Ans: The greenhouse effect is the trapping of heat by greenhouse gases in the atmosphere. The sun's radiation warms the Earth, which emits infrared radiation. Greenhouse gases absorb this IR radiation and re-emit it, warming the atmosphere. Main greenhouse gases: CO₂ (carbon dioxide), CH₄ (methane), N₂O (nitrous oxide), CFCs, and water vapour.

Ans: Nitrogen fixation is the process of converting atmospheric nitrogen (N₂) into ammonia (NH₃) or nitrates, which can be used by plants. Methods: (1) Biological fixation: Rhizobium bacteria (in root nodules of legumes like pea, bean) and free-living bacteria (Azotobacter, cyanobacteria like Anabaena) fix N₂ → NH₃. (2) Atmospheric: Lightning causes N₂ + O₂ → NO → NO₃⁻ (washed into soil by rain). (3) Industrial: Haber process — N₂ + 3H₂ → 2NH₃ (for fertilisers). Importance: Nitrogen is essential for making amino acids (proteins) and nucleic acids (DNA) in all living organisms.

Ans: Ozone depletion is the thinning of the ozone layer in the stratosphere, particularly over Antarctica (ozone hole). The ozone layer absorbs harmful UV radiation from the sun. Depletion causes: Chlorofluorocarbons (CFCs) used in refrigerators, air conditioners, and aerosol sprays release chlorine atoms in the stratosphere which destroy ozone: Cl + O₃ → ClO + O₂. Other chemicals: Bromine compounds (halons), nitrogen oxides. Solution: Montreal Protocol (1987) banned CFCs globally.

Ans: Water cycle (hydrological cycle): (1) Evaporation: Sun heats water in oceans/lakes/rivers → water vapour. (2) Transpiration: Plants release water vapour through stomata. Together: evapotranspiration. (3) Condensation: Water vapour rises, cools → clouds form. (4) Precipitation: Falls as rain, snow, hail. (5) Runoff/Percolation: Water flows to rivers, oceans or seeps into groundwater. The cycle repeats continuously. Importance: Redistributes water across the Earth; regulates temperature and climate.

Ans: The troposphere, the lowest layer of the atmosphere.

Ans: Ultraviolet (UV) radiation.

Ans: Transpiration.

Ans: The troposphere contains most of the air we breathe and nearly all the water vapour of the atmosphere. Clouds, rain, wind and other weather occur here, which supply water to the land and shape climate. The temperature and conditions in this layer support life.

Ans: A trophic level is the position or step of an organism in a food chain based on how it obtains food. Example of a food chain with four trophic levels: grass (producer) → grasshopper (primary consumer) → frog (secondary consumer) → snake (tertiary consumer).

Ans: (1) It releases carbon dioxide, a greenhouse gas that increases global warming. (2) It releases sulphur dioxide and nitrogen oxides, which cause acid rain, and particulates that cause smog and air pollution.

Ans: Water has a high specific heat, so oceans absorb a large amount of heat from the Sun without a big rise in temperature, and release it slowly. Ocean currents carry heat from the equator towards the poles, keeping coastal areas mild. Oceans also absorb carbon dioxide from the atmosphere and supply water vapour through evaporation that forms clouds and rain.

Ans: Weather: the state of the atmosphere (temperature, rainfall, wind, humidity) at a place over a short period such as hours or days; it changes frequently. Climate: the average weather of a region observed over a long period, usually 30 years or more; it changes very slowly. For example, a rainy day is weather, while a region being hot and humid for most of the year describes its climate.

Ans: Increased UV radiation reaching the Earth causes skin cancer, cataracts and weakened immunity in humans. It damages crops and phytoplankton, which form the base of ocean food chains, and so disturbs ecosystems. The Montreal Protocol (1987) was made to phase out the production and use of ozone-depleting substances such as CFCs.

Ans: Carbon dioxide in the atmosphere is taken in by plants during photosynthesis and converted into carbohydrates. Carbon passes to animals when they eat plants. Plants and animals release CO₂ back through respiration, and decomposers release it when dead matter decays. Carbon is also stored in fossil fuels, limestone and oceans, and returns to the air by combustion and volcanic activity. Humans disturb the cycle by burning fossil fuels and by deforestation, which raise atmospheric CO₂ and cause global warming.

Ans: In a pyramid of energy, producers are at the base with the most energy, and each higher level (herbivores, then carnivores) has less. For example, if producers have 10,000 J, primary consumers get about 1,000 J, secondary consumers about 100 J and tertiary consumers about 10 J, by the 10% law. The pyramid is always upright. The flow is unidirectional because producers capture energy from the Sun, and energy passed on cannot return to the previous level; most is lost as heat. So an ecosystem needs a continuous supply of solar energy.