BSCI330 EXAM 1 STUDY GUIDE Chapter 10 Membrane Structure Plasma membrane encloses the cell defines its boundaries and maintains the essential differences between the cytosol and the extracellular environment Membrane bound organelles for specialization Proteins that act as sensors of external signals allow cells to change behavior in response Ion gradients across the membrane used to synthesize ATP to environmental cues and transfer signals and molecules across membrane THE LIPID BILAYER 5nm thick provides fluidity and impermeable barrier to passage of most water soluble Easily seen by electron microscopy Major Lipids in Cell Membrane Phosphoglycerides sphingolipids and sterols o 5x106 lipid molecules in a 1umx1um area or 109 lipid molecules in membrane o All lipid molecules in membrane are amphiphilic have a hydrophilic polar and hydrophobic nonpolar end o Phospholipids one polar head two hydrophobic hydrocarbon tails fatty acids Manufactured in the cytosolic monolayer of ER One tail typically has one more cis double bond unsatured which creates a kink in the tail and makes the membrane more fluid Four major phospholipids in mammalian plasma membrane phosphatidylethanolamine phosphatidylserine phosphatidylcholine sphingomyelin sphingosine Phosphoglycerides have three carbon glycerol backbone two carbons attached to two fatty acid chains through an ester bond and the third carbon is attached to a phosphate group Cholesterol sterol up to one molecule for every phospholipid contains a rigid ring structure attached to a single polar hydroxyl group and a short non polar hydrocarbon chain orient themselves in the bilayer with their hydroxyl group close to the polar head groups of adjacent phospholipids Glycolipids o Also contains Transmembrane proteins serve as structural links that connect cytoskeleton through the lipid bilayer to either the extracellular matrix or an adjacent cell while others serve as receptors to detect and transduce chemical signals in the environment Phopholipids spontaneously form bilayers Shape and amphiphilic nature of phospholipids cause them to form bilayers in aqueous environments lipid molecules aggregate to bury their hydrophobic hydrocarbon tails energetically favorable in the interior and expose hydrophilic heads to water in two ways o spherical micelles tails inward o double layered sheets or bilayers hydrophobic tails sandwiched between heads The lipid bilayer is a two dimensional fluid contains 500 1000 different lipid species phospholipid cholesterol glycolipid bilayers made in the form of spherical vesicle liposomes 25nm 1um in diam planar bilayers black membranes partitioned between two aqueous compartments Techniques to measure the motion of individual lipid molecules fluorescent dye or gold particle attached to polar head group and follow diffusion of individual molecules in membrane o Lipid molecules readily exchange places with neighbors within a monolayer 107 times per second rapid lateral diffusion lipid molecules diffuse length of a large bacterial cell 2um in about 1 second o These studies show that individual lipid molecules rotate very rapidly about their long axis and have flexible hydrocarbon chains The fluidity of lipid bilayer depends on its composition Fluidity of cell membrane must be regulated precisely o Membrane transport process and enzyme activities cease when bilayer viscosity is increased beyond threshold level Bilayer changes from a liquid state to a two dimensional rigid crystalline gel state at a freezing point phase transition o If hydrocarbon chains are short or have double bonds the temperature is lower for this transition phase more difficult to freeze o Shorter chain lengths reduce tendency of the hydrocarbon tails to interact cis bonds create kinks in chains that make it difficult to pack together so membrane remains fluid at lower temperatures o Bacteria yeast and other organisms whose temperatures fluctuate adjust fatty acid composition in membranes to maintain a constant fluidity As temperature falls cells synthesize fatty acids with more cis bonds Cholesterol modulates lipid bilayer properties avoiding the decrease in bilayer fluidity o Mixed with phospholipids it enhances the permeability barrier properties of lipid bilayer o Inserts into bilayer with its hydroxyl group close to polar head of phospholipid the platelike steroid rings interact partially immobilize regions of hydrocarbon chains closest to polar head group makes lipid bilayer less deformable and decrease the permeability of the bilayer to small water soluble molecules o Prevents hydrocarbon chains from coming together and crystallizing The asymmetry of the lipid bilayer is functionally important RBCs phospholipid molecules have choline in head group outer monolayer and terminal primary amino group in the inner layer negative charge Converting extracellular signals into intracellular ones o enzyme protein kinase C PKC is activated in response to various extracellular signals it binds to the cytosolic face of plasma membrane polar heads phosphatidylserine is concentrated and requires this negatively charged phospholipid for activity o Phospholipases activated by extracellular signals to cleave specifc phospholipid molecules generating fragments that act as short lived mediators Glycolipids are found on the surface of all plasma membranes Glycolipids sugar containing lipid molecules found exclusively in the noncytosolic monolayer of bilayer extreme asymmetry 5 of lipid molecules in outer monolayer o Made from sphingosine o Self associate partly through hydrogen bonds between their sugars and partly through van der Waals forces between their long and straight hydrocarbon chains o Asymmetric distribution due to addition of sugar groups to the lipid molecules in the lumen of Golgi apparatus o Most complex ganglioslides net negative charge alters electrical field across membrane and concentrations of ions at membrane surface Ca2 o Confined to the exposed surface of cell so they help protect membrane against harsh conditions low pH and high concentrations of degradative enzymes o Function in cell recognition processes MEBRANE PROTEINS Membrane proteins can be associated with the lipid bilayer in various ways Transmembrane Integral proteins amphiphilic where their hydrophilic region is exposed to water on either side of membrane and hydrophobic region interacts with hydrocarbon chains of lipid molecules in the interior of bilayer Some located entirely in
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