ICSE Class 6 Biology Chapter 6 Our Environment

Unit 6: Our Environment

Learning Objectives

Learning about the planet earth

Conditions on the earth to sustain life

Environment has three components: biotic, abiotic and socio-cultural

Identifying producers, consumers and decomposers

Food chains and food web

Description of flow of energy in nature

Introduction

Our earth is one of the eight planets that revolve around the sun. It is the only planet that is placed at the right distance from the sun and has all the necessary conditions required for the survival of living organisms. It is perhaps the only planet in this Solar System that has life on it.

Structure Of The Earth

The outer, solid layer of the earth which supports life is called lithosphere. This layer is full of minerals and all other nutrients essential for plants. About seventy per cent of the earth surface is covered with water in the forms of seas, rivers, lakes, etc. The aquatic envelope of the earth is called hydrosphere. Surrounding the lithosphere is an envelope of gases (air) which is called as atmosphere. Thus planet earth consists of three layers (i) Lithosphere (ii) Hydrosphere (iii) Atmosphere.

The biosphere includes the lithosphere, hydrosphere and atmosphere together.

The Inner Structure Of The Earth

The interior of the earth is made up of three layers:

Crust: This is the most important layer as it sustains life. This layer provides all necessary materials for the growth of life. It is a compound of rocks, soil, gravel, water, etc. The materials found in the earth crust are called minerals. These minerals are used to extract metals. The metal is used in making tools, machines and other equipments. Fossil fuels like coal, petroleum and natural gas are also found in the earth crust. The coal and oils are the sources of energy.

Mantle: This is the middle layer. It is much hotter than the crust. Iron and magnesium compounds are found abundantly in the molten state in the inner rocks of this layer. The hot molten rock is called magma. The mantle is about 2900 km in thickness.

Core: This is the innermost part of the earth. It is very hot and the temperature inside is about 4000 C. This consists of molten iron and nickel. The innermost core is solid because of the high pressure in that part of the earth.

Teacher's Note

The Earth's structure can be related to an onion - layers upon layers protecting what's inside, much like how soil protects the minerals and nutrients that help grow the vegetables in your garden.

Factors Favourable To Support Life On The Earth

(i) Right Distance from the Sun: Earth is the third planet from the sun and it is the only planet that can support life. The distance of the earth from the sun is such that it is neither very hot, nor very cold. Most of the places on the earth receive right amount of solar heat to sustain life.

(ii) Rotation and Revolution of the Earth: The earth always spins on its axis from west to east. It takes 24 hours to complete one rotation. This spinning of the earth causes day and night.

The earth moves around the sun in an oval shape orbit. It is called the revolution of the earth. The earth takes 365 days to complete one revolution. The earth's axis is tilted at an angle. The revolution of the earth and the tilt of its axis cause the different seasons on the earth.

Teacher's Note

The rotation and revolution of Earth are why we have day and night, and seasons - imagine how different your life would be if it were always summer or always dark outside.

(iii) Presence of Water: Earth has many waterbodies on it. Three fourths of the surface of the earth are covered with water in the form of seas, rivers and lakes. Out of the total water available on the earth, only 2.5% is fresh water. The rest 97.5% of the total water is salt water. Plants take water from the soil to make their food. Water is required for germination of seeds. It helps in digestion, excretion and many other biochemical reactions.

(iv) Just Right Temperature: The temperature on the earth is neither too low nor too high. All life supporting reactions take place at moderate temperature.

(v) Presence of Atmosphere: The cover of gases surrounding the earth is called the earth's atmosphere. The atmosphere extends up to 1000 km from the surface of the earth. It mainly consists of nitrogen, oxygen, carbon dioxide and water vapours. Oxygen is required for respiration. Aquatic animals take in dissolved oxygen from water. Carbon dioxide is important as plants take in carbon dioxide from the air to make their food. Nitrogen is an inactive gas. Presence of nitrogen slows down the rate of combustion. Atmosphere also has a protective layer ozone gas. This gas protects the earth from harmful effects of certain ultraviolet rays from the sun.

(vi) Gravitational Force: The gravitational force of the earth holds the atmosphere above it and does not allow gases like oxygen, carbon dioxide and water molecules to escape from its atmosphere.

(vii) Presence of Minerals: All important minerals are present in the soil which are essential for the growth of the plants, the main source of food found on the earth.

Teacher's Note

All these factors - water, air, temperature, and gravity - work together like a perfectly balanced recipe to create conditions where life can thrive, similar to how a recipe needs the right ingredients in the right proportions.

Environment

If we look around, we come across different kinds of animals, plants, birds, rivers, mountains, sky, clouds etc. If we examine closely, we discover that these things interact with one another. Living organisms do not live in isolation. All living things interact with one another and with their surroundings. Living beings and their surroundings constitute the environment.

Living beings influence their surroundings and are themselves get influenced by the environment they live in. The environment can broadly be divided into three segments, namely biotic, abiotic and socio-cultural. The three components constitute living, non-living and the environment created and influenced by human activities. Human beings have a great influence on the environment. The natural environment in which early men used to live, is no longer natural, as we have modified it to suit our needs and requirements.

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