Biology Outline Ch.7,17,8

Biology Outline Ch.7,17,8 - Chapter 7 Membrane Structure...

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Chapter 7 – Membrane Structure and Function p.125-141 Selective permeability – plasma membrane exhibits this, as it allows some substances to cross it more easily than others. Concept – Cellular membranes are fluid mosaics of lipids and proteins. Lipids and proteins are the staple ingredients of membranes, although carbohydrates are also important. The most abundant lipids in most membranes are phospholipids. A phospholipid is an amphiphatic molecule – it has both a hydrophilic region and a hydrophobic region. Fluid mosaic model – the membrane is a fluid structure with a “mosaic” of various proteins embedded in or attached to a double layer (bilayer) of phospholipids. Membrane Models History: In 1915, membranes isolated from RBCs were chemically analyzed and found to be composed of lipids and proteins. Ten years later, two Dutch scientists, E. Gorter and F. Grendel, reasoned that cell membranes must be phospholipid bilayers. Hugh Davson and James Danielli suggested in 1935 that the membrane were coated in both sides with hydrophilic proteins to explain why a pure phospholipid bilayer adheres less strongly to water than the surface of a biological membrane; proposed a sandwich model of phospholipid bilayer between two layers of proteins. Researchers first used electron microscopes to study the cells in the 1950s to find pictures that support Davson-Danielli model. In 1972, S.J. Singer and G. Nicolson proposed that membrane proteins are dispersed, individually inserted into the phospholipid bilayer with their hydrophilic regions protruding. A method of preparing cells for electron microscopy called freeze-fracture has demonstrated visually that proteins are indeed embedded in the phospholipid bilayer of the membrane. (a) Movement of phospholipids – lipids move laterally in a membrane (~10^7 times per second), but flip-flopping across the membrane is quite rare (~once per month). (b) Membrane fluidity – unsaturated hydrocarbon tails of phospholipids have kinks that keep the molecules from packing together, enhancing membrane fluidity. (c) Cholesterol within the animal cell membrane – Cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipid movement, but at low temperatures it hinders solidification by disrupting the regular packing of phospholipids.
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A membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent bonds. A membrane remains fluid as temperature decreases until finally the phospholipids setle into a closely packed arrangement and the membrane solidifies, much as bacon grease forms lard when it cools. The membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails. Because of kinks in the tails where double bond are located, unsaturated hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails, and this makes the membrane more fluid. Membranes must be fluid to work properly. When a membrane solidifies, its permeability
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Biology Outline Ch.7,17,8 - Chapter 7 Membrane Structure...

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