CELL AND TISSUE

                                                              INTRODUCTION


    A cell is the structural and functional unit of life. It is the building block of which all living organisms are made, and the smallest unit of life capable of all the living functions. It is defined as a mass of protoplasm bounded by a plasma membrane. 
    Cell vary in size. Most cells are very small (microscopic), some may be very large. Some cells may exist as independent units of life. Some such cells like Euglena and Amoeba can change their shape, but most cells have a fixed shape. The number of cells vary from organism to organism. An amoeba is single-celled, while a human body weighing about 60 kg may have as many as 60 × 1015 cells. In unicelluar organisms, e.g. Amoeba, Paramoecium or Chalamydomonas, all the basic functions of a living being are performed in one cell, while multicellular organisms have well-developed division of labour. So, their different functions are performed by different organs. For example, we have a stomach to digest food, a heart to pump blood and a brain to think. 
    An English scientist, Robert Hooke, discovered the cell in 1665 while examining thin sections of cork under his simple microscope. He observed a mass of hexagonal chambers like a honeycomb and called them(compartments) cells. Cell is the Latin word for 'a little room'.

DISCOVERY OF CELL
1.    Robert Hooke (1665) :– An English man and first 
    curator of Royal  society of London.
    Observed a thin transverse section of bark of a 
    tree under self designed microscope.
    He noticed honey - comb like compartments.                               
    He coined the term cell.
    He wrote a book - Micrographia.
    He actually observed dead cells.
2.    Antony Van Leeuwenhoek (1674) was first to observe 
    living cells like bacteria [from tartar of teeth]
    erythrocytes [fish], sperms and protozoans [eg. Vorticella]
3.    N. Grew (1682) :– Proposed cell concept which states that cell is unit of structure of organisms.
4.    Rudolf Virchow (1858) :– Proposed that new cells formed from the pre-existing cells.

MICROSCOPE
    A microscope is an instrument to view small objects by magnifying them. It enables us to see the different types of living cells and the structures they contain.

Types of microscopes
    There are mainly three types of microscopes. They are :
 1.   Light microscope : The light microscope uses light to produce images.
 2.  Transmission Electron Microscope (TEM) : The electron microscope was designed by Knoll & Ruska (1932). A TEM makes use of a beam of highly energetic electrons to examine objects. The image produced is of a very fine scale.
 3.   Scanning Electron M
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