‘Life is a little gleam of time between two eternities’ -Thomas Carlyle (1795-1881). A great deal is known about life. Yet despite the enormous fund of information that exists, there is no generally accepted definition of what life is. One approach would be to say that life is defined approximately as any system capable of performing a certain, specialized, set of functions.
Characteristics of life
In most animals the characteristics that demonstrate being alive are self-evident, although these features are less obvious in some small animals and plants, and even less obvious in micro-organisms such as bacteria and viruses. These features are: . Respiration, which in its widest sense is the obtaining of energy from chemical changes within the organism. Oxygen, for example, is needed in the breakdown of food. . Eating is an essential feature of life because food is required to provide the energy released by respiration. The way that a tree obtains nutrients is much less obvious than that of an animal which moves actively in search of food. . Metabolism, during which substances taken in from the environment are broken down and the pieces used by the organism to make its own molecules. . Excretion, when waste products produced by respiration, metabolism and other processes are eliminated from the organism before they accumulate and become poisonous. . Growth, which in simple terms is an increase in size but usually implies that the organism is becoming more complicated and efficient. . Movement, which is obvious in most animals but in plants is usually restricted to growth towards light or the opening and closing of petals. . Reproduction, whereby life is handed on from generation to generation – usually in groups we call species. . Sensivity, the ability to detect a stimulus and respond to it. Obvious examples are the movements made by animals as a result of seeing an enemy or hearing a noise. Full-grown plants do not show such obvious responses but during growth they react to light and gravity.