Discovery of Cell

The discovery of cells and advancement in its study is very closely related to the invention and modernization of microscope.

Z. Jansen (1590) : invented microscope
Galileo (1610) : modified the microscope for astronomical purposes
Robert Hooke (1655) : discovered dead plant cells under his microscope.
He observed a slice if the bottle cork, which was the made from bark.
Leuwenhoek (1672) : discovered animal cells or living cells

                                        <---- honey comb like compartments

Cell Theory

Proposed by German biologist M.J. Schleiden (Botanist) and T. Schwann (Zoologist).
Cell theory states that, "Cells are the basic structural and functional units of life."

Modern cell theory (Cell principle) was proposed by Carl Nageli, Virchow, Paster (1855)

According to modern cell theory:

1. All living organisms are composed of cell and their product.


2. Cells are the structural and functional units in the body of living organisms.


3. The functions if an organisms as whole is the outcome of the activities and interactions of the cells making the body of that organisms.


4. The cell belonging to different organisms or different regions of the same organisms are basically similar in structure, chemical composition, and metabolic activities.


5. Each cell is made up of mass of protoplasm having a nucleus, a number of organelles, a covering membrane.


6. New cell arise due to the division of pre-existing cells.


7. Life passes from one generation to another in the form of living cell.


8. A cell can survive independently but its organelles cannot do so.


9. Growth of an organism takes place by cellular growth in unicellular organisms and by cellular growth and cell division in multi-cellular organisms.


10. Genetic information is stored and expressed inside the cells.


11. Basically all the cells are totipotent. (i.e a single cell can give rise to whole organism)

Drawbacks of cell theory

1. Viruses have no protoplasm and nucleus but only DNA or RNA surrounded by protein sheath.


2. Prokaryotic like bacteria also have no true cell.
   They lack true nucleus. The genetic material or DNA lies freely in the cytoplasm.


3. In some organism, body isn't differentiate into cells. Their body is made up of undivided mass of protoplasm with numerous nucleus i.e they are coenocytes.
   Example: Rhizopus
   


4. In protozoa, single cells acts as whole body, i.e they are acellular.
   Example: Paramecium
   


5. Connective tissues contain a lot of intracellular material in which the living cells remain embedded.


6. RBCs and sieve tube cells of phloem live without nucleus and other virtual organelles.

Edited by: Rabin Kalikote