Membrane Proteins

Membrane Proteins

The phospholipid bilayer is a fluid structure that is the fundamental property of  biological membranes, but the different characteristics of different membranes comes from the associated proteins. Proteins provide membranes with different functions and play many roles.

Various Functions of Membrane Proteins 

There are two types of membrane associated  proteins according to their locations.

1.Integral membrane proteins

Proteins that are embedded within the membrane are called integral membrane proteins. 

2.Peripheral membrane proteins

Proteins that are associated with the surface of the membrane are called peripheral membrane proteins.

Membrane proteins provide the unique character of the plasma membrane of specific cell types, as well as different internal cell membranes in eukaryotic cells.

Classification of Membrane Associated Proteins on the Basis of Their Functions

We can classify the membrane associated proteins on the basis of their functions into following classes.

1. Transporter Proteins
Plasma membrane or other internal cell membranes are very selectively permeable , allowing only
certain solutes,ions or molecules to enter or leave the cell through channels or
carriers composed of proteins. These proteins are found in
most cells, but we will see specific examples critical to the
functions of the nervous system and the kidney.

2. Enzymes (Biological Catalysts)

Cells carry out many chemical reactions onthe interior surface of the plasma membrane, usingenzymes attached to the membrane.Exmpleof these proteins are the electron transport chain's electron carrier proteinsfound in both mitochondria and chloroplasts that arecritical to cellular respiration and photosynthesis.

3.Cell-surface receptors

Membranes are exquisitelysensitive to chemical messages, which are detected byreceptor proteins anchored to their surfaces. Thenature of cell-surface receptors found on a cell willdetermine what signals the cell can detect and respond.

4. Cell-surface identity markers 

Membranes carry cell-surface markers that identify them to other cells. Most cell types carry their own ID tags, combinations of cell-surface proteins and glycoproteins.For example  vertebrate immune system.

5. Cell-to-cell adhesion proteins

Cells use specific proteins to glue themselves to one another. Some adhere by formingtemporary interactions, while others form a more permanent bond. Most cells secrete an extracellular matrix, whichis a complex mix of proteins and carbohydrate that varies with cell and tissue type.

6. Cytoskeleton anchors

Surface proteins that interact withother cells are often firmly anchored to the cytoskeleton ofthe cell interior by linking proteins.These proteins and the adhesion proteins mentioned in 5 were both important inthe evolution of multicellularity. The presence of cellularjunctions is critical for multicellularity.

Some membrane proteins are attached to the surface of the membrane by special molecules that associate strongly with phospholipids.These anchored proteins are free to move about on the surface of the membrane tethered to a phospholipid. The anchoring molecules are modified lipids that have (1) nonpolar regions that insert into the internal portion of the lipid bilayer and (2) chemical bonding domains that link directly to proteins.