Living and non-living organisms can be differentiated by characteristic features such as growth and reproduction. Living organisms, whether multi-cellular or unicellular, grow by cell division. However, everything that grows is not a living organism. Plants grow throughout their life, but rocks too grow in size if sand gets deposited over it. Growth is thus a non-definitive characteristic.
Reproduction and growth are synonymous in unicellular organisms, as both lead to an increase in the number of cells. But, in multi-cellular organisms, reproduction leads to the creation of an offspring. These organisms reproduce either sexually or asexually. Sexual reproduction can be seen in higher organisms like man and animals. Asexual reproduction can be seen in lower organisms, through budding as seen in yeast or through fragmentation as in flatworm. However, certain organisms, such as sterile worker bees are not capable of reproducing, but it doesn’t mean that they are non-living. Reproduction is thus, a non-definitive characteristic.
The three definitive characteristics of living organisms are metabolism, cellular organisation and consciousness. The conversion of chemicals, produced in the living organisms into biomolecules is called a metabolic reaction.
The sum total of all such reactions is known as metabolism, which is a definitive feature of living organisms. Cellular organisation is not observed in the non-living organisms and hence, it is also a definitive feature in all life forms. All living organisms sense and respond to environmental stimuli such as light, water and temperature. This is known as consciousness and is a characteristic feature of living organisms.