When we try to define ‘living’, we conventionally look for distinctive

characteristics exhibited by living organisms. Growth, reproduction, ability to

sense environment and mount a suitable response come to our mind
immediately as unique features of living organisms. One can add a few more
features like metabolism, ability to self-replicate, self-organise, interact and
emergence to this list. Let us try to understand each of these.
All living organisms grow. Increase in mass and increase in number of
individuals are twin characteristics of growth. A multicellular organism grows
by cell division. In plants, this growth by cell division occurs continuously
throughout their life span. In animals, this growth is seen only up to certain
age. However, cell division occurs in certain tissues to replace lost cells.
Unicellular organisms grow by cell division. One can easily observe this in in
vitro cultures by simply counting the number of cells under the microscope.
In majority of higher animals and plants, growth and reproduction are
mutually exclusive events. One must remember that increase in body mass is
considered as growth. Non-living objects also grow if we take increase in body
mass as a criterion for growth. Mountains, boulders and sand mounds do
grow. However, this kind of growth exhibited by non-living objects is by
accumulation of material on the surface. In living organisms, growth is from
inside. Growth, therefore, cannot be taken as a defining property of living
organisms. Conditions under which it can be observed in all living organisms
have to be explained and then we understand that it is a characteristic of
living systems. A dead organism does not grow.
Reproduction, likewise, is a characteristic of living organisms. In
multicellular organisms, reproduction refers to the production of progeny
possessing features more or less similar to those of parents. Invariably and
implicitly we refer to sexual reproduction. Organisms reproduce by asexual
means also. Fungi multiply and spread easily due to the millions of asexual
spores they produce. In lower organisms like yeast and hydra, we observe
budding. In Planaria (flat worms), we observe true regeneration, i.e., a
fragmented organism regenerates the lost part of its body and becomes, a
new organism. The fungi, the filamentous algae, the protonema of mosses, all
easily multiply by fragmentation. When it comes to unicellular organisms like
bacteria, unicellular algae or Amoeba, reproduction is synonymous with
growth, i.e., increase in number of cells. We have already defined growth as
equivalent to increase in cell number or mass. Hence, we notice that in
single-celled organisms, we are not very clear about the usage of these two
terms – growth and reproduction. Further, there are many organisms which
do not reproduce (mules, sterile worker bees, infertile human couples, etc).
Hence, reproduction also cannot be an all-inclusive defining characteristic of
living organisms. Of course, no non-living object is capable of reproducing or
replicating by itself.
Another characteristic of life is metabolism. All living organisms are made of
chemicals. These chemicals, small and big, belonging to various classes,
sizes, functions, etc., are constantly being made and changed into some other
biomolecules. These conversions are chemical reactions or metabolic
reactions. There are thousands of metabolic reactions occurring
simultaneously inside all living organisms, be they unicellular or
multicellular. All plants, animals, fungi and microbes exhibit metabolism.
The sum total of all the chemical reactions occurring in our body is
metabolism. No non-living object exhibits metabolism. Metabolic reactions
can be demonstrated outside the body in cell-free systems. An isolated
metabolic reaction(s) outside the body of an organism, performed in a test
tube is neither living nor non-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. Hence, cellular
organisation of the body is the defining feature of life forms.
Perhaps, the most obvious and technically complicated feature of all living
organisms is this ability to sense their surroundings or environment and
respond to these environmental stimuli which could be physical, chemical or
biological. We sense our environment through our sense organs. Plants
respond to external factors like light, water, temperature, other organisms,
pollutants, etc. All organisms, from the prokaryotes to the most complex
eukaryotes can sense and respond to environmental cues. Photoperiod affects
reproduction in seasonal breeders, both plants and animals. All organisms
handle chemicals entering their bodies. All organisms therefore, are ‘aware’ of
their surroundings. Human being is the only organism who is aware of
himself, i.e., has self-consciousness. Consciousness therefore, becomes the
defining property of living organisms.
When it comes to human beings, it is all the more difficult to define the living
state. We observe patients lying in coma in hospitals virtually supported by
machines which replace heart and lungs. The patient is otherwise brain-
dead. The patient has no self-consciousness. Are such patients who never
come back to normal life, living or non-living?
In higher classes, you will come to know that 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 result in emergent properties at a higher level of organisation.
This phenomenon is true in the hierarchy of organisational complexity at all
levels. Therefore, we can say that living organisms are self-replicating,
evolving and self-regulating interactive systems capable of responding to
external stimuli. Biology is the story of life on earth. Biology is the story of
evolution of living organisms on earth. All living organisms – present, past
and future, are linked to one another by the sharing of the common genetic
material, but to varying degrees.