The Living World

A Detailed Exploration for Class 11 Biology

1. What is 'Living'?

To understand the living world, we must first distinguish between living and non-living things. While it seems simple, defining 'life' precisely is complex. We identify living organisms based on certain characteristic features. Some are simply features, while others are defining properties (found in all living things and only in living things, without exception).

a) Growth

An irreversible increase in mass and number of cells. In living organisms, growth is intrinsic (from the inside). However, non-living objects like mountains can also "grow" by accumulation (extrinsic growth). Therefore, growth is a characteristic, but not a defining property of life.

b) Reproduction

The process of producing offspring. It can be sexual or asexual. While vital for a species' survival, not all individuals can reproduce (e.g., mules, sterile worker bees). Since there are living organisms that cannot reproduce, it is not a defining property of life.

Defining Property: Metabolism

Metabolism is the sum total of all chemical reactions (Anabolism + Catabolism) occurring inside a living body. All living organisms exhibit metabolism, while non-living objects do not. Therefore, Metabolism is a defining property of life.

Anabolism and Catabolism diagram
Metabolism: The balance of building up (Anabolism) and breaking down (Catabolism).

Defining Property: Cellular Organization

All living organisms are composed of one or more cells. The cell is the basic structural and functional unit of life. Anything less than a complete cell structure does not ensure independent living. Thus, Cellular Organization is a defining property of life.

Defining Property: Consciousness

The ability to sense environmental stimuli and respond to them. All organisms, from prokaryotes to humans, possess this ability. Humans also have a higher level of awareness called self-consciousness. Consciousness is a defining property of life.

Characteristic Defining Property? Reason
Growth No Non-living things can also grow (extrinsic growth).
Reproduction No Sterile organisms (e.g., mules) are living but cannot reproduce.
Metabolism Yes Occurs in all living things and never in non-living things.
Cellular Organization Yes All life forms are composed of cells.
Consciousness Yes All living organisms respond to external stimuli.

2. Diversity in the Living World (Biodiversity)

Our planet is home to an immense variety of life, referred to as biodiversity. The number of known and described species is between 1.7-1.8 million. To study this vast diversity, we need a system of naming and classification, the science of Taxonomy.

Binomial Nomenclature

This standardized naming system was given by Carolus Linnaeus. Each scientific name has two parts: the Generic Name (Genus) and the Specific Epithet (Species). For example, the name for mango is Mangifera indica.

Universal Rules of Nomenclature:

  1. Names are in Latin and written in italics.
  2. When handwritten, they are separately underlined.
  3. The Genus starts with a capital letter; the species starts with a small letter.
  4. The author's name can be abbreviated after the species name (e.g., Mangifera indica Linn.).

3. Taxonomic Categories

Classification involves a hierarchy of categories, where each category is called a taxon. All categories together form the taxonomic hierarchy.

Diagram of Taxonomic Hierarchy
The Linnaean system of classification arranges organisms in a hierarchy of taxa.
Kingdom → Phylum/Division → Class → Order → Family → Genus → Species

A helpful mnemonic: "Keep Pots Clean Or Family Gets Sick"

4. Taxonomical Aids

These are the tools and procedures that help in the identification and classification of organisms.

1. Herbarium

A storehouse of collected plant specimens that are dried, pressed, and preserved on sheets. Each sheet has a label with key information. It serves as a quick referral system.

A herbarium sheet with a pressed plant specimen.
A herbarium sheet provides a physical record of a plant species.

2. Botanical Gardens

Collections of living plants for reference and conservation. Each plant is labeled with its botanical name and family.

A lush botanical garden with pathways.
Botanical gardens are vital for both study and ex-situ conservation.

3. Museum

Collections of preserved plant and animal specimens. Specimens can be kept in preservative solutions, pinned in insect boxes, or stuffed (taxidermy).

4. Zoological Parks (Zoos)

Places where wild animals are kept in protected environments, similar to their natural habitats. They help in studying animal behavior and are centers for ex-situ conservation.

5. Key

An analytical tool for identifying organisms based on contrasting characters, arranged in a pair called a couplet. Each statement in the key is a lead. By choosing one character and rejecting another, one can identify the organism.

Other Aids