Chapter 7

Chapter 7 - Chapter 7 Membrane structural models have...

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Chapter 7 Membrane structural models have evolved as new Concept 7.1 Cellular membranes are fluid mosaics of lipids and proteins . What do we currently know about biological membranes? Membranes are made of lipids, mostly phospholipids. Evidence: Lipids and lipid soluble material enter quicker than substances which are insoluble in lipid. Phospholipids are amphipathic molecules that can form membranes. Amphipathic is a condition where a single molecule has both hydrophobic and hydrophilic regions. Cell membranes are phospholipid bilayers. Evidence: the phospholipids content of red blood cells is twice the content it would take to make one layer of lipids. Biological membranes contain proteins. Evidence: Membranes isolated from red blood cells contain proteins in addition to lipids. Biological Membranes are coated with proteins that generally absorb water. Evidence: Biological membranes absorb more water on their surface than artificial phospholipid membranes containing no proteins. Davson and Danielli This difference could be accounted for if the membrane were coated on both sides with hydrophilic proteins. 1935 – Sandwich model: phospholipid bilayers residing between 2 layers of proteins. Figure on Moodle Two Problems with this model Not all biological membranes are the same. The plasma membrane is ~7 nm in diameter; mitochondrial inner membrane is ~6 nm. Form determines function. Protein placement. Unlike cytosolic proteins, membrane proteins aren’t very soluble in water. (amphipathic) According to the model, these hydrophobic portions of the protein would be exposed to an aqueous environment. S.J. Singer and G. Nicolson 1972 – Proposed that membrane proteins are dispersed, individually inserted into the phospholipid bilayers with their hydrophilic regions protruding. Hydrophobic regions of these proteins could exist in a non-aqueous environment like the inside of the membrane. Fluid Mosaic Model – figure 7.3 Freeze-fracture and freeze-etch micrograph – figure 7.4 Membrane Fluidity Membranes aren’t locked rigidly in place. Held together primarily by hydrophobic interactions, which are much weaker than covalent bolds. Lateral movement (side-to-side) is very common within the membrane (about 10 7 times per second), but a phospholipid actually crossing layers is quite rare, occurring about once a month. Figure 7.5a. Depends on two factors: lipid composition, and temperature. A membrane remains fluid as a lower temperature if it is rich with phospholipids with unsaturated hydrocarbons (containing C=C bonds)
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Kinks in the tails keep the unsaturated hydrocarbons apart more efficiently – figure on Moodle Cholesterol Remains wedges between hydrocarbons in the membrane. Affects fluidity at different temperatures.
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Chapter 7 - Chapter 7 Membrane structural models have...

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