The plasma membrane surrounds the cell. Chemically plasma membrane is composed of:
- Protein: 60 to 80%
- Phospholipids: 20 to 40 %
- Cholesterol in small amount
- Carbohydrates in small amount
Fig: Fluid mosaic model of plasma membrane
STRUCTURE OF CELL MEMBRANE
S. onathan Singer and Garth Nicholson developed the fluid-mosaic model of embrane structure in 1972. According to this ‘model, a membrane is a double lay r (bilayer) of proteins and phospholipids. It is present in fluid form. The phospholipids bilayer forms a fluid “sea”. The proteins float in this sea like ic- eerg (,);‘—j). Therefore, the membrane is in a constant state of shifting and ch nging. But it retains its uniform structure. The word mosaic is used for many di erent kinds of proteins. These proteins are dispersed in the phospholipids bil yer. The following are important points of the fluid-mosaic model:
T e phospholipids have two ends:
- Polar end or head: It has phosphate group. The polar spherical “heeds” are located over the cell surfaces (outer and inner). They are called hydrophilic (“water attracting”).
- Nonpolar end or tails: It has fatty acid molecules. Nonpolar ends face each other in the middle of the bilayer.’ The “tails” of both layers of phospholipids molecules attract each other. So they are repelled by water. Thus they are called hydrophobic, (“water dreading”).
holesterol is also present in the plasma membrane. The cholesterol molecules re embedded in the interior of the membrane. They make the membrane less ermeable for water-soluble substances. Cholesterol molecules make the embrane rigid structure. Thus the cholesterol molecules stabilize the
The membrane proteins are present in the form of individual molecules. So there are two types of proteins:
- Peripheral proteins: They are attached to the inner or outer membrane surface.
- Intrinsic proteins: They are embedded in the lipids bilayer. The intrinsic proteins perform following functions:
(a) Some intrinsic proteins are Links to sugar-protein markers on the cell surface.
(b) Some intrinsic proteins move ions or molecules across the membrane.
(c) Some attach the membrane. to the cell’s inner cytoskeleton or to various molecules outside the cell.
The carbohydrates are present in two forms
- Glycoproteins: The carbohydrates unite with proteins and form glycoproteins.
- Glycolipids: The carbohydrates unite with lipids and form glycolipids on the surface of a plasma membrane.
- Glycocalyx: Surface carbohydrates and portions of the proteins and U.-As make up the glycocalyx or “cell coat”. The glycocalyx shows complex arrangement, They have distinctively shaped groups of sugar molecules These sugar molecules act as a molecular “fingerprint”. The glycocalyx are necessary for the recognition of cell. So they control the behaviour of certain cells. Therefore, they are key component in coordinating cell behaviour in animals.
FUNCTIONS OF CELL MEMBRANES
Cell membranes perform following functions,
- It regulates material moving into and out of the cell, and from one part cif the cell to another.
- It separates the inside of the cell from the outside
- It separates various organelles within the cell.
- It provides a large surface area on which specific chemical reactions can occur.
- It separates cells from one another.
- There are sites for receptors on the cell membrane. These receptors contain specific cell identification markers. These markers differentiate cell type from other cells.
- The plasma membrane allows some substances to moves in and keens others out. It is called selective permeability. It is essential for maintain,hqIlular homeostasis. The maintenance of a constant internal environment espite fluctuations in the external environment is called homoeostasis.