Cell and theory of cell

Cell 

Hey dear student do you know that how every livings are made ? no then i tell you about cell  

Introduction 

Cell is the Greek word that means  a Hollow space. 

Cell is the most basic structural , functional and biological unit of all living organism.

Life can not exist at any level, lower then cellular level .

Discovery of cell

The real history of the cell begins with the publication of the classical work- "Micrographia" by "Robert Hooke" in London (1665) . In his book he described his observation . using a primitive microscope, Hooke investigated the structure of a slice cork .

He observed a honey comb pattern in the slice of cork , which consisted of thick walled, box like, chamber(compartments).Hooke called this compartment as cells. He was first used to the term 'cell'.

Hooke was only discovered the dead cell . He was only see the cell wall .

Leeuwenhoek (1863) studied both living and free living-cells using his own microscope , mainly protozoa , bacteria and red blood cells were seen in them .

Alfonso Corti (1772 ) reported the presence of living matter within cells.

Robert Brown named it nucleus after seeing some round structures in the cells of orchid roots.

                                      Cell theory 

At the beginning of the 19th century , several discoveries were made . These finally led the German botanist Mathias Jacob Schleiden (1838) and German zoologist Theodore Schwann (1839) to establish the cell theory .

Schleiden observed large varieties of plant tissues and found all were them of composed of cells . He concluded that the cells are the ultimate and basic unit that form the structure of the plant tissue .

Schwann at the same time critically examined many types of animal tissues .He was able to see the nuclei but not the cell walls . (as animal cell lack cell wall , they have a only very thin limiting membrane which we call today as plasma membrane ). Schwann found that interior of both animal a plant cells were almost same but had a nucleus surrounded by clear substance around it .

Both Schleiden and Schwann compared there finding in 1839 an jointly proposed cell theory in their paper - "Microscope investigations on the similarity of structure and growth in animals and plants". It started that-  

Cell theory of Schleiden and Schwann (1839)

• All living  beings composed of cell.
• All cell are basically similar in chemical composition and metabolic activities.
• The functions of the organisms are due to the interactions of the constituent cell.
• New cells develop inside the old ones ( cytoblast )
• Cell could be generated spontaneously like a crystal

Objections against cell theory

• The work of  Redi, Spallanzani and L. Pasteur confirmed that a cell can not be generated spontaneously.
• The theory was no sufficient to explain the origin of new cell .
• Cell theory said nothing about intercellular material.
• Schleiden view regarding the origin of new cells by the budding off of the nucleus was also found to be wrong in the light of the work , proposed by Von Mohl.

                     Modern cell theory ( Cell principle )

This theory was not perfect . It had many shortcomings . Hence it was modified time to time with new discoveries.

• One important modification was proposed by Rudolf Virchow (1855).
• he stated that omnis cellula e cellula (a new cells always arise from pre-existing cells).
• This proposal was called ' Theory of  lineage ' .
•  This fact was also proved experimentally by Louis Pasteur (1862 ).

Two most important feature of modern cell theory are-

1. All living organism are composed of cells and their products.
2. All cells arise from pre-existing cells.

 Cell principle -

• Cell is the basic, structural and functional unit of life.
• Cell is the smallest unit in which life can exist.
• All living beings are composed of cell.
• A cell is able to maintain its vitality and individuality.
• A cells are closely similar in their structure, chemical composition and basic activities .
• A cell can not arise de novo . New cell always arse from pre existing cell.
• The functions of the organisms are regulated by interactions of the constituent cells . An organism can not exhibit those activities , which are absent in its cell.
• A cell can be specialized to perform a particular function according to the need of the individual .
• A cell contain all the required information in genetic material .
• All present day cell have a common origin as they have arisen from a single cell that was evolved for the first time of the earth .

Exceptions of cell theory

• There are many cells which do not have nucleus , e.g. , Bacteria , Mycoplasma etc.
• Viruses do not have cell structure, in spite of this, it can show some living properties.
• Mammalian RBCs, sieve tube cell of plant do not have any nucleus .
• Many fungi and algae have cells that contain many nuclei .
• Highly evolve organism can show even those activities, which are not found in their cells , e.g. , man can speak but the constituents cell can not.
• Unicellular organisms have only one cell and hence their organisation in acellular or subcellular.(i.e. , the body consist of subcellular or acellular componets