms decomposed by bacteria/fungi → CO₂ released. Combustion: burning of fossil fuels (coal, petroleum) and wood releases CO₂ stored over millions of years. Ocean absorption: oceans absorb large amounts of CO₂, dissolving it (carbonic acid). Significance: maintains atmospheric CO₂ levels. Human disruption: burning fossil fuels releases excess CO₂ → enhancing greenhouse effect → global warming. Currently atmospheric CO₂ ≈ 420 ppm (pre-industrial: 280 ppm).

Nitrogen (N₂) makes up 78% of air but most organisms cannot use it directly. Nitrogen cycle: (1) Nitrogen fixation: converting N₂ → NH₃ (ammonia). Done by nitrogen-fixing bacteria (Rhizobium in legume root nodules, Azotobacter free in soil), lightning. Industrial: Haber process (N₂ + H₂ → NH₃). (2) Nitrification: Nitrosomonas bacteria convert NH₃ → NO₂⁻; Nitrobacter convert NO₂⁻ → NO₃⁻ (nitrates). (3) Assimilation: plants absorb nitrates → proteins/DNA. Animals eat plants → proteins. (4) Ammonification: decomposers break down dead organic matter → NH₃. (5) Denitrification: Pseudomonas bacteria convert NO₃⁻ → N₂ (returns nitrogen to atmosphere). Importance: nitrogen is essential for amino acids, proteins, DNA — building blocks of life.

Oxygen cycle: O₂ released to atmosphere by photosynthesis. O₂ consumed by respiration (all organisms), combustion, weathering of rocks. O₂ in atmosphere maintained at ~21% by balance of photosynthesis and respiration. Ozone (O₃) in stratosphere absorbs UV radiation (very important for life). Greenhouse Effect: certain gases (CO₂, CH₄, N₂O, H₂O vapour, CFCs) trap outgoing infrared radiation from Earth, keeping it warm (natural greenhouse effect maintains Earth at ~15°C; without it: −18°C). Enhanced greenhouse effect: human activities increase greenhouse gases → more heat trapped → global warming → climate change. Consequences: rising sea levels, melting glaciers, extreme weather, habitat loss, species extinction. Solutions: renewable energy, reforestation, reducing emissions.

Soil is the uppermost layer of the Earth's lithosphere and is essential for terrestrial life. Formation: rocks are broken down by weathering (physical — temperature changes, wind; chemical — acid rain, oxidation; biological — roots, lichens, bacteria) over thousands of years → parent material → soil. Composition: mineral particles (sand, silt, clay), organic matter (humus), water, air, living organisms. Soil horizons (layers): O (organic), A (topsoil with humus — most fertile), B (subsoil), C (weathered rock), R (bedrock). Soil types: sandy (large particles, low water retention), loamy (best for farming — balanced), clayey (fine particles, high retention). Humus: decomposed organic matter that makes soil fertile, improves structure, and retains water. Soil erosion: removal of topsoil by wind/water — caused by deforestation, overgrazing. Prevention: trees, terracing, contour ploughing.

Energy from the sun drives all life on Earth. Energy flow: Sun → Producers (plants fix solar energy via photosynthesis → stored as chemical energy in glucose) → Primary consumers (herbivores eat plants) → Secondary consumers (carnivores eat herbivores) → Tertiary consumers → Decomposers. Only ~10% of energy is transferred from one trophic level to the next (Lindeman's 10% law); 90% lost as heat. Unlike matter (which cycles), energy flows unidirectionally — from the sun through the ecosystem and is eventually lost as heat. Food chain: linear energy pathway. Food web: multiple interconnected food chains. Pyramid of energy: always upright (base = producers with most energy). Decomposers are critical — they release energy locked in dead matter and return nutrients to soil.

Human activities have dramatically altered Earth's systems in recent centuries. Air pollution: burning fossil fuels releases CO₂, SO₂, NOₓ → acid rain, smog, global warming. Deforestation: forests absorb CO₂; cutting them releases stored carbon and reduces carbon fixation. Agriculture: excessive fertiliser use → nitrate runoff → eutrophication of water bodies. Plastic pollution: non-biodegradable; persists for centuries; enters oceans, kills marine life, enters food chains. Water overuse: groundwater depleted faster than recharged. Ozone depletion: CFCs destroy ozone layer → increased UV radiation. Solutions: renewable energy transition, sustainable agriculture, afforestation, recycling, international climate agreements (Paris Agreement 2015 — limit warming to 1.5°C), SDGs (Sustainable Development Goals). India's commitments: 500 GW renewable energy by 2030, net zero by 2070.

The Earth works as a system of interacting spheres. Lithosphere: the solid outer part (rocks, soil, mountains) including the crust. Hydrosphere: all water on Earth — oceans, rivers, lakes, groundwater, glaciers, and water vapour. Atmosphere: the envelope of gases around the Earth held by gravity. Biosphere: the zone where life exists, formed by the overlap of the other three spheres. These spheres exchange energy and matter continuously. Example: rain (atmosphere) falls on land (lithosphere), fills rivers (hydrosphere) and supports plants and animals (biosphere).

Composition: nitrogen about 78%, oxygen about 21%, argon about 0.9%, carbon dioxide about 0.04%, plus water vapour and traces of other gases. Layers from the ground upward: (1) Troposphere — up to about 8–18 km; weather, clouds and rain occur here; temperature falls with height; holds most of the air mass and nearly all water vapour. (2) Stratosphere — up to about 50 km; contains the ozone layer; temperature rises with height. (3) Mesosphere — up to about 80 km; coldest layer; meteors burn here. (4) Thermosphere — up to about 600 km; temperature rises sharply; contains the ionosphere. (5) Exosphere — outermost layer merging into space.

Ozone (O₃) is a form of oxygen with three atoms. A layer of ozone in the stratosphere absorbs most of the harmful ultraviolet (UV) radiation from the Sun and protects life. Depletion: chlorofluorocarbons (CFCs) used in refrigerators, air conditioners and aerosol sprays rise to the stratosphere, where UV breaks them down and releases chlorine atoms. One chlorine atom can destroy many ozone molecules, creating the ozone hole over Antarctica. Effects: skin cancer, cataracts, weakened immunity, crop and plankton damage. The Montreal Protocol (1987) was agreed to phase out ozone-depleting substances.

The Sun heats the Earth's surface, which then heats the air above it. Land heats up and cools down faster than water because of its lower specific heat. Sea breeze (day): land becomes hotter than the sea, the air above land rises creating low pressure, and cooler air from the sea moves in to take its place. Land breeze (night): land cools faster than the sea, so warmer air rises over the sea and cooler air blows from land to sea. Winds are generally the horizontal movement of air from a high-pressure area to a low-pressure area.

A food chain shows the transfer of food and energy from producers to consumers, e.g. grass → grasshopper → frog → snake → eagle. Each step is a trophic level: producers (first), herbivores/primary consumers (second), carnivores (third and higher). A food web is a network of interconnected food chains. Decomposers (bacteria, fungi) break down dead matter and return nutrients to the soil. Energy flow is one-way, from the Sun to producers to consumers. Only about 10% of the energy is passed to the next trophic level (Lindeman's 10% law); the rest is lost as heat in life processes, so food chains are short (3–4 levels).

Global warming is the gradual rise in Earth's average surface temperature caused by increased greenhouse gases (carbon dioxide, methane, nitrous oxide, water vapour, CFCs) that trap heat. Causes: burning of fossil fuels, deforestation, industrial processes, agriculture and livestock. Effects: melting of glaciers and polar ice, rise in sea level, extreme weather, changes in rainfall, loss of biodiversity and threats to crops. Measures: use renewable energy, plant trees, save energy, reduce, reuse and recycle, and use public transport.

Earth's four spheres: lithosphere, hydrosphere, atmosphere, biosphere. Atmosphere gases: N₂ 78%, O₂ 21%. Weather happens in the troposphere; the ozone layer is in the stratosphere. Biogeochemical cycles (water, carbon, nitrogen, oxygen) move matter between living and non-living parts. Energy comes from the Sun and flows one way through food chains (10% law). Human activities such as burning fuels, deforestation and CFC use disturb the system, causing global warming, ozone depletion and pollution. Key terms: trophic level, producer, consumer, decomposer, greenhouse effect, albedo, sea breeze, Montreal Protocol.