Question

How many (in number) of the following properties are the defining characteristics
of living organisms?
Growth, reproduction, metabolism, cellular organization, consciousness
(a) 2
(b) 3
(c) 4
(d) 5

Answer 1

Hint: Defining character refers to those characters only which are applicable exclusively for living organisms and in no way are observed in non-living organisms.

Complete answer

Characters observed in both living and non-living organism:

1. Growth: Growth can be described as an increase in the number of cells and an increase in
   the mass. These are the twin characteristics of growth. Among the living organisms, growth
   may be definite or indefinite. Non-living substances can also show growth, for example,
   accumulation of snow continuously can increase the overall mass of a snow-covered
   mountains or landslides can decrease the mass of a mountain. Hence it is not a defining
   character.

2. Reproduction: It refers to the production of new progeny. But in unicellular organisms like
   bacteria, amoeba, etc cell division is the mode of reproduction and it also leads to an
   increase in their numbers. Hence reproduction for unicellular organisms becomes
   synonymous to growth. Also, many organisms are sterile and cannot reproduce like mules,
   and worker bees, but they are living organisms. So it is not a defining character of living
   beings.

Characters observed only in living beings:

1. Metabolism: It feature is only seen in living cells and organisms. It is the total of all the
   biochemical reactions occurring within the cells which are necessary for ensuring the
   survival of the organism. It is not observed in non-living objects.

2. Cellular Organization: It is a defining feature of living cells only. Metabolic reactions occur
   within the living cells continuously, other processes like replication, transcription,
   translation, respiration, etc are highly organized stepwise processes that involve several cell
   organelles. This is not observed in non-living things and hence is a defining character.

3. Consciousness: The most complex feature in living beings in consciousness or being aware
   of their surroundings. All living beings respond to an external stimulus which may be
   physical, chemical, or biological.
   Hence we can conclude that metabolism, cellular organization, and consciousness are the
   defining characters of living organisms.

So, the correct answer is,” Three of the following characters including metabolism, cellular organization, and consciousness are the defining characters of living beings.”

Note: Even though consciousness is the defining character of living beings, self-consciousness is the property of only humans and no other organism exhibits self-consciousness. One exceptional case is when a human in a coma or when an individual is brain dead, then that an individual has no self-consciousness or consciousness. One thing we should remember that the properties exhibited by cells are not due to the constituent molecules but the interaction
between those molecules.