What is living

The beings or objects showing growth, development, responsiveness, adaptation, and reproduction are called living beings.

Important characteristics of living beings

Cellular structure

All living beings are made up of a number of cells. These cells contain protoplasm or living matter.
The cellular structure is the defining property of living beings as all the processes are related to the structure of cells.
Protoplasm and cellular structure are absent in viruses.

Metabolism

It refers to all the chemical reactions, occurring within the body of living organisms. These reactions can be grouped into two types i.e. anabolism and catabolism.
Anabolism: Processes or reactions including the formation of complex substances from simpler ones. E.g. photosynthesis
Catabolism: Processes or reactions including the breakdown of complex substances into simpler ones. E.g. respiration

Growth

It refers to an irreversible increase in the mass of an individual.
In plants, growth continues throughout life due to the presence of meristematic tissue. In animals, growth is definite.
Growth in unicellular organisms is through cell division.

Reproduction

Organisms can produce new individuals that are similar to them through a process called reproduction. It is required for the perpetuation of living organisms.

Consciousness/ Response to stimuli

It refers to the awareness of surroundings by showing a response to an external stimulus. All organisms can sense and respond to the environment.
The ability of organisms to react to an external stimulus is called irritability.
Higher animals possess sense organs for sensing the stimuli. Plants do not have sense organs but they respond to stimuli by showing some movement.

Movement

All organisms show some kind of movement.
Animals show locomotion.
Plants, though do not show locomotion, but exhibit movements.

Homeostasis

It refers to maintaining a favorable internal environment despite fluctuations in the external environment.
Livings beings have a self-regulated system to adjust and maintain the internal environment.

Adaptations

Adaptations refer to changes acquired by an organism over a period of time in order to survive in a particular environment.

Death

It refers to the stoppage of all vital activities due to ageing, disease, accident, etc. Complete death is absent in unicellular organisms.
They multiply by fission and the parent cell continues living as a daughter cell however, it loses its own identity.
Unicellular organisms are thus immortal.

Biodiversity and Need for classification

There is a wide variety of living organisms differing in their size, colour, behaviour, habits etc. This diversity is referred to as biodiversity.
The term biodiversity was coined by Edward Wilson.
Currently, there are some 1.7-1.8 million species known to mankind.
It is believed that this is 2 % of all the species occurring on earth.

Need for classification

Due to such a large diversity of life forms, a proper system of classification is required because it is not possible to study every organism.
We can study, some members of a group to ultimately know about the whole group.
Without any classification system, organisms cannot be identified.
classification also helps in knowing the relationships amongst different groups of organisms.