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What is Living: Following are the main characters of ‘‘living’’ – 1. GROWTH 2. REPRODUCTION Not the defining properties/characters/features
3. METABOLISM 4. CELLULAR ORGANISATION Defining properties/characters/features 5. CONSCIOUSNESS The character which has no exception is called as defining property of life.
~ Increase in mass or overall size of a tissue or organism or its parts is called growth. l Increase in mass and increase in a number of individuals are twin characters of growth.
~ Growth is an irreversible permanent increase in the size of an organ or its parts or even of an individual cell. l Growth is the result of metabolism when the synthetic reactions or anabolism is more than that of the destructive processes of catabolism, growth occurs.
~ In the reverse situation (Less anabolism, more catabolism) there will be ‘Degrowth’ or negative growth. l Growth is of two types :(a) Intrinsic growth:- This growth is from inside of the body of living organisms. (b) Extrinsic growth:- This growth is from outside i.e. accumulation of material on any body surface Nonliving exhibits this type of growth. l Growth is of two types — (a) Indeterminate growth = Unlimited growth. Growth which occurs continuously throughout their life span is indeterminate growth or unlimited growth. It occurs in plants and not in animals. (b) Determinate growth = Limited growth ® Growth which occurs only up to a certain age is determinate growth or limited growth. It occurs in animals. However, cell division occurs in certain tissues to replace lost cells.
~ In the majority of higher plants and animals, growth and reproduction are mutually exclusive events.
~ Because both living and nonliving exhibit growth so it can not be taken as defining property. l Growth from the inside (intrinsic growth) can be taken as defining property.
2. Reproduction :
Production of new individuals or progeny is called reproduction. l Reproduction in the case of multicellular organisms is the production of progeny possessing features more or less similar to those of parents. l Reproduction in the case of unicellular organisms like bacteria, unicellular algae, or Amoeba is increased in a number of cells. Means in unicellular organisms the growth and reproduction are synonyms or same.
~ Reproduction is not found in any nonliving object.
~ There are many living organisms that can/do not reproduce like mules, sterile human couples, worker bees.
~ So the reproduction also can not be taken as a defining characteristic of living organisms.
l Reproduction is of two types.
(A) Asexual Reproduction:- Reproduction in which gametic fusion or fertilization and meiosis are not involved is Asexual Reproduction. Many methods of asexual reproduction are there. (a) By Asexual spores:- In algae and fungi (b) By Budding:- In Yeast and Hydra (c) By Fragmentation or true regeneration :
(i) In filamentous algae, fungi, and the protonema of moss plants
(ii) Fragmented organisms regenerate the lost part of its body and become a new organism.
e.g. Planaria (Flatworms)
Note: Regeneration is a process in which only the lost part of the body is repaired or regained e.g., Starfish, Lizard.
(B) Sexual Reproduction:- Reproduction in which gametes are formed by meiosis and fertilization also takes place to form progeny is called sexual reproduction.
3. Metabolism :
~ The sum total of all the chemical reactions occurring in a living body is metabolism.
~ All living organisms, both unicellular and multicellular exhibit metabolism.
~ No non-living exhibits metabolism.
~ In this way metabolism is a defining character of living organisms because it has no exceptions.
~ Now we have known many of the chemical or metabolic reactions occurring in our body so we can demonstrate many of them in a cell-free medium or in a test tube in the lab.
~ The isolated metabolic reaction outside the body of an organism, performed in a test tube (in-vitro) is neither living nor nonliving.
~ These isolated reactions can not be regarded as living things, but they are definitely living reactions because they are similar to the reactions performing in our body.
~ Here we should not forget the fact that metabolism is the total sum of all the chemical reactions performing in the living body, it is not the sum of few or more living reactions.
~ All organisms are made of small or big chemicals. These chemicals perform thousands of reactions and form some other chemicals also in the bodies of living organisms.
~ All plants, animals, fungi, and microbes exhibit metabolism.
4. Cellular Organisation :
~ Cell is the basic unit of life. l All living organisms are composed of cells. Some are composed of a single cell and are called Unicellular Organisms while others like us composed of many cells 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, a cell is the fundamental structural and functional unit of all living organisms.
~ In this way the cellular organization is proved to be defining property of all living organisms.
5. Consciousness :
~ Ability to sense the surrounding environments and respond to these environmental stimuli is called consciousness.
~ 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, other organisms, pollutants, etc.
~ All organisms from the prokaryotes to complex eukaryotes show consciousness to environmental cues.
~ Some common examples of consciousness can be seen in organisms, like – Plants performs flowering in a particular season (photoperiodism), Some animals perform breeding in a particular season only (seasonal breeders), and all organisms handle the chemicals entering their bodies, etc.
~ When human is concerned a very high level of consciousness is found in human because of our very well developed nervous system and supreme level of skill of communication which is called self-consciousness.
~ Human is very fast to respond towards the external stimuli and even it can think or predict possible changes of surroundings also so it can prepare itself according to the surrounding situations. Further humans can even change its surrounding situations up to a limit so this topmost or climax level of consciousness is regarded as self-consciousness, which can not be seen elsewhere.
~ Self-consciousness is thought to be present only in humans.
~ The brain dead coma patient who is supported by machines that replace heart and lungs also has consciousness so it is living but it does not have self-consciousness because it has lost the co-ordination of organs of different body parts.
~ Means all the living phenomena are due to underlying interactions between different components of an individual or organ or tissue or cell.
~ Living organisms are self-replicating, evolving, and self-regulating interactive system capable of responding
to external stimuli. Adaptations and homeostasis are also very important characters of living organisms