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The following points highlight the top six characteristics of a living body. The characteristics are: 1. Complex Organization 2. Maintenance of Steady State 3. Growth 4. Reproduction 5. Adaptation 6. Evolution.
Characteristic # 1. Complex Organization:
Man has built up many complicated machines which work with great precision. But the working of his own body machine has exceeded all of them in complexity and precision (Fig. 1.2). With the aid of his tools, man has learnt about the organisation of living bodies. It is known that all living bodies are made up of units called Cells.
Some possess only one cell while other have many. In the multi-cellular forms, the cells with similar forms and functions are grouped as Tissues. The tissues make up an Organ and several organs participate to form a System which performs some vital functions. Combination of a few systems constitutes a multicellular organism.
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Up to the beginning of this century man was busy in unravelling the different systems in the different animals. But with the advent of different techniques in this century, man is continuing his probe to understand the organisation at subcellular level. Considerable amount of facts is known about the atoms and molecules present there and the mode of their action, their properties at the functional level.
Characteristic # 2. Maintenance of Steady State:
All activities like the running of an automobile, burning of a candle, jumping of a frog or the flying of a bird involves expense of energy. Non-living bodies like a battery, after certain period of work lose all its energy. It has no way to maintain its steady state.
A living body, too, loses energy during work, but is able to carry on complicated chemical activities to maintain a steady state. It must be remembered that this equilibrium is a dynamic steady state and not a static one. Living system procures energy-yielding substances from the environment in the form of food (Fig. 1.3) and converts them into its own body substance.
This process is called nutrition. These body substances are burnt by oxygen which is taken in from the environment, resulting in the release of energy and production of carbon-dioxide and water. The energy is utilised by the body during its activities while the carbon-dioxide and water are given out. The entire process of energy release and the gaseous exchange is called respiration (Fig. 1.4).
Thus nutrition works for the building up of body substances (anabolism) and respiration is involved in its breakdown (katabolism). The anabolism and katabolism are together called metabolism. When anabolism exceeds katabolism, new materials are added to the body. Such a phenomenon of replenishment is not seen in non-living objects like rocks, which always retain the same old atoms and cannot replenish the loss.
Another phenomenon which helps in the maintenance of steady state is the response to stimuli. Any change in the external environment or any alarm from the internal parts, is at once detected and attended to (Fig. 1.5). For example, sight of a snake on the road immediately alarms us and we speed up to reach a safe place.
Similarly, stimuli may come as sound, odour, touch, pain, heat and cold; we always react to them. The process of metabolism and the power of responsiveness are thus two unique features in the living systems, which help in the maintenance of equilibrium at dynamic level.
Characteristic # 3. Growth:
When anabolism exceeds katabolism, the individual grows. Some non-living crystals in solution may grow in bulk, but here the growth is by the addition of identical materials available as such. Whereas in living body growth takes place by the conversion of materials which are dissimilar to its body (Fig. 1.6).
Characteristic # 4. Reproduction:
After certain period of growth, all living beings tend to multiply. This multiplication is known as reproduction which may be either by participation of entirely one individual (asexual) or by the contribution of the two (sexual). Such multiplication in number by self-duplication is not seen in the non-living world (Fig. 1.7).
Characteristic # 5. Adaptation:
All living forms have the wonderful ability to adapt themselves to their particular environment. Such adaptive behaviour permits certain forms to flourish well and allows them to survive in the course of evolution. Land, air, water, whatever it may be, are invaded by some or other forms of life that have perfectly adjusted to the environment through suitable changes in their organisation (Fig. 1.8).
Characteristic # 6. Evolution:
The living world, as we see it today, was not the same in the past. Many species of plants and animals that lived in distant past exist no longer and have now become extinct. At the same time many new forms have emerged.
Life first originated as a very simple form. From that starting point it has marched through thousand million years and has reached the present level of complexity and diversity. It involved gradual changes and such changes occurred from generation to generation. It is known as evolution.
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The most remarkable feature in the evolution of living beings is that only those which could adapt themselves to continuous changes in the environment survived, while others which failed became extinct. Such production of variation to meet the ordeal of evolution is not seen in the inorganic world. Figure 1.9 shows a few stages through which the horse passed before attaining the present form.
