MEMBRANE PROTIENS

 

                                          2. Cell membrane Proteins

 

       Embedded in the fluid lipid bilayer

Types: Peripheral & integral or transmembrane proteins

   1.Peripheral proteins :

·         Loosely bound to the membrane

·         Non-covalently bound to integral proteins

·         Provides structural integrity to the cell membrane

      Types        

                      1. Intrinsic - anchored to cytoskeleton of the cell

                      2. Extrinsic - act as cell adhesion molecules (CAM)

  2. Integral proteins (Transmembrane proteins ) :

·         Covalently bond

·         Penetrate lipid bilayer

Functions of integral proteins:

·         Channels: Provide channels for the ions to diffuse in both directions e.g., Na+ Channels (allow sodium ions to diffuse inside). Diffusion of ions through channels is simple diffusion

·         Carriers: Transport the substances along the concentration gradient. Diffusion with the help of carrier proteins is called facilitated diffusion. E.g., transport of g

3.  Glycocalyx

Glycocalyx is a coat on the external surface of the plasma membrane. This coating consists of several carbohydrate moieties (glycolipids and glycoproteins)

Functional significance:

 

contribute to cell-cell recognition, communication, and intracellular adhesion

 lucose by a carrier protein GLUT

·         Pumps: Transport the substances against the electrochemical gradient e.g., Na+-K+ ATPase pump. The pumping of substances with the help of transport protein is called primary active transport

·         Receptors: Receive the chemical signals from outside e.g., Hormones & neurotransmitters

·         Antigens: Differentiates the self from non-self e.g Human Leukocyte Antigen (HLA) & Blood group Antigens

·         Enzymes: Catalyze the reactions at surface of the membranes e.g Adenylate cyclase

Cell Adhesion Molecules: Help to anchor the cells to neighboring cells and to the basal lamina  

Lipid Bilayer as semipermeable membrane::

The phospholipid bilayer structure with specific membrane proteins accounts for the selective  permeability of the membrane 

Small molecules and larger hydrophobic molecules move through easily.  e.g. O₂, CO₂, N₂ and alcohol

Hydrophilic molecules have lower solubility to penetrate the membrane slowly. E.g. Ions, glucose and urea ,

Importance of Semipermeability:

Semipermeability of plasma membrane determines the concentration difference between ECF & ICF