Characteristics of Growth, Metabolic Reactions, and Response to Stimuli in Living Organisms

(i) Increase in mass and increase in number of individuals are twin characteristics of growth.

- Growth is a fundamental characteristic of living organisms that involves an increase in size, mass, and complexity.
- The increase in mass and number of individuals are two aspects of growth that are interrelated but not identical.
- Mass growth refers to an increase in the amount of material that constitutes an organism, such as proteins, lipids, and carbohydrates.
- Number growth, on the other hand, refers to an increase in the number of individuals in a population, which can occur through reproduction, immigration, or colonization.

(ii) Metabolic reactions can be demonstrated outside the body in isolated cell-free systems.

- Metabolism is the set of chemical reactions that occur within an organism to maintain life, such as energy production, nutrient utilization, and waste removal.
- Metabolic reactions can be studied in vitro, which means outside the body in a controlled environment.
- Cell-free systems are experimental models that use purified components of metabolism, such as enzymes, substrates, and cofactors, to mimic the reactions that occur in living cells.
- Cell-free systems are useful tools for studying the kinetics, regulation, and pathways of metabolism, as well as for developing drugs and therapies.

(iii) Response to stimuli is a defining property of living organisms.

- Living organisms are characterized by their ability to respond to changes in their environment, both internal and external, through various physiological and behavioral mechanisms.
- Stimuli can be physical, chemical, or biological factors that trigger a response in an organism, such as light, temperature, pH, nutrients, toxins, or pathogens.
- The response to stimuli is a dynamic process that involves signal transduction, gene expression, and cellular adaptation, and is essential for survival, growth, and reproduction.