Notes On Membrane Structure And Function

1153 words - 5 pages

*Membrane Structure and Function*Selective permeability- allows some substances to cross it more easily than others*Phospholipids are the most abundant lipids in membranes *An amphipathic molecule (has both hydrophilic and hydrophobic region), proteins too*Overton hypothesized that membranes are made of lipids*Langmuir made artificial membranes by adding phospholipids dissolved in benzene to water*Benzene evaporated, phospholipids remained as a film covering the surface of water with only the hydrophilic heads immersed in the water*Gorter | Grendel reasoned that cell membranes were two molecules thick (bilayers)*Davson | Danielli proposed the model of a bilayer sandwiched between two layers of protein*Singer | Nicolson proposed that proteins are dispersed and individually inserted into the bilayer*Fluid mosaic model- the membrane is a fluid structure with various proteins embedded in or attached to a bilayer of phospholipids*Freeze-fracture prepares cells for electron microscopy*Splits a membrane along the middle of the phospholipids bilayer into a P face and E face*Etching enhances the topography (removal of frozen water by direct evaporation; sublimation)*A mist of platinum is sprayed on the fracture surface of the cell with a coat of carbon for strength*The original specimen is digested away*The replica is examined through the electron microscope*Membranes are fluid, held together primarily by hydrophobic interactions*Lateral movement of phospholipids within the membrane is rapid*Flip-flopping across the membrane is rare*Membrane remains fluid as temperature decreases until the phospholipids settle into a closely packed arrangement and the membrane solidifies; temperature of solidification depends on the types of lipids it is made of; membranes remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails*Cholesterol at body temperature in humans makes the membrane less fluid by restraining the movement of phospholipids; lowers the temperature required for the membrane to solidify*A membrane is a mosaic; a collage of different proteins embedded in the fluid matrix of the bilayer *Integral proteins penetrate the hydrophobic core of the lipid bilayer; transmembrane proteins completely span the membrane; hydrophilic ends, hydrophobic midsection*Peripheral proteins are appendages loosely bound to the surface of the membrane, often to the exposed parts of integral proteins; others are held in place by attachment to the cytoskeleton; those on the exterior side are attached to fibers of the extracellular matrix*Carbohydrates are restricted to the exterior surface*Molecules that start out on the inside face of the endoplasmic reticulum end up on the outside face of the plasma membrane*Carbohydrates on the plasma membrane allow cell-cell recognition--a cell's ability to distinguish one type of neighboring cell from another*Are usually branched oligosaccharides (cell markers) with fewer than 15...

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