Model web note

1. THE LIVING WORLD

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

A living organism is self-replicating, evolving and self-regulating interactive system capable of responding to external stimuli.

PROPERTIES OF LIVING ORGANISMS

1. Growth

• It is the increase in number & mass of cells by cell division.
• In plants, growth continues throughout their lifespan.
• In animals, growth is only up to a certain age. However, cell division occurs to replace lost cells.
• Basically, growth is the increase in mass & size. Thus non-living objects also grow (surface accumulation of material). So growth is not a defining property of living organisms.
• In living organisms, growth is from inside.

2. Reproduction

• It is the production of progeny having features similar to those of parents.
• Organisms reproduce asexually and sexually.
• In unicellular organisms, growth & reproduction are same because they reproduce by cell division.
• Many organisms do not reproduce (e.g. mules, worker bees, infertile human couples, etc). Hence, reproduction is not a perfect defining property of living organisms.

3. Metabolism

• It is the sum total of all biochemical reactions taking place inside a living system.
• It is the defining feature of living organisms.
• Metabolic reactions can be demonstrated outside the body in cell-free systems. Isolated metabolic reactions in vitro are not living things but are living reactions.

4. Cellular organization

• Organisms are made up of one or more cells.
• It is the defining feature of living organisms.

5. Consciousness

• It is the ability of organisms to sense their environment and respond to environmental stimuli (like light, water, temperature, other organisms, chemicals, pollutants, etc).
• All organisms are ‘aware’ of their surroundings. So, it is the defining property of living organisms.
• Human is the only organism having self-consciousness.

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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 is the study of identification, classification & nomenclature of organisms. Systematics (Latin ‘systema’ = systematic arrangement) deals with evolutionary relationships among organisms. Systema Naturae is the book written by Linnaeus. Basic processes of taxonomy Characterization: It is the understanding of characters of organisms such as external & internal structure, structure of cell, development process, ecological information etc. Identification: It is the correct description of the organism so that the naming is possible. Classification: It is the grouping of organisms into convenient categories (taxa) based on characters. Nomenclature (naming): It is the standardization of names of the organisms such that an organism is known by the same name all over the world. The system of naming with two components is called Binomial nomenclature. It is proposed by Linnaeus. Botanical names are based on the rules 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 in Latin or Latinised and written in italics. When handwritten, they are underlined separately. The first word is genus name (Generic name) and second word is the species name (specific epithet). E.g. Homo sapiens- Homo represents the genus name and sapiens represents the species name. The Genus name starts with capital letter and the species name starts with small letter. Name of the author (in abbreviated form) appears at the end of the biological name. 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 taxonomic category (rank). All categories together constitute a taxonomic hierarchy. A group of organisms occupying a particular category is called a taxon (pl. taxa). E.g. Class Mammalia. Each category or taxon represents a unit of classification.