What is Living?

• When we try to define ‘living’, we conventionally look for distinctive characteristics exhibited by living organisms.

• Growth, reproduction, ability to sense the environment and mount a suitable response come to our mind immediately as unique features of living organisms.

• One can add a few more features like metabolism, ability to self-replicate, self-organize, interact and emergence to this list.

Characteristics of Living Organisms:

Growth:

• All living organisms grow.

  • It is regarded as an intrinsic property of all living organisms.

    • A multicellular organism grows by cell division.

    • In plants, this growth by cell division occurs continuously throughout their life span.

    • In animals, this growth is seen only up to a certain age.

  • However, cell division occurs in certain tissues to replace lost cells.

    • Unicellular organisms grow by cell division.

• In the majority of higher animals and plants, growth and reproduction are mutually exclusive events.

• One must remember that increase in body mass is considered growth.

  • Non-living objects also grow if we take an increase in body mass as a criterion for growth.

    • Mountains, boulders and sand mounds do grow.

      • However, this kind of growth exhibited by non-living objects is by the accumulation of material on the surface.

  • In living organisms, growth is from the inside.

• Growth, therefore, cannot be taken as a defining property of living organisms.

Reproduction:

• In multicellular organisms, reproduction refers to the production of progeny possessing features more or less similar to those of parents.

• Invariably and implicitly we refer to sexual reproduction.

• Organisms reproduce by asexual means as well.

  • Fungi multiply and spread easily due to the millions of asexual spores they produce.

  • In lower organisms like yeast and hydra, we observe budding.

  • In Planaria (flatworms), we observe true regeneration, i.e., a fragmented organism regenerates the lost part of its body and becomes a new organism.

  • The fungi, the filamentous algae, and the protonema of mosses, all easily multiply by fragmentation.

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

• But, there are many organisms which do not reproduce (mules, sterile worker bees, infertile human couples, etc).

Hence, reproduction also cannot be an all-inclusive defining characteristic of living organisms.

Metabolism:

• All living organisms are made up of chemicals.

• These chemicals, small and big, belonging to various classes, sizes, functions, etc., are constantly being made and changed into other biomolecules.

  • Such thousands of chemical reactions which occur inside the living organisms during various processes like photosynthesis, respiration etc. help them to maintain their living state.

• The sum total of all the chemical reactions occurring in our body is Metabolism.

  • All living organisms from unicellular to multicellular i.e. from bacteria to plants, and animals possess metabolism of their own kind.

  • The metabolic reactions which involve the synthesis of molecules are called Anabolism.

• Example: synthesis of proteins from amino acids.

  • The metabolic reactions in which large molecules are broken down into smaller ones are known as Catabolism.

• Example: Sugars are broken down into molecules of water and carbon dioxide to liberate energy i.e. ATP.

  • No non-living object exhibits metabolism.

• Metabolic reactions can be demonstrated outside the body in cell-free systems.

  • An isolated metabolic reaction(s) outside the body of an organism, performed in a test tube is neither living nor non-living.

• Hence, while metabolism is a defining feature of all living organisms without exception, isolated metabolic reactions in vitro are not living things but surely living reactions.

• Hence, the cellular organisation of the body is the defining feature of life forms.

Consciousness:

• The most obvious and technically complicated feature of all living organisms is this ability to sense their surroundings or environment and respond to these environmental stimuli which could be physical, chemical or biological.

• Plants respond to external factors like light, water, temperature, other organisms, pollutants, etc.

  • All organisms, from the prokaryotes to the most complex eukaryotes can sense and respond to environmental cues.

• Photoperiod affects reproduction in seasonal breeders, both plants and animals.

  • A human being is the only organism who is aware of himself, i.e., has self-consciousness.

• Consciousness, therefore, becomes the defining property of living organisms.

Diversity in Living Organisms:

• The number of species that are known and described ranges from 1.7-1.8 million.

• This refers to biodiversity or the number and types of organisms present on earth.

  • Each different kind of plant, animal or organism represents a species.

Nomenclature:

• The naming of living organisms is such that a particular organism is known by the same name all over the world.

  • Nomenclature or naming is only possible when the organism is described correctly and we know to what organism the name is attached. This is identification.

    • For plants, scientific names are based on agreed principles and criteria, which are provided in International Code for Botanical Nomenclature (ICBN).

    • Animal taxonomists have evolved the International Code of Zoological Nomenclature (ICZN).

  • The scientific names ensure that each organism has only one name.

    • Description of any organism should enable the people (in any part of the world) to arrive at the same name.

    • They also ensure that such a name has not been used for any other known organism.

Principles to provide scientific names to known organisms:

• Each name has two components – the Generic name and the specific epithet.

• Biological names are generally in Latin and written in italics.

  • The first word in a biological name represents the genus while the second component denotes the specific epithet.

  • Both the words in a biological name, when handwritten, are separately underlined or printed in italics to indicate their Latin origin.

  • The first word denoting the genus starts with a capital letter while the specific epithet starts with a small letter.

• Example: Mangifera indica.

This system of providing a name with two components is called Binomial nomenclature.

• This naming system given by Carolus Linnaeus is being practised by biologists all over the world.

  • The name of the author appears after the specific epithet, i.e., at the end of the biological name and is written in an abbreviated form, e.g., Mangifera indica Linn.

  • It indicates that this species was first described by Linnaeus.

Classification:

• Classification is the process by which anything is grouped into convenient categories based on some easily observable characteristics.

  • The scientific term for these categories is taxa.

  • Taxa can indicate categories at very different levels.

• Examples: ‘animals’, ‘mammals’ and ‘dogs’ represent taxa at different levels.

  • Hence, based on characteristics, all living organisms can be classified into different taxa.

  • This process of classification is taxonomy.

• External and internal structure, along with the structure of cells, development process and ecological information of organisms are essential and form the basis of modern taxonomic studies.

• Systematics is the study of different kinds of organisms and their diversities, including the relationships among them.

  • The word systematics is derived from the Latin word ‘systema’ which means the systematic arrangement of organisms.

  • Linnaeus used Systema Naturae as the title of his publication.

Taxonomic Categories:

• Here, all categories together constitute the taxonomic hierarchy.

  • Each category, referred to as a unit of classification, represents a rank and is commonly termed a taxon.

  • These taxonomic groups/ categories are distinct biological entities and not merely morphological aggregates.

• Example: Insects represent a group of organisms sharing common features like three pairs of jointed legs.

• Taxonomical studies of all known organisms have led to the development of common categories such as kingdom, phylum or division (for plants), class, order, family, genus and species.

Species:

• A group of individual organisms with fundamental similarities collectively form a species.

  • One can distinguish one species from the other closely related species based on the distinct morphological differences.

• Examples: Mangifera indica, Solanum tuberosum (potato) and Panthera leo (lion).

  • Panthera has another specific epithet called tigris

  • Solanum includes species like nigrum and melongena.

  • Human beings belong to the species sapiens which is grouped in the genus Homo.

    • The scientific name thus, for human beings, is written as Homo sapiens.

Genus:

• A group of related species which has more characters in common in comparison to species of other genera (genera are aggregates of closely related species).

• Example:

  • Potato and Brinjal are two different species but both belong to the genus Solanum.

  • Lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) with several common features are all species of the genus Panthera.

Family:

• A group of related genera with still less number of similarities as compared to genus and species.

• Families are characterized on the basis of both vegetative and reproductive features of plant species.

• Example -

  • Among plant genera Solanum, Petunia and Datura are placed in the family Solanaceae.

  • Genus Panthera, comprising lion, tiger, and leopard is put along with genus, Felis (cats) in the family Felidae.

    • But a cat and a dog are separated into two different families Felidae and Canidae, respectively.

Order:

• It is the assemblage of families which exhibit a few similar characters.

• Example:

  • Plant families like Convolvulaceae, Solanaceae are included in the order Polymoniales mainly based on the floral characters.

  • The animal order, Carnivora, includes families like Felidae and Canidae.

Class:

• A group of related orders constitute a class.

• Example:

  • Order Primata comprising monkey, gorilla and gibbon is placed in class Mammalia along with order Carnivora which includes animals like tiger, cat and dog.

Phylum:

• Classes comprising animals like fishes, amphibians, reptiles, birds along with mammals constitute the next higher category i.e. Phylum.

  • They have common features like the presence of notochord and dorsal hollow neural system and constitute Phylum Chordata.

• Classes with a few similar characters are assigned to a higher category called Division.

Kingdom:

• Examples:

  • All animals belonging to various phyla are assigned to the highest category called Kingdom Animalia in the classification system of animals.

  • The Kingdom Plantae, is distinct and comprises all plants from various divisions.

Taxonomical Aids

Herbarium:

• It is a storehouse of collected plant specimens that are dried, pressed and preserved on sheets.

  • These specimens, along with their descriptions on herbarium sheets, also carry a label providing information about the date and place of collection, English, local and botanical names, family, collector’s name, etc.

• Herbaria also serve as quick referral systems in taxonomical studies.

Botanical Gardens:

• These have collections of living plants for reference.

• Plant species in these gardens are grown for identification purposes and each plant is labelled indicating its botanical/scientific name and its family.

• Examples:

  • Kew (England)

  • Indian Botanical Garden (India)

  • National Botanical Research Institute (India).

Museum:

• They have collections of preserved plant and animal specimens for study and reference and are set up in colleges, educational institutions and schools.

  • Specimens are preserved in containers or jars in preservative solutions.

  • Plant and animal specimens may also be preserved as dry specimens.

  • Insects are preserved in insect boxes after collecting, killing and pinning.

  • Larger animals like birds and mammals are usually stuffed and preserved.

• Museums often have collections of skeletons of animals too.

Zoological Parks:

These are the places where wild animals are kept in protected environments under human care and which enable us to learn about their food habits and behaviour.

Key:

• The keys are based on the contrasting characters generally in a pair called a couplet.

  • Each statement in the key is called a lead.

  • Separate taxonomic keys are required for each taxonomic category such as family, genus and species for identification purposes.

• Keys are generally analytical in nature.