NCERT Book Class 11 Biology Chapter 7 Structural Organisation in Animals

STRUCTURAL ORGANISATION IN ANIMALS 79

C HAPTER 7
S TRUCTURAL O RGANISATION IN
ANIMALS
7.1 Organ and Organ In the preceding chapters you came across a large variety of organisms,
System both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
7.2 Frogs
are performed by a single cell. In the complex body of multicellular
animals the same basic functions are carried out by different groups of
cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? As you have already learnt in your earlier classes, in
multicellular animals, a group of similar cells alongwith intercellular
substances perform a specific function. Such an organisation is called
tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.

7.1 O RGAN AND ORGAN SYSTEM
The basic tissues as you have learnt in earlier classes, organise to form
organs which in turn associate to form organ systems in the multicellular
organisms. Such an organisation is essential for more efficient and better
coordinated activities of millions of cells constituting an organism. Each