CBSE Class 10 Science Our Environment Notes

Environment: Our surrounding is called environment.

These CBSE Class 10 Science notes on Chapter 15, Our Environment, explain the meaning of environment, pollution and pollutants, the classification of wastes into biodegradable and non-biodegradable, the components of an ecosystem, food chains, food webs and trophic levels, the unidirectional flow of energy, the cyclic movement of nutrients, ozone layer depletion, and the disposal and recycling of hazardous and non-hazardous wastes. The intext and exercise questions of the chapter, with answers, are included in the notes.

Chapter 15: Our Environment

Environment

Our surroundings are called the environment. Living organisms live in different surroundings. Some plants and animals live completely in water, and others live on land. Human beings also live in different surroundings, in cities, towns and villages.

Plants, animals and human beings survive through interaction with each other and with non-living things such as air, water and land. Human beings depend on the resources of nature, which include soil, water, coal, electricity, oil and gas. These resources improve the lifestyle of human beings.

Environmental science can be defined as the study of organisms in relation to their surroundings.

Pollution and Pollutants

Human activities related to livelihood and welfare generate waste. All wastes are pollutants, and they create pollution in one way or another. Air, land and water are affected by the improper disposal of wastes, which creates an imbalance in the environment.

Pollution: any undesirable change in the physical, chemical or biological characteristics of air, land and water that affects human life adversely.
Pollutant: a substance released into the environment, due to natural or human activity, that affects the environment adversely. Examples: sulphur dioxide, carbon monoxide, lead and mercury.

Classification of Wastes

1. Biodegradable wastes
2. Non-biodegradable wastes

Biodegradable wastes: substances that are broken down by biological or microbial action. Examples: wood, paper and leather.
Non-biodegradable wastes: substances that are not broken down by biological or microbial action. Examples: plastic and mineral wastes.

Intext Questions (Page No. 257)

Q1. Why are some substances biodegradable and some non-biodegradable?

Answer:
1. Substances such as metal, glass and plastic cannot be decomposed by living organisms, so they are non-biodegradable.
2. Micro-organisms do not have enzymes that can digest these substances.
3. Other substances, such as paper and vegetable wastes, can be easily broken down by enzymes, so they are biodegradable.

Teacher's Note:
a) Enzymes of micro-organisms act only on certain chemical structures.
b) Plastic lasts for hundreds of years in the environment because no natural process breaks it down quickly.

Q2. Give any two ways in which biodegradable substances would affect the environment.

Answer:
1. Biodegradable substances return nutrients to the environment, as they are broken down.
2. Their degradation may release certain gases into the atmosphere, which pollutes the environment.

Teacher's Note:
a) Nutrients returned to the soil support the growth of new plants.
b) Large amounts of decaying waste can produce methane and other gases.

Q3. Give any two ways in which non-biodegradable substances would affect the environment.

Answer:
1. They make the environment poisonous and unfit for survival.
2. They block the transfer of energy and minerals in the ecosystem.

Teacher's Note:
a) Non-biodegradable wastes stay in the soil and water for a very long time.
b) Animals can be harmed when they accidentally eat such wastes.

Ecosystem: What Are Its Components?

A community of organisms that interact with one another and with the environment is called an ecosystem. An ecosystem has two types of components: biotic components and abiotic components. Producers, consumers and decomposers are the biotic factors.

Abiotic component: all the non-living things, such as air, water and soil.
1. Air provides oxygen for respiration, carbon dioxide for photosynthesis and other gases needed by living beings.
2. Water is essential for all living beings, because all metabolic activities take place in the presence of water.
3. Soil is the reservoir of various nutrients used by plants. Through plants, these nutrients reach other living beings.

Biotic component: all living beings.
1. Green plants are producers, because they prepare food by photosynthesis.
2. Animals and other living beings are consumers, because they take food directly or indirectly from plants.
3. Bacteria and fungi are decomposers, because they decompose the dead remains of plants and animals, so that the raw materials can be returned to the environment.

[Figure: Flow of energy in an ecosystem. The Sun gives energy to the Producers. Energy flows from the Producers to the Consumers, and from the Consumers to the Decomposers. Heat is lost from the Producers, the Consumers and the Decomposers. Decomposers return inorganic nutrients to the Inorganic Nutrient Pool, which goes back to the Producers.]

Balance in Ecosystem

A balanced ecosystem is an ecological community, together with its environment, functioning as a complex unit. An ecosystem is maintained by a balance in nature, such as the balance between hawks and mice. If the hawk population is larger than the mouse population, the balance is lost. Similarly, a banana tree and monkeys depend on each other: if the banana trees stop growing, the monkeys will not get bananas.

An ecosystem maintains a balance between the number of resources and the number of users, or between prey and predators.

Food Chain and Food Web

The ultimate source of energy in an ecosystem is the Sun. Producers, like green plants, trap solar energy and convert it into the chemical energy of food. When a primary consumer eats the producer, part of this energy is passed on to it. The primary consumer is then eaten by a secondary consumer, which may be eaten by a tertiary consumer, and so on. In this way energy is transferred from one consumer to the next higher level. A series of organisms through which food energy flows in an ecosystem is called a food chain.

A food chain in an ecosystem is a series of organisms in which each organism feeds on the one below it in the series.

In a forest ecosystem, grass is eaten by a deer, which in turn is eaten by a tiger. The grass, deer and tiger form a food chain. Energy flows from the grass (producer) to the deer (primary consumer) to the tiger (secondary consumer).

A food chain in a grassland ecosystem may consist of grasses and other plants, grasshoppers, frogs, snakes and hawks.

In a freshwater aquatic ecosystem like a pond, the organisms in a food chain include algae, small animals, insects and their larvae, small fish, big fish and a fish-eating bird or animal.

[Figure: (a) Forest food chain: grass or tree, deer, tiger. (b) Grassland food chain: grass, grasshopper, frog, snake, hawk. (c) Pond food chain: algae, small animal (insect), small fish, big fish, fish-eating bird.]

A food chain always begins with producers. Herbivores (plant-eaters) come next in the chain. They are consumed by carnivores (flesh-eaters). A few food chains can be long and may extend to the fourth, fifth or even sixth order of consumers.

Some common food chains are:
1. Plants → Deer → Lion
2. Plants → Worm → Bird → Cat
3. Plants → Grasshopper → Frog → Snake → Hawk
4. Algae → Small animal → Small fish → Big fish → Bird

Food web: a graphical depiction of the feeding connections among the species of an ecological community. A food web consists of the food chains of a particular ecosystem, which are interconnected and overlapping. It defines the flow of energy through the species of a community as a result of their feeding relationships. A food web increases the stability of the ecosystem, provides other sources of food, and allows endangered species to grow.

[Figure: Food web consisting of many food chains, showing green plants at the bottom, then herbivores such as insects, rabbits and deer, and carnivores such as frogs, snakes, foxes, owls, eagles and tigers at the top, with arrows showing the direction of food flow.]

Trophic Levels of Food Chains

The levels of a food chain, which form a food pyramid, are called trophic levels. The trophic level of an organism is the level it occupies in a food pyramid.

1. Producers: green plants (autotrophs), which absorb the Sun's light energy and make their own food by photosynthesis. They form the first trophic level.
2. Primary consumers: herbivores that eat green plants, such as insects, sheep, caterpillars and cows.
3. Secondary consumers: carnivores that eat primary consumers and other animals, such as lions, snakes and cats.
4. Tertiary consumers: animals that eat secondary consumers.
5. Quaternary consumers: animals that eat tertiary consumers.
6. Predators: animals at the top of the levels, which have little or no natural enemies. They are the 'bosses' of their ecosystems. Examples: owls, snakes, wild cats, crocodiles and sharks. Humans can also be called predators. A prey is an animal that a predator hunts and feeds on.
7. Detritivores and decomposers: when an organism dies, detritivores (such as vultures, worms and crabs) eat it up. The rest is broken down by decomposers (mostly bacteria and fungi), and the exchange of energy continues. Decomposers start the cycle again.

[Figure: Food pyramid with Green Plants at the base, then Primary Consumers (cow), Secondary Consumers (snake and lion), Tertiary Consumers, Quaternary Consumers and Predators (owl, shark and others) at the top, with the Sun shown above.]

Energy Flow is Unidirectional and Requires Continuous Inputs

Ecosystem energy processes follow the laws of thermodynamics:
- Energy cannot be created or destroyed.
- Energy transformations are not perfect, and energy is lost as heat in each transformation.

The flow of energy in the ecosystem is unidirectional. Energy enters the plants from the Sun through photosynthesis, and then passes from one organism to another in a food chain. Energy given out by the organisms as heat is lost to the environment, and it does not return to be used by plants again. So the energy flow is unidirectional, because the energy lost as heat cannot be reused by plants in photosynthesis.

During the transfer of energy through successive trophic levels, energy is lost all along the path. No transfer of energy is 100 per cent efficient. The energy available at each successive trophic level is about 10 per cent of the previous level. So there is a progressive decline in the amount of energy available as we go from the producer level to the higher trophic levels.

Cyclic Movement of Nutrients

The movement of nutrients is cyclic, because nutrients revolve round within an ecosystem. Nutrients pass from abiotic nutrient stores, such as the soil and the atmosphere, into biotic stores, the plants and animals (the biomass). The nutrients are then recycled within the ecosystem following death and decomposition.

Intext Questions (Page No. 261)

Q1. What are trophic levels? Give an example of a food chain and state the different trophic levels in it.

Answer:
1. Each stage of a food chain, through which food energy passes, is known as a trophic level.
2. Example of a food chain: Trees → Rabbit → Snake → Hawk.
3. Trees are the producers (first trophic level), rabbit is the I consumer (second trophic level), snake is the II consumer (third trophic level), and hawk is the III consumer (fourth trophic level).

Teacher's Note:
a) The printed key gives only the first sentence; the complete answer names the example and each level.
b) The number of trophic levels is usually small, because only about 10 per cent of the energy passes to each level.

Q2. What is the role of decomposers in the ecosystem?

Answer:
1. Decomposers, such as bacteria and fungi, obtain their nutrients by breaking down the remains of dead plants and animals.
2. They convert the organic matter of dead bodies into simple inorganic raw materials, such as CO₂, H₂O and nutrients.
3. These raw materials return to the soil and air, where plants can use them again.

Teacher's Note:
a) Without decomposers, dead matter would pile up and nutrients would stay locked in it.
b) Decomposition completes the nutrient cycle.

Ozone Layer Depletion

Ozone (O₃) is a molecule formed by three atoms of oxygen. O₂, which we normally refer to as oxygen, is essential for all aerobic forms of life. Ozone, however, is a deadly poison. At the higher levels of the atmosphere, ozone performs an essential function: it shields the surface of the earth from ultraviolet (UV) radiation from the Sun. This radiation is highly damaging to organisms, and it is known to cause skin cancer in human beings.

Ozone at higher levels of the atmosphere is formed when UV radiation acts on oxygen (O₂) molecules. The higher energy UV radiation splits some molecular oxygen (O₂) into free oxygen (O) atoms. These atoms then combine with molecular oxygen to form ozone:
O₂ → (UV) O + O
O + O₂ → O₃ (Ozone)

The amount of ozone in the atmosphere began to drop sharply in the 1980s. This decrease has been linked to synthetic chemicals such as chlorofluorocarbons (CFCs), which are used as refrigerants and in fire extinguishers. In 1987, the United Nations Environment Programme (UNEP) succeeded in forging an agreement to freeze CFC production at 1986 levels.

Intext Questions (Page No. 264)

Q1. What is ozone and how does it affect any ecosystem?

Answer:
1. Ozone is the molecule O₃, and the ozone layer is present in the stratosphere.
2. It absorbs most of the ultraviolet radiation coming from the Sun, which protects organisms in the ecosystem from harmful radiation.
3. Ozone gas is poisonous if inhaled, so it is harmful at ground level.

Teacher's Note:
a) Ultraviolet radiation can damage the DNA of living cells.
b) The same gas that protects us high up is harmful when it is near the ground.

Q2. How can you help in reducing the problem of waste disposal? Give any two methods.

Answer:
1. Using biodegradable materials, which are broken down naturally.
2. Using recyclable materials, which can be reprocessed and reused.

Teacher's Note:
a) Carrying a cloth bag reduces the use of plastic bags.
b) Reusing containers also reduces the amount of waste generated.

How to Protect Us from These Hazardous Wastes?

The following methods are adopted for the disposal of harmful waste materials.

1. Land fills: there are permanent storage facilities in secured lands for military-related liquid and radioactive wastes. High-level radioactive wastes are stored in deep underground storage.
2. Deep well injection: a well is drilled into dry porous material below the ground water. Hazardous waste liquids are pumped into the well, where they soak into the porous material and remain isolated indefinitely.
3. Incineration: the burning of materials. Hazardous bio-medical wastes are usually disposed of by incineration. Bio-medical wastes include human anatomical wastes, discarded medicines, toxic drugs, blood, pus, animal wastes, and microbiological and bio-technological wastes.

Management of Non-Hazardous Wastes: Solid Waste Management

Reuse and recycling technique: separating out materials such as rubber, glass, paper and scrap metal from refuse, and reprocessing them for reuse, is called reclamation of waste or recycling.

Paper (54% recovery): can be repulped and reprocessed into recycled paper, cardboard and other products.
Glass (20% recovery): can be crushed, remelted and made into new containers, or crushed and used as a substitute for gravel or sand in construction materials such as concrete and asphalt.
Food waste and yard waste (leaves, grass and so on): can be composted to produce humus, a soil conditioner.

Exercise Questions (Page No. 159)

Q1. Which of the following groups contain only biodegradable items?
(a) Grass, flowers and leather
(b) Grass, wood and plastic
(c) Fruit-peels, cake and lime-juice
(d) Cake, wood and grass

Answer: (a), (c) and (d)

1. Grass, flowers and leather are broken down by biological processes, so option (a) contains only biodegradable items.
2. Fruit-peels, cake and lime-juice are all biodegradable, so option (c) contains only biodegradable items.
3. Cake, wood and grass are all biodegradable, so option (d) contains only biodegradable items.
4. Option (b) contains plastic, which is non-biodegradable.

Teacher's Note:
a) The printed key lists only (c) and (d). Leather is listed as biodegradable in the chapter notes, so option (a) also qualifies.
b) Plastic is the non-biodegradable item in option (b).

Q2. Which of the following constitute a food-chain?
(a) Grass, wheat and mango
(b) Grass, goat and human
(c) Goat, cow and elephant
(d) Grass, fish and goat

Answer: (b) Grass, goat and human

1. Each step of a food chain forms a trophic level.
2. Grass is the producer (first trophic level), goat is the herbivore (second trophic level) and human is the carnivore (third trophic level).
3. Hence, the correct option is (b).

Teacher's Note:
a) Each organism in a food chain feeds on the one before it.
b) Options (a), (c) and (d) mix plants with plants or animals with animals, so they do not form a food chain.

Q3. Which of the following are environment-friendly practices?
(a) Carrying cloth-bags to put purchases in while shopping
(b) Switching off unnecessary lights and fans
(c) Walking to school instead of getting your mother to drop you on her scooter
(d) All of the above

Answer: (d) All of the above

1. Cloth bags are biodegradable and reduce the use of plastic bags.
2. Switching off unnecessary lights and fans saves electricity and reduces pollution from power generation.
3. Walking instead of using a scooter reduces the use of petrol, which causes less pollution.
4. Hence, all three practices are environment-friendly, and the correct option is (d).

Teacher's Note:
a) Small daily actions add up to a large reduction in pollution.
b) Reducing the use of petrol and diesel lowers the release of smoke and carbon monoxide.

Q4. What will happen if we kill all the organisms in one trophic level?

Answer:
1. If the organisms of one trophic level are killed, the organisms of the lower trophic level will grow in number.
2. The organisms of the higher trophic level will not survive, because their food supply is lost.
3. The flow of energy from one trophic level to the next will stop.

Teacher's Note:
a) The balance between prey and predators is disturbed.
b) This effect spreads through the whole food chain.

Q5. Will the impact of removing all the organisms in a trophic level be different for different trophic levels? Can the organisms of any trophic level be removed without causing any damage to the ecosystem?

Answer:
1. The impact is different for different trophic levels, because the lower trophic levels have more individuals than the higher trophic levels.
2. Removing the producers affects all the organisms, and they will not survive.
3. Removing a higher trophic level leads to an increase in the organisms of the lower trophic level, and these organisms may die due to a shortage of food.
4. Hence, the organisms of no trophic level can be removed without damage to the ecosystem.

Teacher's Note:
a) Producers are the base of every food chain, so their loss is the most serious.
b) Each trophic level depends on the levels below it for energy.

Q6. What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?

Answer:
1. Biological magnification is the accumulation of chemicals in the bodies of organisms at higher trophic levels.
2. The levels of magnification are not the same at all trophic levels. The chemicals are not degraded, so they accumulate progressively at each trophic level.
3. Hence, biomagnification is greater in the organisms of higher trophic levels.

Teacher's Note:
a) Pesticides such as DDT are classic examples of chemicals that show biological magnification.
b) A top predator such as an eagle can have the highest concentration of such chemicals.

Q7. What are the problems caused by the non-biodegradable wastes that we generate?

Answer:
1. Non-biodegradable substances cannot be broken down, so they accumulate and contaminate the soil and the water resources.
2. When accidentally eaten by stray animals, they can harm them and even cause their death.
3. They occupy more space in landfills and require special disposal techniques.
4. They can accumulate in the environment and enter the food chain.

Teacher's Note:
a) Plastic bags are a common example of non-biodegradable waste.
b) Recycling and reducing the use of such materials help to limit these problems.

Q8. If all the waste we generate is biodegradable, will this have no impact on the environment?

Answer:
1. If all the waste is biodegradable, there will be no accumulation of waste, and the earth would be a cleaner place to live.
2. However, if the amount of biodegradable waste is very large, its degradation may lead to air pollution, because gases are released.

Teacher's Note:
a) Even natural processes need to be managed when they happen on a very large scale.
b) Composting and controlled disposal reduce the gases released from decaying waste.

Q9. Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?

Answer:
1. The thinning of the ozone layer in the stratosphere is called depletion of the ozone layer.
2. Because of this depletion, harmful ultraviolet radiation reaches the surface of the earth, which may cause skin diseases and cancer.
3. To reduce the depletion of the ozone layer, the use of chlorofluorocarbons (CFCs) has been minimised, and CFC production has been frozen at 1986 levels through an agreement of the United Nations Environment Programme.

Teacher's Note:
a) The ozone hole over Antarctica is the most widely known sign of this depletion.
b) Substitutes for CFCs are now used in refrigerators and fire extinguishers.

1. Bio–degradable wastes

2. Non–bio-degradable wastes

Ans: (i) Biodegradable substances act as a medium to return back the nutrients to the environment.

(ii) Their degradation may release certain gases in the atmosphere thereby polluting the environment.

Fig. Flow of energy in an ecosystem

BALANCE IN ECO-SYSTEM

♦ The sun is the source of all the energy in food chains. Green plants, usually the first level of any food chain, absorb some of the Sun’s light energy to make their own food by photosynthesis. Green plants (autotrophs) are therefore known as ‘Producers’ in a food chain.

♦ The second level of the food chains is called the Primary Consumer. These consume the green plants. Animals in this group are usually herbivores. Examples include insects, sheep, caterpillars and even cows.

♦ The third in the chain are Secondary Consumers. These usually eat up the primary consumers and other animal matter. They are commonly called carnivores and examples include lions, snakes and cats.

♦ The fourth level is called Tertiary Consumers. These are animals that eat secondary consumers.

Ecosystem energy processes conform to the thermodynamic laws.

♦  Energy cannot be made or destroyed

Question. Ecosystem consists of
(A) producers
(B) consumers
(C) decomposers
(D) all of these
Answer : D

Question. What will happen, if all the green plants disappear from the earth ?
(A) all animals will die
(B) only carnivores will die
(C) only herbivores will die
(D) it will have no effect on animals
Answer : A

Question. In an ecosystem
(A) Primary producers are more than that of primary consumers
(B) Secondary consumers are largest, because they are powerful
(C) Primary consumers are out of number
(D) Primary consumers are least dependent upon primary producers
Answer : A

Question. In grassland ecosystem the pyramid of biomass is
(A) Linear
(B) upright
(C) inverted
(D) inverted upright
Answer : B

Question. Primary consumers in detritus food chain are
(A) Herbivores
(B) Bacteria and fungi
(C) Insect larvae, nematodes
(D) Carnivoroes
Answer : B

Question. A food chain starts with
(A) decay
(B) respiration
(C) photosynthesis
(D) nitrogen fixation
Answer : C

Question. Which is not an harmful effect of ozone depletion ?
(A) cataract
(B) skin cancer
(C) Decrease immunity
(D) Respiratory troubles
Answer : D

Question. Which one of the following pyramid is upright always ?
(A) Pyramid of number
(B) Pyramid of energy
(C) Pyramid of biomass
(D) All of the above
Answer : B

Question. The pyramid of number in tree ecosystem-
(A) Inverted
(B) Upright
(C) Both
(D) None
Answer : A

Question. Decomposers of an ecosystem include
(A) Microscopic animals
(B) Bacteria, fungi and microscopic organism
(C) Algae and virus
(D) Bacteria, fungi and cyanobacteria
Answer : B

Question. In terrestrial ecosystem , maximum energy is present in
(A) T₃
(B) T₂
(C) T₁
(D) T₄
Answer : C

Question. The life sustaining zone of the earth, where lithosphere, hydrosphere and atmosphere interact is called
(A) ecosystem
(B) stratosphere
(C) biosphere
(D) ozonosphere
Answer : C

Question. Pyramids that is never inverted
(A) energy
(B) mass
(C) number
(D) size
Answer : A

Question. The most rapidly increasing and much harmful waste, today is
(A) Household waste
(B) Paper
(C) municipal sewage
(D) electronic waste
Answer : D

Question. Incineration is the disposal of waste by ;
(A) sedimentation
(B) burning
(C) Scrubers
(D) none of these
Answer : B

Question. Our atmosphere is made of many layers. The layer nearest to the surface of the earth is known as :
(A) Geosphere
(B) Stratosphere
(C) Troposphere
(D) none of above
Answer : C

Question. e-waste is
(A) emergeney waste
(B) energy waste
(C) electronic waste
(D) None of these
Answer : C

Question. Which one of these are non degradable waste ?
(A) Vegetable waste
(B) Fruit peel
(C) Plastics
(D) none of these
Answer : C

Question. Type of effect produce by coolants
(A) Golbal warming
(B) Green house effect
(C) Ozone depletion
(D) All of the above
Answer : A

Question. The human made synthetic chemical used in refrigerator is
(A) LPG
(B) CFC
(C) CH₄
(D) PVC
Answer : B

Question. Biological O₂ demand measures
(A) industrial pollution
(B) dissolved oxygen needed by microbes
(C) polluting capacity of effluents
(D) air pollution
Answer : B

Question. Example of omnivorous animals is -
(A) Sheep
(B) Deer
(C) Lion
(D) Cockroach
Answer : D

Question. Biogeochemical cycling means cycling of
(A) water
(B) energy in an ecosystem
(C) nutrients in an ecosystem
(D) gases between plants and atmosphere
Answer : C