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Origin of plasma membrane

Origin of plasma membrane

Origin of this plasma membrane is hardly available of cell structure is very important to the immediate health of the all types of living beings cell than the  plasma membrane. If it is weakened as well as injured, the  living beings cell loses its ability to maintain gradients of cell, to carry out the selective transport of  best types of nutrients, and to contain the pool of  the plasma membrane enzymes and organelles essential for the homeostasis of cell. 

In consequence of cells which is new membranes may be added to existing membrane without altering the function as a barrier  for cell membrane and  the selective transporter. Also for maintaining the characteristic membrane asymmetry of cell and its plasma membrane must be assembled with precisely the correct molecular topography of this plasma membrane.

Thus, all types of living beings cellular membranes that grow from pre-existing membranes which act as tempelates for the addition of new precursors of this membrane. All types cells divide, daughter cells receive a full complement of  plasma membrane systems which undergo growth until the next division that may be passed on to subsequent progeny of cell membrane while the molecules within the plasma membrane undergo continuous replacement with cell.

The protein molecules of the plasma membrane that are synthesized on both attached as well as free ribosomes of this cell. Proteins synthesized through free ribosomes may be inserted into the plasma membrane following heir completion and release from the ribosomes of cell membrane. Proteins of plasma membrane which is synthesized on attached ribosomes of rough ER are inserted first into the membrane of RER and then transferred to the Golgi apparatus and its processed there and ultimately are dispatched to the plasma membrane via the secretory vesicles of plasma membrane. 



Likewise, the synthesis of phospholipid molecules of the plasma membrane takes place through the smooth ER (SER). Like the proteins, newly synthesized lipid are inserted into SER membranes, then they are passed to Golgi apparatus for the processing and ultimately are dispatched to the plasma membrane via small secretory vesicles. The cytosol also containts a number of phospholipid transport proteins that function to transfer phospholipid molecules from one cellular membrane to another.

In fact, the process of glycosylation is completed at the level of Golgi apparatus. However, some sugars are added to the proteins in the lumen of RER.

Functions of Plasma Membrane

The plasma membrane acts as a thin barrier which seperates the intra-cellular fluid or the cytoplasm from the extra-cellular fluid in which the cell lives. In case of unicellular organism the estra-cellular fluid may be blood, lymph or interstitial fluid. Though the plasma membrane is a limiting barrier around the cell but it performs various important physiological fanctions which are as follows:

1. Permeability 

The plasma membrane is a thin, elastic membrane around the cell which usually allows the movement of small ions and molecules of various substances through it. This nature of plasma membrane is  termed as permeability.

According to permeability following types of the plasma membranes have been recognized:

(i)Impermeable plasma membranes. 

The plasma membrane of the unfertilized eggs of certain fishes allows nothing to pass through it except the gases. Such plasma membranes can be termed as impermeable plasma membranes.

(ii)Semi-permeable plasma membranes. 

The membranes which allow only water but no solute particle to pass through them are known as semi-permeable membranes. Such membranes have not so far been recognized in animal cells.

(iv)  Selective permeable plasma membranes.

The plasma membrane and other intra-cellular membrane are very selective in nature. Such membranes allow only certain selected ions and small molecules to pass through them.

(v)   Dialysing plasma membranes. 

The plasma membranes of certain cells have certain extraneous coats around them. The basement membranes of endothelial cells are the best examples of extraneous coats. This type of plasma me having extraneous coats around it, acts as a dialyzer. In these membranes the water molecules and crytalloids are forced through them by the hydrostatic pressure forces.

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