Cell Theory | Types Of Cell | Cell Wall

• Cell is a basic membrane - bound unit that contains the fundamental molecules of life and of which all life begins .
• Robert Hooke was the first to observe dead cells in cork and Anton von Leeuwenhoek first saw and described live cells.
• Robert Brown later discovered the nucleus in the cells of orchid roots .
• Study of cell structure is called cytology 

CELL THEORY

• The cell theory was jointly put forward by Matthias Schleiden and Theodor Schwann in 1839. This theory includes following three principles
• All living organisms are composed of one or more cells and all the life processes ( metabolism and heredity ) occur within these cells .
• Cells are the basic units of structure and function in living beings .
• All cells arise by division of previously existing cells .

TYPES OF CELL

• Cells can be mainly classified into two types on the basis of nuclear organisation : cells that have membrane bound nuclei are called eukaryotic and cells that lack a membrane bound nucleus are called prokaryotic .

• Cells differ greatly in size , shape and activities .
• Mycoplasma is the smallest cell ( 0.15-0.3 μm in length ) while the largest single cell is the egg of ostrich .
• Cells also vary greatly in their shape . They may be disc - like polygonal , columnar , cuboidal , thread - like or even irregular . The shape of the cell may vary with the function they perform.

CELL STRUCTURE

• Cell wall is absent in animal cell . A plant cell consists of cell wall and protoplast .
• Protoplasm includes cytoplasm and nucleus.
• Various cell organelles like mitochondria ,Golgi bodies , etc. , and many cytoplasmic inclusions are found in the cytoplasm .
• In primitive cells of prokaryotes , genetic material is not organised in the form of nucleus and other membrane bound cell organelles are also absent .
• Eukaryotic cells , in general comprise of following main components

Cell Wall

• Structure
• It is the outer rigid , protective covering of plant cells , fungi and some protists . It is composed of cellulose microfibrils running through a matrix of other complex polysaccharides .
• Bacterial cell wall is made up of peptidoglycan . Algae have cell wall made up of cellulose , galactans , mannans and minerals like calcium carbonate .

• In other plants , it consists of cellulose , hemicellulose , pectins and proteins . Cell wall has upto three parts :
• ( i ) Middle lamella - Cementing layer between the adjacent cells and is the first layer to be formed during cytokinesis.
• It is made up of calcium and magnesium pectates 
• ( ii ) Primary wall - It is formed inner to middle lamella and is capable of extension It grows by addition of  .  materials within the existing wall .
•  ( iii ) Secondary wall - It is produced in mature cells that have stopped growth . It is formed by deposition of materials over surface of existing structure .
• Plasmodesmata are connections that link the cytoplasm of adjacent plant cells .

Functions

• Provides mechanical support and protection from mechanical injury .
• Provides shape and rigidity to the cell .
• Counteracts osmotic pressure and prevents bursting of plant cells by inhibiting excessive endosmosis .

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