The five most important indications of life are Cellular organization, Metabolism, Growth, Movement and Reproduction. If we examine more carefully and more deeply into the phenomena of life processes, the following characteristics of living beings will be evident.
The tiny mass of living substance inside cells is called protoplasm. It is a soft semifluid viscous, transparent, colourless jelly-like substance and I colloidal in nature. The famous biologist, T.H. Huxlay described it as the physical basis of life. All the vital functions of a living organism have their seat in the protoplasm. The living protoplasm exhibits a series of complex processes which constitute life.
(b) Cellular organization:
The cellular plan of organization is an exclusive property of living objects. Protoplasm lies within the cell which is the structural unit of living being. A group of cells of same origin and carrying similar functions is known as tissue. Tissues are aggregated into tissue-systems, which together form an organ. The interdependence and harmonious working of different organs make the life of an organism possible.
Living organisms carry on metabolism, which is necessary for the formation, maintenance and repair of protoplasm and for the continuity of life. Life is not possible without the complex physiological processes of construction (anabolism) and breakdown (catabolism) of food materials. In anabolism (e.g. Photosynthesis, nitrogen assimilation) the organism gais energy in potential form and reserves it for further use. In catabolism (or Katabolism) (e.g., Respiration, digestion) there is breakdown of food materials, so that the potential energy is converted into kinetic form. the dual process of construction and destruction is collectively called metabolism. The principal metabolic processes of living organisms are;
(iv) Digestion and
Living organisms are capable of growth. Growth involves not only increases size but also the processes of division of labour, co-ordination of activities and development according to a definite plan. Foos substances remaining in excess after the catabolic reactions use used towards the building up of fresh protoplasm and cells. These new cells result in the growth of the organism. The growth is form within, and this type of growth is termed as intussusception. Non-living objects also grow but the growth is by means of deposition on its surface (accretion), i.e., by the addition of materials from outside, just as a stone grows in size by deposition of materials from outside.
(e) Movement :
All living organisms exhibit movement.
(i) Automatic or spontaneous when the movement is due to its own accord as observed in case of animals and in the protoplasm of aquatic plants like Vallisneria. This type of movement occurs without any external influence.
(ii) Induced movement or Irritability involves the sensitivity of protoplasm to changes in the environment, i.e., external conditions. The protoplasm reacts specifically, usually in the form of movement, to those changes. The the power of responding to external stimuli i.e.., Irritability is an inherent character of living bodies. The stimuli may be due to physical factors (heat, light, moisture or drought), mechanical factors (tough or contact, wounding, tension) and even chemicals (presence of acids and bases) or force of gravity etc. irritability is more pronounced in animals than in plants.
Living organism possess the power of reproduction of similar kind i.e., offspring of the same kind as themselves. Non-living objects can mechanically breakdown into a number of irregular parts but cannot give rise to new young ones like themselves. In living organisms individually within a species tends to be transmitted from the parent to the offspring. Instruction for production is contained in DNA, the molecules of heredity.
Living organisms show a distinct life-cycle. After birth it takes food and grows actively in the young stage. When the organisms attains maturity it reproduces and in course of time it grows old or senescence sets in, finally it dies,. Hence all living organism must pass through these stages in a life-cycle.