Our Environment

Everything around us, living and non-living, is our environment. The chapter opens by asking what happens to the things we throw away, and the answer splits every substance into two groups.

Biodegradable substances are those that can be broken down by biological processes, chiefly by the enzymes released by bacteria and other saprophytes. Vegetable peels, dead leaves, paper, cotton, wool, cow dung, leather and wood are all broken down in weeks or months and their material goes back into the soil.

Non-biodegradable substances are not broken down this way. Plastics, polythene, glass, metal, DDT and synthetic fibres like nylon and polyester persist for a very long time. The reason is one sentence the examiner likes: enzymes are specific in their action. The bacterial enzymes that digest cellulose or protein simply do not fit the man-made molecules of a plastic, so nothing in nature can take those molecules apart. Some of these materials may also be broken down only physically (a plastic bag tears into smaller pieces) which is not decomposition at all.

Why does this matter? Non-biodegradable wastes pile up, choke drains, are eaten by animals, and some, like pesticides, enter food chains and harm other organisms. Biodegradable wastes are not harmless either: a heap of rotting garbage breeds flies, smells, releases methane and can spread disease. The chapter's point is that each type needs its own disposal, which is why we keep two separate dustbins.

In NCERT's Activity 13.1 you bury pieces of both kinds of waste and dig them up after three weeks. The vegetable matter has darkened and started to break down; the plastic looks exactly the same. That observation is the whole idea in one picture.

An ecosystem is all the organisms living in a given area, interacting with each other and with the non-living constituents of that area. Write both halves. The biotic components are the plants, animals and micro-organisms; the abiotic components are the physical factors such as temperature, rainfall, wind, soil and minerals.

Ecosystems can be natural, like a forest, a pond, a lake or the ocean, or artificial (man-made), like a garden, a crop field or an aquarium. A garden is still an ecosystem because its plants, insects, birds, earthworms and soil bacteria all interact with each other and with sunlight, soil and water, even though a human put it together.

Within the biotic part there are three roles. Producers are the green plants and some blue-green algae that make their own food from sunlight by photosynthesis. Consumers depend on producers directly or indirectly: herbivores eat plants, carnivores eat animals, omnivores eat both, and parasites feed on a living host. Decomposers are the bacteria and fungi that break down the dead remains and waste products of organisms into simple inorganic substances, which go back into the soil and are used again by plants.

The decomposer point earns marks in three different questions, so learn the role clearly: they clean the environment of dead matter, they recycle nutrients, and they keep the soil fertile. Without them, dead bodies and waste would pile up, nutrients would stay locked inside them, and the soil would soon be unable to support new plants.

The favourite comparison is the pond and the aquarium. A pond is a complete natural ecosystem with producers, consumers and decomposers, so its waste is broken down and its nutrients are recycled on their own. An aquarium is an incomplete artificial ecosystem: it usually has no decomposers and no natural inflow, so fish waste and uneaten food accumulate and the tank must be cleaned. To make an aquarium self-sustaining you would need aquatic plants for oxygen and food, decomposer bacteria and a balanced number of fish.

A food chain is a series of organisms in which each one feeds on the one before it. The simplest example: grass → deer → lion. Each step or level of the chain is called a trophic level.

The first trophic level is always the producers (autotrophs), because they are the only organisms that can fix the sun's energy into food. Herbivores, the primary consumers, form the second trophic level. Small carnivores that eat the herbivores are the secondary consumers at the third level, and the larger carnivores that eat them are the tertiary consumers at the fourth level. In grass → grasshopper → frog → snake → hawk, the frog is at the third level and the hawk at the fifth.

Decomposers are not shown as a step in the chain because they act at every level, feeding on the dead remains of producers and consumers alike.

In nature an organism rarely eats only one kind of food. Grass is eaten by grasshoppers, rabbits and deer; a snake may eat frogs, rats and birds. When these overlapping food chains are drawn together they form a food web. A food web makes the ecosystem more stable: if one prey species declines, the predator can switch to another, so a single loss does not collapse the whole system.

What if we remove one trophic level? Remove the producers and every level above starves. Remove the herbivores and the plants grow unchecked while the carnivores die out. Remove the top carnivores and the herbivores multiply, overgraze and eventually destroy the vegetation. So the impact differs with the level, but no level can be removed without damage. The examiner also expects you to say that in a food web the damage is smaller than in a simple chain because alternative pathways exist.

One more fact: the number of individuals is usually greatest at the producer level and falls as you go up, though this is not always true, since one large tree can support thousands of insects.

Energy enters an ecosystem from the sun. Green plants in a terrestrial ecosystem capture only about 1 per cent of the solar energy falling on their leaves and convert it into food energy. So if 10,000 J of sunlight falls on a plant, it stores about 100 J.

When a herbivore eats the plant, most of the energy it takes in is used up in digestion, in movement and in keeping warm, and a part is lost as heat. On average only 10 per cent of the food eaten is turned into the eater's own body and so becomes available to the next level. This is the 10 per cent law. Grass with 1000 J of stored energy gives the deer 100 J, and the lion that eats the deer gets only 10 J.

Two consequences follow. First, since so little energy passes each step, food chains generally have only three or four trophic levels; by the fifth level there is too little energy to support a population. Second, energy flow is unidirectional. The energy that a plant captures does not go back to the sun, and the energy that a deer takes from the grass does not return to the grass; at each step some is lost as heat to the surroundings and cannot be recovered. Matter, by contrast, is recycled by decomposers.

Worked calculation, the way the examiner wants it: in plants → rats → snakes → hawks, if the plants have 10,000 J, the rats get 10 per cent of that, 1000 J; the snakes get 100 J; the hawks get 10 J. Going the other way, if the third level receives 50 kJ, the second level had 500 kJ and the producers had 5000 kJ.

Biological magnification is the other half of this section. Pesticides and other non-degradable chemicals sprayed on crops wash into the soil and water and are taken up by plants. They cannot be broken down or excreted, so they stay in the body. When a herbivore eats many plants, it collects all their pesticide; when a carnivore eats many herbivores, it collects all of theirs. The concentration therefore increases progressively at each trophic level, and the organism at the top of the chain gets the maximum concentration. Humans are at the top of many chains, which is why our grains, vegetables, fruits, milk and meat all carry pesticide residues that washing cannot remove.

Humans are part of the environment, not observers of it. Every activity, from farming to travelling to buying packaged goods, changes the environment in some way. NCERT picks two problems to study in detail: the depletion of the ozone layer, and the garbage we produce. Before those, it is worth having a clear picture of the general link between lifestyle and environmental damage, because the board paper asks about it in 1- and 2-mark questions.

An affluent lifestyle means more consumption: more packaging, more disposable items, more electricity, more vehicles, more chemicals in farming. Each of these adds waste or pollution. Industrialisation has deteriorated the environment in at least four ways the examiner accepts: it pollutes air with smoke and gases, pollutes water with effluents, produces mountains of solid waste, and cuts down forests for land and raw material. Excessive use of fertilisers and pesticides on crops adds a fifth: chemicals that enter food chains and reach us by biological magnification.

On the other side, some everyday choices are genuinely environment-friendly and the chapter's own exercise lists them: carrying cloth bags when shopping, switching off unnecessary lights and fans, walking or cycling to school instead of being dropped by scooter, using both sides of paper, and separating biodegradable from non-biodegradable waste. Questions built on this section are simple; they just want you to name the harm and suggest a practical alternative. Organic farming, which avoids synthetic pesticides, is the one option in a recent MCQ that does not damage the environment.

Read this section as a bridge. The two sub-sections that follow are the ones that carry the marks, and both are examples of the same idea: a convenient product (CFC refrigerants, plastic cups) whose waste nature cannot handle.

Ozone (O₃) is a molecule made of three atoms of oxygen. Ordinary oxygen (O₂) has two atoms and is essential for all aerobic life; ozone at ground level is a deadly poison. But high in the atmosphere ozone does a vital job: it absorbs the harmful ultraviolet (UV) radiation from the sun, which would otherwise reach the earth and cause skin cancer, cataracts and damage to plants and animals.

How ozone forms. At the higher levels of the atmosphere, high-energy UV radiation splits molecular oxygen into free oxygen atoms. These very reactive atoms then combine with molecular oxygen to make ozone.

O₂ → (UV) → O + O
O + O₂ → O₃

Write both steps; the first alone is half the answer.

Depletion. The amount of ozone in the atmosphere began to drop sharply in the 1980s. The decrease was linked to synthetic chemicals called chlorofluorocarbons (CFCs), used as refrigerants (in fridges and air-conditioners), in aerosol sprays and in fire extinguishers. CFCs are very stable; they drift up to the upper atmosphere, where UV breaks them and releases chlorine atoms that destroy ozone molecules. The thinning was most dramatic over Antarctica, the so-called ozone hole.

What was done. In 1987 the United Nations Environment Programme (UNEP) forged an agreement, the Montreal Protocol, to freeze CFC production at 1986 levels. It is now mandatory for all manufacturers to make CFC-free refrigerators and air-conditioners throughout the world.

Why it is a cause for concern. Less ozone means more UV reaching the ground: higher rates of skin cancer and cataracts in humans, weakened immune systems, damage to crops and plankton, and hence disturbance of food chains. A 3-mark answer needs the definition, the formation equations and the CFC link; a 5-mark answer adds the effects and the UNEP action.

Improvements in our lifestyle have changed the amount and the kind of waste we make. Packaging, disposable items, plastics and electronic goods have made non-biodegradable waste the bigger problem. NCERT tells the story of the tea cup on Indian trains. Tea was once served in kulhads, disposable clay cups. These were biodegradable but making millions of them stripped fertile topsoil. They were replaced by paper cups, which need trees to be cut. Then came plastic cups, which are cheap and hygienic but non-biodegradable and pile up along every railway line. Each solution created a new problem, and the exam asks you to explain the harm at each step.

Why plastic bags are harmful. They are non-biodegradable, so they persist for centuries; they choke drains and cause flooding; they are swallowed by cattle and marine animals, which die; burning them releases toxic gases; and their litter spoils soil and water. This is why many states have banned polythene bags. The alternatives are cloth, jute and paper bags, which are reusable or biodegradable.

Disposal methods. Sort waste at home into two dustbins. Biodegradable kitchen and garden waste goes to composting or a vermicompost pit, which turns it into manure. Non-biodegradable waste is recycled where possible (paper, glass, metal, some plastics) or sent to a sanitary landfill. Some cities burn waste in incinerators, which reduces volume but can pollute the air. Sewage is treated at sewage treatment plants before the water is released.

How you can help. The three Rs: Reduce (avoid disposable items and excess packaging), Reuse (cloth bags, refillable bottles, using old jars for storage) and Recycle (send paper, plastic, metal and glass for reprocessing). Add the practical habits: carrying your own bag, composting kitchen waste, not littering, and buying products with less packaging.

If a question asks why biodegradable and non-biodegradable wastes should go in separate bins, the answer is that they need different disposal: one can be composted, the other must be recycled or safely dumped, and mixing them makes both impossible and spreads the non-degradable waste into the soil.