The Living World

Life is a unique, complex organization of molecules, expressing through chemical reactions which lead to growth, development, responsiveness, adaptation & reproduction.

PROPERTIES OF LIVING ORGANISMS

1. Growth

It is the increase in mass and number of cells. In plants, growth by cell division occurs continuously throughout their life span. In animals, growth is only up to a certain age. However, cell division occurs in certain tissues to replace lost cells. Non-living objects grow by accumulation of material on the surface. In living organisms, growth is from inside.

2. Reproduction

It is the production of progeny similar to those of parents. Organisms reproduce asexually and sexually. 

Growth is the increase in cell number or mass. Hence in unicellular organisms, growth & reproduction are same. 

Many organisms do not reproduce (e.g. mules, worker bees, infertile human couples, etc). Hence, reproduction is not a perfect defining characteristic of living organisms.

3. Metabolism

It is the sum total of all biochemical reactions taking place inside a living system.
Isolated metabolic reactions in vitro are not living things but are living reactions.

4. Cellular organization

They are made up of cells (unicellular or multicellular). The organization is as follows:

Molecules → cell organelles → cells → tissues → organs → organ system → organism.

5. Consciousness

It is this ability of organisms to sense their environment and respond to the environmental stimuli (like light, water, temperature, other organisms, chemicals, pollutants, etc).

All organisms therefore, are ‘aware’ of their surroundings.

Human is the only organism having self-consciousness. 

Living organisms are self-replicating, evolving and self-regulating interactive systems capable of responding to external stimuli.

DIVERSITY IN THE LIVING WORLD

The number and types of organisms present on earth refer to biodiversity. 

Number of species described is 1.7-1.8 million.

Taxonomy (Systematics) is the study of identification, classification, nomenclature & documentation of organisms.

Systematics (Latin ‘systema’) means systematic arrangement of organisms.

Systema Naturae is the book written by Linnaeus.

Processes of taxonomy

Characterization: It is the understanding of characters of organisms such as external and internal structure, structure of cell, development process, ecological information etc.

Identification: Nomenclature is only possible when the organism is described correctly and we know to what organism the name is attached to. This is identification.

Classification: It is the grouping of organisms into convenient categories (taxa) based on characters.

Nomenclature (naming): It is the providing of standardized names to the organisms such that a particular organism is known by the same name all over the world.

The system of naming with two components (Binomial nomenclature) is proposed by Carolus Linnaeus.

Botanical names are based on the rules provided in International Code for Botanical Nomenclature (ICBN).

Zoological names are based on International Code for Zoological Nomenclature (ICZN).

Universal rules of Binomial nomenclature

• Scientific names are generally in Latin and written in italics.
• The first word is genus name (Generic name) and second word is the species name (specific epithet).
• When handwritten, the names are underlined.
• The names are printed in italics.
• The first name (Genus) starts with capital letter and the second name (species) starts with small letter. E.g. Homo sapiens- Homo represents the genus name and sapiens represents the species name.
• 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.

TAXONOMIC CATEGORIES

Classification involves hierarchy of steps in which each step represents a rank (taxonomic category or taxon). All categories together constitute the taxonomic hierarchy.

Each taxon represents a unit of classification.

Taxonomic hierarchy with example

Kingdom               -          Animalia

    ↑

Phylum                 -           Chordata

(Division in case of plants)

    ↑

Class                     -          Mammalia

    ↑

Order                     -          Primata

    ↑

Family                   -          Hominidae

    ↑

Genus                   -          Homo

    ↑

Species                -          sapiens

Species: It is a group of individual organisms with fundamental similarities. It is the lowest category. E.g.

Generic name	Specific epithet	Common name
Mangifera	indica	Mango
Solanum	tuberosum	Potato
Solanum	nigrum	Nightshade
Panthera	leo	Lion
Panthera	tigris	Tiger
Homo	sapiens	Modern man

Genus: It is the aggregates of closely related species.

E.g. Potato, tomato & brinjal are species of genus Solanum. Lion (Panthera leo), leopard (P. pardus) & tiger (P. tigris) are species of genus Panthera. This genus differs from another genus Felis which includes cats.

Family: It is a group of related genera with less number of similarities as compared to genus and species. E.g.

Family Solanaceae includes Genus Solanum, Genus Petunia and Genus Datura.

Family Felidae includes Genus Panthera and Genus Felis.

  Order: It is the assemblage of related families. E.g.

Order Polymoniales includes Family Convolvulaceae and Family Solanaceae.

Order Carnivora includes Family Felidae & Family Canidae. 

Class: It is the assemblage of related orders. E.g.

Order Primata, Carnivora etc is placed in class Mammalia.

Phylum: It is the assemblage of related classes. E.g.

Classes Amphibia, Reptilia, Aves, Mammalia etc come under phylum Chordata.

Kingdom: The assemblage of various phyla. It is the highest category. E.g. Kingdom Plantae, Kingdom Animalia etc.

Organisms with their taxonomic categories

Common name	Man	Housefly	Mango	Wheat
Biological name	Homo sapiens	Musca domestica	Mangifera indica	Triticum aestivum
Genus	Homo	Musca	Mangifera	Triticum
Family	Hominidae	Muscidae	Anacardiaceae	Poaceae
Order	Primata	Diptera	Sapindales	Poales
Class	Mammalia	Insecta	Dicotyledonae	Monocoty-ledonae
Phylum/Division	Chordata	Arthropoda	Angiospermae	Angiospermae

TAXONOMIC AIDS

a. Herbarium

It is a store house (repository) of collected plant specimens that are dried, pressed and preserved on sheets and are arranged according to universally accepted classification.

The herbarium sheets are labelled with information about date and place of collection, English, local and botanical names, family, collector’s name etc.

b. Botanical gardens

These are specialized gardens having collections of living plants for reference and identification purposes.
Each plant is labeled with its botanical name and family.

Famous botanical gardens:

• Royal Botanical Garden at Kew (England).
• Indian Botanical Garden, Howrah (India).
• At National Botanical Research Institute, Lucknow (India).

c. Biological Museum

Museum is a collection of preserved plants and animals for study and reference. 

A museum contains

• Specimens preserved in preservative solutions in containers or jars.
• Plant and animal specimens preserved as dry specimens.
• Insects preserved in insect boxes after collecting, killing and pinning.
• Stuffed larger animals like birds and mammals.
• Collections of animal skeletons.

d. Zoological Parks (Zoos)

These are the places where live wild animals are kept in protected environments under human care. It enables to learn about their food habits and behaviour.

e. Key

It is the device used to identify each species in a group of organisms based on similarities and dissimilarities.

The keys are based on the contrasting characters generally in a pair called couplet. It represents the choice made between two opposite options. This results in acceptance of only one and rejection of the other. Each statement in the key is called a lead.

Flora, manuals, monographs & catalogues

These are some other means of recording descriptions. They also help in correct identification.

Flora contains the actual account of habitat and distribution of plant species of a given area.

Manuals help in providing information for identification of names of species found in an area.

Monographs contain information on any one taxon.