Cells are the basic unit of life. All cells have a common feature known as outer selective permeable membrane called as cell membrane or plasma membrane. Almost all eukaryotic cells contain more complex and complicated system of internal membranes. These internal membranes give rise to membrane covered various compartments within each cell. Cell membranes are mainly composed of lipids and proteins.
The Plasma Membrane:
The plasma membrane acts as the boundary between the interior of the cell and also the extra cellular fluid that surrounds each and every cell. The lipids mainly present in the plasma membrane are phospholipids. These phospholipids are amphiphilic with the hydrocarbon tail of the molecule being hydrophilic, where as the polar heads are hydrophilic in nature.
1. Most common phospholipids such as cholesterol and phosphatidyl ethanolamine are present in the plasma membrane.
2. Plasma membrane has got watery surface on both the sides that are both inside and outside of the cell. Therefore the phospholipid present in the cell membrane forms a phospholipid bilayer structure with the hydrophobic tails facing each other.
Even though all membrane proteins are present in the membrane, they are structurally as well as functionally diverse from one another. All the biological membrane has got the same basic phospholipid bilayer structure, which are associated with a set of membrane proteins. This lipid and membrane protein structures enables the plasma membrane to carry out all its biological activities.
Some proteins which are present in the plasma membrane are only bound to the surface of the plasma membrane, where as others have one region masked within the membrane and also domains on one or the both the sides of it.
Mostly, protein domains on the extracellular membrane surface, and also they help in cell signalling mechanism. Protein domains which are formed within the membrane, more specifically which form channels and pores in the membrane help in the transportation of biomolecules across the membranes.
Protein domains which face the cytosolic face of the membrane deal with a variety of biological functions, such as they trigger the intracellular signalling pathways or they also act as cytoskeletal proteins.
Membrane proteins can be classified into two main types of proteins such as integral proteins or in other words as intrinsic and peripheral or extrinsic proteins depending upon the nature of the membrane protein interactions. Most of the biomembrane or plasma membranes or cell membrane contains both types of membrane proteins.
Integral Membrane Proteins:
Integral membrane proteins are also called as intrinsic proteins, as one or more parts of these proteins are embedded in the phospholipid bilayer of the cell membrane. Many of the proteins which are associated with the plasma membrane or cell membrane are tightly bound to it. Integral proteins also contain some residues with hydrophobic side chains that interact with fatty acyl groups of the phospholipids which are present in the membrane. This helps in enabling the anchoring of the proteins strongly into the cell membrane.
Most of the integral proteins span the entire phospholipid bilayer. The transmembrane proteins present in the plasma membrane contain one or more membrane-spanning domains. These domains are of four to several hundred residue long, which also extend into the aqueous medium on either side of the bilayer.
Two types of membrane-spanning domains such as one or more α helices or multiple β strands are found in transmembrane proteins. Proteins that contain seven membrane-spanning α helices form a very important and also major class that includes many cell-surface receptor and also bacteriohodopsin.
Some of the transmembrane proteins span the bilayer cell membrane several times and form a hydrophilic channel through which certain ions and also molecules can enter or leave the cell. For example, all G-protein-coupled receptor like receptors of peptide hormones. All the receptors span the cell membrane or plasma membrane seven times.
Transmembrane protein which form the portions within the lipid bilayer are made up of hydrophobic amino acids. Some portions of the transmembrane protein that are projected out from the phospholipid bilayer are mostly made upon hydrophilic amino acids. Proteins that project into the aqueous surrounding of the cell are usually made up of glycoprotein, which also contains many of the hydrophilic sugar residues that are attached to the part of the polypeptides exposed at the cell surface.
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