What is Life (Biology) NEET 2024 CBSE CLASS 12th

What is Life ?

It is the ulimate question that has troubled us since the time immemorial. In fact, the answer of thisquestion is a philosophical one.

 According to a scientist:

Life is a unique, complex organisation of ions and molecules which perform certain chemical reactions in order to bring about life activities.

In the living world, the organism as a whole is something more than the sum of its individual parts. That means, lite is not the property of any single part of an organism. It is the property of the organism as a whole.

For example, we know the exact chemical composition of the protoplasm but we can not create it by simpiy putting together all the elements in a mixture in the right proportion and supplying it with energy. 

mystery of life can be understood by comparing the living organisms with non-living matter.

 What is "Living'?

In order to have an understanding of living', we conventionally look for the characteristic  features exhibited by the living organisms which distinguish them from non-living matter.

All living organisms and non-living matter have one thing in common that they are made up of some basic elements (about 99% of the living matter is composed of C, H, O and N probably because these elements were abundant when life first began). The living and non-living things are also subjected to the same physical laws suoh as gravitation. magnetism, radiation, action and reaction, etc. Still, the living organisms are dfferent from non-living matter (eg, a rabbit differs from a stone) on the basis of some fundamental  characteristics such as, presence of protoplasm, cellular organisation, nutrition, metabolism growth reproduction, responses to environment, etc. Presence of protoplasmisthemosti mportantcharacacteristic feature of living  organisms which acts as the site of all vital activities. Therefore, Huxley (1863) called it pysical basis of life".

 Characteristics of iving Organisms

All living organisms have some common features which are listed below

1. Definite shape and size: All living organisms have a definite shape and size which may change within a very narrow.range. For example, a rat is a small animal which looks like a rat whereas a whale is a large animal which looks like a whale. Due to this property, the living organisms are called morphous (having morphological forms). On the other hand, the non-living matter is called amorphous (formless).

2.Organisation : Living organisms have different organ systems to perform different functions in a cordinated manner .Each organ system consists of different organs. Each organ consists of different kinds of

tissues which perfonm specific functions. The tissues are made up of similar or dissimilar cells. Each cell is regarded as basic structural unit of life. Furthermore, each cell is made up of different types of cellular organelles and sub-cellular inclusions to perform different intracellular functions. The differentiation of a

body into smaller units and sub-units arranged in an orderly manner in such a way to form a larger part is called organisation. Due to presence of an organisation, a living being is called an organism.

3. Growth and development (All living organisms grow): All living organisms show growth and development. These two life processes may be defined as follows:

Growth is a permanent and irreversible change in the size of a cell, an organ or whole organism usually accompanied by an increase in dry weight, increase in mass and increase in number of individuals.

Development is the sum of changes that occur in an organ or an organism during its life time.

Growth of living organisms occurs by cell division, cell enlargement and cell differentiation. Plant growth is somewhat different from growth of animals. In case of plants, the cell division occurs throughout

their life span, i.e., growth occurs throughout the life of plants. In case of animals, the growth stops after a Certain age, However, cell division occurs in certain tissues to replace lost cells. Unicellular organisms also grow by cell division. It can be easily observed by culturing unicellular organisms in artificial cultures and by counting the number of cells under the microscope per unit time. The majority of multicellular organisms

grow by cell division, cell differentiation and by the methods of reproduction. Thus, the processes of growth reproduction are mutually exclusive events.

if growth is considered simply increase in body mass as the only critiera, then non-iving objects also exhibit growth as they also show increase in body weight. For example, mountains, boulders and sand mounds

increase in size. However, such an increase of non-living objects is due to deposition of matter from outside In case of living organisms, the growth occurs from inside. Thus, growth alone cannot be considered as the only criteria defining the property of living organisms. It must be accompanied by the specific conditions under which it can be observed in all living organisms. It must be taken as granted that only 1iving organisms

exhibit growth the dead organism does not grow.

4. Reproduction. Living organisms do not survive indefinitely on this Earth. Every individual dies after attaining a certain age or due to predation, accident, disease or failure to replace molecules to run the machinery

of life. Death causes reduction in the number of individuals of a population which is restored by the addition of new individuals through reproduction. Thus, reproduction is not only production of a new copy like itself,

but it is a means for the survival of the population of species on this earth. It is well known that life comes from life. All the living organisms have a property to produce more of its own kind by the process called

reproduction. Bacteria, protozoa and other unicellular organisms mostly reproduce by splitting of their cells. It is a kind of asexual reproduction. Fungi multiply and spread very easily by producing millions of

asexual spores. The lower organisms reproduce asexually by budding and fission. Yeast and Hydra reproduce by budding. Bacteria reproduce by fission. In Planaria (flat worms), reproduction occurs by true regeneration  i.e the fragmented Organism regenerates the lost part of its body and becomes a new organism. Breaking of

filaments or mycelia and development of new organisms fron fragmented pieces is a type of reprouduction called fragmentation. It occurs in many filamentous algae, fungi and protonema of mosses. In case of unicellular

organisms Such as bacteria, unicellular algae (Chalmydomonas, Chlorella) or protozoans (Amoeba), the reprouction is synomymous with growth, i.e., increase in number of cells. However, most of the multcellular

Organisms produce sexually in which the sperm (male gamete) produced from male partner fuses with the egg (female gamete) of the female partner resulting in the development of new individual.

Furher more, there are certain organisms which do not reproduce. For example, mules, sterile workerbees, intertile human couples, etc., do not reproduce, Thus, reproduction alone can not be considered as the critena ol living organism. Of course, non-living organisms do not show reproduction or repiication.

5. Metabolism: Within cells, an increase in the number and diversity of molecules provides a greater opportunity for a vast array of biochemical reactions. It is estimated that the cells of living organisms possess about 5000 different types of chemicals which take part in many times more kinds of chemical reactions. All these chemical reactions going on in living cells are collectively termed as metabolism. Metabolism may be defined as the exchange of materials and energy within an organism or between an organism and its environment.

The metabolism is further divided broadly into two categories.

(a) Anabolism and (b) Catabolism.

(a) Anabolism: These are the biochemical reactions concerned with the 'building up' or 'synthesis' ofcomplex substances from simple ones. During this process, the free energy is stored in the form of potentialenergy, eg., synthesis of carbohydrates from CO² and H²0 in presence of light and chlorophyll during

photosynthesis.

6 CO²+ 12 H²O      light           C⁶H¹²O⁶+6O²+6H²O

                           ________>  Glucose

                           Chlorophyll

      

Other examples of anabolism are synthesis of starch from glucose, synthesis of lipids from fatty acidsand alcohols, synthesis of proteins from amino acids, etc.

(b) Catabolism: These are the biochemical reactions concerned mainly with the break down' oforganic nutrient molecules into simple substances. During this break down, the potential energy is converted into kinetic energy, The energy released during the process is usedup in various activities of the organism,

e.g, Oxidative break down of glucose (or other respiratory substrates) to release energy (i .e.respiration)

                        Enzymes

C⁶H¹²O⁶+60² __________> 6CO²+6H²O+Energy

Homeostasis .All living organism keep themselves stable by maintaining their internal conditions within certain boundaries under adverse temperature moisture level acidity and other physiological factors in order to continue their metabolic processes .It is called homeostasis.