WHAT IS LIVING? – Class 11 Biology

Life is a unique, complex organization of molecules expressing itself through chemical reactions (metabolism) which lead to growth, development, responsiveness, adaptation and reproduction.

Hence, “the object by itself exhibiting the growth, development, death, consciousness, reproduction etc. is designated as living being.”

Characteristics of Living Beings

All the living beings share certain unique and basic characteristics which set them apart from non-living objects.

    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.
• Growth is the result of difference between anabolism (building up reactions) and catabolism (breakdown reactions).
• 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.
• Unicellular organisms also grow by cell division. In majority of higher animals and plants, growth and reproduction are mutually exclusive events, but in unicellular organisms like Amoeba, reproduction is synonymous with growth, i.e., increase in number of cells.
• Non-living objects also grow. 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 and protonema of mosses multiply by fragmentation.
• Hence, reproduction also cannot be an all inclusive property of living organisms. Still, no non living object is capable of reproducing or replicating by itself. Further, there are some organisms which do not reproduce at all, e.g., worker bees, mules etc.

     3. Metabolism:

• The energy is 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.
• 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.
• Despite vast differences occurring in structure and functioning of cells in different organisms, metabolic reactions are unusually similar.

• 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.

    4. Cellular Organisation :

• Body of all living organisms consists of cells and their products. Hence, cellular organisation of the body is the defining feature of life forms.

    5. Consciousness:

• Every living beings, are aware of their surrounds. All living organisms are able to detect changes, i.e., sence their surroundings and can also respond to them.
• All organisms from prokaryotes like bacteria to complex eukaryotes like plants, animals and fungi can sense various changes up to variable degrees in their surroundings. They can also respond by showing movements or behavioural changes in respect to stimuli.
• Human being is the only organism, who is aware of himself. He has “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.
• Other characteristics of living beings are adaptation, life span, homeostasis, healing and repair, movement and variation.