Notes on Membrane Structure and Function

Essay by starcraftgoddessHigh School, 12th grade October 2004

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*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...