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PHSL233 lecture 02.doc

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University of Otago
Kirk Hamilton

PHSL233 Lecture 02 Structure and Function of Epithelia I So cellular membranes acts as a barrier for a cell and some organelles so there are many different varieties of membranes. Membrane of mitochondria have electron transport chain and thus it is useful for ATP synthesis. The cellular membrane is flexible and asymmetric with hydrophilic head facing the outside and lipid tails on the inside, thus like a mirror image of lipid bilaye. The membrane have both lipids and proteins but protein population will vary between membranes. Lipid components of plasma membrane The plasma membrane is a lipid bilayer of mainly phospholipids (75%). There are different types of phosphlipids, mainly phosphatidylcholine (PC) is present in large amount, but other types are phosphatidylserine, phosphatidylethanolamine and phosphatidylinositol. Phosphatidylinositol present in very small amounts because it can be released by cell as a signalling molecule. Phospholipids have a head and 2 fatty acid tails and it is a amphipathic structure which just means 2 different characteristic in one molecule. The choline head group is hydrophilic (polar) and the fatty acid chain which is hydrophobic and is non-polar. Phospholipids in a aquous environment can spontaneously form a vesicle or lipid bilayer structure. Although phospholipids are a very stable structure, it can also be broken down such as soap which breaks down the phospholipids so cell can be penetrated. Cholesterol is another hydrophobic structure and is present as 20% in a plasma membrane. Glycolipids are lipids with a sugar chain on top or a sugar associated group, they only make up about 5% of the lipids in a plasma membrane composition. Fluid mosaic model Phospholipids are continously moving, and the fluidity of the membrane will depend on the type of membrane so some can be quite stiff while other are very fluid like. Lipid molecules can move sideways in their own half of the bilayer, exchanging places with their neighbours. This fluidity can occur because of the chemical structure of lipids, on the fatty acid tails, if there is a lot of double carbon bonds (unsaturated fatty acids) this will un- straighten the tail and giving it a kink in the fatty acids so the phospholipids cannot pack as tightly together thus they can move around more freely and faster. But with saturated fatty acids and cholesterol present, they have straight structure so they can pack together and tightly giving very little room for movement thus very stiff and lipids move slower. Because lipids and protein can move, this means that membrane can break apart and re-seal such as exocytosis. Proteins are embedded in the lipid bilayer, but can move within the bilayers. Lipid molecules can move sideways in their own half of the bilayer and exchange places with their neighbour. They are flexible for cell growth, secretion and self sealing. Integral (intrinsic) membrane proteins are transmembrane proteins that span through out the lipid bilayer, many are glycoproteins. Integral membrane proteins help to form glycocalyx which is the glycoprotein on extracellular material. Peripheral proteins are proteins which only associate with one leflet of the lipid bilayer. These proteins have a hydrophobic domain which allowing it to insert and anchore itself on a side of the lipid layer into the fatty acid tails within. There are many other ways in which proteins can associate with the membrane. So different populations of protein are expressed will depend on the cell type, proteins are associated with membrane because that's normally where they function. Membrane need to be flexible to allow sealing and growing. Example insuline is a polar hormone so it targets liver and skeletal muscle and is a polar hormone, this means that insuline receptors must be embedded within the plasma membrane of those cells but not necessaryly in other cells. Ion channels and carrier proteins are integral proteins which provide ways for substance to move across the membrane barrier layer of lipid bilayer which are impermeable. Receptor is when a ligand just binds onto it which then will sent a signal or second messenger cascade into the cell. Enzyme also works similar in which substrate binds to the outside of the enzyme and catalytic activity occurs and products is released. Linker or adhersion proteins are not necessarily integral proteins but they link to a integral membrane protein to help form a stable complex of proteins. Because epithelia cells are specilised for transport of ions and molecules, so the population of proteins must be different between the apical and the basolateral membranes. So this shows a sodium glucose co- transporter (SGLT) which maybe found in the proximal tubule of kidney or small intestine, this will allow Na and glucose to come into the cell. On the basolateral side, there are Na-K ATPase which will pump Na back out of the cell and K into the cell with the use of ATP, glucose will be transported in the glucose transporter into the capillary. So this is a transcellular pathway which means transport of substance from external to internal environment via the epithelia cell. Depending on the epithelia, some molecules may enter the internal environment between the epithelia cells called paracellular pathway, in this pathway the molecule must penetrate via the tight junction. In some epithelia nothing goes via the paracellular pathway while some a lot of things goes through. So here shows 4 epithelial cells packed together with 4 different type
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