The Living World

The Living World: The world is full of living and non-living things. But biology deals with living beings and their functions. There is a great diversity of living beings. The difference between living beings and non-living matter was perceived by an early human being. A detailed system of identification, nomenclature began later. This led to the recognition of sharing similarities among organisms. The man could recognize that living organisms are related to each other. 

Various forms of living organisms are found in different types of habitats in the world like the ocean, air, fresh-water, forests, cold mountains, deserts, hot water springs, etc. which make us think about the vividness of living organisms and makes us differentiate between the living and the non-living organisms. 

It is very difficult to define ‘living organisms’ and to characterize them, few of these characteristics are as under: 

  1. Living things have cellular organization. 
  2. They undertake the metabolism of food. 
  3. Living organisms can grow. 
  4. Living organisms give rise to the next progeny (Generation). 
  5. Living organism’s response to stimuli (Irritability) 

It is estimated that more than 5 million species are present on earth. Of these, about 1.7 million species are known and described. Every year several new species are described and added to the list. The study of all these organisms is nearly impossible, they are classified for convenience.

Diversity In The Living World 

It is known that more than 5 million living species exist on earth out of these approximately 1.5 million species are known and are described. Every year several new living species are reported by scientists in the world and are added to the list.

They are coming from all corners of all sources and to study all of them is nearly impossible. 

For convenience, depending upon their occurrence and physical forms they are classified.  Their diversified sources like deserts, oceans, forests and cold zones, etc. make their study very difficult, but without their detailed study.  We may not be able to utilize them suitably for our proper survival. 

Binomial Nomenclature 

It is universally accepted that plants and animals around use are known various local names for convenience, it is also agreed that they should be known by titles uniform throughout so that there is no confusion about their identity. Scientists have finalized the procedure to assign their names ICBN (International Code for Botanical Nomenclature) and ICZN (International Code for Zoological Nomenclature) were entrusted to name the plants and animals respectively. The scientific name’s the scientist entrusted have two components i.e. Generic name and specific epithet. 

Examples: Saraka indica (Ashok); 

Here, Saraka is a generic name; and indica is its specific epithet. 

Rules of Nomenclature: 

  1. Each scientific name has two parts. The first word represents the genus and the second represents the specific epithet. 
  2. The words of the name should be separately underlined when handwritten and should be in italics when printed.
  3. The generic name should start with a capital letter and the specific epithet should start with a small letter. 
  4. The names should be either Latin or Latinized. 
  5. The name of the author appears at the end of the scientific name is an abbreviated form e.g. Saraca indica Linn. It indicates that this species is first described by Linnaeus.


 It is the arrangement of organisms in specific groups or categories based on certain characters. These categories are called taxa


It is the science of identification, nomenclature, and classification of organisms based on an external and internal structure with cell structure, development process, and ecological information.


It is the study of organisms concerning identification, nomenclature. classification and evolutionary relationship. 

The descending order of taxa used in classification is – Kingdom, Phylum or Division, Class, Order, Family, Genus, and Species. 

  1. Kingdom: Kingdom comprises various phyla of animals and various divisions of plants. 
  2. Phylum/Division: Phylum in animals and Division in plants includes related classes. 
  3. Class: Several related orders are induced in a class. 
  4. Order: The order includes several related families. 
  5. Family: Family is a group of related families. 
  6. Genus: Genus is a group of related species which have co-related characters. 
  7. Species: It is the basic unit in classification. The members of a species are closely related, derived from a common ancestor, and can interbreed to produce fertile offspring. 

This system was finalized by Carolus Linnaeus and is known as Binomial nomenclature. It is followed by all biologists of the world. 

Besides the above categories, taxonomists developed sub-categories in the hierarchy to help more precise placement of various taxa.

Five Kingdoms of Life 

Five kingdoms of life and their salient features are as follows, namely Monera, Protista, Fungi, Animalia, Plantae, and Viruses. 

(1) Monera: 

The monerans are the simplest form of the entire living organism. They are extremely small.  These organisms are prokaryotic. The organisms are simple unicellular and microscopic.  The cell wall is present in some organisms and absent in others. Their cell wall is not made up of cellulose. They do not possess a definite nucleus and lack cell organelles.  Some organisms can synthesize their food while others show the heterotrophic mode of nutrition. Examples: Bacteria, mycoplasma, blue-green algae, etc. 

(2) Protista: 

A kingdom or large grouping that comprises mostly single-celled organisms such as the protozoa, simple algae and fungi, slime molds, and bacteria. They are now divided into thirty phyla, and some have both plant and animal characteristics. This group consists of many types of unicellular eukaryotic organisms. Salient features of Protista are as under: 

They are simple and unicellular eukaryotic organisms.

  • They have a defined nucleus and membrane-bound organelles.
  • Locomotion occurs with the help of hair-like structures called cilia e.g. Paramecium, whip-like structures called flagella e.g. Euglena, or Pseudopodia e.g. Amoeba.
  • Few protests synthesize their own food i.e. they are autotrophic, while others show the heterotrophic mode of nutrition. 

(3) Fungi: 

Fungi are simple eukaryotic, lacking chlorophyll with salient features such as:

  • The cell wall is present. It is made up of tough complex sugar called chitin.
  • They are non-photosynthetic. Their mode of nutrition is heterotrophic. Most of them are saprophytes.
  • Most fungi are multicellular (exceptionally yeast is a unicellular fungus).
  • The body of the multicellular filamentous fungus is called mycelium, which is composed of several thread-like structures called hyphae.
  • Few fungi live in a symbiotic relationship with blue-green algae as Lichens.  Examples: Mucor, Aspergillus, Penicillium, Rhizopus. etc. 

(4) Animalia: 

This group consists of all multicellular eukaryotes which do not possess a cell wall. Their salient features are:

  • These are heterotrophic they do not prepare their food.
  • They are multicellular eukaryotes.
  • The cell wall is absent.
  • They show very limited growth which stops after maturity.
  • Kingdom Animalia is further classified into Vertebrates and Invertebrates. 

(5) Plantae:

This group consists of multicellular Eukaryotic organisms,

  • They are autotrophic i.e. they prepare their food in presence of sunlight, Chlorophyll, water, and carbon dioxide by photosynthesis.
  • They possess a cell wall made up of cellulose.
  • Kingdom planate are further classified into Thallophyta, Bryophytes, Pteridophyta, Gymnosperm, and Angiosperm. 

(6) Viruses: 

Viruses are ultra-microscopic, non-cellular living particles, composed solely of a nucleic acid (DNA or RNA) core, surrounded by a protein envelope called a capsid. 

Viruses are devoid of the sophisticated enzymatic and biosynthetic machinery essential for independent activities of cellular life. Therefore, they can grow only inside suitable living cells. That is why; they are cultivated in the laboratory only inside living cells. 

Viruses do not increase in size. They can pass through filters, through which bacteria cannot pass. 

A virus is called either ‘DNA virus’ or ‘RNA virus’ depending on whether it contains the nucleic acid DNA or RNA. A virus cannot have both DNA and RNA. 

Structure of Viruses: 

The viruses, which infect bacteria, are called ‘bacteriophages’ or ‘phages’. The bacteriophages were first described by Twort and d’Herelle in 1915. 

The term bacteriophage, (means to eat bacteria), was coined by d’Herelle because of the ability of these viruses to destroy the infected bacteria cells through lysis. Bacteriophages exhibit notable variability in their size, shape, and complexity of structure.

Structure of T-even bacteriophage
Fig.1: Structure of T-even bacteriophage

The functions of structural components of the virus are: 

  1. Capsid (Protein Coat): Protection of nucleic acid from destruction by DNase. 
  2. Nucleic Acid Core: Phage genome carrying genetic information necessary for replication of new phage. 
  3. Spiral Protein Sheath: Retracts, so that nucleic acid can pass from capsid into host cell’s cytoplasm. 
  4. End Plate and Tail Fibres: Attachment of phage to specific receptor sites on a susceptible host’s cell wall.
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