Ex

  Biology (NTSE/Olympiad)  

1. The cell

Plasma Membrane

Introduction :
•Cell surface in all the cells is enclosed by a living membrane which is called cell membrane by C. Nageli and C. Kramer (1855).
Historical Account :
J.Q. Plower (1931) coined the term Plasmalemma for cell membrane.
Ultrastructure :
• Plasma membrane forms outer covering of each cell.
• It is present in both plant and animal cells.
• Plasma membrane is a living, thin, delicate elastic, selectively permeable membrane.
• It separates contents of a cell from the surrounding medium.
Fluid Mosaic Model :
• In 1972, Singer Nicolson proposed this model. According to this, cell membrane consists-two layers of phospholipid molecules, phospholipid & protein molecules are arranged as a mosaic.
• Phospholipid molecules have their polar heads directed outward non polar tail pointing inward.
• The proteins are of two types ¬:
• Peripheral and integral. Peripheral proteins are located superficially while integral proteins are embeded in the phospholipid matrix. The protein monolayer have elasticity & mechanical support to the lipid matrix.
Functions of Plasma Membrane :
• The main function of plasma membrane is to regulate the movement of molecules inside and outside the cell.
• It allow the movement of gaseous substance from high concentration to low concentration which known as diffusion.
• Water also obeys the low of diffusion. The movement of water molecule through a selectively permeable membrane is called osmosis.
• The flexibility of cell membrane also enables the cell to engulf in food, which is also known as endocytosis. For example – in Amoeba

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