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* The body of all the organisms (except viruses) is made of small units called cells. * Cell is the fundamental structural unit of all living organisms and it is considered as “building block of life”. * A cell can be defined as structural and functional unit of all the living organisms. * Study of all aspects of cell is considered as cytology or cell biology * The term cytology was coined by Hertwig (1893).

* In unicellular organisms, a single cell performs all the functions ex: bacteria and viruses whereas in multicellular organisms ex: fungi, plants and animals have various levels of organization in them. All the cells co-ordinate each other to perform specific task. * Multicellular organisms have various levels of organizations ranging from simplest to most complex. (a) Cells are the basic units of living organism helps to carry out specific task. (b) Tissues are made of group of cells (similar or dissimilar) which will work together to perform a specific activity. (c) Different tissues combine together to form an organ.

(d) One or more organs combine to form organ systems.
(e) All the organ systems combine to form an independent unit that is capable of carry out the basic life processes called organism. Atoms → Molecules → Cells → Tissues → Organs → Organ systems → Organism

* The study of cell evolved into a branch of science with the invention of an operational compound microscope by Zacharias Janssen (1590). * He constructed the first compound microscope with a magnification of 10x and 30x to examine small organisms. * The first observation of cells was made in 1665 by English scientist Robert Hooke. He observed plant cells while observing thin slices of cork tissue of Quercussuber(oak tree)He actually observed only cell walls which are arranged in tiny compartments like honey comb. * The name cell was given by Robert Hooke. The term was derived from a Greek word cellulae which means empty space. * Robert Hooke was also considered as father of cytology. * He also wrote a book called Micrographia.

* Later Anton von Leeuwenhoek described the structure of living cell such as bacteria (first observed by him), red blood cells and protozoa. * Dutch scientists Corti and Fontana (1772) identified the presence of jelly like substance in the cell. * Dujardin (1835) named it as sarcode and this term was replaced by protoplasm by Purkinje (1840). * Robert brown (1831) discovered the nucleus in a plant called Tridescantia. He called it as cell brain. CELL THEORY:

* Cell theory was put forward by M.J.Schleiden and Schwann. * Schleiden (1838),a German Botanist examined a large number of plants and found that all plant tissues were composed of cells of various types. * Schwann (1839), a British Zoologist studied different types of animal cells and found that cells had outer layer, which we call as cell membrane; but no cell wall. Then He concluded, presence of cell wall is the unique feature of plant cells. He also defined cell as a membrane-bounded, nucleus containing structure. * Schleiden and Schwann together formulated the cell theory. However, they did not explain how new cells are formed. * Later Rudolf Virchow (1855) showed that new cells develop by the division of pre-existing cells (Omnis cellula-e-cellula). * Thus, the cell theory comprises the following postulates : (a) All the living organisms are composed of cells their products. (b) All cells arise from pre-existing cells.

Shapes of Cells:
* The shape of cells varies greatly; amoeboid or irregular shape occurs in Amoeba and Leucocytes, round or oval shape ex: eggs of many animals, spindle shape ex: smooth muscle fibres branched ex: nerve cell and so on. Size:

* The smallest cell is Myoplasma, a prokaryote, which measures about 0.1 – 0.3µ. * The egg of an ostrich is the...
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