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Biology1107 Chapter 7 NotesMembrane Structure and FunctionOverview:1. The plasma membrane is the boundary that separates the living cell from its surroundings.2. The plasma membrane exhibits selective permeability, allowing some substances to cross more easily than others. 7.1 Cellular membranes are fluid mosaics of lipids and proteins1. Lipids and proteins are the main ingredients of membranes(carbs are also important) 2. Phospholipids are the most abundant lipids in the plasma membrane3. A phospholipid is an amphipathic molecules, meaning it has both a hydrophilic region and a hydrophobic region (most proteins within membranes have both hydrophobic and hydrophilic regions as well)a. Hydrophobic region:b. Hydrophilic region:4. How are phospholipids and proteins arranged in the membranes of cells?a. Fluid mosaic model: states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it or attached to a double layer (bilayer) of phospholipidsMembrane Models: Scientific Inquiry:1. Membranes have been chemically analyzed and found to be made of proteins and lipids2. Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayera. Such a double layer of molecules could exist as a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water. 3. Hugh Davson and JAMES Daniello proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins. a. Had problems: 1. Inspections of a variety of membranes revealed that membranes with different functions differ in structure and chemical compositions. 2. Membrane proteins are not very soluble in water because they are amphipathic: the placement of membrane proteins which have hydrophobic and hydrophilic tails 4. Singer and Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water5. A method for preparing cells for electron microscopy called freeze-fracture: demonstrated visually that proteins are indeed embedded in phospholipid bilayer of the membrane6. It splits a membrane along the middle of the bilayer (pulling apart a pb&j sandwichFluidity of Membranes1. Membrane is held together primarily by hydrophobic interactions (Weaker than covalent bonds)2. Phospholipids in the plasma membrane can move within the bilayer3. Most of the lipids and some proteins drift laterally (rapid movement)-switch positions about 10^7 times per second4. Proteins are larger than lipids and move slowly, but some membrane proteins do drift5. Rarely does a molecule flip-flop transversely across the membrane switching from one phospholipid layer to another6. As temp cools, membranes switch from a fluid state to a solid state7. Temp at which a membrane solidifies depends on the types of lipids8. Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids9. Membranes must be fluid to work properly; usually as fluid as salad oil10. Steroid cholesterol is wedged between phospholipids molecules in the plasma membrane of animal cells-has different effects on membrane fluidity at diff temps11. Warm temp (37 C) cholesterol restrains movement of phospholipids12. Cool temp: maintains fluidity by preventing tight packing13. Permeability changes when a membrane solidifies- enzymatic proteins in the membrane become inactive if their activity requires them to be able to move within the membrane (too fluid can’t support protein function either)Evolution of Differences in Membrane Lipid Composition1. Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions (fish that live in extreme cold have membranes with high proportion of unsaturated hydrocarbon tails-enabling their membranes to remain fluid2. Bacteria thrive at temperatures greater than 90 C in hot springs- their membranes include unusual lipids that may prevent excessive fluidity at such high temperatures3. Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures varya. Winter wheat- percentage of unsaturated phospholipids increases in autumn: keeps membranes from solidifying in the winterb. Bacteria and archea change proportion of unsaturated phospholipids in their cell membranetoo depending on temp at which they are growingMembrane Proteins and their functions:1. Membrane is a collage of different proteins often grouped together and embedded in the fluid matrix of the lipid bilayers2. Phospholipids from the main fabric of the membrane, but protein determine most of the membrane’s functions3. Diff types of cells contain diff sets of membrane proteins and the various membranes within a cell each have a unique collection of proteins4. 2 major populations of membrane proteins:a. Integral proteins: penetrate the hydrophobic core- majority span the membrane and are called transmembrane proteins; other extend only partway into the hydrophobic interiori. Hydrophobic regions of an integral protein consist of one or more stretches of non-polar amino acids, often coiled into alpha helicesii. Some proteins also have a hydrophilic channel through their center that allows passage of hydrophilic substancesb. Peripheral proteins: loosely bound to the surface of the membrane i. Not embedded in the lipid bilayer at all; often to exposed parts of integral proteins5. Cytoplasmic side of the plasma membrane: some membrane proteins are held in place by attachment to cytoskeleton6. Extracellular side: certain membrane proteins are attached to fibers of the extracellular matrix7. These combine to give animal cells a stronger framework than plasma membrane could alone provide. 8. 6 MAJOR FUNCTIONS PERFORMED BY PROTEINS OF THE PLASMA MEMBRANE PAGE 1291. Transport:2. Enzymatic activity:3. Signal transduction:4. Cell-cell recognition:5. Intercellular joining6. Attachment to the cytoskeleton and extracellular matrix (ECM):The Role of membrane carbohydrates in cell-cell recognition:1. Cell-cell recognition: Cell’s ability to distinguish one type of neighboring cell from another (crucial to the functioning of an organism)2. Cells recognize each other by binding to surface molecules, often containing carbs, on the extra cellular surface of the plasma membrane3. Membrane

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UGA BIOL 1107 - Chapter 7 Notes

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