Notes-NCERT-Class 10-Science-Chapter-13-Our Environment-CBSE Board

Topics to be learn  Introduction Eco-system — what are its components? How do our activities affect the environment?

Introduction :

Environment : Environment is defined as sum total of all conditions that affect the development and life of organisms.

• The environment of any organism has two components, abiotic and biotic.
• Abiotic components are air, water and soil.
• Biotic components include all living organisms which interact with each other and with abiotic components.

Eco-system — what are its components?

Ecosystem : Ecosystem is the whole biotic community (living organisms) in a given area plus its abiotic environment (physical factors like temperature, rainfall, wind, soil and minerals).

• Ecosystem can be as large as a forest, or small as a pond or lake.
• This functional system may be natural as forest or may be artificial (man-made), e.g., a crop field or an aquarium.
• Thus the ecosystem has biotic and abiotic components.

Artificial ecosystem : Aquarium :

• An aquarium comprises green aquatic plants, fish and other aquatic animals. These biotic factors form the community. Water, sand (or gravel etc.), temperature, light, etc., are the abiotic factors.
• There is a constant exchange of materials between living and non-living components. Aquatic green plants are the producers of the aquarium. Animals are the consumers. The green plants give out oxygen which is taken up by the animals.
• Animals give out carbon dioxide which plants utilise during photosynthesis. Animals excreta is decomposed by the decomposers and the simple compounds produced are taken up by the plants.
• Thus, an aquarium is self-perpetuating functional unit. Therefore, it is known as ecosystem. An aquarium is a man-made ecosystem, so it is known as artificial ecosystem.

Producers: All green plants and certain blue-green algae can produce food by photosynthesis and are called producers or autotrophs.

Producer is always a green plant. It synthesises its own food by trapping light energy (sunlight).

Consumers : The organisms which consume food produced either directly from producers or indirectly by feeding on other organisms are called consumers.

• A consumer is an animal or a non-green plant which cannot synthesise its own food by photosynthesis.
• A consumer directly or indirectly obtains its food from producers (green plants).

• Consumers can be herbivores (deer, horse, cattle, rabbit, hen) omnivores (bear, monkey, human), carnivores (tiger, lion, leopard, wolf) and parasites (flea, ticks, lice, malarial parasite, tapeworm).

Decomposers : Decomposers are microorganisms comprising of bacteria and fungi that breakdown the dead remains and waste products of organisms. They breakdown the complex organic substances into simple inorganic substances that go into the soil and are used up once again by the plants.

If there were no decomposers :

• The organic dead remains and waste would pile up and the nutrients would not be recycled i.e., complex compounds would not break to form simple compounds that can be taken up from the soil by the plant for their use.
• Natural replacement of the soil would not take place. All the nutrient cycles have decomposers (bacteria and fungi) as their components.
• Without decomposers, the dead remains of the other organisms in a community would accumulate.
• Eventually the world would run out of carbon dioxide or nitrate or phosphates or other inorganic materials essential for life.
• Thus existence of life on this earth will become impossible if there are no decomposers.

Food chains and webs :

Food chains : The sequential process of one organism consuming the other is known as food chain.

• Trophic level of each organism is fixed in a particular food chain.
• Each organism at a trophic level receives food from one group of organisms.

Food webs : Network of food chains with intercrosses and inter-linkages is called the food web.

• In a food web, several food chains are inter-connected. So, each organism in one trophic level receives its food from more than one group of organisms.

Trophic level:

• The different steps through which food energy passes into an ecosystem is known as trophic levels.
• The producers are at the first trophic level who fix up the solar energy and make it available for heterotrophs or consumers.
• Herbivores come at the second level, small carnivores at third level and large carnivores at fourth level as depicted in the figure.

Flow of energy: The energy trapped by the autotrophs or producers supports all the activities of the living world. From autotrophs the energy travels to heterotrophs and decomposers. At each level some energy is lost to the environment. It has been found that

• Green plants in a terrestrial ecosystem capture about 1% of the sun energy that falls on the leaves and convert it into food energy. (A fraction i.e., about 1/50 millionth of the total solar radiation reaches the earth’s atmosphere.) Plants absorb red and blue light. Solar radiations energy is converted into chemical form by the producers.
• An average of 10% of the food eaten by the primary consumers is turned into its own body and is available for the next level of consumers.
• 10% can be taken as an average value for the amount of organic matter that is present at each step and reaches the next trophic level.

Food chains generally consist of three or four steps only :  

Reason : Because after these steps, the energy becomes very less. Only 10% of the energy stored in one trophic level (i.e., producers) is actually transferred to the next trophic level (i.e. herbivores). This is called the pyramid of energy. Pyramid of energy is always erect. There is very less energy left for the top trophic level. This is why there are very few (if any) fourth order consumers in any ecosystem.

Example :

The energy flows from the sun which is the only continuous source of energy for the earth. Green plants trap a part of solar energy reaching the earth. If the solar energy reaching the earth is 1000 J, the green plants trap only 1% i.e., 10 J of the energy as biomass. This 10 J energy as plant matter is available to the available as food herbivore. Out of this 10 J energy 90% is lost as heat by the herbivore and only 10%, i.e., 1J of energy is available to the carnivore, as shown in figure.

Biological magnification :

• Biological magnification is a phenomenon which explains the increasing concentration of harmful chemicals (like DDT) with each increase in trophic level.
• From the soil the chemicals (mainly pesticides, heavy metals, etc.) are absorbed by the plants. The primary consumers eat these plants and the harmful chemicals come to reside in their bodies.
• As these chemicals are not degradable, they accumulate in the bodies of the organisms and the top level of the food chain gets the highest concentration of these harmful chemicals.
• Most of the plant products which we eat are grown in fields in which pesticides and fertilisers have been used. These are absorbed by the plants and cannot be removed by washing or other means.
• As humans are at the top level of the food chain, these chemicals get accumulated in our bodies and cause various disorders.

How do our activities affect the environment?

Ozone layer and how it is getting depleted :

Ozone layer: There is layer of ozone gas in the stratosphere (a part of atmosphere). Ozone at higher levels of the atmosphere is a product of UV radiation acting on oxygen molecule.

The higher energy UV radiations split apart some molecular O, into free oxygen ‘O’ atoms. These atoms then combine with O, to form Oy (ozone) as depicted below:

O₂ + UV radiations → O + O

O + O₂ → O₃ (Ozone)

Ozone layer protects us from Sun’s harmful UV radiations by cutting them off.

Ozone depletion :

Ozone hole : Due to the depletion of stratospheric ozone, a seasonal hole in the ozone layer at polar regions is formed due to the vortex in the atmosphere. This is known as the ozone hole.

• Areas of human habitation under this hole are more vulnerable to the effect of UV rays from the sun.
• Recently, satellite data has indicated that there is a reduction of the ozone layer over the Antarctic, causing a hole of the size of a big continent.

Depletion of the ozone layer could cause:

• temperature changes and rainfall failures on earth.
• cancer in human beings, especially skin cancer, eye cancer or cataracts.
• destruction of aquatic life and vegetation.
• loss of immunity in humans.
• destruction of microbes and those bacteria which are very useful for us.

Causes of ozone depletion : Major air pollutants responsible for the depletion of ozone layer are chlorofluorocarbons (CFCs), nitrogen oxides and hydrocarbons. CFC is used in air conditioners and refrigeration, as cleaning solvents, aerosol propellants, in foam insulation and in certain fire extinguishers.

Two gases which have replaced CFCs are:

• Hydrochlorofluorocarbons (HCFCs)
• Hydrofluorocarbons (HFCs)

Managing the garbage we produce :

• The waste we generate may be biodegradable or non-biodegradable.
• The disposal of the waste we generate is causing serious environmental problems.

Solid waste (Garbage) : Solid waste (Garbage) includes glass bottles, crockery, plastic containers, polythene and other packing materials, automobile spares, machines, cycle parts, construction wastes, sludge, dead animal skeletons, crop residues, etc.

• These pile up at public places and cause obstruction in daily life.
• Solid wastes are causing much problem in developed countries like United States and European countries. Disposal of these wastes is difficult in these developed countries because labour is very expensive.
• In India also several million tonnes of solid waste is dumped at different places.
• To solve the problem of solid wastes, technologies have been developed to recycle most of the solid waste items such as paper, cans, newspapers, metallic vehicle parts.
• To control solid waste pollution, methods like (i) recycling, (ii) burning of waste and (iii) composting of organic waste for preparation of manure and bio-gas are suggested.
• Changes in our attitude and lifestyle are essential to save the earth’s environment. For example, drastic reduction in the use of non-biodegradable items and encouragement to sustainable development.

Biodegradable substances : Substances which can be broken down with the help of enzymes secreted by the bacteria and other saprophytes over a period of time are biodegradable. These substanceshave short molecular organisation and can be assimilated by the saprophytes.

• Biodegradable wastes in large quantities would contribute to the pollution of the environment
• They serve breeding ground to flies which are carrier of many diseases such as cholera.

Nonbiodegradable substances : Nonbiodegradable substances cannot be acted upon by the enzymes in nature as they consist of polymers which are strongly bonded with each other.

• Non-biodegradable substances cannot degrade and do not return the minerals to the environment. Instead of a cycle, these materials follow a linear path. They are produced in a factory and then end up in the soil. They pile up and cause harm to various members of the ecosystem.
• They may cause bio-magnification in the food chain that may end up in humans. For example, pesticides like DDT enter the food chain, biomagnified and affect human beings and other organisms.

Safe disposal of non-biodegradable waste:

• Some of the non-biodegradable substances can be used for land refill.
• Some of non-biodegradable substances can be recycled, for example—broken glass bottles, cans, other metal items.

Effect of our life on environment :

• Excessive use of fossil fuel (like petrol or diesel), even when we have to go nearby we use car, motorcycle, etc., instead of going on foot or by bicycle.
• Fossil fuels, on burning produce oxides of carbon, nitrogen and sulphur which cause air pollution.

What practices we can follow to change our lifestyles so that there is less waste generation?

• First step is to use only those materials which come packed in biodegradable packs such as paper, coir, jute, etc.
• We can separate the waste generated into recyclable and non-recyclable parts.
• We may reduce our use of certain products which are not so essential for our life.
• We may recycle plastic, paper, glass and metal items.
• We can reuse certain things like paper, plastic and glass bottles.

Harmful effects of using plastic bags on the environment :

• Plastic or polythene bags when buried in the soil make that area barren and leads to soil pollution.
• Plastic bags often block drains leading to overflow of drains causing foul smell and spread of diseases.
• On burning, they release toxic gases that cause air pollution.

Alternatives to plastic bags:

• You can use cloth, jute and paper bags to carry your purchased items.
• Paper bags can be recycled. Their recycling does not produce poisonous gases like recycling of plastic bags.
• Paper bags are biodegradable and do not pollute the environment like nonbiodegradable plastic bags.