Ans: A biogeochemical cycle is the cyclic movement of chemical elements between the living (biotic) and non-living (abiotic) components of the environment. The main biogeochemical cycles are: water cycle, carbon cycle, nitrogen cycle, and oxygen cycle.

Ans: Bacteria play multiple roles in the nitrogen cycle: (1) Nitrogen-fixing bacteria (e.g., Rhizobium in root nodules of legumes, Azotobacter free-living) convert atmospheric N₂ into ammonia (NH₃). (2) Nitrifying bacteria convert NH₃ → NO₂⁻ → NO₃⁻ (nitrates usable by plants). (3) Denitrifying bacteria (e.g., Pseudomonas) convert nitrates back to N₂ gas.

Ans: The water cycle is the continuous circulation of water between the Earth's surface and the atmosphere. Processes: (1) Evaporation: Heat from the sun evaporates water from oceans, lakes, rivers into water vapour. (2) Transpiration: Plants release water vapour through stomata. (3) Condensation: Water vapour rises, cools, and condenses to form clouds (water droplets/ice crystals). (4) Precipitation: Water falls as rain, snow, sleet or hail. (5) Runoff/Infiltration: Water flows back into rivers, lakes, and groundwater, completing the cycle. Driving force: Solar energy for evaporation; gravity for precipitation and runoff.

Ans: The carbon cycle involves continuous exchange of carbon between atmosphere, living organisms, oceans and rocks. Key processes: (1) Photosynthesis: Plants absorb CO₂ and convert it to organic carbon (glucose). 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. (2) Respiration: Organisms oxidise food, releasing CO₂ back. (3) Decomposition: Decomposers break down dead organisms, releasing CO₂. (4) Combustion: Burning of fossil fuels and wood releases CO₂. (5) Carbon locked in fossil fuels (coal, petroleum) and limestone (CaCO₃) for millions of years. Human disturbance: Burning fossil fuels and deforestation releases excess CO₂ into the atmosphere, causing the greenhouse effect and global warming. This is disrupting the natural balance of the carbon cycle.

Ans: The greenhouse effect is the natural phenomenon by which certain gases in Earth's atmosphere trap heat from the sun, keeping the planet warm enough to support life. The sun's radiation (short-wave) passes through the atmosphere and warms the Earth's surface. The Earth re-radiates this energy as long-wave infrared radiation. Greenhouse gases absorb this infrared radiation and re-emit it, trapping heat. Main greenhouse gases: CO₂ (carbon dioxide), CH₄ (methane), N₂O (nitrous oxide), H₂O (water vapour), CFCs (chlorofluorocarbons). Enhanced greenhouse effect: Due to excess CO₂ and other gases from human activities, more heat is trapped → global warming → climate change, melting glaciers, rising sea levels.

Ans: Ozone (O₃) is a form of oxygen found in the stratosphere (20–50 km above Earth's surface) as the ozone layer. Importance: Absorbs harmful ultraviolet (UV) radiation from the sun (especially UV-B and UV-C), preventing it from reaching the Earth's surface. Without it: increased UV → skin cancer, cataracts, immune suppression, damage to crops and marine ecosystems. Depletion: Chlorofluorocarbons (CFCs) — used in refrigerators, air conditioners, aerosol sprays — release chlorine atoms in the stratosphere that destroy ozone: Cl + O₃ → ClO + O₂. One Cl atom can destroy thousands of O₃ molecules. Ozone hole forms mainly over Antarctica. Montreal Protocol (1987) phased out CFCs globally.

Ans: Nitrogen constitutes 78% of the atmosphere as N₂ gas, but most organisms cannot use it directly. The nitrogen cycle returns nitrogen to the atmosphere after it has been used by living things. Stages: (1) Nitrogen Fixation: N₂ → NH₃/NH₄⁺. Methods: (a) Biological: Rhizobium (in legume root nodules), Azotobacter/Cyanobacteria (free-living). (b) Industrial: Haber process — N₂ + 3H₂ → 2NH₃. (c) Atmospheric: Lightning converts N₂ + O₂ → NO → NO₂ → HNO₃ (washed into soil by rain). (2) Nitrification: Bacteria convert NH₄⁺ → NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter). Plants absorb NO₃⁻. (3) Assimilation: Plants use nitrates to make amino acids, proteins, nucleic acids. Animals eat plants and assimilate nitrogen. (4) Ammonification: Decomposers (bacteria, fungi) break down dead matter → NH₃/NH₄⁺. (5) Denitrification: Pseudomonas converts NO₃⁻ → N₂ back into atmosphere. Importance: Nitrogen is essential for proteins and nucleic acids (DNA/RNA) that make up all living organisms.

Ans: Biotic components: All living organisms in an ecosystem — producers (plants), consumers (animals), decomposers (bacteria, fungi). They carry out life processes and interact with each other and the environment. Abiotic components: All non-living physical and chemical factors — sunlight, temperature, water, soil, air, minerals, pH, wind. They provide the conditions and raw materials for life. Interaction: Biotic components depend on abiotic factors for energy and matter; they in turn influence abiotic components (e.g., plants affect soil pH, decomposers release nutrients).

Ans: Lithosphere, hydrosphere, atmosphere and biosphere.

Ans: Nitrogen, which makes up about 78% of the atmosphere by volume.

Ans: The ozone layer is found in the stratosphere, roughly 15–35 km above the Earth.

Ans: According to Lindeman's 10% law, only about 10% of the energy of one trophic level is passed on to the next level. The rest is lost as heat in respiration and life processes.

Ans: Food chain: a single linear sequence showing who eats whom, e.g. grass → grasshopper → frog → snake. Food web: a network of many interconnected food chains in an ecosystem. A food web is more stable, since if one food source disappears an organism can switch to another.

Ans: Decomposers are organisms such as bacteria and fungi that break down dead plants and animals and other waste. They release nutrients back into the soil, which producers reuse, and so help recycle matter and keep the environment clean.

Ans: A sea breeze is a wind that blows from the sea to the land during the day. Land heats faster than water, so air over land becomes warm and rises, creating low pressure. Cooler, high-pressure air from the sea moves in to fill the space.

Ans: CFCs (chlorofluorocarbons) are compounds of chlorine, fluorine and carbon used in refrigerators, air conditioners and aerosol sprays. In the stratosphere, UV light breaks them down and releases chlorine atoms, which destroy ozone molecules. This thins the ozone layer and lets more UV radiation reach the Earth.

Ans: (1) Troposphere (up to about 8–18 km): weather and clouds occur here; temperature falls with height. (2) Stratosphere (up to about 50 km): contains the ozone layer that absorbs UV rays. (3) Mesosphere (up to about 80 km): the coldest layer; meteors burn up here. (4) Thermosphere (up to about 600 km): temperature rises sharply and the ionosphere reflects radio waves. (5) Exosphere: the outermost layer, which gradually merges into space.

Ans: Energy enters an ecosystem as sunlight, which producers convert into chemical energy by photosynthesis. It then passes to herbivores and carnivores along the food chain. The flow is one-way and the energy cannot be reused. At each step only about 10% of energy moves to the next level and the rest is lost as heat. After 3–4 levels too little energy is left to support another level, so food chains are short.

Ans: Global warming is the rise in Earth's average temperature due to an increase in greenhouse gases that trap heat. Causes: burning of fossil fuels (CO₂), deforestation, industries, livestock and rice fields (methane), fertilisers (nitrous oxide), CFCs. Effects: melting glaciers and polar ice, sea-level rise, flooding of coastal areas, erratic weather, droughts and loss of biodiversity. Measures: (1) use solar, wind and other renewable energy, (2) plant more trees, (3) reduce, reuse and recycle, and cut the use of fossil fuels.

Ans: Lithosphere: solid rocky outer layer, including soil. Hydrosphere: all water bodies. Atmosphere: the layer of gases around the Earth. Biosphere: the region where life exists, formed by the overlap of the other three. Interactions: (1) Evaporation of water from oceans (hydrosphere) forms clouds in the atmosphere, and rain falls on land (lithosphere), where it erodes rocks and forms soil. (2) Plants (biosphere) take in CO₂ from the atmosphere and water and minerals from the soil, and release oxygen. Together these processes show that the Earth works as one connected system.