The Living World

In this class we will learn about

1) What is living ?

2) Salient features of living entities:

• Growth
• Reproduction
• Tactile sensation, Ability to sense the environment
• Metabolism
• Self-organize
• Mortality

3) Growth
Living things grow by increase in mass and increase in number of individuals/cells. In multi cellular organisms in particular, growth occur by cell division or increase in number of cells. Growth occurs continuously throughout life in plants, whereas, in animals, it occurs up to a certain age only. However, growing in certain body parts like nails, hair and replacement of lost cells, occurs throughout the life.
In unicellular organisms, growth can be observed under the microscope by simply counting the number of cells via in vitro experiment.
Non-living things like mountains, boulders, sand dunes also grow in size, but just by accumulating the material on their external surface. Thus, growth in living things is internal, while in non-living things, it is external. It is to be noted that a dead organism do not grow.
2. Reproduction
Reproduction, a characteristic of living organisms is the process of producing offsprings, possessing features similar to those of parents. In multicellular organisms, the mode of reproduction is generally sexual. Living organisms also reproduce by asexual means.
Some examples are given below
(i) Fungi spread and multiply fast by producing millions of asexual spores. Some fungi, the filamentous algae and the protonema of mosses multiply by fragmentation.
(ii) In yeast and Hydra, budding occurs to produce new organisms. While, in Planaria (flatworm),
regeneration of fragmented body parts occur. These parts inturn grow as a new organism.
(iii) Unicellular organisms like bacteria, algae and Amoeba reproduce by increasing the number of cells, i.e., through cell division (growth is synonymous with reproduction).
Some organisms like mules, sterile worker bees, infertile human couples, etc., do not reproduce. Hence, reproduction also cannot be an all-inclusive defining characteristic of living organisms.
3 Metabolism
Metabolism is an another characteristic and defining feature of all living things. The sum total of anabolic or constructive reactions (anabolism) and catabolic or destructive reactions (catabolism) continuously occurring inside the body is called metabolism.
Metabolism —> Anabolism + Catabolism Metabolism occurs in all unicellular and multi cellular organisms. Its two stages include, i.e., anabolism, the process of building up or synthesis of complex substances from simpler ones, e.g., Photo synthesis and catabolism, the process of breakdown of complex substances into simpler substances, e.g., Respiration, releasing waste outside.
Metabolic reactions can also be demonstrated outside the body in cell free systems, which are neither living nor non-living. Thus, these reactions in vitro are surely living reactions not living things. Hence, metabolism can be considered as a defining feature of all living organisms without exception.