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Unformatted text preview: Cell membrane acts as a selective barrier o it shields the cell from the outside environment or it acts as a barrier within the cell Membranes are important for: • Cell communication • Transport of molecules (import and export of molecucles) • Cell growth and motility Lipid bilayers are the universal basis for cell membrane structure Phospholipids are the most abundant lipid in the cell membrane • Phosphatildycholine is the most common phospholipid Lipids have two very different properties • Hydrophilic head (water loving) (soluble in water) • Hydrophobic tail ( water hating ) (insoluble) o These two properties make lipids amphipathic Phospholipids, sterols, and glycolipids are all amphipathic molecules • Subject to two conflicted forces o Hydrophilic head (attracted to water) o Hydrophobic tail (shuns water) • Conflict resolved by formation of lipid bilayer which is most energetically favorable • In lipid bilayer, hydrophilic heads face the water, and hydrophobic tails are shielded from the water o Figure 11.11 Phospholipids spontaneously close on themselves to form sealed compartments • A sealed compartment is more energetically favorable than a phospholipid bilayer with its edges exposed to water (exposure of hydrophobic hydrocarbon tails to water is energetically unfavorable, making the closed structure more stable) The lipid bilayer acts as a two dimensional fluid • Fluidity of lipid bilayers can be studied using artificial lipid bilayers • Two are commonly used o Liposomes, closed spherical vesicles o Flat phospholipid bilayers • Phospholipids very rarely tumble from one monolayer (1/2 of the bilayer) to the other • Lipid molecules within their monolayer continuously change places with their neighbor • A drop in temperature or thermal energy decreases the rate of lipid movement making the bilayer less fluid • The fluidity of a cell membrane, or the ease at which lipids travel within the plane of a bilayer, is important for membrane function Composition Affects Fluidity • The closer the packing of the tails, the less fluid the bilayer will be • Two properties of hydrocarbon tails affect fluidity o The chain length o Degree of unsaturation (number of double bonds) • The shorter the chain length – increases fluidity (vary from 14 – 24 carbon atoms, with 18 – 20 being the most common) • Unsaturated tails (double bonds) make it harder to pack, making the bilayer more fluid • The addition of cholesterol or sterol, tends to stiffen the bilayer, making it...
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studyguide - Cell membrane acts as a selective barrier o it...

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