BIOMEMBRANE IN CELL BIOLOGY
All cells and cell organelles are made up of a basic structure that is called as biomembrane. eg. endoplasmic reticulum and golgi apparatus. All membranes consist of three substances :- Lipids, Proteins, Carbohydrate.
The ratio of lipids and proteins depends on the function of cell. Every cell has a different combination of protein, lipid and carbohydrate as follows :-
The biomembrane is made up of phopholipids. It is impermeable for water soluble molecules, ions and biological molecules. Membranes are formed by a lipid bilayer of phospholipids. Phospholipids are made up of a polar heads (water) and fatty acid tails (oil). Polar heads are hydrophilic and tails are hydrophilic.
Natural phospholipids fatty acids have one or more double bonds. It causes kink formation in the interior of the membrane and make the hydro carbon chains difficult to be packed together. The polar head region is variable in charge and fatty acids (R) chains also very is length from C12 to C22 as well as in their degree of unsaturation. i.e. double bond.
1.1. Biomembrane Components :
Biomembrane has mainly three components.
All biomembranes consist of proteins, glycolipids steroids and cholesterol. Due to the presence of different types of proteins in biomembranes, the cell may vary in it's functions. Membrane receptor proteins transfer the signals between the cells internal and external environments. Transport proteins involve in transport of molecules across the membrane. Many types of proteins are present in membrane. Some are attached to the surface of membrane and many proteins are embedded in the membrane, that's why the cell membrane is not smooth, it seems to be rough. Membrane enzymes may play a crucial roles in many cellular activities like oxidoreductase, transferase or hydrolase. Cell adhesion molecules gives a platform to the cells to identify and intract with each other. For eg. protein involved in immune response. There are mainly two types of proteins, found in the biomembrane.
(A) Interinsic or Integral protein
(B) Extrinsic or Peripheral protein
(C) Protein anchor proteins
(D) Lipid anchor proteins
- Integral Proteins : These proteins are embedded in the membrane. These are attached permanently in a bio-membrane. These can be separated only by using nonpolar solvents, denaturing agents and detergents. Integral proteins can be single pass, double pass and multi pass proteins. The protein which crosses single time a membrane and having a-helix structure is called as single pass. If a proteins a-helix structure passes twice in the membrane, it is called as double pass integral proteins and if a protein passes multiple time in the membrane, it is termed as multi pass intrinsic protein.
(i) Helix bundle proteins : present in all types of biomembranes
(ii) -Barrel proteins (Porins) : Porins are beta barrel proteins that cross a cellular membrane and act as a pore, through which molecules can diffuse. They are large enough to allow passive diffusion. i.e. they act as channels that are specific to different types of molecules. Porins are present in the other membrane of gram-negative bacteria such as E.Coli, mitochondria, chloroplast and some gram positive bacteria of the group Mycoleta.
E.Coli has double membrane, the cell wall (outermembrne) & plasma-membrane (inner membrane). The outer membrane are more permeable than inner due to this porins channels, which form aqueous channels through the lipid bilayer. The outer membrane protect the intestinal bacterium from harmful agents (eg-antibiotics, bile salts & proteases) but permits the uptake and disposal of small hydrophilic molecules including nutrients and waste products. It also allows passage of disaccharides and other small molecules as well as phosphate.
A-(ii) Integral monotopic proteins : These proteins attach to only one side of biomembrane and do not span the whole way across.
Other examples of integral proteins are glycophorins and band 3 proteins.
(A) Glycophorin :- It is one of the example of integral proteins in red blood cells. It is rich in sialic acid. This acid gives a very hydrophilic charged coat to RBC. This makes them to circulate without disturbing the other cells. These are distributed in 5 types. Glycophorin A, Glycophorin B, Glycophorin C and Glycophorin E.
(B) Band - 3 - It is 14 trans membrane protein. This gene protein is encoded by the SLC4A1 (Solute carrier family 4 member 1) gene in humans. This is called as Band 3 anion transport protein or anion exchanger 1 (AE1). Many activities like protein anchor, transporter activity, protein and ankyrin binding, inorganic anion exchanger activity, regulation of intracellular pH, chloride transport, sodium transport, anion transport are occured by this protein.
(B) Peripheral membrane proteins or Extrinsic proteins : These proteins are temporarily attached to the biomembrane or to the integral proteins by a non covalent, electrostatic or hydrophobic interactions. These can be dissociate by a polar reagent treatment. Integral and peripheral proteins may be post-transtionally modified, with the addition of glycosyl phospho tidylinositol (GPI) prenyl chains and fatty acid. Which may be anchored in the lipid bilayer. Examples of these proteins are ankyrin and band 4.1.
B-(i) Ankyrin : Ankyrins are the mediator protein between spectrin and integral protein band 3. Thus it links between the plasma membrane and cytoskeleton. These have binding sites for -subunit of spectrin and at least 12 families of integral membrane protein. This linkage plays a very important role in the integrity of plasma membrane, ion channels, ion exchangers and ion transporters in the plasma membrane.
B-(ii) Band 4.1 : This protein is associated with the cytoskeleton and encoded by the EPB41 gene in the human. This protein plays a very crucial role in maintaining the structure and skeleton of erythrocytes.
(C) Protein anchor proteins :- These type of proteins are involved in Apoptosis. These are water soluble proteins but these can aggregate and associate irreversibly with the lipid bilayer and become reversibly or irreversibly membrane associated.
(D) Lipid anchor proteins : LAP are also known as lipid - linked proteins. They are located on the surface of the cell membrane. They are covalently attached to lipids. These proteins are embeded within the cell membrane. These proteins are linked to a certain fatty acid like mysistate and palmitate. These anchor the protein to the cell membrane these protein to the cell membrane. These play an important role in the protein functions.
The involvement of phospholipids, cholesterol and proteins is occured in the fluidity of membrane. Lipid packaging and it's components influence the fluidity of the membrane. The fluidity of membrane is crucial for membrane functions. The membrane behaves as two dimensional fluids.
1.2. Membrane Fluidity
The ratio of saturated and unsaturated fatty acids determines the fluidity in the membrane at cold temperature. Factors affecting the fluidity of biomembrane
- Temperature : At low and high temperatures cholesterol increases and decreases the fluidity of membrane respectively. When fluidity increases, it prevent membrane lipids packing close together. Temperature decides what can enter or leave the cell. Cell functions it's best at normal physiological temperature, which is 98.6 degrees Fahrenheit in warm-blooded animals like human.
- Lipid Composition : If saturated fatty acids are compressed by decreasing temperatures, they press in on each other, making a dense rigid membrane. If unsaturated fatty acids are composed, the kinks push adjacent phospholipid molecules away, which helps maintain fluidity in the membrane.
- Different form of fatty acid : Trans form of fatty acid are more packed than cis form of fatty acid. Thus trans fatty acid are solid and cis fatty acid are liquid.
- Cholesterol : Cholesterol serves as a buffer, preventing lower temperatures from inhibiting fluidity and preventing higher temperatures from increasing fluidity. Cholesterol helps to separate the phospholipids in our cell plasma membranes at the high concentrations so that the chains of fatty acids cannot come together and crystalize. So, cholesterol prevents extremes wheather too fluid or too firm in the consistency of the cell membrane.
1.3. Lipid Rafts :
Some micro domains of cholesterol and spingolipids from discrete membrane are known as lipid rafts. These clusters move laterally within the plasma membrane and get associate with specific membrane proteins. Two types of lipid rafts are (1) planar (Non coveolar) (2) Caveolae. Caveolae are flask shaped invagination of plasma membrane that contain caveolin protein. Caveolin is an intrinsic cholesterol binding membrane protein. Although the function of lipid rafts remain to be fully understood, they play an important role in processes such as cell movement (Cholesterol trafficking) and the uptake of extracellular molecules by endocytosis as well as in cell signaling. The lipid composition of a bilayer also influences its thickness which in turn may play a role in localizing proteins to a particular membrane. It is found that spingomyelin associates into a more gel-like & thicker bilayer than phospholipids do. Similarly, cholesterol and other molecules that decrease membrane fluidity increase membrane thickness. Because Spingomyelin tails are already optimally stabilized, the extra addition of cholesterol has no effect on the thickness of a spingomyelin bilayer. Lipid rafts are the centers of organizing for the assembly of signaling molecules, influencing membrane fluidity and membrane protein trafficking and regulating neurotransmission and receptor trafficking lipid rafts are more ordered and tightly packed than it's surrounding stsucture of biomembrane. Lipid rafts are present in plasma membranes and other parts of cells like Lysosomes and Golgi complex.