Something which is ‘living’ (not dead) is said to be ‘alive’. In most simple terms, ‘alive’ means ‘having life’. Alive is called ‘jeevit’ or ‘zinda’ in Hindi. We are alive and you are also alive. Those things which are alive are called ‘living things’. All the plants and animals (including human beings) are alive or living things. Now, an important question arises: What criteria do we use to decide whether something is alive? This is discussed below.

The most important criterion to decide whether something is alive (or not) is the movement. Movement is one of the most important signs of life in an organism. All the living things (which are alive) move by themselves without any external help. In some cases the movements of living things are quite fast which can be easily observed by us but in other cases the movements are very slow and hence observed with difficulty.

For example, the movements in most of the animals are fast and can be observed easily but the movements in plants are usually slow and observed with difficulty. Animals and plants move in different ways. This will become clear from the following discussion.

Animals can move from one place to another or they can move their body parts. For example, a frog moves when it jumps into a pond, a bird moves when it flies in the sky, an athlete moves when he runs and a fish moves when it swims in water. We move our hands when we clap and our chest moves up and down when we breathe. And a dog can wag its tail. All these movements’ show that a frog, bird, first, dog and human beings are alive (or living things).

ADVERTISEMENTS:

The plants are fixed in the soil at a place, so they cannot move like animals from place to place. The plants can only move parts of their body such as leaves, flowers, shoots and roots. The plant parts move towards a stimulus such as sunlight, gravity or water, etc. For example, the shoot, the leaves and flower of a sunflower plant move by bending towards the sun so as to face the sunlight. The leaves of a Mimosa pudica plant (sensitive plant) move by folding up when touched with a finger. Plants also show movement by growing their roots and shoots bigger.

Non-living things (which are not alive) cannot move by themselves. For example, a stone is a non-living thing which cannot move by itself from one place to another or show any other type of movement. We will have to move it by applying force from outside.

All the living things (plants and animals) are made up of tiny living units called cells. The cells themselves are made up of still smaller particles called molecules. The movements over very small scale (as those in the molecules of living things) are invisible to the naked eye. The invisible molecular movement is, however, necessary for the existence of life.

In fact, viruses do not show any molecular movement in them (until they infect some cell) and this has created controversy about whether they are truly alive or not. In addition to movement, the living things also show some other characteristics. These are discussed below.

ADVERTISEMENTS:

All the living things (which are alive) have some common characteristics (or features) which make them different from non-living things. The characteristics of living things are as follows:

(i) Living things can move by themselves.

(ii) Living things need food, air and water.

(iii) Living things can grow.

ADVERTISEMENTS:

(iv) Living things can respond to changes around them. They are sensitive.

(v) Living things respire (release energy from food).

(vi) Living things excrete (get rid of waste materials from their body).

(vii) Living things can reproduce. They can have young ones.