Necessary characteristics of living organisms

The Characteristics of Living Organisms

Before commencing our study of Botany it is necessary for us to inquire how we may distinguish living organisms from dead matter; in other words, how we may limit the territory designated as Biology.

 In the present state of scientific knowledge it is impossible to answer the question" What is life? " though it is possible to describe those character­istics which distinguish the living from the non-living. In living matter we find only those elements which are among the common constituents of the earth, and these elements obey the ordinary laws of Chemistry and Physics. 

All living organisms are built up of a substance known as protoplasm. This may be termed" living matter," or, in the words of the great zoologist, T. H. Huxley (1825-95), " The physical basis of life." One of the most striking features of living organisms is that their protoplasm is never stable, but is constantly undergoing change. All changes in matter imply the performance of work, which in turn can only be carried out by the use of energy. Every activity, therefore, on the part of a living organism demands the expenditure of energy, and it is for the purpose of obtaining this energy that many of the vital functions are performed. 

Though some of this energy is dissipated, usually as heat, much is utilized in the manufacture of fresh material to take the place of that which has been broken down. The sum total of this breaking down and building up again is spoken of as metabolism. This cycle is not perpetual, however, since there is an ineyitable wastage of energy which must be made good from without. Thus every organism requires an external source of energy to keep its metabolism going. 

In adult life these two processes are fairly equally balanced. If building­up is in excess of breaking-down, as in young organisms, there is a definite increase in the amount of protoplasm, resulting in growth. On the other hand, if the reverse should occur, as is common in old age, there is a gradual increase of destruction over repair, which will finally result in death, though for various other reasons death may occur even when large quantities of food reserves are present. 

Growth, of course, may be observed in non-living matter. A familiar example is the growth of crystals in the chemical laboratory. But this kind of growth is by accretion, in which layers of new molecules of the same kind are superimposed upon the existing ones. It differs fundamentally from the type of growth found in organisms, where the fresh molecules, not necessarily of the same kind, are interspersed among those already present, and trans­formed in such a way that they can be built into the substance of the organism throughout its body. This type of growth is characteristic of living organisms, and is knon as intussusception. 

Break-down is essentially a process of oxidation, as a result of which oxygen is used up and carbon dioxide is formed. This gaseous interchange, with the liberation of energy, constitutes the essential feature of respiration. All living organisms, whether animals or plants, continuously respire through­out life. The chief sources of energy are carbohydrates, such as sugars, which are found in the protoplasm and are easily broken down by respiration. 

Occasionally substances are formed in the course of metabolism which are put to some special use outside the usual metabolic cycle. These sub­stances are termed secretions. 

All living organisms exhibit irritability, that is they respond to external influences which disturb the metabolic balance. This response generally takes the form of movement. The phenomenon of movement varie greatly in degree, and though very marked in animals is not so readily observed in plants owing to their slower reaction to stimuli. 

The power of reproduction is common to all living organisms, that is to say, the formation of new individuals which are similar to those already existing. In its simplest form reproduction is merely an outcome of the power of growth, involving the separation of part of the body. In this way the race of organisms is perpetuated. 

We shall see that all plants exhibit phenomena which are characteristic of life, but that the ways in which these are manifested are often very different. 

The protoplasm of even the simplest organism is divided into different parts, a phenomenon spoken of as differentiation; hand in hand with this goes the allotting of special duties to the several parts so differentiated; this is called division of physiological labour. Probably there is no organism without any differentiation at all and with a body consisting of a homogeneous mass of protoplasm, though we shall have occasion to describe some in which there is apparently very little. At the other end of the scale there are organisms, all the parts of whose bodies are differentiated for various functions. 

It is this progress towards specialization which forms the basic principle of modern Biology. We can conceive that life first originated in the far­distant past, by some means unknown, as a minute undifferentiated mass of protoplasm. From this start there has been a gradual increase in com­plexity in the descendants, resulting in highly specialized organisms. This idea of the transition from the less specialized to the more specialized is called evolution The whole world has been populated by evolution with animals and plants which are specialized for their particular surroundings-that is to say, they are adapted to their environment. 

Posted in Botany