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Structure of Plasma Membrane
This Sqadia video is the demonstration of Membrane Structure and Function. Plasma membrane encloses the cell, defines its boundaries and maintains the essential difference between the cytosol and extracellular environment. Despite of small differences, all biological membranes have same structure: composed of a very thin film of lipid and protein molecules, held together by non-covalent interactions. Lipid molecules serve as an impermeable barrier to the transport of water soluble molecules. Protein molecules embedded in the membrane, have both structural and functional role. Membrane Lipids are amphipathic molecules. They have a hydrophilic or polar end and a hydrophobic or nonpolar end. Hydrophobic tails are buried in the interior of the membrane while the hydrophilic heads are exposed to water. Lipid rafts are small, specialized areas in membranes where some lipids (primarily sphingolipids and cholesterol) and proteins are concentrated. Because the lipid bilayer is somewhat thicker in the rafts, certain membrane proteins accumulate.
Membrane phospholipids play a crucial role in the cell signaling. Extracellular signals can activate PI 3-kinase, which phosphorylates inositol phospholipids in the plasma membrane. Various intracellular signaling molecules then bind to these phosphorylated lipids. Other extracellular signals activate phospholipases that cleave phospholipids. The lipid fragments then act as signaling molecules to relay the signal into the cell. Most trans-membrane proteins are thought to extend across the bilayer as: a single α helix, as multiple α helices, or as a rolled-up β sheet. Some of these "single-pass" and "multipass" proteins have a covalently attached fatty acid chain inserted in the cytosolic lipid monolayer. The covalent attachment of either type of lipid can help localize a water-soluble protein to a membrane after its synthesis in the cytosol. A fatty acid chain is attached via an amide linkage to an N-terminal glycine. A prenyl group is attached via a thioether linkage to a cysteine residue that is initially located four residues from the protein's C-terminus. Single‐pass transmembrane proteins participate in signalling in a variety of ways, either as ligands, receptors, enzymes coreceptors and/or adaptors. β-Barrels (beat-barrels) from eukaryotic organelles are thought to facilitate the translocation of precursor proteins through the membranes as well as to aid in assembly of other OMPs.
Membrane Associated Proteins and Channels
While converting a single-chain multi-pass protein into a two-chain multi-pass protein, proteolytic cleavage of one loop creates two fragments that stay together and function normally. Expression of the same two fragments from separate genes gives rise to a similar protein that functions normally. An experiment was conducted that demonstrated the mixing of plasma membrane proteins on mouse-human hybrid cells. The mouse and human proteins were initially confined to their own halves of the newly formed heterocaryon plasma membrane, but they intermix with time. The cell coat is made up of the oligosaccharide side chains of glycolipids and integral membrane glycoproteins and the polysaccharide chains on integral membrane proteoglycans. In addition, adsorbed glycoproteins and adsorbed proteoglycans contribute to the glycocalyx in many cells. Open channels in membrane create a water filled pore. Carriers never form an open channel.
The smaller the molecule and the more soluble it is in oil, the more rapidly it will diffuse across a lipid bilayer. Small nonpolar molecules, such as O2 and CO2, readily dissolve in lipid bilayers and therefore diffuse rapidly across them. Small uncharged polar molecules, such as water or urea, also diffuse across a bilayer, albeit much more slowly. There are two types of memebrane transport protein: Carrier proteins and Channel proteins. Carrier proteins, also called carriers, permeases, or transporters, bind the specific solute to be transported and undergo a series of conformational changes to transfer the bound solute across the membrane. Channel proteins, in contrast, interact with the solute to be transported much more weakly. They form aqueous pores that extend across the lipid bilayer. Binding of Na+ and glucose is cooperative that is, the binding of either ligand induces a conformational change that greatly increases the protein's affinity for the other ligand. Since the Na+ concentration is much higher in the extracellular space than in the cytosol, glucose is more likely to bind to the carrier. The overall result is the net transport of both Na+ and glucose into the cell. Because the binding is cooperative, if one of the two solutes is missing, the other fails to bind to the carrier.
The concentration of K+ is typically 10 to 20 times higher inside cells than outside, whereas the reverse is true of Na+. These concentration differences are maintained by a Na+ K+ pump (sodium potassium pump), or Na+ pump, found in the plasma membrane. The pump operates as an antiporter. Because the pump hydrolyzes ATP to pump Na+ out and K+ in, it is also known as a Na+ , K+ ATPase. The binding of Na+ and the subsequent phosphorylation by ATP of the cytoplasmic face of the pump induce the protein to undergo a conformational change that transfers the Na+ across the membrane and releases it on the outside. Protein has many fuctions such as it plays role in cell-cell recognition, intracellular joining. In The auxiliary transport system associated with transport ATPases, the solute diffuses through channel-forming proteins (porins) in the outer membrane and binds to a periplasmic substrate-binding protein. As a result, the substrate-binding protein undergoes a conformational change. The gating of ion channels shows different kinds of stimuli that open ion channels.