What is
Living?
Life is a unique, complex organisation of molecules that
expresses itself through chemical reactions which lead to growth, development,
responsiveness, adaptation and reproduction. The objects exhibiting growth,
development, responsiveness and other characteristics of life are designated as
living beings. They have their own specific form and structure. Based on these
characteristics, the living objects can be recognised as trees, shrubs, cattle,
birds, fungi, bacteria, etc. However, a specific shape, size and structure is
also present in many non-living objects such as brick or rock. No single trait
of life can distinguish a living being from a non- living object. Therefore, a
number of traits are examined simultaneously to differentiate living beings.
Characteristics
of Living Beings
1. Cellular
Structure: It is defining property of living beings. Each living
being is a complex entity which is formed of one or more cells. The cells are
made of protoplasm, popularly called living matter. Composition of living
matter is known. However, we have not yet been able to create protoplasm
because of lack of organisation of biomolecules. Protoplasm and cellular
structure are absent in viruses.
2.
Metabolism: All organisms operate a network of thousands of
chemical reactions. 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 is called metabolism (Gk. metabolic change).
Metabolism is defining property of living beings. All activities of an organism
including growth, movements, development, responsiveness, reproduction, etc.
are due to metabolism. No non-living object 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.
Types. Metabolism is of two kinds, catabolism and anabolism. Anabolism
includes all the "building up" reactions. It is also called
constructive metabolism since it involves the synthesis of complex substances
from simpler ones, e.g., synthesis of organic compounds from CO₂ and H₂O during
photosynthesis, formation of starch from glucose, production of proteins from
amino acids, formation of lipids from fatty acids and alcohols. Energy is
stored (as potential energy) in the process.
Catabolism (=
katabolism) constitutes "breakdown reactions". It is also known as
destructive metabolism because it involves breaking of complex substances into
simpler ones. Potential energy present in the complex substances is converted
into kinetic energy. Respiration is an example of catabolism. It releases
energy for performing different body activities.
3. Growth:
Growth is irreversible increase in mass of an individual. A multicellular
organism increases its mass by cell division. In plants growth continues
throughout life as they have meristematic areas where cell divisions occur
regularly. In animals, growth occurs to a certain age after which cells divide
only to replace worn out and lost cells. Unicellular organisms also grow by
cell division. However, cell division is also a means of reproduction in them.
In higher animals and plants, growth and reproduction are mutually exclusive.
Living organisms show internal growth due to addition of
materials and formation of cells inside the body. Such a method is called
intussusception (L. intus-- within, suscipere- to receive). A dead organism
does not grow; However, some non-living articles can increase in size, e.g.,
mountains, boulders, crystals, stones. It is due to addition of similar
materials to their outer surface. The process is called accretion (L.
accrescere-- to increase). In living beings growth producing substances are of
two types, protoplasmic and apoplasmic. Protoplasmic substances are components
of living matter like cytoplasm and nucleus. Apoplasmic substances (Gk. apo
away, plastos-- formed) are non-living materials formed by the cells which
become component of tissues, e.g., cell wall, fibres of connective tissue,
matrix of bone and cartilage.
Chemically growth is a result of difference between
anabolism and catabolism. Growth occurs when anabolism exceeds catabolism. There
will be no growth if anabolism and catabolism are equal. Degrowth or negative
growth can occur when catabolism exceeds anabolism.
4.
Reproduction: It is the formation of new individuals of the similar
kind - life arises from pre-existing life. Reproduction is not essential for
survival of the individuals. It is required for perpetuation of a population.
Ability for reproduction develops when a young individual becomes mature.
Reproduction is of two types, asexual and sexual. Asexual reproduction is uniparental
while sexual reproduction is generally biparental. Asexual reproduction is the
formation of new individuals from specialised or unspecialised parts of a
single parent without the formation and fusion of gametes. It occurs by spores,
binary fission, multiple fission, fragmentation and regeneration. Sexual
reproduction involves the formation and fusion of two types of sex cells or
gametes. The fusion product or zygote gives rise to an offspring.
In unicellular organisms, growth and reproduction are
synonyms. Many organisms do not reproduce, e.g., mules, sterile worker bees,
infertile human couples. Therefore, repro-duction is not an all-inclusive
characteristic of living organism. However, no non-living object has the power
to reproduce or replicate.
5.
Consciousness: It is awareness of the surroundings and responding to
external stimuli. The external stimuli can be physical, chemical or biological.
The stimuli are perceived by sense organs in higher animals, e.g., eyes, ears,
nose. Plants do not possess such sophisticated sense organs. However, they do
respond to external factors such as light, water, temperature, pollutants,
other organisms, etc. Photoperiods (duration of daily exposure to light)
influence reproduction in those animals and plants which breed during
particular season (seasonal breeders).
All organisms, from primitive prokaryotes to most advanced
and complex eukaryotes, are able to sense and respond to environmental factors.
Organisms also handle chemicals entering their bodies. Human beings have an
additional faculty of self-consciousness (aware-ness of self). Consciousness is
said to be the defining property of living organisms. If a patient is lying in
coma and is supported by machines for various functions, self-consciousness and
consciousness to external environment are supposed to be absent. Some of these
patients never come back to normal life. They can neither be called living nor non-living
or dead.
6.
Organisation: A living being has an organisation, that is, the
living being consists of several components and subcomponents which cooperate
with one another for the well-being of the whole organism. A living being has
multiple level organisation. Each level of organisation has its own properties
which are not found in its constituents. A cellular organelle develops a
property not found in its interacting molecular components. A living cell has
its own characteristics not found in its organelles. A tissue is able to have a
trait not found in its constituent cells.
7. Energy:
Living beings constantly require energy not only to perform various activities
of the body but also to overcome entropy or tendency to randomness. The source
of energy is food. It is required by every cell of the body.
8.
Homeostasis (Homoeostasis): A favourable internal environment
suitable for the functioning of body organs is present in every living being.
It is quite different from the external environment. Changes in external
environment do not have much impact on the internal environment as the living
beings have a self-regulated system to adjust and maintain the internal
environment. The phenomenon is called homeostasis (Gk. Homois- alike, stasis--
standing). Homeostasis is also present in each cell of a multicellular organism.
alike.
9.
Variations: Living beings possess variations and have the ability
to evolve with time.
10.
Adaptations (L. ad- toward, apt-adjust): Useful inheritable
variations or changes in form, function and behaviour which help an organism to
adjust well and successfully in its environment are called adaptations. An
organism is considered best adapted to an environment when it possesses
inherited traits that enhance its survival and ability to reproduce in that
environment. Adaptations allow the organisms to overcome seasonal and other
changes in the environment. They are of two types, short term adaptations
(e.g., hibernation in most amphibians and reptiles and some mammals) and long
term adaptations (e.g., the claws of different birds are well adapted to suit
their perching habits).
11. Healing
and Repair: Living beings can repair and heal the broken and injured
parts.
12.
Disposal of Wastes: Wastes generated by living beings are
regularly disposed off.
13.
Movements: Living beings show movements of their parts. Some are
able to move from place to place. The phenomenon is called locomotion.
14. Life
Span: Every living organism has a definite life span of birth,
growth, maturity, senescence and death.
15. Death: The
stoppage of various life activities by an individual organism accompanied by
increase in entropy is called death. Death occurs due to ageing, disease,
accident and predation. Ageing normally occurs in all organisms after a period
of reproductive maturity. It is, however, absent in some cases where the
organism multiplies by binary fission, e.g. Amoeba, bacteria. A fully grown
Amoeba, or a bacterium divides into two daughters. In the process it loses its
independent existence. Here, natural death is absent and the organism is
immortal.
Living organisms are, therefore, self-replicating, evolving
and self-regulatory interactive systems capable of responding to external
stimuli, sharing a common genetic material to varying degree both horizontally
and vertically.