All organisms have certain characteristics by which they can be differentiated from non-living things. Let us study the characteristics of living organisms.
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1. Cell Structure
The body of every organism is made up of tiny compartments called ‘cells’. The term cell was coined by Robert Hooke. The cell is the basic unit of life. Just as a wall is made up of bricks, the bodies of living organisms are made up of cells. If we examine a thin slice of any part of a plant or an animal under the microscope, we will find numerous tiny compartments separated by parturitions. Each of these compartments is a cell.
The cell contains a complex substance called protoplasm which is the living substance found in all living organisms. Protoplasm performs all the important functions of a cell.
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A living organism may be made of one or many cells. Living organisms that are made up of a single cell are said to be unicellular. Amoeba, paramecium, euglena, bacteria and yeast are some examples of unicellular organisms. These organisms are very small and can be seen only under a microscope. Most organisms are however, made of many cells. They are said to be ‘multicellular’. Dogs, cats, human beings, sunflower and mango tree are multicellular. Can you see a cell with your naked eyes? Most cells are very small and can be seen only under a microscope. However, some cells like eggs of birds are large and can be easily seen with the naked eyes.
2. Form and Size
All living organisms have a definite form or shape and size. This property of living organisms helps us to identify them. Thus, a rose plant can always be differentiated from a sunflower plant because of the size and shape of its leaves, the color, size and fragrance of its flowers. An elephant can be identified as a huge creature bearing a big trunk. In contrast to this, non-living things like air and water do not have definite shape and size.
3. Growth
Growth is defined as an irreversible increase in the size of an organism. Sow a seed in soil. In a few days the seed will germinate and a tiny seeding will emerge from it. In a few weeks’ time, the seeding becomes bigger and changes into a mature adult. In simple words, we say that the plant has grown.
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A bay changing into an adult, a puppy changing into a dog, a kitten changing into a cat and a seeding changing into a plant are all examples of growth in living organisms. Growth generally occurs when cells present in an organisms divide to form more cells. As the number of cells increase, the organisms grow in size. Animals grow only up to a certain age in their life. They stop growing after having grown to their full size. However, plants continue to grow throughout their life. A non-living object like a ‘crystal’ can also grow in size. However objects the growth is external because it takes place due to the deposition (addition) of more particles on the outer surface of the crystal. In contrast to this, growth in a living organism is internal because it occurs when new particles are secreted by the protoplasm of the cells.
Life-span:
All organisms follow a definite life-cycle. They are born; they grow, reproduce, become old and die. Each organism remains alive for some time and then dies. The time period for which organisms is expected to survive. All organisms have a definite life-span.
Nutrition:
Living organisms require food for:
- Growth
- Repair and replacement of worn-out cells in the body
- Energy to perform vital activities
Green plant makes their own food from carbon dioxide and water in the presence of sunlight by the process of photosynthesis while animals depend directly or indirectly on plants for food.
Non-living things require no food to live.
Movement:
Movement is generally regarded as an important sing of life. Movement is restricted in most plants, as they remain fixed to the ground whereas most animals move freely from place to place.
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Many unicellular and filament-like algae like chlamydomonas, volvox and spirogyra move about freely in water. In higher plants the movement is restricted to certain parts only like the movement due to growth observed in rots in the downward direction and in shoot in the upward direction. Most animals however, move freely from one place to another in search of food and shelter.
Response to Stimuli:
This refers to movement of organisms or any of its parts in response to a change in its surroundings. A change in the surrounding environment is called a stimulus. An action which occurs as an answer to a stimulus is called a response. The ability of living organisms to respond to stimuli is called irritability or sensitivity.
If you accidentally touch a hot object, you quickly pull your hand away. In this case, heat is the stimulus and the act of ‘pulling away the hand’ is the response. Blinking of eyes when you go from a dark-room to a well-lit one is a response shown to the stimulus ‘light’. Your pet dog begins to wag its tail on seeing you. Animals run for safety when a lion approaches. These are examples of responses shown to specific stimuli. Plants too respond to various stimuli such as light, gravity, touch and temperature. Plant shoots bend towards the source of light while leaves of touch-me-not plant fold up when touched.
Non-living things do not respond to stimuli.
Respiration:
You know that we require energy to stay alive. Walking, running, sleeping and beating of heart are some of the activities that require energy. From where does the energy required to perform all these activities come? This energy is obtained from the food you eat. The chemical process by which energy is released from the food you eat is called respiration. Respiration takes place in all organisms both plants and animals. All living organisms give out carbon dioxide during respiration. Non-living things do not respire.
Excretion:
Removal of waste products from the body of organisms is called excretion. Several chemical processes like digestion and respiration take place in our body. During the course of these chemical processes, harmful waste substances like urea, uric acid, faces and carbon dioxide are formed in the body. Since these wastes are harmful to the body, we must get rid of them. These waste products are removed from our body in the form of urine, sweat, faces and exhaled air. Animals have special organs like kidneys, sweat glands, gills and lungs for excretion.
Plants have no special organs for excretion. Tannins, resins and gum are the major plant wastes. These wastes collect in the bark or old leaves which are periodically shed by the plants. Carbon dioxide is excreted through tiny holes in the leaves called stomata. Non-living things do not excrete.
Reproduction:
Reproduction is the ability of an organism to duplicate itself or to give rise to new individuals of its own kind. Reproduction is a unique property possessed only by living organisms. Non-living thing cannot reproduce.
Some animals lay eggs from which the young ones hatch out after a certain period of time, while others give birth to young ones. Fish, birds, snakes, lizards, frogs and insects lay eggs. You must have seen baby sparrows and pigeons emerging from their eggs in and around your home. Animals like dog, cat, lion, cow and human beings give birth to young ones.
Plants reproduce by producing seeds. The seeds germinate into seedlings which grow into full-grown plants in due course of time. However, some plants like mint, rose, sugarcane and potato can be grown from different parts of their body without the formation of seeds.