CBSE NOTES CLASS 10 SCIENCE CHAPTER 15

OUR ENVIRONMENT

Environment

The surroundings in which we live are called environment.

Substances which can be decomposed by microorganisms using biological processes are called biodegradable substances. For example cow dung, compost, leather, bones etc., are biodegradable.

Substances which cannot be decomposed by microorganisms using biological processes are non-biodegradable. Many synthetic substances like plastics, polythene bags etc., are non-biodegradable. They are inert and persist for long time in the environment and may harm the eco-system.

Ecosystem

All the interacting organisms in an area together with the non-living constituents of the environment form an ecosystem.

Components of Ecosystem

An ecosystem has two types of components, viz. biotic component and abiotic component.

Abiotic Component

All the non-living things comprising physical factors like temperature, rainfall, wind, soil and minerals, make the abiotic component of ecosystem.

• Air provides oxygen (for respiration), carbon dioxide (for photosynthesis) and other gases for various needs of the living beings.

• Water is essential for all living beings because all the metabolic activities happen in the presence of water.

• Soil is the reservoir of various nutrients which are utilised by plants. Through plants, these nutrients reach other living beings.

Biotic Component comprises of all the living organisms

Examples of Ecosystems

Garden, pond, river, sea, forest etc..

For ecosystem in the garden grasses, trees, insects etc. are biotic components and sand, terrain, air, temperature etc. are abiotic components.

Producers

The organisms which can make organic compounds like sugar and starch from inorganic substances using the radiant energy of the Sun in the presence of chlorophyll are called producers. All green plants and certain blue green algae which can produce food by photosynthesis come under this category.

Consumers

The organisms which consume the food produced, either directly from producers or indirectly by feeding on other consumers are the consumers.

Consumers can be classified as,

• Herbivores - animals that feed on plants

• Carnivores - animals that feed on other animals.

• Omnivores - animals or persons that eat food of both plant and animal origin.

• Parasites - organisms which live in or on another organism (its host) and benefit by deriving nutrients at the host’s expense.

Decomposers

Organisms, that break-down the complex organic substances into simple inorganic substances, which go into the soil and are used up once more by the plants. Bacteria and fungi play the role of decomposers.

Importance of decomposers

Decomposers break down the waste and bodies of dead plants and animals into simple inorganic substances and return it to the ecosystem (soil, air, water), which can be reused by plants and other living beings. If they weren't in the ecosystem, the plants would not get essential nutrients, and dead matter and waste would pile up.

Food Chains

A food chain is a sequence of organisms feeding on one another.

Each step or level of the food chain forms a trophic level.

The autotrophs or the producers are at the first trophic level. They fix up the solar energy and make it available for heterotrophs or the consumers.

The herbivores or the primary consumers come at the second trophic level.

Small carnivores or the secondary consumers come at the third trophic level.

Larger carnivores or the tertiary consumers form the fourth trophic level.

Producer → Primary Consumer → Seconadry Consumer → Tertiary Consumer

For example, Plant → Mice → Snake → Eagle

Food Web

Each organism is generally eaten by two or more other kinds of organisms which in turn are eaten by several other organisms. So instead of a straight line food chain, the relationship can be shown as a series of interlinked food chains, called a food web.

Transfer of Energy Through a Food Chain

Energy is transferred from one trophic level to another. Interactions among various components of the environment involve flow of energy from one component of the system to another.

• Sun is the ultimate source of energy.

• The autotrophs capture the energy present in sunlight and convert it into chemical energy. This energy supports all the activities of the living world.

• From autotrophs, the energy goes to the heterotrophs and decomposers.

• The flow of energy is unidirectional.

The energy that is captured by the autotrophs does not revert back to the solar input and the energy which passes to the herbivores does not come back to autotrophs. As it moves progressively through the various trophic levels it is no longer available to the previous level.

Not all the energy is passed on the next level

• Autotrophs capture around 1% of the sun’s energy.

• Out of the energy consumed by an organism at a particular trophic level, 90% is utilised for its own need and rest 10% is left for the organism of the next trophic level.

• Very little energy is left for the organism which is at the tertiary level.

• Let us assume that a green plant makes 100% energy in the form of chemical energy. 90% of this energy would be utilised for its own purpose.

This would leave just 10% energy for the primary consumer. Now, primary consumer shall also utilize 90% of energy which was consumed by it.

This would leave just 1% energy for (10% of 10 = 1) for the secondary consumer. By this logic, the tertiary consumer would get just 0.1% of energy which was originally made by the green plant.

• That is why there can be just one or two organisms at the top of the food pyramid.

This explains why the population of producers is always the largest in an ecosystem, followed by the population of herbivores and then that of carnivores. Moreover, an herbivore needs to eat many plants in its lifetime to fulfill its energy need. Similarly, a carnivore needs to eat many herbivores in its lifetime.

Balance in the Ecosystem

There is a delicate balance in an ecosystem as far as number of organisms at a particular trophic level is concerned. An increase or decrease in population of any organism can disturb this balance.

Let us take a hypothetical example to understand this. If all the deer are killed in a jungle, the lions would be left with no food. This would endanger the existence of lions. Once the lions and deer would be finished, it would result in population explosion of green plants.

If all the lions die in a jungle, it would create another problem. Since no lion would be left to kill the deer, the population of deer would increase substantially. This will finish off all the green plants and finally even the deer would be left with no food for them.

Biological Magnification

Unknowingly some harmful chemicals enter our bodies through the food chain.

Several pesticides and other chemicals are used to protect our crops from diseases and pests. These chemicals are either washed down into the soil or into the water bodies.

From the soil, these are absorbed by the plants along with water and minerals, and from the water bodies these are taken up by aquatic plants and animals.

Also our food grains such as wheat and rice, vegetables and fruits, and even meat, contain varying amounts of pesticide residues. They cannot always be removed by washing or other means.

As these chemicals are not degradable, these get accumulated progressively at each trophic level. As human beings occupy the top level in any food chain, the maximum concentration of these chemicals gets accumulated in our bodies.

The phenomenon, by which the harmful chemicals get accumulated progressively at each trophic level, is known as biological magnification.

Ozone Layer Depletion

Ozone (O3) is a molecule formed by three atoms of oxygen.

O2 (oxygen) is essential for all aerobic forms of life.

Ozone is a deadly poison. However, at the higher levels of the atmosphere, it shields the surface of the earth from ultraviolet (UV) radiation from the Sun. This radiation is highly damaging to organisms. It (UV radiation) is known to cause skin cancer in human beings.

The higher energy UV radiations split apart some molecular oxygen (O2) into free oxygen (O) atoms.

Ozone layer is also known as stratosphere. Ozone at the higher levels of the atmosphere is a product of UV radiation acting on oxygen (O2) molecule.

${\mathrm{O}}_{2}\stackrel{\mathrm{UV radiation}}{\to }\mathrm{O}+\mathrm{O}$

O2 + O → O3

Effect of CFCs

CFCs are used in refrigerators and aerosol spray.

Use of CFCs (Chlorofluorocarbon) has damaged the ozone layer. As a result, the ozone layer has become thinner at certain parts.

In 1987, the UNEP (United Nations Environment Programme) succeeded in forging an agreement among different nations to freeze the CFC production at 1986 level. Later, an agreement was signed among different nations to phase out CFCs. India is also a signatory of that agreement.

Due to efforts by the United Nations and different environmentalists, the CFC emission has been put under some control.

Problems of Waste Disposal

During our day to day activities, we produce lot of waste.

Improvements in our life-style have resulted in greater amounts of waste material generation.

Changes in attitude also have a role to play, with more and more things becoming disposable.

Changes in packaging have resulted in much of our waste becoming nonbiodegradable.

For minimizing the effect of nonbiodegradable substances like plastic we need to use the 3R (reduce, reuse, recycle).

Disposable Cups in Trains

The introduction of disposable cups in trains helped a lot in hygiene, but it resulted in waste generation caused by the disposal of millions of these cups on a daily basis.

Some time back, kulhads, that is, disposable cups made of clay, were suggested as an alternative. But a little thought showed that making these kulhads on a large scale would result in the loss of the fertile top-soil.

These days, disposable paper-cups are being used. Since paper is biodegradable, it can be recycled or it will degrade easily by nature.