The living has certain features that distinguish them from the non-living. All living things share some key characters like –growth, reproduction, metabolism, cellular organization and respond to external stimuli.
· Growth: It is defined as the increase in mass and the number of individuals. Amoeba grows by binary fission and higher organism by cell division.
· Reproduction: It is defined as the production of the off springs having features more or less similar to their parents. It is of two types -
Asexual reproduction: It does not involve two sexes. Eg: Fungi reproduce by asexual spore production and yeast (Saccharomyces cerevisiae) by budding.
Sexual reproduction: It involves the two sexes. Eg: Higher animals and human beings.
· Metabolism: The sum total of all the chemical reactions occurring in our body is called as metabolism. It is of two types-
Anabolism: Constructive phase of metabolism. Eg: Photosynthesis.
Catabolism: Destructive phase of metabolism. Eg: Respiration.
· Cellular organization: All living organism is made up of cell having protoplasm and nucleus (exceptional virus). The cell organelles and various components of body act in coordination. Group of similar cells gives rise to tissue, group of tissue forms organ, group of organ form the organ system and group of organ system the organism or individual.
Cell--- Tissue --- Organs--- Organ system---- Organism (individual).
· Respond to external stimuli: Living organism respond to change in environment or their surroundings. They respond to various stimuli like chemical, physical, mechanical etc. plant respond to light, water, temperature etc. and animals respond by their five sense organs (eyes, nose, tongue, ears and skin).