The following points highlight the ten main characteristics of living body. The characteristics are: 1. Size and Form of the Living Body 2. Composition of the Living Body 3. Organization of the Living Body 4. Metabolism 5. Growth 6. Movement 7. Sensitiveness and Adaptation to Environment 8. Reproduction 9. Rhythmicity or Periodicity 10. Life-Cycle, Senescence and Death.
Characteristic # 1. Size and Form of the Living Body:
Every kind of organism has its own peculiar form and size of the body which may vary in different individuals within narrow limits. But lifeless inanimate objects have no definite size or form of the body.
Water, for instance, may exist as a drop of rain or an ocean. It may be moulded into a mass of cloud or an iceberg or a small icicle. It is the same H2O in all the cases, though varying in physical form and size to such a wide extent.
Characteristic # 2. Composition of the Living Body:
A living body is mainly composed of the living matter known as protoplasm first; plasma = mould or form). It is the essence of life. Protoplasm is a soft, semifluid substance resembling glue and comas of a mixture of complex organic compounds such as carbohydrates proteins and fats along with inorganic salts in a watery medium.
This living matter is never idle, because its ingredients are actively engaged in undergoing a series of chemical changes during the entire lifetime of the individual to which it belongs.
An organism is said to be alive as long as its protoplasm remains active. Protoplasm is essentially the same in all living organisms whether it is a tiny microbe, a tree, a man or a huge elephant.
The living parts of the body such as roots, stems, leaves of plants, and heart, lungs, liver of animals may differ widely in form and function, but these are all composed of the basic living material, the protoplasm, which is responsible for the vital activities that characterise life.
It must, however, be admitted that some component parts of the living body such as the cork and wood of plants or the horns and hooves of cattle are dead substances produced by the activity of the protoplasm. These parts, though dead, are useful to their owners.
Characteristic # 3. Organization of the Living Body:
A living body is highly organized. It consists of closely correlated parts or organs such as the leaves of a plant or the lungs of an animal. Each organ is assigned to a particular vital function and there is a definite division of labour amongst the various organs in a living body.
An organ is composed of small protoplasmic units known as the cells (Fig. 6). In fact, the entire protoplasm of a living individual is arranged and organized as small unit masses called cells.
A cell, then, is an organized mass of protoplasm fit enough to carry on all the vital activities peculiar to life. It is not merely the structural unit comparable to the separate bricks in a building, but it is a functional or working unit as well.
Whatever a plant or an animal does, is what its cells do. Groups of similar cells which perform a particular function are organized to form tissues, and likewise, collections of tissues constitute an organ.
The organs are interrelated and act together in a spirit of co-operation. A living body is, therefore, a community of cells—a co-operative society—where the individual members (cells) as well as the organs (groups of cells and tissues) act in unison for the benefit of the entire organism.
In the lowest form of life the entire protoplasm is organized into a single cell which by differentiation of its parts carry on the essential life functions.
Characteristic # 4. Metabolism:
It has been pointed out that living protoplasm is constantly undergoing a series of chemical changes, and that cessation of protoplasmic activities results in disorganization and death. The sum total of these changes is collectively known as metabolism.
The metabolic changes are of two kinds. A formative or constructive kind, in which new materials are added to the protoplasm, is known, as anabolism. In this process the individual gains energy in potential “form to be kept in reserve for future use.
In the other kind, known as katabolism, the protoplasm is destroyed and lost. This is a means for conversion of the potential energy stored in the body of an individual into the kinetic form. If life is to remain in the protoplasm, both processes must continue side by side.
By anabolism protoplasm is built up and potential energy is accumulated. Katabolism involves the conversion of the potential energy into the kinetic form. By katabolism the organic constituents of protoplasm are destroyed and reduced to inorganic materials.
If anabolism exceeds katabolism, new materials accumulate in the cell and growth takes place. If, on the other hand, katabolism gains over anabolism, death is inevitable. This is life. In course of the life-processes new protoplasm is constantly built by synthesis of inorganic materials to replenish the wear and tear of the old organic matter which is thrown out and rejected.
It is very interesting to note that construction and destruction of protoplasm, a dual process of waste and repair, are going on simultaneously and side by side without rest. The metabolic changes find expression in the various vital activities such as nutrition, respiration, secretion, and excretion.
In fact, life may be defined as the external manifestation of the internal chances which are constantly occurring in the protoplasm of the organism. Metabolism is the means by which a living individual gains energy and spends the same to carry on its own day-to-day activities. The living machine is fuelled and cleansed automatically by its own metabolism.
(a) Nutrition:
A living body requires energy to carry on its vital activities just as a machine requires fuel to run smoothly. The food is the fuel of the living body. The energy locked up in food molecules is utilised by plants and animals for this process of feeding, but in the strict sense it is a much more complicated affair and involves several stages running successively.
(i) Firstly, the food is taken within the body. This is ingestion. Plants ingest the raw materials of their food either in-the form of a liquid or in the form of a gas. The liquid is absorbed through their roots and the gases enter mainly through pores present on the surface of their leaves.
These simple inorganic raw materials are then converted within the plant body into complex organic food matters, the carbohydrates, proteins and fats, and remain stored up there for future use. The animals cannot manufacture carbohydrates, proteins, and fats in their bodies. They get these food matters ready-made from the plants and ingest them in a solid or liquid state by their mouth.
(ii) The ingested food matters are not suitable for immediate utilisation. The complex substances are broken down into simpler forms and transformed into a soluble state by the process of digestion. Digestion of food is effected by certain chemical substances known as enzymes which are produced in the body cells of the organism for this purpose.
(iii) The digested food materials diffuse intimately from cell to cell. They are carried now to parts of the body where they are most needed. This is absorption.
(iv)The absorbed food is either utilised immediately for repair of the body cells and as a source of energy or they may be stored within a storage organ for future use. This is assimilation which is the ultimate goal of nutrition. It may be noted that in the course of assimilation, the food matters are ultimately transformed into the essential living substance, the protoplasm.
(v) All the ingested food may not be digested, absorbed, and assimilated completely. The residue is thrown out of the body by the process of egestion. Nutrition, then, means energy-intake and storage in a potential form for future use. It is an anabolic process and is concerned with the building and repair of protoplasm.
(b) Respiration:
This word implies much more than mechanical breathing, involving taking in of oxygen and giving out of carbon dioxide gas. It has been said that food provides the material from which energy can be extracted. Respiration is the process by which energy is released from food and other materials in the living body.
It is the motive force by which the potential energy stored in the living body is converted into an utilisable kinetic form. Respiration occurs spontaneously throughout the life period of all cells. Its complete cessation means death, and, therefore, it is one of the constant and invariable characteristics of living matter.
Air or oxygen is usually required for respiration, but in exceptional cases it may proceed even in the absence of oxygen. In fact, free oxygen is injurious to certain microbes. In the majority of organisms respiration means combustion or oxidation of the complex food substances, particularly carbohydrates and fats.
The result is the liberation of energy in the form of heat and production of carbon dioxide gas (CO2) and water vapour (H2O).
The process of oxidation of glucose, which is a carbohydrate, may be represented by the following equation:
C6 H12O6 + 6O2 = 6CO2 + 6H2O + Heat energy.
(Glucose) (Oxygen) (Carbon dioxide) (Water).
In this respect, respiration is almost identical with the burning or oxidation of a lump of coal or a log of wood which also yields similar products along with release of heat and light energy. But respiration is a vital process.
It takes place at a much lower temperature and is regulated by the vital activity of the protoplasm. The energy released by respiration is utilised by plants and animals to carry on their routine activities, whereas CO2 and H2O are breathed out.
Respiration, therefore, is essentially a destructive or katabolic process. It is a means for energy-release by breaking down the organic materials of the living body and changing them back into inorganic materials.
(c) Secretion and Excretion:
The protoplasm produces certain new chemical substances during its anabolic and katabolic activities. Such substances, when useful to the organism, are called secretions.
If, on the other hand, the new materials are useless or harmful to the organism, they are called waste products or excretions. Secretions and excretions are formed in specialised organs called glands within cavities called alveoli, and are finally released through tubes or ducts (Fig. 7).
Secretions often contain ‘enzymes’ or ‘ferments’, useful for digestion and other purposes. They may also contain chemical substances, called hormones, which have profound influence over the growth of the body as well as that of the mind. Enzymes and hormones are responsible for the chemical co-ordination of the living body, and control most of its metabolic activities.
The waste products or excretions are mostly formed during the combustion or oxidation of proteins, just as carbon dioxide and water are produced during oxidation of carbohydrates and fats. They are usually nitrogenous substances such as alkaloids, ammonia, urea, etc.
As the organism has no use for the waste products, they are thrown out of the body along with other such substances. Secretion and excretion, like respiration, are katabolic processes. The living body is automatically cleansed by excretion of waste products.
Characteristic # 5. Growth:
When anabolism exceeds katabolism, new organic substances are added between already existing particles of protoplasm, and the living body increases in bulk. This method of growth by wedging in of new substances between matters already in existence is known as growth by intussusception.
A non-living body may grow in bulk by the mere deposition of identical matter on its outer surface. Thus a lump of sugar may be made to gain in size by keeping it suspended in a concentrated sugar solution when fresh particles of sugar are deposited on the outer surface of the original lump.
This is growth by accretion, as opposed to growth by intussusception seen in plants and animals. It is to be noted further that a living body never grows in bulk at the expense of substances identical with its own protoplasm.
A plant increases in size and builds its own protoplasm by utilizing organic food matters manufactured out of carbon dioxide gas absorbed from the air and water with dissolved mineral matters from the soil.
A man increases in balk by taking plant products such as wheat and rice which are by no means identical with its own protoplasm. A non-living lump of sugar, on the other hand, increases in bulk at the expense of substance chemically identical with its own matter.
Characteristic # 6. Movement:
Most of the living objects have the power of movement. Spontaneous movement of protoplasm is characteristic of plants as well as animals. This protoplasmic movement occurs within the cells and helps to distribute food and eliminate waste products.
It can readily be observed in the cells of the water-plant Vallisneria (B. Patashaola). In a few of the lower forms of plants and animals are seen long whip-like protoplasmic structures called flagella or short hair-like processes called cilia which help the organisms to swim about in their watery habitat.
The lowly Amoeba creeps about by means of pseudopodia, which are temporary outgrowths from the surface of its own protoplasm.
This inherent property of the protoplasm may be observed in a more pro—degree in majority of the higher animals which have developed specialised organs for locomotion such as fins for the purpose of swimming, wings for flying and legs for walking. Thus the living body is a perfectly automatic machine.
Characteristic # 7. Sensitiveness and Adaptation to Environment:
Organisms are very sensitive and possess wonderful power of responding to external influences. Such influences are known as stimuli. A stimulus may be physical, such as heat or light, mechanical such as touch or contact, and even chemical, such as presence of acids or bases. A moth flies into a flame under the influence of light, and the leaves of the delicate sensitive plant shrink down modestly to touch (Fig. 8).
The roots of a plant go deep into the soil under the influence of the force of gravity, and a muscle contracts when stimulated by an electric shock. The entire life of an individual is a series of reflexes or reactions between it and its surrounding environment.
The organism responds to external stimuli with the definite purpose of maintaining its own life as well as for perpetuation of the whole race. Purposive behaviour is a feature of the living body.
Characteristic # 8. Reproduction:
Plants and animals have the power of producing young ones like themselves. This is reproduction. It is a unique process which maintains the continuity of life and occurs in all organisms usually after attaining maturity. The non-living can never reproduce its own kind.
There are various methods by which a living body reproduces its own kind. Whatever may be the method of reproduction, the young ones resemble the parents in the long run. So it is ‘like from like’—referred to as law of biogenesis (Omne vivum ex vivo).
Characteristic # 9. Rhythmicity or Periodicity:
Vital activities such as nutrition, respiration, secretion, etc., are all periodic in occurrence. A stage of intense activity is always followed by a distinct pause, however small this might be. In the meantime, the protoplasm gathers the vital force necessary for the next active phase.
The heart, lungs, etc., have a rhythm of their own which is strictly followed, unless disturbed by some external factors. Growth and reproduction are also periodic in nature. Rhythm is one of the important signs of life and may be considered as a basic property of the protoplasm.
Characteristic # 10. Life-Cycle, Senescence and Death:
An organism has a definite life-cycle. In the young or trophic stage it takes food and grows actively. When fully grown, it reproduces its own kind. In course of time it grows old and finally dies out.
The trophic or growing stage of an individual always culminates in reproduction and is ultimately terminated by senescence and death. These four important events occur in the order mentioned, and constitute the life-cycle of every living organism.
Life is always limited in duration. The limit of life varies widely in different kinds of plants and animals. The tiny ‘Diwali’ insect completes its life-cycle and dies out in the course of a few hours, but the gigantic banyan tree lives for more than one hundred years. During the latter part of life the vital activities decrease in intensity. The metabolism gradually slows down.
This sluggishness is the stage of senescence or old age which spares none. During age the co-operation between the various cell groups organs gradually becomes feeble. Ultimately, death ends all. It is the gross failure of the co-operative society and is always preceded by the bankruptcy of the individual cells as well as the cell-communes.