Biology What is Life

### Topics Covered

star The Definition of Living
● Growth
● Reproduction
● Metabolism and Cellular Organisation
● Consciousness and Response to Stimuli
star Biodiversity

### The Definition of Living

color{blue} ul(mathtt ("Living Organisms")) : Living organisms are self-replicating, evolving and self-regulating interactive systems capable of responding to external stimuli.

\color{green} ✍️ \color{green} \mathbf(KEY \ CONCEPT)

● Conventionally, the definition of living includes those distinctive characteristics that are exhibited only by living organisms. Such characteristics distinguish the living from the non living.

●The basic characteristics involve growth, reproduction, ability to sense environment and mount a suitable response.

● Additional characteristics include metabolism, ability to self-replicate, self-organise, interact with each other and the environment and emergence.

color{purple}♣ color{Green} " Just for Fun"

mathtt "NASA" astrobiologists after much debate have agreed to the common definition of mathtt "living organisms" as " self-sustaining chemical systems capable of Darwinian evolution ".

### Growth

color{blue} ul(mathtt (GROWTH")) :The irreversible increase in the mass of the living body or/ and the number of cells.

'\color{green} ✍️ \color{green} \mathbf(KEY \ CONCEPT)

● Growth can be understood as an increase in mass or an increase in number or both.

● Increase in cell number occurs by means of color{Violet}"cell division", in both unicellular and multi cellular organisms. In plants, this growth by cell division occurs continuously throughout their life span (color{Violet}"Indeterminate Growth)". In animals, this growth is seen only up to a certain age color{Violet}"(Determinate growth)". Although in certain tissues, cell division does occur throughout the life to replace lost cells.

● In majority of higher animals and plants, growth and reproduction are color{Violet}"mutually exclusive events". Here cells stop growing during cell division and vice versa.

● If increase in body mass is considered as a criterion for growth, then non living objects like mountains, boulders and sand mounds grow too. However, this kind of growth exhibited by non-living objects is by accumulation of material on the surface (color{Violet}"Extrinsic Growth)". In living organisms, growth is from inside (color{Violet}"Intrinsic Growth)".

● Therefore, color{Brown}"Growth, cannot be taken as a defining property of living organisms". Only after carefully observing the conditions in which growth occurs, it is understood that it a characteristic of only living organisms as dead organisms do not grow.

### Reproduction

\color{green} ✍️ \color{green} \mathbf(KEY \ CONCEPT)

● In multicellular organisms, reproduction mostly refers to the production of gametes by meiosis and fertilization of gametes to form progeny possessing features more or less similar to those of parents, invariably referring to color{Violet}"sexual reproduction"; though some reproduce asexually too.

● In fungi and algae, multiplication and dispersal occurs by the means of the millions of color{Violet}"asexual spores" they produce. Whereas in lower organisms like yeast and hydra, reproduction occurs by means of color{Violet}"budding".

● The fungi, the filamentous algae, the protonema of mosses, all easily multiply by color{Violet}"fragmentation",

●In Planaria (flat worms), color{Violet}"true regeneration" results in the generation of a new organism from a lost body part.This regeneration is of color{Violet}"Morphallaxis type".

● In unicellular organisms like bacteria, unicellular algae or Amoeba, reproduction is synonymous with growth, i.e., increase in number of cells.

● Thus, in such organisms, the usage of such terms is not much clear. Moreover, there are many living organisms like mules, sterile, ,worker bees, infertile human couples, etc. which do not reproduce at all.,etc. Hence, color{Brown}"reproduction also cannot be an all-inclusive defining characteristic of living organisms".

color{purple}♣ color{Green} "Just for Curiousity"

● Regeneration (color{Violet}"Epimorphosis type") is a process in which only the lost part of the body is repaiired or regained e.g., Star fish, Lizard.

### Metabolism and Cellular organisation

color{blue} ul(mathtt ("METABOLISM")) :The sum total of all the chemical reactions occurring in a living organism or cell.

'\color{green} ✍️ \color{green} \mathbf(KEY \ CONCEPT)

● All living organisms are made of small chemicals and large polymers called color{Violet}"biomolecules"; of various classes, sizes and function. Thousands of chemical conversions are constantly and simultaneously being carried out among these molecules, called as color{Violet}"metabolic reactions".

● All multicellular and unicellular organisms exhibit metabolism which is defined as the sum total of all the chemical reactions occurring in a living organism or cell. Moreover, no non-living object exhibits metabolism.

● These consist of color{Violet}"catabolic" (destructive) reactions and color{Violet}"anabolic" (constructive) reactions.

● However, metabolic reactions can be demonstrated in cell-free systems outside the body.

● In this case, such isolated metabolic reactions, performed in a test tube outside the body are neither living nor non-living.

color{Brown}"Cellular organisation"

●AII living organisms are composed of cells.

● Some are composed of a single cell and are called as color{Violet}"Unicellular Organism" while other like us composed of many cells, are called color{Violet}"multicellular organisms".

●Unicellular organisms are capable of independent existence and performing essential functions of life .

● Anything less than a complete cell structure, fails to ensure independent living.

●Thus, cell is the color{Violet}"fundamental structural and functional unit" of all living organisms.

● Hence, color{Brown}"Metabolism and Cellular organisations are unexceptionally a defining feature of all living organisms".

### Consciousness or Response to Stimuli

color{blue} ul(mathtt ("CONSCIOUSNESS")) :It is the ability of all living organisms is their to sense their surroundings or environment and respond accordingly.

\color{green} ✍️ \color{green} \mathbf(KEY \ CONCEPT)

● The most obvious and technically complicated feature of all living organisms is their ability to color{Violet}"sense" their surroundings or environment and color{Violet}"respond" accordingly.

● These environmental stimuli could be physical, chemical or biological.

● We sense our environment through our color{Violet}"sense organs". Plants respond to external stimuli like light, water, temperature, other organisms, pollutants, etc. Animals use their sense organs to respond to their external environments.

● color{Violet}"All organisms", from the prokaryotes to the most complex eukaryotes can sense and respond to environmental
cues. So, all only organism can be said to be aware of their surroundings, thus making color{Brown}"consciousness the defining property of living organisms".

●Human beings are the only organisms who have color{Violet}"self consciousness"; the ability to be aware of himself/ herself.

●This does not hold true in case of the brain dead patients, who are lying in coma in hospitals virtually supported by machines which replace heart and lungs. Such patients have no self-consciousness