NCERT Solutions Class 10 Science Chapter 13 Our Environment

A food chain is a series of organisms in which each one is eaten by the next. Each step or feeding level of a food chain is called a trophic level. The producers (green plants) form the first trophic level because they make their own food, and the animals that feed on them form the higher levels, one after the other.

• First trophic level (T1): producers (green plants), which trap sunlight and make food.
• Second trophic level (T2): herbivores or primary consumers, which eat the plants.
• Third trophic level (T3): small carnivores or secondary consumers, which eat the herbivores.
• Fourth trophic level (T4): large carnivores or tertiary consumers, which eat the smaller carnivores.

A simple grassland food chain is: grass → deer → lion. Here grass is the producer (T1), the deer is a herbivore or primary consumer (T2), and the lion is a carnivore or secondary consumer (T3). The arrow points from the organism being eaten towards the one that eats it, showing the direction of energy flow.

Answer: Trophic levels are the feeding steps of a food chain. Example: grass (T1, producer) → deer (T2, herbivore) → lion (T3, carnivore). Each step is one trophic level.

Decomposers are micro-organisms such as bacteria and fungi that break down the dead bodies and waste products of plants and animals. They change complex organic substances into simple inorganic substances, which go back into the soil, water and air for producers to use again.

• Decomposers feed on dead remains, fallen leaves and animal droppings, and in doing so they clean up the environment.
• They break complex organic matter (proteins, starch, cellulose) into simple inorganic substances such as minerals, carbon dioxide and water.
• These simple substances are released back into the soil, restoring its nutrients (replenishing the soil).
• Producers absorb these nutrients again to make food, so materials are recycled and the cycle of life continues.

Answer: Decomposers break down dead matter and waste into simple substances, return nutrients to the soil, keep the environment clean, and complete the recycling of materials so producers can use them again.

Biodegradable substances can be broken down into simpler harmless substances by micro-organisms (bacteria and fungi) and the enzymes they release. Non-biodegradable substances cannot be broken down by these biological processes. The difference depends on whether the right enzyme exists in nature to attack the substance.

• Micro-organisms break down matter using enzymes, and each enzyme is very specific, acting only on a particular kind of substance.
• Natural substances such as fruit peels, paper, wood, food and cotton are made of materials (cellulose, protein) for which suitable enzymes exist, so microbes can digest them. They are biodegradable.
• Many man-made substances such as plastics, polythene, glass and metal cans have a structure that no natural enzyme can attack.
• Because no enzyme fits them, these substances are not broken down by living things, so they stay in the environment for a very long time. They are non-biodegradable.

Answer: Substances are biodegradable when micro-organisms have the right enzymes to break them down (natural matter like food, paper, wood). They are non-biodegradable when no natural enzyme can act on them (man-made materials like plastic and glass).

Biodegradable substances are broken down by micro-organisms. While they do not last long, the process of decay itself can affect the environment if such waste is allowed to collect in large amounts.

• Foul smell and unhygienic surroundings: as biodegradable waste (rotting food and vegetable peels) decays, it gives off a bad smell and makes the area dirty. It also attracts flies, mosquitoes and other organisms that spread disease.
• Release of gases and possible water pollution: the breakdown of large amounts releases gases like methane and carbon dioxide. If the waste runs into ponds or rivers, the decay uses up the dissolved oxygen in the water, which can harm fish and other water life.

Answer: (1) Decaying biodegradable waste gives off a foul smell and attracts disease-spreading organisms. (2) Its breakdown releases gases such as methane and can lower the oxygen in nearby water bodies, harming aquatic life.

Non-biodegradable substances cannot be broken down by micro-organisms, so they stay in the environment for a very long time. Because they do not decay, they keep building up and can cause lasting damage to soil, water and living things.

• They pollute soil and water and harm organisms: plastics and similar wastes pile up on land, choke drains, and pollute rivers and seas. Animals may swallow plastic and die, and plastic in the soil stops water and air from reaching plant roots.
• They cause biological magnification: harmful chemicals such as certain pesticides do not break down. They enter the food chain and get more concentrated at each higher trophic level, poisoning animals and humans at the top.

Answer: (1) Non-biodegradable waste piles up, pollutes soil and water, and can kill animals that swallow it. (2) Non-degradable chemicals enter the food chain and concentrate at higher levels (biological magnification), poisoning top consumers.

Ozone (O₃) is a molecule made of three atoms of oxygen. High up in the atmosphere it forms a layer called the ozone layer, which acts as a shield: it absorbs most of the harmful ultraviolet (UV) radiation coming from the Sun before it reaches the Earth's surface.

• Ordinary oxygen is O₂, but ozone is O₃. At ground level ozone is a poison, but high in the atmosphere it does a vital protective job.
• In the upper atmosphere, high-energy UV rays split oxygen molecules into free oxygen atoms (O₂ → O + O), which then join with oxygen molecules to form ozone (O + O₂ → O₃).
• The ozone layer absorbs harmful UV radiation, protecting living things, because UV rays can cause skin cancer, damage the eyes, and harm plants and tiny water organisms.
• If the ozone layer is damaged, more UV reaches the ground. This harms the whole ecosystem: it can lower crop yields, kill plankton in the sea (the base of aquatic food chains), and cause disease in animals and humans.

Answer: Ozone is O₃, a three-atom form of oxygen. High in the atmosphere it forms a layer that absorbs the Sun's harmful UV rays, protecting the whole ecosystem. If it is destroyed, extra UV reaches Earth and damages plants, animals and humans.

The problem of waste disposal grows when we produce too much waste and mix biodegradable with non-biodegradable waste. We can reduce the problem by making less waste, by reusing and recycling, and by separating the two kinds so each can be handled properly.

• Reduce, reuse and recycle: use fewer disposable items, carry a cloth bag instead of plastic bags, reuse bottles and containers, and send paper, glass and metal for recycling. This cuts down the total amount of waste we throw away.
• Separate the waste and compost it: keep biodegradable waste (kitchen and garden waste) apart from non-biodegradable waste (plastic, glass, metal). Turn the biodegradable part into compost or manure, and send the non-biodegradable part for proper recycling or safe disposal.

Answer: (1) Reduce, reuse and recycle: make less waste, carry cloth bags, reuse containers, and recycle paper, glass and metal. (2) Separate biodegradable from non-biodegradable waste, and compost the biodegradable part.

Biodegradable items can be broken down by micro-organisms. A group counts as only biodegradable if every item in it can decay, so we check each group and reject any that contains a non-biodegradable item.

• Group (a): grass, flowers and leather are all natural and can be broken down (leather decays slowly but is still biodegradable). Fully biodegradable.
• Group (b): grass and wood are biodegradable, but plastic is non-biodegradable, so this group is rejected.
• Group (c): fruit-peels, cake and lime-juice are all natural food items and are fully biodegradable. Accepted.
• Group (d): cake, wood and grass are all natural and fully biodegradable. Accepted.

The textbook marks options (c) and (d) as containing only biodegradable items, since both contain no man-made non-biodegradable substance.

Answer: The correct answer is (c) Fruit-peels, cake and lime-juice and (d) Cake, wood and grass. Both groups contain only biodegradable items. Option (b) is wrong because plastic is non-biodegradable.

A food chain must start with a producer and then move through organisms in a who eats whom order, so that each organism is food for the next. We check which option follows this producer-to-consumer order.

• Group (a): grass, wheat and mango are all producers (plants). One plant does not eat another, so they cannot form a chain. Rejected.
• Group (b): grass (producer) → goat (herbivore eats grass) → human (eats the goat). This follows the correct eating order, so it is a valid food chain.
• Group (c): goat, cow and elephant are all herbivores. None eats the other, so there is no chain. Rejected.
• Group (d): grass, fish and goat do not form a feeding sequence (a goat does not eat fish), so this is rejected.

Answer: The correct answer is (b) Grass, goat and human, because it follows the order producer (grass) → herbivore (goat) → consumer (human), with each organism eaten by the next.

An environment-friendly practice is any habit that reduces pollution, saves energy or cuts down waste. We check each option to see whether it helps the environment.

• Option (a): carrying a cloth bag reduces the use of plastic bags, which are non-biodegradable. Environment-friendly.
• Option (b): switching off unnecessary lights and fans saves electricity, and saving energy reduces the burning of fuels and pollution. Environment-friendly.
• Option (c): walking to school instead of using a scooter saves fuel and avoids the smoke and gases a vehicle gives out. Environment-friendly.
• Since all three habits help the environment, the correct choice is all of the above.

Answer: The correct answer is (d) All of the above. Carrying cloth bags, switching off unnecessary lights and fans, and walking to school are all environment-friendly practices.

In a food chain, every trophic level depends on the levels above and below it. The level below is its food; the level above feeds on it. Removing one whole level breaks these links and upsets the balance of the whole ecosystem.

• The organisms in the level just above the killed level lose their food. They will starve and their numbers will fall.
• The organisms in the level just below the killed level are no longer eaten. Their numbers will rise sharply (overpopulation), and they may strip away their own food and damage the environment.
• These changes pass on along the whole chain, so the balance of the entire ecosystem is disturbed.

For example, in the chain grass → deer → lion, if all the deer are killed, the grass grows out of control because nothing eats it, while the lions starve because they have lost their prey.

Answer: Killing all organisms in one trophic level starves the level above (it loses its food) and lets the level below overpopulate (nothing eats it). This breaks the food chain and disturbs the whole ecosystem.

Different trophic levels play different roles, so removing one level does not always cause the same amount of damage. The producers are the base of every food chain, while the top consumers sit at the end. Removing the base is far more serious than removing the top.

• Yes, the impact is different for different levels. Producers are the most important, because they make the food for everyone else. If all producers are removed, the entire ecosystem collapses, since no other organism can survive without food.
• Removing herbivores would leave producers without control (they overgrow) and carnivores without food (they starve). The damage is large, but not as complete as removing producers.
• Removing the top carnivores causes the least damage at first, but the herbivores below them may then increase too much and overgraze the plants.
• No level can be removed without some damage, because every trophic level is linked to the others, so removing any one disturbs the balance in some way.

Answer: Yes, the impact is different: removing producers is the most damaging (the whole ecosystem collapses), while removing top carnivores does less harm at first. But no trophic level can be removed without causing some disturbance, because all levels are linked.

Biological magnification is the increase in the concentration of harmful, non-degradable chemicals (such as certain pesticides) in the bodies of organisms as we move up the trophic levels of a food chain. Because these chemicals are not broken down or thrown out, they keep building up at each higher level.

• Harmful chemicals like pesticides enter the soil and water, and from there are taken in by plants (producers) in small amounts.
• When a herbivore eats many plants, the chemical from all those plants collects in its body, so its level is higher than in any single plant.
• When a carnivore eats many herbivores, the chemical builds up even more. At each step up the chain the concentration increases.
• The organism at the top of the food chain (often humans) ends up with the highest concentration of the harmful chemical.

Yes, the level of biological magnification is different at different trophic levels. It is lowest in the producers and increases at each higher level, becoming highest at the top of the food chain.

Answer: Biological magnification is the build-up of harmful non-degradable chemicals to higher and higher concentrations as we move up the food chain. Yes, the magnification differs by level: it is lowest in producers and highest in the top consumers.

Non-biodegradable wastes (plastics, polythene, glass, metal cans and some chemicals) cannot be broken down by micro-organisms. Because they last for a very long time, they keep building up and cause several lasting problems.

• They pile up and pollute: since they do not decay, they accumulate on land and in water. They make surroundings dirty, choke drains and rivers, and pollute the soil so plants grow poorly.
• They harm living things: animals may eat plastic by mistake and die, and plastic bags can block drains and cause water-logging, which helps mosquitoes breed and spread disease.
• They cause biological magnification: non-degradable chemicals like certain pesticides enter the food chain and get more concentrated at each higher level, finally poisoning the animals and humans at the top.
• They are hard to get rid of: burning plastic releases poisonous gases, and burying it pollutes the soil, so disposal itself is a problem.

Answer: Non-biodegradable wastes do not decay, so they pile up and pollute soil and water, harm or kill animals, cause biological magnification of toxins up the food chain, and are very hard to dispose of safely.

Biodegradable waste does break down into harmless substances, so it is better than non-biodegradable waste. But biodegradable does not mean no impact. If such waste is produced in very large amounts or not handled properly, the process of decay itself can still affect the environment.

• No, it will still have an impact. Even biodegradable waste, in huge amounts, takes time to decay and piles up in the meantime, making surroundings dirty and smelly.
• Decaying biodegradable waste gives off a foul smell and attracts flies, rats and germs, which can spread disease.
• The breakdown of large amounts releases gases like methane (a greenhouse gas) and carbon dioxide into the air.
• If the waste enters ponds or rivers, micro-organisms use up the dissolved oxygen to decompose it, and the fall in oxygen can kill fish and other water organisms.

Answer: No. Even fully biodegradable waste affects the environment if it is in large amounts or poorly managed: it causes foul smell and disease, releases gases like methane, and can remove oxygen from water and kill aquatic life.

The ozone layer high in the atmosphere absorbs most of the Sun's harmful ultraviolet (UV) radiation. If this layer is damaged, more UV reaches the Earth, which is dangerous for all living things. The main cause of the damage is man-made chemicals called CFCs (chlorofluorocarbons).

• Why it is a concern: a damaged ozone layer lets more UV reach the ground. UV rays cause skin cancer and cataracts (eye damage), weaken the immune system, harm crops, and kill the tiny plankton in the sea that form the base of aquatic food chains.
• The cause: the thinning of the ozone layer in the 1980s was linked to CFCs, used in refrigerators, air conditioners, spray cans and fire extinguishers.
• Step 1, international agreement: in 1987, countries signed an agreement (through the United Nations Environment Programme) to freeze and then reduce CFC production. This is the Montreal Protocol.
• Step 2, CFC-free products: it is now compulsory to make CFC-free refrigerators and air conditioners, and safer chemicals are used in their place all over the world.

Answer: Damage to the ozone layer is a concern because more harmful UV radiation then reaches Earth, causing skin cancer, eye damage and harm to crops and sea life. Steps taken: a global agreement (1987 Montreal Protocol) to cut CFC production, and a worldwide shift to CFC-free refrigerators and other products.