Home » Lakhmir Singh Class 10 Biology » Lakhmir Singh Class X Biology Chapter 4 – Our Environment

Lakhmir Singh Class X Biology Chapter 4 – Our Environment


Exercise : Solution of Questions on page Number : 226


Answer: 1
The functional unit of the environment consisting of living and non-living components is called an ecosystem.


Answer: 2
Two natural ecosystems are the pond ecosystem and the forest ecosystem. Two artificial ecosystems are the crop field ecosystem and the aquarium ecosystem.


Answer: 3
Among the given ecosystems, an aquarium is not a terrestrial ecosystem.


Answer: 4
Plants are known as producers because they prepare their food themselves by the process of photosynthesis.


Answer: 5
Organisms that break down the complex organic compounds present in dead animals and plants are called decomposers.


Answer: 6
The tiny herbivorous plants and animals present in water are known as plankton. Plankton are classified as phytoplankton (plants) and zooplankton (animals).


Answer: 7
(a) False. Green plants are known as producers.
(b) True.


Answer: 8
Producers (green plants) form the starting point of a food chain.


Answer: 9
Goats and rats belong to the same trophic level as they are both herbivorous animals.


Answer: 10
Trees and grass belong to the same trophic level as they are both producers.


Answer: 11
A food chain that operates in a water body, such as a pond, a lake or an ocean, is known as an aquatic food chain.


Answer: 12
The second trophic level consists of insects, and the fourth trophic level consists of snakes.


Answer: 13
The various steps of a food chain are called trophic levels.


Answer: 14
Plants → Rats → Snakes → Hawks


Answer: 15
Algae → Small animals → Fish → Big fish


Answer: 16
Snakes belong to the third trophic level and hawks belong to the fourth trophic level in the food chain.


Answer: 17
Environment is the term that includes the others, i.e., air, flora, fauna, water, sunlight and soil.


Answer: 18
A food chain represents a unidirectional flow of energy.
X is energy.


Answer: 19
(a) Decomposer organisms are specific in their action.
(b) In nature, all green plants are producers, whereas animals are consumers.
(c) A series of organisms, each of which feeds on the next organism, the beginning of which is a green plant, is called a food chain.
(d) The science that deals with the inter-relationships of living things with one another and their environment is called ecology.
(e) Plastic is a non-biodegradable material, whereas paper is a biodegradable material.


Answer: 20
Organisms that produce their own food from simple inorganic substances such as water and carbon dioxide are called producers. Examples are green plants and blue-green algae.
Organisms that consume the food prepared by producers are called consumers. Examples are cows and human beings.


Answer: 21
(a) Micro-organisms that break down the complex organic compounds present in dead plants and animals and their products, such as faeces and urine, into simpler substances are called decomposers. Bacteria are decomposers.
(b) Decomposers decompose dead plants and animals and hence act as cleansing agents of the environment.


Answer: 22
Herbivores that feed on plants are called primary consumers. Example: Insects.
Small carnivores that feed on other animals, especially herbivores, are called secondary consumers. Example: Frogs.
Large carnivores that feed on other animals, especially secondary consumers, are called tertiary consumers. Example: Birds.
Insects → Frogs → Birds
(Primary consumers) (Secondary consumers) (Tertiary consumers)


Exercise : Solution of Questions on page Number :227


Answer: 23
Grass →Insect → Frog → Eagle
The secondary consumer in this food chain is the frog.


Answer: 24
(a) The various steps in a food chain at which the transfer of food and energy takes place are known as trophic levels.
A food chain with four trophic levels can be shown as follows.
Grass → Insect → Frog → Eagle
(b) If we kill all the organisms in one trophic level, there will be no food for the organisms of the next trophic level, and they will die of starvation. Also, the organisms of the previous trophic levels will increase in number as there will be no organism to consume them as food.


Answer: 25

First trophic level Third trophic level
Producers belong to the first trophic level. Carnivores belong to the third trophic level.
They prepare their own food by photosynthesis. They consume the flesh of other animals.
Example: Green plants. Example: Lions.

Answer: 26
No, the organisms of no trophic level can be removed without causing damage to the ecosystem, as removing them can disturb the food chain.
Yes, the impact of removing all the organisms in a trophic level will be different for different trophic levels. Suppose the producers, who belong to the first trophic level, are removed, no organism belonging to the higher trophic levels will be able to exist, as the food and energy for the higher trophic levels are provided by the producers.
If consumers such as deer are removed from their trophic level, consumers such as lions, which eat deer, will die because of the unavailability of food. Hence, the population of lions will be reduced.
If consumers such as lions are removed from their trophic level, then there will be no predator control over the population of deer. Thus, the deer population will increase, leading to an excessive grazing of grass.


Answer: 27
If all the lions are removed from the above food chain, the population of deer will increase greatly, as there will be no lion to consume them as food. Also, the density of grass will decrease as the high population of deer will lead to excessive grazing of grass.


Answer: 28
As frogs were exported from the village, the mosquito population increased, because there were no frogs to consume the mosquitoes. Hence, the increased number of mosquitoes led to an increase in the number of malaria patients.


Answer: 29

Biodegradable waste Non-biodegradable waste
Waste materials that can be broken down into non-poisonous substances in due course of time, by the action of natural micro-organisms such as certain bacteria, are called biodegradable wastes. Waste materials that cannot be broken down into non-poisonous or harmless substances naturally are called non-biodegradable wastes.
Examples: Paper and vegetable peels. Examples: Plastic and iron nails.

Answer: 30
Biodegradable: Paper, hay, wheat, cake, wood, jute bag, cotton cloth, grass, vegetable peels.
Non-biodegradable: Glass bottle, ballpoint pen refill, DDT, polythene bag.


Answer: 31
(a) An activity to prove that paper is biodegradable and plastic is non-biodegradable is described below.
1) Take a piece of paper and a polythene bag.
2) Dig a hole in the ground about 15 cm deep and place both the paper and the polythene bag separately in it. Cover them with soil.
3) Leave them for about one month.
4) On uncovering the buried materials, it will be observed that the paper has been partially eaten up by micro-organisms, whereas the polythene bag has remained unaffected. This proves that paper is biodegradable, whereas plastic is non-biodegradable.
(b) Some materials are biodegradable, whereas some are non-biodegradable, because decomposers break down natural materials such as paper but do not break down man-made materials such as plastics. Non-biodegradable materials consist of compounds that are not degraded by decomposers, and they persist in the environment.


Answer: 32
(a) Algae →Protozoa → Small fish → Large fish
(b) Grass → Goat → Man
(c) Grass → Grasshopper → Frog → Snake → Eagle


Answer: 33
(a) Third trophic level
(b) Second trophic level
(c) Third trophic level
(d) Second trophic level
(e) Fourth trophic level


Answer: 34
(a) Weed → Tadpole → Water beetle
(b) There are three links in this food chain.


Answer: 35
(a) Phytoplankton (algae) → Zooplankton (protozoa) → Small fish → Big fish → Man
(b) Plants → Man


Answer: 36
(a) Jute bags are biodegradable, and hence they are degraded by micro-organisms. Plastic bags are non-biodegradable, and hence they are not degraded by micro-organisms. Therefore, the advantage of using jute bags over plastic bags is that jute bags are environment friendly.
(b) Algae → Protozoa → Small fish → Big fish


Answer: 37
Ponds and lakes contain decomposers, which act as cleansing agents, whereas an aquarium does not have decomposers that cleanse it. Hence, an aquarium needs to be cleaned periodically.


Answer: 38
Decomposers help in decomposing dead plants and animals, and hence act as cleansing agents of the environment. They also help restore to the soil, air and water the various elements that plants and animals are made of, for reuse by the producers. If there were no decomposers in the ecosystem, then dead plants and animals would remain as such, and their elements would never be returned to the soil, air and water.


Answer: 39

Food chain Food web
The sequence of living organisms in a community in which one organism consumes another for the transfer of food energy is called a food chain. The inter-connected food chains operating in an ecosystem, which establish relationships among various species, make up a food web.
It represents a unidirectional flow of energy. It represents the numerous pathways along which food energy flows.

Answer: 40
(a) Trophic level
(b) Abiotic components
(c) Consumers
(d) Environment
(e) Ecosystem


Answer: 41
(a) Waste materials that can be broken down into non-poisonous substances in nature in due course of time, by the action of micro-organisms such as certain bacteria, are called biodegradable wastes. Examples are paper and vegetable peels.
(b) Aluminium wire and synthetic fibre are non-biodegradable.


Answer: 42
(a) Waste materials that cannot be broken down into non-poisonous or harmless substances in nature are called non- biodegradable wastes. Examples are plastic and iron nails.
(b) Animal bones and leather belts are biodegradable wastes.


Exercise : Solution of Questions on page Number :228


Answer: 43
(a) An ecosystem is a self-sustained unit of living things (plants, animals and decomposers) and their non-living environment (air, water and soil).
Examples: A pond is an ecosystem that includes the aquatic life (aquatic animals and plants) that live in the pond water.
A forest is an example of a terrestrial ecosystem that includes the physical environment of the forest along with the plants and animals that live in that forest.
(b) The biotic components of an ecosystem are the plants, animals and decomposers present in it.
The abiotic components of an ecosystem are air, water and soil. The abiotic components also include physical factors such as light, temperature, pressure and humidity. Inorganic substances such as carbon dioxide, oxygen, water, nitrogen, sulphur and phosphorus are also abiotic components of an ecosystem.


Answer: 44
(a) The sequence of living organisms in a community in which one organism consumes another organism to transfer food energy is called a food chain. An example of a simple food chain is that operating in a grassland:
Grass                    →                 Deer           →            Lion
(Producer)                           (Herbivore)                  (Carnivore)

(b) The inter-connected food chains operating in an ecosystem that establish a network of relationships among various species comprise a food web.
A food web is formed by many food chains operating together. A food web operating in a grassland is shown in the figure.
Five food chains are possible in this food web, which are as follows.

1) Grass → Grasshopper → Frog → Hawk
2) Grass → Grasshopper → Rat → Fox
3) Grass → Rabbit → Fox
4) Grass → Rat → Snake →Hawk
5) Grass → Rat → Hawk


Answer: 45
(a) Our physical surroundings, which include the air, water bodies, the soil and all living organisms such as plants, animals, human beings and micro-organisms, constitute our environment.
(b) Non-biodegradable substances are the major pollutants of the environment. Two such substances are plastic articles and glass articles.
(c) Human beings are the organisms whose uncontrolled activities are damaging the environment.
(d) It is better to use paper bags than plastic bags because paper bags are biodegradable and can easily be degraded by micro-organisms, whereas plastic bags are non-biodegradable and cannot be degraded by micro-organisms.


Answer: 46
(b) Grass, goat and human
Grass is eaten by goats, and goats are eaten by humans.


Answer: 47
(a) photosynthetic
The initial organism of a food chain is the producer, which produces food through photosynthesis.


Answer: 48
(d) green algae, mosquito larvae, fish
In a pond, green algae are the primary producers. Green algae are consumed by mosquito larvae, which therefore become the primary consumers. Mosquito larvae are consumed by fish, which then become the secondary consumers.


Answer: 49
(c) yes, yes, no
Bacteria and fungi are decomposers of dead organisms.


Answer: 50
(b) Crop field
A crop field is an artificial ecosystem, because it is made by man.


Answer: 51
(d) they are made of non-biodegradable materials
Disposable plastic plates cannot be degraded by micro-organisms and hence should not be used. They will remain as such in the environment and pollute it.


Answer: 52
(a) carnivores
The third trophic level is always occupied by carnivores. The first trophic level is occupied by producers, the second by herbivores and the fourth by decomposers.


Answer: 53
(c) biomagnification
The accumulation of non-biodegradable pesticides in the food chain in an increasing amount at each higher trophic level is known as biomagnification.


Answer: 54
(c) primary consumer to secondary consumer
A grasshopper eats grass; hence, the grasshopper is the primary consumer. A frog eats the grasshopper, so the frog becomes the secondary consumer. The energy transfer is from the primary consumer to the secondary consumer, i.e., from the grasshopper to the frog.


Answer: 55
(c) both living organisms and non-living objects
Both living organisms and non-living objects are components of an ecosystem.


Answer: 56
(b) convert organic materials to inorganic forms
Decomposers convert the organic matter present in the bodies of dead organisms into inorganic forms and mix them with the soil.


Exercise : Solution of Questions on page Number : 229


Answer: 57
(d) The population of tigers decreases and the density of grass increases.
If deer are removed from the given food chain, the population of tigers will decrease as they will not have deer for their food and will starve to death. The density of grass will increase as there is no organism to consume it as food.


Answer: 58
(b) producers
Green plants are producers that prepare carbohydrates as food from inorganic compounds, such as carbon dioxide and water, by using sunlight as the radiant energy.


Answer: 59
(b) food web
Organisms of a higher trophic level that feed on several types of organisms belonging to a number of lower trophic levels constitute a food web.


Answer: 60
(d) (ii) and (iv)
These materials cannot be degraded by micro-organisms and hence are non-biodegradable.


Answer: 61
(b) Green plants get their food from readymade organic compounds.
This is an incorrect statement as green plants prepare their food themselves; they do not get readymade organic compounds as their food.


Answer: 62
(c) (ii) and (iii)
A food chain is a sequence of living organisms in a community or ecosystem, in which one organism is consumed the other. The organisms given in options (i) and (iii) cannot comprise a food chain.


Answer: 63
(c) T1
At T1, producers, which make food, are present, and hence they are at the bottom of the trophic levels. As they get energy directly from the sun, they have the maximum energy. This energy keeps on reducing as it is passed on to the next trophic levels.


Answer: 64
(c) aluminium foil
Aluminium foil cannot be degraded by the action of micro-organisms and hence is non-biodegradable.


Answer: 65
(c) jute carry bag
A jute carry bag is made from jute, which is a plant fibre. Hence, it is biodegradable.


Answer: 66
(c) nylon cloth carry bags
Nylon cloth is non-biodegradable and pollutes the environment.


Answer: 67
(c) algae
Algae are producers, which prepare their food themselves through photosynthesis. They are not consumers.


Answer: 68
(b) zooplankton
Zooplankton are herbivorous micro-organisms such as protozoa that are found in water bodies. They depend upon producers for food and hence are not producers.


Answer: 69
(c) fungi
Fungi are micro-consumers, which consume micro-producers.


Answer: 70
(c) Putrefying bacteria
Putrefying bacteria decompose dead organisms, and hence act as decomposers in an ecosystem.


Answer: 71
(b) saprotrophs
Saprotrophs such as bacteria and fungi recycle the organic matter present in the bodies of dead organisms by mixing it with the soil.


Answer: 72
(c) frog
The grass is eaten by the insect, which is eaten by the frog, which is eaten by the snake. Hence, the grass is the producer, the insect is the primary consumer, the frog is the secondary consumer and the snake is the tertiary consumer.


Answer: 73
(c) excessive killing of large carnivores
Excessive killing of lions resulted in an increase in the number of herbivores, which in turn consumed all the vegetation. Hence, the Sahara Desert was formed.


Exercise : Solution of Questions on page Number : 230


Answer: 74
(a) Weeds → Tadpole → Water beetle → Fish
(b) (i) The tadpole is a herbivore, and (ii) the water beetle and the fish are carnivores.
(c) (i) The water beetle and the fish are predators, and (ii) the tadpole and the water beetle are prey.
(d) Weeds can trap solar energy to make food.
(e) The water beetle is the secondary consumer in the given food chain.


Answer: 75
(a) The nectar present in the flowers of plants serves as food for bees. Bees suck nectar from flowers.
(b) Humans obtain honey made by bees and consume it as food.
(c) This food chain differs from an usual one, as humans, here, consume honey made by bees, whereas, usually, the food chain resembles Plants → Goats → Humans, in which humans consume the meat of goats directly.
(d) No, it cannot be regarded as a food chain, as humans do not consume bees directly but only consume the food made by them.


Answer: 76
(a) Algae are phytoplankton.
(b) Jelly fish and copepods are examples of zooplankton.
(c) X is a small fish.
(d) Y could be a big fish.
(e) (i) Zooplankton are primary consumers, and (ii) big fish are tertiary consumers.


Answer: 77
(a) The following is the food chain that provides food to the lion in the given case:
Plants → Deer → Man → Lion
(b) The lion could have looked for a herbivore, such a wild buffalo or a zebra, if it he had not got the hunter as prey.
(c) Any top consumer, such as a tiger or a python, could have been in the place of the lion in the forest.
(d) In the food chain,
Plants → Goat → Man
man is the top consumer whereas, in the food chain,
Plants → Deer → Man → Lion,
the top consumer is lion.


Answer: 78
(a) If the number of foxes increases, it will lead to foxes consuming more rabbits as food, which will lead to a decrease in the number of rabbits. Hence, grass plants will grow denser as there will be fewer rabbits to consume them.(b) If the number of foxes decreases, then there will be fewer foxes to consume rabbits as food, so the number of rabbits will increase, which will in turn lead to rabbits consuming more grass plants as food. Hence, grass plants will become sparser.


Answer: 79
(a) If the number of rabbits increases, it will lead to rabbits consuming more grass plants, and hence grass plants will become sparser. As foxes consume rabbits, any increase in the number of rabbits will lead to an increase in the number of foxes.
(b) If the number of rabbits decreases, then there will be less food for foxes. Hence, they will not be able to survive, because of the shortage of food, and their number will decrease. A decrease in the number of rabbits will lead to an increase in the density of grass plants, as there will be fewer rabbits to consume them.


Exercise : Solution of Questions on page Number : 231


Answer: 80

Column I Column II Column III
(i) Secondary consumer Carnivore 3rd trophic level
(ii) Primary consumer Herbivore 2nd trophic level
(iii) Producer Autotroph 1st trophic level
(iv) Tertiary consumer Large carnivore 4th trophic level

(b)   Grass        →             Grasshopper                     →          Frog                    →                            Bird
 Producer                      Primary consumer                    Secondary consumer                          Tertiary consumer
Autotroph                   Herbivore                                    Carnivore                                             Large carnivore
1st trophic level           2nd trophic level                         3rd trophic level                                   4th trophic level


Exercise : Solution of Questions on page Number : 240


Answer: 1
One per cent of solar energy is trapped and utilised by plants.


Answer: 2
per cent of the energy available at the producer level is transferred at successive trophic levels in a food chain.


Answer: 3
Biomagnification is the process by which a harmful chemical enters the food chain and gets concentrated at each trophic level.


Answer: 4
The bird in the food chain will have the maximum concentration of harmful chemicals among the given organisms, because it lies at the highest trophic level.


Answer: 5
The cat in the food chain will have the maximum concentration of harmful chemicals in its body because it lies at the highest trophic level.


Answer: 6
Ultraviolet radiations are absorbed by the ozone layer.


Answer: 7
The group of chemical compounds that damages the ozone layer is known as chlorofluorocarbons (CFCs).


Answer: 8
Paper and plastic are two waste materials that can be recycled.


Answer: 9
Landfill is the process by which the volume of solid wastes can be reduced.


Answer: 10
The energy that will be transferred to the lion in the given food chain is 0.05 J.


Answer: 11
False. Only one per cent of light energy is trapped by plants, and of this, only 10 per cent is transferred at each trophic level in a food chain.


Answer: 12
All the energy in living organisms originates from the sun.


Answer: 13
There are rarely more than five links (or five organisms) in a food chain because, according to the 10 per cent law, only 10 per cent of the available energy is transferred to the next trophic level. So, at the higher trophic levels, the amount of energy is reduced to such an extent that it becomes insufficient for an organism to sustain itself.


Answer: 14
Hawks and peacocks are two predators of snakes in a food web operating in a forest ecosystem.


Answer: 15
(a) Ultraviolet rays can cause skin cancer.
(b) Pesticides enter the food chain at the producer level.
(c) Grass → Goat → Human
(d) Lettuce → Rabbit → Fox
(e) Plants → Antelope → Lion


Exercise : Solution of Questions on page Number : 241


Answer: 16
According to the 10 per cent law, only 10 per cent of the energy entering a particular trophic level of organisms is available for transfer to the next higher trophic level.
In the following food chain, only 10 per cent of energy is transferred to the next trophic level. The flow of energy is shown below.
Plants → Grasshopper → Frog → Snake → Hawk
100,000 J    10,000 J    1,000 J    100 J    10 J
When the energy available at the producer level is 100,000 J, only 10 J of energy is available to the hawk.


Answer: 17
The full form of CFC is chlorofluorocarbon.
Chlorofluorocarbons react with the ozone in the upper atmosphere and destroy the ozone layer, making the layer thinner and allowing ultraviolet (UV) radiation to pass through it and fall on the earth’s surface. This UV radiation can cause skin cancer and other ailments in humans and also affect plants and animals.


Answer: 18
Harmful radiations of sunlight are prevented from reaching the earth’s surface by the ozone layer in the upper atmosphere, which absorbs them.


Answer: 19
Chlorofluorocarbons, which are widely used in refrigerators, fire extinguishers and aerosol sprayers, are released into the air and react with the ozone present in the ozone layer and destroy it gradually. This causes the depletion of the ozone layer. If the ozone layer becomes thinner, harmful ultraviolet radiations will enter the earth and may cause skin cancer and other ailments in humans and also affect plants and animals.


Answer: 20
Chemical substances such as pesticides are sprayed over crop plants to protect them from pests and diseases. When herbivorous animals eat plant food, these poisonous chemical pesticides go into their bodies. Man, being an omnivore, eats plant food as well as the flesh of herbivores. Thus, the pesticides present in plant food and in the bodies of herbivores are transferred to the bodies of humans through food. In this way, harmful chemicals enter our bodies.


Answer: 21
Chemical substances such as pesticides are sprayed over crop plants to protect them from pests and diseases. When herbivorous animals eat the plant food, these poisonous chemical pesticides go into their bodies. Man, being an omnivore, eats plant food as well as the meat of herbivores. Thus, the pesticides present in plant food and herbivores get transferred to man’s body through food. Pesticides are non-biodegradable chemicals, so they get accumulated at each trophic level. Since humans occupy the top level in any food chain, the maximum amount of harmful chemical pesticides gets accumulated in our bodies, damaging our health gradually.
Hence the statement ‘excessive use of pesticides to protect crops from diseases may cause long-term damage to mankind’ is justified.


Answer: 22
The increase in the concentration of harmful substances, such as pesticides, in the body of living organisms at each level of a food chain is called biological magnification.
Chemical substances such as pesticides are sprayed over crop plants to protect them from pests and diseases. When herbivorous animals eat plant food, these poisonous chemical pesticides go into their bodies. Man, being an omnivore, eats plant food as well as the meat of herbivores. Thus, the pesticides present in plant food and herbivores get transferred to man’s body through food. Pesticides are non-biodegradable chemicals, so they get accumulated at each trophic level. Since humans occupy the top level in any food chain, the maximum amount of harmful chemical pesticides gets accumulated in our bodies, damaging our health gradually.
The following food chain explains how biological magnification of harmful chemicals can occur.

                                             Plants → Herbivore → Man


Answer: 23
The increase in the concentration of harmful chemical substances, such as pesticides, in the body of living organisms at each trophic level of a food chain is called bioconcentration of pesticides.
The common pesticide that has accumulated in the human body in considerable amounts is DDT.


Answer: 24
Household waste or rubbish is called garbage. Garbage consists of leftover food, fruit, vegetable peels, fallen leaves of potted plants, waste paper, unwanted plastic objects, glass articles, metal objects, old wooden objects, rags, discarded shoes, sewage, etc.


Answer: 25
The various modes of waste disposal are as follows.
1) Recycling
2) Preparation of compost
3) Incineration
4) Landfill
5) Sewage treatment


Answer: 26
Wastes such as paper, plastic and metal objects can be disposed of through recycling. This is done in the following ways.

1) Waste paper is sent to paper mills, where it is reprocessed to make new paper once again.
2) Broken plastic articles, such as plastic bags, buckets, bowls, cups and plates, are sent to plastic-processing factories, where they are melted and remoulded to make new articles.
3) Waste metal articles are sent to metal industries, where they are melted into solid metal and recycled for various purposes.


Answer: 27
Leftover food, fruit and vegetable peels, and leaves of potted plants can be converted into compost by burying them in a pit dug in the ground. The compost thus prepared can be used as manure. In this way, household wastes can be disposed of.


Answer: 28
Incineration means reducing something to ashes. In this method, waste material is burnt and converted to ashes. It is used to destroy household waste, chemical waste and biological waste (hospital waste).


Answer: 29
Most of the solid wastes in urban areas are disposed of by dumping them in low-lying sites. These sites are then covered with earth to level the uneven ground. This process is called landfill.


Answer: 30
Two advantages of using disposable paper cups over disposable plastic cups are as follows.
(a) Disposable paper cups decay naturally and become harmless after some time, but disposable plastic cups do not.
(b) Disposable paper cups usually do not pollute the environment, but disposable plastic cups pollute the environment.


Answer: 31
The dirty drain water containing urine and faeces that is carried from our homes by underground pipes (called sewers) is called sewage. It is disposed of by treating it at a sewage treatment plant.


Answer: 32
The following are the harmful effects of ozone depletion.
1) It can lead to skin cancer in humans.
2) It can lead to cataract in humans.
3) It can lead to damage to our immune system by lowering the body’s resistance to diseases.
4) It can lead to many diseases in animals.
5) It can lead to damage to plants.


Answer: 33
If the ozone layer in the atmosphere completely disappears, then the harmful ultraviolet radiations from the sun would enter the earth’s surface and cause skin cancer and many other ailments in humans and animals, and may also damage plants.


Answer: 34
(a) Energy from the sun flows through various trophic levels in the following steps.
1) Green plants trap solar energy with the help of their green pigment, chlorophyll, and convert it into chemical energy. They store this energy in the form of carbohydrates. On an average, about one per cent of the sun’s energy falling on the leaves of green plants is used by the plants in the process of photosynthesis and stored as the chemical energy of food.
2) When plants are eaten by herbivores, the chemical energy stored in the plants is transferred to them.
3) The herbivores are then consumed by carnivores, and the chemical energy stored in the flesh of the herbivores is transferred to the carnivores.
4) Some of the energy from producers and consumers is also utilised for the life processes of micro-organisms called decomposers. Decomposers, in turn, release the unutilised energy into the environment as heat. The energy that is not utilised by producers, consumers and decomposers is lost in the environment as heat. This heat is called community heat.

(b) Energy enters plants from the sun during food production through photosynthesis. This energy is then passed on from one organism to another in a food chain. The energy given out by organisms as heat is lost in the environment and does not return to be used by plants again. This makes the flow of energy in an ecosystem unidirectional. Thus, the flow of energy in an ecosystem is said to be unidirectional because the energy lost as heat from the organisms in a food chain cannot be reused by plants in photosynthesis.


Answer: 35
(a) Ozone is made up of three atoms of oxygen combined together. It is formed high up in the atmosphere by the action of ultraviolet (UV) radiation on oxygen gas. The UV radiation from the sun splits the oxygen gas into free oxygen atoms.
O2 → O + O + O
Oxygen (UV radiation Oxygen Oxygen
molecule from the sun) atom atom
An oxygen atom reacts with an oxygen molecule to form an ozone molecule.
O2 + O → O3
Oxygen Oxygen Ozone
molecule atom molecule
(b) The ozone layer protects us from the harmful effects of the UV radiation from the sun by absorbing it and preventing it from reaching the earth’s surface.
(c) The UNEP is United Nations Environment Programme. In 1987, in an attempt to protect the ozone layer, the UNEP forged an agreement among its member countries to freeze the production of chlorofluorocarbons at 1986 levels.


Answer: 36
(a) Plants trap solar energy to make their food. In this way, energy is introduced into the ecosystem by the sun.
(b) According to the 10 per cent law, only 10 per cent of the energy entering a particular trophic level of organisms is available for the next higher trophic level. If the energy at the producer level is 100 J, the energy at the hawk level in both the food chains can be calculated as under.
(i) Plants → Mice → Snakes → Hawks
100 J → 10 J → 1 J → 0.1 J
(ii) Plants → Mice → Hawks
100 J → 10 J → 1 J
So, it is clear that hawks will get 10 per cent more energy in the second food chain compared with the first food chain.


Answer: 37
(a) At each trophic level in a food chain, a large portion of the energy available is utilised for the maintenance of organisms in that trophic level and is lost as heat. Food chains usually cannot have more than three or four steps because the energy available for organisms in any additional step will be so small that it will be insufficient to sustain the life of those organisms.
(b) The amount of energy available to big fish will be 1 J. It can be calculated as follows-
Sun → Small algae → Zooplankton → Fish → Big fish
10000 J → 1000 J → 100 J → 10 J → 1 J


Answer: 38
(a) The law given by Lindeman is called the 10 per cent law. It states that only 10 per cent of the energy entering a particular trophic level of organisms is available for transfer to the next trophic level.
(b) Ten per cent of the energy will be available to hawks.
In this food chain, the energy transfer will take place as follows.
Sun → Paddy → Mice → Snakes → Hawks
10,000 J    1,000 J    100 J    10 J    1 J


Exercise : Solution of Questions on page Number : 242


Answer: 39
(d) Sunlight
Sunlight provides energy to plants for preparing food, and this energy is then passed on to the next trophic levels through a food chain.


Answer: 40
(c) CFC

Chlorofluorocarbons (CFCs) are released into the air during refrigeration and air conditioning and are the greatest contributor to the depletion of the ozone layer.


Answer: 41
(c) 5,000 kJ
According to the 10 per cent law, only 10 per cent of the energy available in a trophic level is passed on to the next level. Hence, if the amount of energy available at the fourth trophic level is 5 kJ, the energy available at the producer level will be 5,000 kJ: 5 → 50 → 500 → 5,000.


Answer: 42
(b) Decrease in energy at higher trophic levels
According to the 10 per cent law, the amount of energy decreases in each trophic level, and hence the number of trophic levels in a food chain is limited.


Answer: 43
(c) 1 per cent
Only one per cent of the sun’s energy falling on the leaves of green plants is utilised by the plants in the process of photosynthesis and stored as the chemical energy of food.


Answer: 44
(b) chlorofluorocarbons
Chlorofluorocarbons emitted by air-conditioners, refrigerators and jet planes cause the depletion of the ozone layer.


Answer: 45
(c) chemical energy
The food prepared by plants is in the form of chemical energy, which is passed on to the next trophic level in an ecosystem.


Answer: 46
(a) unidirectional
Energy always flows in one direction in an ecosystem, from a lower trophic level to a higher trophic level.


Answer: 47
(c) (i) and (iii)

Excessive exposure of humans to ultraviolet rays results in skin cancer and damage to the immune system.


Answer: 48
(b) large carnivore
As large carnivores are at the upper trophic levels, they get the minimum energy through the food chain, according to the 10 per cent law.


Answer: 49
(c) dog
The concentration of harmful pesticides in the bodies of birds and animals increases at each trophic level, and hence the dog in the given food chain will have the maximum concentration of harmful pesticides.


Answer: 50
(d) fish
As fish are in the top trophic level in an aquatic food chain, they get the minimum energy, according to the 10 per cent law.


Answer: 51
(c) larvae
Larvae feed on algae for their nutrition, so they get the energy they require directly from the producers. Thus, they get the maximum energy among all the consumers.


Answer: 52
(d) 0.15 J
According to the 10 per cent law, only 10 per cent of the energy available in one trophic level is transferred to the next trophic level.


Answer: 53
(c) 1,000 J
According to the 10 per cent law, only 10 per cent of the energy available in one trophic level is transferred to the next trophic level.


Answer: 54
(c) mice
As mice are in the lowest trophic level, they will have the minimum concentration of pesticides in their body.


Exercise : Solution of Questions on page Number : 242


Answer: 55
(c) Incineration
Incineration, which means burning harmful wastes into ashes, is the best method to dispose of biological wastes from hospitals.


Answer: 56
(d) (i) and (iii)
In an ecosystem, the flow of energy is unidirectional and the flow of materials is cyclic.


Answer: 57
(c) transfer of chemical energy from one organism to another
The 10 per cent law is associated with the transfer of chemical energy from one organism to another.


Answer: 58
(b) dichlorodiphenyltrichloroethane
The harmful pesticide that accumulates in the bodies of human beings through the food chain, through a process known as biomagnification, is dichlorodiphenyltrichloroethane (DDT).


Answer: 59
(b) ultraviolet radiations
Oxygen is converted into ozone by ultraviolet radiations.


Answer: 60
(c) flowers of plastic
Plastic is a non-biodegradable substance, and hence it cannot be decomposed by composting.


Answer: 61
(a) (i) A is oxygen (O2), and (ii) B is ozone (O3).
(b) C is the ozone layer.
(c) (i) X is ultraviolet (UV) radiations.
(ii) Y is the sun.
(iii) Z is chlorofluorocarbons (CFCs).
(d) D is Antarctica.
(e) Skin cancer and cataract are two human ailments that may be caused by X (ultraviolet radiations).


Answer: 62
(a) X could be phytoplankton.
(b) Phytoplankton → Tadpole → Water beetle → Fish
(c) ‘Aquatic food chain’ is the general name of food chains such as the one described above.
(d) (i) In the given food chain, the water beetle is the secondary consumer.
(ii) The phytoplankton is the producer.
(iii) The fish is the tertiary consumer.
(iv) The tadpole is the primary consumer.

(e) Sun → Phytoplankton → Tadpole → Water beetle → Fish
20,000 J → 200 J → 20 J → 2 J → 0.2 J


Answer: 63
(a) Green plants → Rabbit → Fox → Lion
(b) (i) The rabbit is the herbivore and (ii) the fox and the lion are the two carnivores in the food chain.
(c) The fox is both a predator of the rabbit and a prey of the lion.
(d) (i) The rabbit forms the second trophic level, and (ii) the fox forms the third trophic level.
(e) The lion can feed on both the rabbit and the fox independently. Hence, it can feed on animals in both the second and third trophic levels.
(f) The energy available at the trophic level of the fox is 0.1 J.
Sun → Green plants → Rabbit → Fox → Lion
1,000 J → 10 J → 1 J → 0.1 J → 0.01 J


Exercise : Solution of Questions on page Number : 244


Answer: 64
(a) The pesticides enter the phytoplankton from the polluted water. This level of organisms is known as producers.
(b) The birds will have the maximum amount of pesticides accumulated in them through the food chain. This process is known as biomagnification.
(c) Phytoplankton → Larvae → Fish → Birds
(d) Humans also consume fish, and hence they can replace the birds in the above food chain.


Answer: 65
(a) Material A is garbage.
(b) Incineration is the name of the method of waste disposal named B.
(c) Structure C is known as an incinerator.
(d) (i) D is carbon dioxide.
(ii) E is water vapour.
(iii) F is ash.
(e) The method of disposing of wastes by burning them is especially suitable for the disposal of hospital wastes.


 

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