Chapter 7 - Chapter 7 Membrane structure and Function...

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Chapter 7- Membrane structure and Function Plasma Membranes: These structures are about 8nm thick and are selectively permeable: allow some substances across it more easily than others. Cellular membrane models: Lipids and proteins are key constituents of membranes, although carbohydrates are by far the most important. The most abundant lipids are phospholipids, which are amphipathic molecules, containing both hydrophobic and hydrophilic regions. Most proteins have hydrophobic and hydrophilic regions too. The membrane itself consists of phospholipid bilayers. In this arrangement the hydrophilic heads shelter the hydrophobic tails from the water in the cell. 1. Sandwich model: “Membrane was coated on both sides by hydrophilic proteins.” A phospholipid bilayer between two layers of proteins. REJECTED! Because: Electron micrographs show that this model’s membrane is too thin and does not have enough layers Hydrophobic parts of the membrane protein would be exposed to water 2. Our model: Membrane proteins are dispersed evenly and individually into the phospholipid bilayer, with only their hydrophilic regions protruding far enough from the bilayer to be exposed to water. This arrangement maximises the contact of the hydrophilic regions with water and provides the hydrophobic regions with a non-aqueous environment. The membrane becomes an assortment of protein molecules bobbing in a fluid bilayer of phospholipids. PROOF!
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Electron microscopy has demonstrated that proteins are in fact embedded in the phospholipid bilayer of the membrane. Fluidity of Membranes: A membrane is held together primarily by hydrophobic interactions. The lipids and some proteins drift about laterally and in some rare instances, a molecule can flip flop (move transversely across a membrane, from one phospholipid layer to the other). Adjacent phospholipids switch positions about 10 million times per second. Proteins are much larger than phospholipids and move much more slowly. However some membrane proteins do in fact drift while the rest are virtually immobile. A membrane remains fluid as temperature decreases, until the phospholipids settle into a closely packed arrangement and the membrane solidifies. For example bacon grease forms lard when it cools. The temperature at which the membrane solidifies is specific to the type of lipid. Saturated Hydrocarbon Tails
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Chapter 7 - Chapter 7 Membrane structure and Function...

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