Plasma membrane and role of plasma membrane

 

Plasma membrane

Plasma membrane or cell membrane is the outer most boundary of the animal cells and inner to cell wall in plant cells. Cell membrane is chemically composed of lipids (20-40%) and proteins (60.80%). In addition there is small quantity of carbohydrates present.Many biologists contributed to establish the structural organization of cell membrane. IL modern technology has revealed that lipid bilayer is not sandwiched between two protei layers. In 1972, Singer and Nicholson proposed a most acceptable model for membrane called Fluid Mosaic Model. This model is in agreement with photograph of cell member by electron microscope. This model explains that "the membrane is like a sea of lipids which protein are floating".

a.Proteins and their role

The proteins are not arranged in sheet but as globes of proteins which are floa about in the sea of lipid molecules. Some proteins extend completely through the double layer of lipids from one side to the other and are called intrinsic proteins. Some other proteins are smaller and are placed between the phospholipids molecules. These are side of the membrane and are called extrinsic proteins.Carbohydrates extend out from the outer surface of the membrane an attached either to membrane lipids as glycolipids or to proteins as glycoprotein. These protiens act as permmeases. 

For your information

Industrial use of cell wall components i.e. cellulose

1) Nitrocellulose (Explosives)

2) Rayan (Textile fiber)

3) Cellophane (Partially permeable membrane)

4) Plastics including celluloid's & cinematography

5) Paper making

b::Role of Glycolipids and Glycoproteins 

Lipids, proteins and carbohydrates are responsible for functional diversity of the

membranes. Lipid bilayer makes the membrane differentially permeable barrier that allows the transport of non-polar materials across it and prevents ionic

materials. Membrane proteins on the other hand makes it selectively permeable barrier that select materials according to cell's

need. If glucose concentration inside the cells proper, no more glucose can enter the cell.Extrinsic proteins function as receptor that receives the stimuli from the environment and thus inform the cell to respond. Integral proteins also called as "permeases" regulate diffusion, osmosis and active transport of ionic materials.

Membrane carbohydrates such as glycolipids and glycoproteins provide receptor sites that

receives different types of stimuli like hormone receptor sites (HRS), receives nerve

impulses, recognition antigen and food materi thus inform the cell response. Membran carbohydrates are a

responsible for Endocyto?i.c. phagocytosis (eating the cell) and pinocytos (drinking of the cell). Thus glycolipids and

glycoproteins act as cell  surface markers.

Role of Plasma membrane

Plasma membrane is a dynamic structure only about 7 nm wide but it present

barrier to the movement of ions and molecules, particularly polar (water soluble) molecules such as glucose and amino acids, which are repelled by the non-polar, hydrophilic lipids.This prevents the aqueous contents of the cell from escaping. However, transport or maintenance across membranes must still occur for a number of reasons, for example

(1) To obtain nutrients.

(2) To excrete waste substances (urea, uric acid etc).

(3)To secrete useful substances (hormones, enzymes, etc).

(4)To generate the ionic gradients essential for nervous & muscular activity.

(5)To maintain a suitable pH and ionic concentration with in the cell for enzyme

activity..