Which of the following aspects is an exclusive characteristic of living organisms?

The correct answer is: increase in mass is from inside only

To understand why this is an exclusive characteristic of living organisms, let us analyze the concept of growth and how it differs between living beings and non-living objects:
1. Growth as a Parameter: Growth is defined as an irreversible increase in mass, volume, or the number of cells. Both living organisms and certain non-living things can exhibit an increase in mass.
2. Growth in Non-Living Objects: Non-living structures, such as mountains, crystals, sand dunes, and snowballs, can increase in mass. However, this growth is external, occurring due to the accretion or accumulation of material on their outer surface.
3. Growth in Living Organisms: In contrast, living organisms grow from within (a process known as intussusception). This internal growth occurs because of cellular division, cell enlargement, and the synthesis of new protoplasmic and apoplasmic materials inside the cells. Therefore, an increase in mass that occurs strictly from the inside is a unique and exclusive defining feature of living systems.

Let us briefly review why the other options do not fit the description of an exclusive characteristic of living organisms:
- Isolated metabolic reactions occur in vitro: While metabolic reactions are characteristic of life, isolated metabolic reactions carried out in a test tube (in vitro) are not living organisms themselves; they are merely living reactions in a non-living environment.
- Increase in mass by accumulation of material both on surface as well as internally: Living organisms do not grow by accumulating material on their outer surface.
- Perception of events happening around their environment and their memory: While responsiveness to environmental stimuli (consciousness) is a defining feature of life, not all living organisms (such as simple unicellular organisms or plants) possess a complex nervous system with "memory" in the conventional sense. Thus, the specific structural growth from within remains the most fundamental physical differentiator for growth.