Class 11 Biology

Living Beings

Characteristics of Living Beings


All living beings grow in size. Some non-living beings also grow in size. While growth in a non-living thing happens because of accumulation of matter from outside, growth in living beings happens because of internal processes, i.e. due to formation of new cells. While plants usually show indeterminate growth, the animals show definite growth. Growth of an animal stops after a certain size.


Reproduction is an important feature of living beings. This is the process by which a living being produces its own kind. Reproduction helps in continuing the lineage of a species. There are two types of reproduction, viz. sexual reproduction and asexual reproduction.


Various biological processes are called metabolism. Metabolism involves continuous change in the chemicals within a living organism. Metabolism is composed of two processes, viz. anabolism and catabolism.

  1. Anabolism: The process of synthesis of any substance is called anabolism, e.g. photosynthesis.
  2. Catabolism: The process of breaking up of a substance is called catabolism, e.g. respiration.

Response to External Stimuli: All living organisms respond to external stimuli. Light, heat, chemicals, other organisms, etc. are examples of external stimuli. Response to external stimuli is important for the survival of an organism. Let us take the example of an animal when it sees its food. Food is external stimulus and the organism’s response is to take up the food. Since nutrition is essential for survival hence response to the sight of food is essential for survival of an organism.

Nomenclature and Identification:

An organism is known by different names in different languages. It would be impossible for any person to remember the names of an organism in all the languages. Hence, there is need for a uniform system of nomenclature of organisms. A uniform system of nomenclature and identification helps the scientists in systematic study of living beings. ICBN (International Code for Botanical Nomenclature) applies to the plants and ICZN (International Code for Zoological Nomenclature) applies to the animals.

General Rules for Nomenclature:

Taxonomic Categories

different levels of biological classification

Various steps of the classification hierarchy are called taxonomic categories. Each category represents a particular rank and is usually called the taxon.


A group of individuals in which the individuals can interbreed among themselves is called species. Members of a species have a large number of similar characters. For example; all the tigers are called Panthera tigris. Since all of them can interbreed hence, they are kept under one species.


A group of closely related species is called genus. Examples: Lion (Panthera leo), leopard (Panthera pardus) and tiger (Panthera tigris) are members of the genus Panthera. Similarly, potato, tomato and brinjal belong to the genus Solanum.


A group of closely related genera is called a family. For example: potato and chili belong the family Solanaceae. Similarly, the genus Panthera and the genus Felis belong to the family Felidae.


A group of closely related families is called order. For example: Convolvulaceae and Solanaceae are plant families which belong to the order Polymoniales. Similarly, Felidae and Concidae belong the order Carnivora.


The group of closely related orders is called class. For example: orders Primata and Carnivora belong to the class Mammalia.


A group of closely related classes is called phylum. In the Plant Kingdom, the term phylum has been replaced with division. For example: pisces, amphibia, reptilia, aves and mammalia belong to the Phylum Chordata. All members of chordate have notochord at some stage of life.


The group of all the related phyla is called the Kingdom. For example: all autotrophic organisms which are eukaryotic and contain chloroplast are kept under the Plant Kingdom. Similarly, all heterotrophic organisms which are eukaryotic and lack a cell wall are kept under Animal Kingdom.