Characteristics of Living Beings
All the living beings share certain unique and basic characteristics which set them apart from non-living objects these characteristics are listed below :
1. Growth : increase in mass and increase in number of cells are twin characters of growth. Growth refers to irreversible increase in mass or overall size of a tissue, an organism or its parts. It occurs due to synthesis of two types of substances, protoplasmic and apoplasmic. Protoplasmic substances are components of living matter like cytoplasm and nucleus. Apoplasmic substances are non- living materials formed by the cells and which become components of tissues. Growth is the result of difference between anabolism (building up reactions) and catabolism (breakdown reactions). Growth occurs when anabolism or synthetic processes exceeds catabolism. If anabolism is just equal to catabolism, there will be no increase in mass of cells or organism. Hence, growth will be absent. Degrowth or negative growth will occur when catabolism exceeds anabolism. It will decrease the mass of body.
Growth by cell division occurs continuously in plants. While in animals this can be seen only upto a certain age. Unicellular organisms also grow by cell division. in majority of higher animals and plants, reproduction is synonymous with growth, i.e., increase in number of cells.
Non-living objects also grow if we take increase in body mass as a criterion of growth. But in these objects extrinsic growth is present i.e. increase in the mass of body from outside e.g., mountains, boulders and sand mounds. Growth, therefore , cannot be taken as a defining property of living organisms.
2. Reproduction : Reproduction is the formation of new individuals of similar kind. It is, however, required for survival of the population as it compensates for the loss of life due to death. Reproduction is of two types, asexual and sexual. Asexual reproduction is uniparental multiplication that occurs through binary fission, multiple fission, spore formation, fragmentation and vegetative multiplication. Fungi multiply by asexual spores, yeast and Hydra show budding, Planaria exhibits true regeneration. Fungi, filamentous algae, protonema of mosses easily multiplies by fragmentation. But , when we notice single – called organisms like bacteria, unicellular alga and Amoeba, we are not clear about the usage of these two terms – growth and reproduction, i.e., increase in number of cells. Hence, reproduction also cannot be an all – inclusive property of living organisms. Still, no non-living object is capable of reproducing by itself. Further, there are some organisms which do no reproduce at all, e.g., worker bees, mules etc.
3. Metabolism : life is a never ending flow of energy and materials. The latter are required by all living cells for building and functioning of their living matter. Metabolism is the sum total of all chemical reactions occurring in an organism due to specific interactions amongst different types of molecules within the interior of cells. It involves exchange of matter and energy between an organism and its environment and transformation of matter and energy within an organism. Despite vast differences occurring in structure and functioning of cells in different organisms, metabolic reactions are unusually similar.
Metabolic reactions can be demonstrated outside the body in cell free systems. An isolated metabolic (s) in test – tube is neither living nor-living. Hence, while metabolism is a defining feature of all living organisms without exception, isolated metabolic reactions in-vitro are not living things but surely living reactions.
4. Cellular structure : Body of all living organisms consists of cells and their products. Hence, cellular organization of the body is the defining feature of life forms.
5. Consciousness : all living beings , plants and animals, are aware of their environment. All organisms sense their surroundings and respond to sudden or smooth changes in it. All organisms from prokaryota to eukaryota can sense and respond. Humans also have “self- consciousness” hence, this becomes the defining property of living beings. It is most obvious and technically complicated feature of all living organisms.
The foregoing discussion, nevertheless concludes that living beings are organized, self – replicating, evolving and self- regulating interactive systems, with consciousness at the head of all.
A question may arise whether a man lying in coma on the life support systems is living or non-living? The answer lies in the quantitative presence of consciousness in the living being in that particular state. Infect, the person in coma ahs lost the requisite quantity of consciousness to exhibit the features of living being, but still exhibits life. If the requisite consciousness is restored, the person may again start to be as living being or die otherwise.
Other characteristics of living beings are adaptation, life span, homeostasis, healing and repair, movement and variation.