What Is Living? – Defining Factors of a Living Thing
Living organisms show some unique and distinctive characteristics which help us in recognising and differentiating them from other non-living things.
The characteristics which we observe commonly in all living beings are growth, reproduction, sensitivity. There are some other features which are not seen from outside but we know that they are taking place inside their bodies like metabolic reactions, self-replication, self-organisation, etc. These living organisms also have capability to interact and evolve, which can be observed and studied in them.
Characteristics of Living Beings
Growth is a fundamental characteristic of all living organisms. It is regarded as an intrinsic property of living organisms through which they can increase both in mass and in number of cells, in their body.
All living organisms whether unicellular or multicellular grow by cell division. The pattern and duration of growth is distinct in different organisms, like in plants, growth can occur throughout their life span whereas animals have only a limited period of growth, in their life span.
In unicellular organisms like bacteria, Amoeba, growth occurs by cell division and such cell division also leads to the growth of their population. Although, by such process of cell division reproduction of the individual also occurs. Hence, growth and reproduction are mutually inclusive events, in unicelled organisms. One can observe, cell division in unicellular organisms like bacteria, in-vitro (outside the body of an organisms), i.e., In a test tube or petridish in an artificial medium, under microscope and can even count number of cells, increased during it, manually. In higher animals and plants, growth and reproduction are linked but are mutually exclusIve events. However, cell division not only occursin living organisms at time of growth and reproduction but also during maintenance to replace lost cells due to some injury, etc. from the body. Hence, to maintain original size, shape and structure of a body, new cells are formed by division in various living beings.
Hence, growth involves both increase in mass and number of cells which takes place from inside the body of living organisms and is irreversible. On the other hand, non-iving objects can also be seen growing like snow mountains grow by addition of snow on them, crystals increase in size by addition of molecules on its surface, sand-dunes increase by frequent transport of more material from its native bed by natural agencies like river, etc. But such growth in non-living objects happens externally by addition of some material from any outside source. So, we can say in non-living objects growth is extrinsic as compared to intrinsic growth in Iiving organisms. Therefore, non-living objects can increase their mass by accumulation of material on surface by any external agency which can be reversed.
Growth, therefore, cannot be taken as defining property of living organisms, though it takes place in all living organisms and is absent in dead organisms. Growth is a characteristic of all living organisms, when all the conditions are well-understood and properly examined from a scientific point of view.
ReproduGtion is one of the fundamental characteristics of living organisms. It can be defined as the production of new individuals of same kind by the grown up individuals.
It is the characteristic exhibited by living organisms which can produce new young ones of their own kind. There are two modes of reproduction- asexual and sexual. In asexual mode, new individuals are produced from specalised or any unspecialised part of a single parent (i.e., with or without the involvement of gamete formation). For instance, unicellular organisms like bacteria, algae or Amoeba divide by fission to produce new individuals. In such processes, parent body undergoes division to form two or more individuals, ie., number of cells increases. Hence, in unicellular organisms reproduction is synonymous with growth.
In lower organisms like yeasts and Hydra, budding takes place in which new individuals are produced by the formation of an outgrowth known as “bud“. These buds first grow on parent’s body, and then separates from it, to give rise new individual. Fragmentation is also an another modé of asexual reproduction, as in this, body of an organism (parent body) breaks up into two or more parts (known as fragments) each of which grows into a new individual. It is also quite common in filamentous algae, fungus, bryophytes (at protonema stage which occurs during life cycle in mosses). Planaria (flat worms) exhibit an extraordinary ability to regenerate its lost body parts completely (which is known as true regeneration). This is a method of reproduction as new planarians develop by splitting of parent planarian body either lengthwise or transversely. In higher organisms like plants, animals sexual mode of reproduction is quite common which involves formation of gametes (i.e., sex cells) from two parents of opposite sexes but same species. These gametes then fuse to form zygote (2n) which develops to form a new organism of same kind.
Hence, reproduction is shown by all living organisms except a few which are either sterile or infertile, like mule, worker-bees, infertile human couples, etc., do not reproduce at al. So, these cannot produce their ofspring but show all other characteristics of living organisms. Although, no non-living object can replicate itsef by its own, i.e., power of replication or production of new individual of their own kind, is totally absent in them.
Hence, reproduction can be regarded as characteristic of living organisms but it is not their exclusive defining characteristic.
All living organisms are made up of chemicals. These chemicals may be small and large, belonging to various classes, sizes, functions etc. These biomolecules are constantly synthesized and broken down into some other biomolecules in the body of living organisms through various kinds of reactions.
Such thousands of chemical reactions which occur inside living organisms during various processes like photosynthesis, respiration, etc., help them to maintain their living state. The sum total of all the chemical reactions occurring in the body of living organisms is known as “Metabolism‘”. All living organisms from unicellular to multicellular, i.e., bacteria to multicellular fungi, plants and animals, possess metabolism of their own kind. The metabolic processes which involve the synthesis of molecules are called “anabolism, e.g synthesis of proteins from amino acid, whereas those metabolic processes in which large molecules are broken down into smaller are known as “catabolism“, e.g., sugars broken down into molecules of water and carbon dioxide, to liberate energy, i.e., ATP
On the other hand, non-living objects do not show metabolism. So, metabolism can be regarded as defining feature of all living organisms. Although, some of these reactions can be made to occur outside the body (in-Vitro) in cell free system. These reactions occurring outside the body is not living but are living reactions.
Hence, the way cell performs all-its functions or processes to organise or constitute the body of an organism (cellular organisation) is unique and that can be regarded as defining feature of all life forms.
Most obvious and technically complicated feature of all living organism. All living organisms are able to deted changes, i.e., sense their surroundings and can also respond to them. This is known as sensitivity which s defined as the ability to detect changes in the environment and to give response towards it accordingly. Any change that can be detected by an organism is called stimulus. This can be physical (like intensity, duration, direction of light, sound, change in temperature, duration of day length, i.e., photoperiod, etc.), chemical (ike acids, pollutants, etc.) or biological (like other organisms).
All organisms from the prokaryotes like bacteria to the complex eukaryotes like plants, animals and fungi, can sense various changes upto variable degrees in their surroundings. They can also respond by showing movements or behavioural changes in respect to stimuli. For instance, some plants like soyabean, radish etc. or animals like sheep, goat, horse, etc. breed or reproduce at specific seasons only, like in winters, summers, autumn. Hence, they are known as seasonal breeders as their reproductive behaviour changes with length of day, ie., photoperiod. Therefore, they mate or reproduce during their specific breeding season only. Plants are sensitive to external factors like light, water, temperature, pollutants and other organisms etc.
Besides, human being is the only organism, who is aware of himself. He has self-consciousness too with awareness of the surroundings. He can relate his mind to the changes taking place in the world. He is an intelligent animal with thoughts, feelings and self-hood.
Sensitivity or awareness is regarded as defining property of living organisms as non-living things do not have power of sensing their surroundings and give response according to it. However, patients lying in coma in hospitals virtually supported by machines which replace heart and lungs are neither living nor dead otherwise brain-dead.
All living phenomena are due to underlying interactions. Properties of tissues are not present in the constituent cells but arise as a result of interactions among the constituent cells. Similarly properties of cellular organelles are not present in the molecular constituents of the organelle but arise as a result of interactions among the molecular components comprising the organelle. These interactions resuit in emergent properties at a higher level of organisation. This phenomenon is true in the hierarchy of organisational complexity at all levels.
Therefore, the living organisms can be said to be self-replicating, evolving and interactive systems capable of responding to external stimuli.
All living organisms – present, past and future are linked to one another by the sharing of the common genetic material, but to varying degrees.