ICSE Class 6 Biology Chapter 1 Life on Earth

Life on Earth

Learning Objectives

After completing this chapter you will be able to

- define a life process.

- distinguish between living and non-living things.

- identify and list the life processes common to all living things.

- recognise the cell as the basic unit of life.

- differentiate between unicellular and multi-cellular organisms.

- recognise the vast variety in the shape and size of living beings.

- differentiate between autotrophs and heterotrophs.

- recognise that all living beings reproduce to give rise to their own kinds.

- differentiate between plants and animals.

Introduction

You see many things around you - plants, animals, stones, soil, clothes and utensils. You also see television, radio, computers and telephones. Do all these things look alike? Do all of them show activity? The answer is 'No'. All these things look different and some of these things show activity while others do not.

If you look closely, you will notice that humans, plants and animals show activity while others do not. This is because humans, plants and animals are living beings.

Millions of plants and animals inhabit the earth. They differ in their shape, size, habits and in many other ways. An oak tree and a blade of grass are different in their form but they are both plants. An elephant and a mosquito do not look alike, but they are both animals.

Although all these look different from each other, they have some common characteristics. For example, all of them grow, take in oxygen, show movement, need nutrition, give out waste

Teacher's Note

Living things are all around us - from the plants in gardens to the pets we care for at home. Understanding what makes something alive helps us appreciate the diversity of life.

Living Beings and Non-Living Things

Living beings are made up of cells. They show movement and growth, take in oxygen and food, give out waste products, and give birth to young ones to continue their race. All plants and animals are living beings.

On the other hand, the things which do not show these characteristics are called non-living things. For example, stones, tables, chairs, cars, soil, cycles, clothes, utensils, televisions, radios, computers and telephones are all non-living things.

Characteristics of Living Beings

Living beings possess the following characteristics.

- They are made up of cells.

- All living beings grow.

- They need food.

- They respire.

- They move on their own.

- They throw out waste.

- They respond to stimuli.

- They give birth to young ones or reproduce.

- They have a definite lifespan.

Let us read about these characteristics in detail.

Living Beings Are Made Up of Cells

All plants and animals are made up of cells. A cell is made up of molecules. Though these molecules do not have life, the cell is living.

Teacher's Note

Cells are the building blocks of all life, from tiny bacteria to large animals. Learning to observe cells with a microscope shows us the hidden structure of living things.

Activity 1

To make a wet mount on a slide

A microscopic slide made with water is called a wet mount. You can practise making wet mounts by following these steps.

1. Take a clean glass slide.

2. With a dropper put a drop of water in the middle of the slide (Fig. 1.1a).

3. Gently put the object to be observed on the slide with the help of a brush or a needle.

4. With the help of a needle or a pair of forceps hold the coverslip such that it touches the edge of the drop of water. Gently lower the coverslip on the water (Fig. 1.1b).

5. Extra water may come out from under the coverslip (Fig. 1.1c). Wipe it carefully with the help of a blotting paper. Your slide is ready (Fig. 1.1d).

Unicellular and Multicellular Organisms

Some living beings like Paramecium, Chlamydomonas and Amoeba (Fig. 1.2) are made up of a single cell. These are called unicellular organisms.

Other organisms like ants, houseflies, birds, rats, frogs and rose plants (Fig. 1.3) are made up of millions of different kinds of cells. They are called multicellular organisms. The body of a human being may consist of trillions of cells. Human beings are multicellular organisms.

Let us try to see some plant and animal cells under the microscope.

Teacher's Note

Observing cells under a microscope reveals the intricate structure of life. Students can see how both plant and animal cells are organized in similar yet distinct ways.

Activity 2

To observe cells of an onion peel

Take an onion. Pull out a thin peel from the inner lining of the fleshy leaves. This is an onion peel. Prepare a wet mount of onion peel following the procedure mentioned in Activity 1 and observe under a microscope.

You will observe brick-like cells (Fig. 1.4).

Activity 3

To observe cells from the inner lining of your cheek

Gently scrape the inner lining of your cheek with the blunt end of a toothpick. Put a drop of iodine solution on a slide and transfer the scrapings onto the slide. Now make a wet mount and observe it under the microscope. You will observe flat and irregularly shaped cells (Fig. 1.5).

Can you observe a distinct round part inside each cell? This is the nucleus.

Teacher's Note

Examining cells from everyday sources like onions and our own bodies demonstrates that living matter is fundamentally cellular at the microscopic level.

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