Here I’ll be explaining the defining properties of living organisms in detail, these characteristic features are the ones that differentiate the living from the non-living.
It’s included in the first chapter “The Living World” of Class 11 NCERT Biology which is also repeatedly asked in NEET exam as well as boards.
What is ‘living’?
When we try to define the term ‘living’, we look for certain distinctive characteristics exhibited only by living organisms. Such characteristics or properties distinguish the living from the non-living. They include:
- Growth
- Reproduction
- Metabolism
- Response to the stimuli
- Ability to sense the environment
- Ability to self-replicate
- Ability to self-organise
- Interaction and emergence
These are characteristics of living organisms but they aren’t defining properties of living organisms. What do you mean by characteristic of the living beings? There is a line between living and non-living which includes special features for each of them. Living organisms as a whole have certain unique and basic characteristic that sets them apart from non-living world.
Defining Property of Living Organisms
Defining property means that it should be an all-inclusive defining characteristic unique feature of all the living organisms without any exceptions. Some examples of defining properties of living organisms are:
- Metabolism
- Cellular organisation
- Consciousness
Let’s cover each feature in detail.
Growth
Growth is one of the most important characteristics of living organisms.
For example, kittens grow into cats, puppies grow into dogs and a human baby grows to become an adult.
Growth is defined as an irreversible increase in mass and an increase in the number. These are the twin characteristics of growth.
An increase in mass refers to an intrinsic increase (increase in the protoplasmic matter of the cell) and an increase in number is by the cell division.
Growth is the result of metabolism when the anabolic (synthetic) processes are more than that of the catabolic (destructive) processes in an organism.
Growth is of two types, intrinsic (from inside) and extrinsic (from outside).
If we consider only an increase in mass as growth, non-living objects grow too. This type of growth where there is an accumulation of material on the surface is known as extrinsic growth. (example: mountains, boulders, and sand mounts).
Growth in plants is an indeterminate type of growth (unlimited), it occurs continuously throughout their life span. Certain regions like shoot tip, root tip, and cambium have meristematic tissue that allows them to grow continuously throughout their life span.
Growth in animals is a determinate type of growth (limited), it occurs only up to a certain age. However, cell division occurs in certain tissues to replace lost cells.
Hence, growth cannot be taken as a defining property of living organisms, though it is a characteristic of only living organisms.
Reproduction
Production of new individuals or progeny is known as reproduction.
Reproduction is the only way that living things can perpetuate themselves. It is one of the most important life functions.
Living organisms reproduce asexually as well as sexually.
Lower organisms (example: yeast and hydra) reproduce by budding.
True regeneration is seen in planaria or flatworms where a fragmented organism regenerates the lost part of its body and becomes a new organism.
Unicellular organisms (example: bacteria, amoeba, and unicellular algae) reproduce or multiply by cell division. Reproduction in this case is basically an increase in the number of cells. Hence, in unicellular organisms, reproduction is synonymous with growth.
Fragmentation is seen in fungi, filamentous algae, and protonema of mosses.
In multicellular organisms, reproduction mostly refers to sexual reproduction; some of them reproduce asexually as well.
Reproduction is of two types:
- Asexual Reproduction: Reproduction in which gametic fusion or fertilisation and meiosis are not involved. There are many methods of asexual reproduction.
By Asexual spores: Algae and Fungi
By Budding: Yeast and Hydra
By Fragmentation: Filamentous algae, Fungi and the Protonema of Moss Plants
True Regeneration (Morphallaxis type): Fragmented organisms regenerate the lost part of its body and become a new organism. Seen in Planaria.
NOTE: Regeneration (Epimorphosis type) is a process in which only the lost part of the body is repaired or regained. Seen in Starfish and Lizard.
- Sexual Reproduction: Reproduction in which gametes are formed by meiosis and fertilisation also takes place to form progeny.
Reproduction cannot be considered an all-inclusive defining characteristic of living organisms, because of 2 main reasons:
- In single-celled organisms, growth and reproduction are synonymous, as an increase in the number of cells is considered as growth as well as reproduction.
- Many living organisms do not reproduce or give rise to offsprings (example: mules, sterile worker bees, and infertile human couples).
Metabolism
The sum total of all the chemical reactions occurring in a living cell/organism is called metabolism.
Metabolism includes catabolism (reactions that break down the materials) and anabolism (reactions that build the materials).
All living organisms (unicellular and multicellular) exhibit metabolism without any exception.
No non-living thing shows metabolism. However, metabolic reactions can be carried out outside the body of an organism in cell-free systems. Such reactions are neither living nor non-living. The isolated in vitro metabolic reactions can, however, be called biological reactions or living reactions as they involve biochemicals.
Hence, metabolism is considered a defining property of living organisms. Since metabolism occurs only inside the cells, the cellular organisation of the body is also a defining feature.
Cellular Organisation
A cell is the basic unit of life. All living organisms are composed of cells.
Some are composed of a single cell and are called unicellular organisms while others are composed of many cells and are called multicellular organisms.
Unicellular organisms are capable of independent existence and performing essential functions of life.
Anything less than a complete structure of a cell does not ensure independent living. Hence, the cell is the fundamental, structural, and functional unit of all living organisms.
Hence, the cellular organisation is a defining property of all living organisms.
Response to stimuli/Consciousness
It is the ability of all living organisms (prokaryotes or eukaryotes), simple or complex in organisation, to sense the conditions in their surroundings or environment and respond to these stimuli, which may be physical, chemical, or biological.
Consciousness is the most obvious and technically complicated feature of all living organisms.
We sense these physical, chemical or biological stimuli through our sense organs.
Plants also sense and respond to external factors like light, water, temperature, humidity, etc in a variety of ways by increasing and decreasing their rate of transpiration, opening and closing of stomata, or by turgidity and wilting, etc.
Some common examples are plants flowering in a particular season (photoperiodism) and animals breeding in a particular season (seasonal breeders).
All organisms, from prokaryotes to eukaryotes show consciousness to environmental cues.
Therefore, consciousness is a defining property of all living organisms.
NOTE: Human beings are the only organisms that have self-consciousness (is aware of himself/herself). This is due to their well developed nervous system and communication skills.
Humans are very fast to respond to external stimuli and even think or predict possible changes in the surroundings to prepare themselves.
In the hierarchy of organisational complexity of organisms, the property of the living organism is due to the interactions occurring at the organ system, organ, tissue, and the cellular level. In conclusion, we can say that living organisms are self-replicating, self-regulating, and evolving interactive systems that are capable of responding to external stimuli.
Hope you found it helpful.
Good luck Medicoholics! Until next time.