The following points highlight the fifteen essential characteristics of living beings. The characteristics are: 1. Cellular Structure 2. Metabolism 3. Growth 4. Reproduction 5. Consciousness 6. Organisation 7. Energy 8. Homeostasis 9. Variations 10. Adaptations 11. Healing and Repair 12. Disposal of Wastes 13. Movements 14. Life Span 15. Death.
Characteristic # 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.
Characteristic # 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. metabole— 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 bio-chemicals.
Characteristic # 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.
Characteristic # 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 bi-parental.
Asexual reproduction is the formation of new individuals from specialised or un-specialised 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, reproduction is not an all inclusive characteristic of living organism. However, no non-living object has the power to reproduce or replicate.
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 HO2 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.
Differences between Anabolism and Catabolism:
1. It is the sum total of building up or constructive processes.
2. Anabolism produces complex materials from simpler ones.
3. It stores energy.
4. Kinetic energy is changed into potential energy.
5. Anabolism is required for growth, maintenance and storage.
6. Fewer types of precursors form diverse products (reactions diverge).
1. Catabolism is the sum total of breakdown or destructive processes.
2. It forms simple substances from complex ones.
3. It releases energy.
4. Potential energy is changed into kinetic energy.
5. Catabolism is required for performing various activities of living beings.
6. Many types of larger substances breakdown to form fewer types of simple molecules (reactions converge).
Characteristic # 5. Consciousness:
It is awareness of the surroundings and response 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 (awareness 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.
Characteristic # 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 organisations.
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.
Characteristic # 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.
Characteristic # 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.
Characteristic # 9. Variations:
Living beings possess variations and have the ability to evolve with time.
Characteristic # 10. Adaptations (L. ad— toward, apt— adjust):
Useful inheritable variations or changes inform, 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).
Characteristic # 11. Healing and Repair:
Living beings can repair and heal the broken and injured parts.
Characteristic # 12. Disposal of Wastes:
Wastes generated by living beings are regularly disposed off.
Characteristic # 13. Movements:
Living beings show movements of their parts. Some are able to move from place to place. The phenomenon is called locomotion.
Characteristic # 14. Life Span:
Every living organism has a definite life span of birth, growth, maturity, senescence and death.
Characteristic # 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.