• Life can be defined as unique complex organization of molecules expressing itself through chemical reactions (metabolism) which lead to growth, development, responsiveness, adaptation and reproduction.
• Living things possess certain characteristics, which makes them different from non-living things.
(i) Catabolic activities release energy
(ii) Energy liberating reactions are termed as ‘exergonic’
(iii) These are sum total of breakdown or destructive process.
(iv) These reactions from simple substances from complex ones
(i) Anabolic reactions store energy.
(ii) Energy absorbing reactions are termed as ‘endergonic’.
(iii) These are sum total of building up or constructive process.
(iv) These reactions produce complex molecules from the simpler ones.
► However, some of the metabolic reaction can be carried outside living system. Isolated metabolic reactions ‘in vitro’ are not living things, but are living reactions.
• All living organisms show growth either by multiplication or by increase in size. It is an irreversible increase in mass of individual.
• For larger organisms, growth is related to the development of new parts either in between or within the older ones. Thus, a sort of internal growth is visible in living beings.
• Two types of cells are produced by cells, i.e. apoplastic and protoplastic.
Protoplastic substances are components of living matter like cytoplasm and nucleus.
Apoplastic substances are non-living material formed by the cells which becomes components of tissue, e.g. cell wall, fibres of connective tissue, matrix of bone and cartilage.
So, we can say growth is common characteristic feature of all living beings.
• Plants grow throughout their life while animals grow for a certain period only. Although some non-living things like mountains, sand mounds and crystal also grow, but their growth is due to the addition of matter from outside. This process is called
• It is formation of new individuals of similar kind either by asexual (uniparental) or sexual (bi-parental) mode. This is required for perpetuation of a population/ species and also help in passing on traits from one generation to the next.
Although some organisms live for a very long time, no organisms lives forever. Because all organisms dies one day, ongoing life is impossible without reproduction. Thus the process of reproduction is essential for the continuity of life on the earth.
• Fungi reproduces asexually producing millions of asexual spore, while yeast, Hydra multiply by building.
• Planaria exhibit true regeneration by fragmentation. In amoeba, growth and reproduction are synchronized. When repeated reproduction is there in life cycle of any organism at regular interval it is called itereoparity.
• Although reproduction and metabolism are most important features of living beings, but when we compare these two the importance of metabolism comes first. It is due to this reason that we do not include viruses in living beings inspite of the fact that viruses have power of reproduction.
• All organisms consists of one or more cells, i.e. complex organized assemblage of molecules in the form of cell organelles enclosed within the biological unit membranes. So cellular structures is a defining property of living beings cells are made of living matter called protoplasm. Cell work together in hierarchical manner to form tissues organ and organ systems.
• All organisms respond to external stimuli which can be physical, chemical or biological and this property is called irritability. The stimulus response may be either simple (e.g. movement of an organism towards alight source) or quite complex such as responding to a complicated series of signals in mating ritual.
• Stimuli are perceived by sense organs in animals, but plants can response to external factors like light, water, temperature, pollutants etc. Both plants and animals respond to photo periods which influence their reproduction cycles as well. Actually, all living phenomena are due to underlying interaction.
•The properties of tissues are not present in the constituent cells, however, arise as a result of interactions among component cells. The appearance of new characteristic at a given level of organization is called emergence and these properties are called emergent properties.
• All life processes require energy and much of it is food obtained by the nutrition which is used as a source of this energy. The energy is released during the breakdown of certain energy rich compounds in the process of respiration.
• The energy is stored in ATP (Adenosine tri phosphate), a compound known to occur in all living cells and is referred to universal energy carrier.
Animals and some unicellular forms have the ability to move from place to place, called as locomotion. This is necessary to obtain their food, shelter and mate. Plants lacks locomotion. Nevertheless, some movements of part of body structure can be seen in plants. E.g. roots move in search of water under the soil or flower closes at night etc.VIII) EXCRETION
• Excretion is the removal of waste products from the body. Every living cell, whether it exists independent or as part of a multicellular organism, must eliminate waste products otherwise it might poison the body, if stored inside. • For example, the process of aerobic respiration produces a waste product CO2( carbon dioxide) and must be eliminated because it can be harmful in excess. Animals take in food during nutrition, this material breaks down during metabolism and need to be excreted.IX) HOMEOSTASIS
• It is a property of all living organisms. (homois–alike, statis – standing). Maintenance of a favorable dynamic constancy of internal environment despite changes in the external environment is called homeostasis.
• It is carried out by regulatory mechanisms which coordinate internal functions such as providing nutrients to cells and transporting substances. Some organisms attain, homeostasis be adapting to change in temperature, salinity and other aspects of environment.
• This process occurs at all levels i.e. from cellular level to ecosystem level. The term ‘homeostasis’ are used by Cannon ( 1932)
• Most of the known modes of homeostasis occur through
1. Maintenance of an internal environment.
2. Self –regulatory mechanisms using genetic clock.
3. Feedback system (in which the rate of the product formation is regulated by gathering information about the amount left out at any time). They generally involve switching on and switching off mechanism operating at different levels in all organism.
• Ageing is a progressive deterioration of structure and function of cells, tissues and vital organs so, that the ability to repair and resist disease declines. It leads to death. Death is stoppage of life activities in an individual due to degeneration of body parts and increase in entropy.
• In nature, death occurs due to ageing predation, accident or disease.
• Biological death occurs when brain and other body parts begin to degenerate due to non-availability of nutrients and oxygen.
• Clinical death is characterized by stoppage of vital functions like pulse, heart beat and breathing etc. Death is essential for keeping a population under check and recycling of minerals.