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CHAPTER 11 I II CELL STRUCTURE AND FUNCTION EXAM 2 Membrane Structure a Plasma membrane fatty film that holds the cells and all of its organelles It serves as a barrier to prevent the contents of the cell from escaping and mixing with the surrounding medium it also allows for nutrients to pass inward and waste products to pass out via highly selective channels and pumps Other functions of a cell membrane cell communication transport of small molecules and energy generation Lipid Bilayer a Membrane Lipids Form Bilayers in Water each lipid has a hydrophilic head and one or two hydrophobic tails The most abundant lipids are phospholipids molecules in which the head is linked to the rest of the lipid through a phosphate group Phosphatidylcholine is the most common phospholipid Molecules with both hydrophobic and hydrophilic properties like glycolipids and sterols are called amphipathic To avoid the conflict amphipathic molecules face as they are subject to two conflicting forces the formation of a lipid bilayer formed to satisfy all parities It is also the most energetically favorable The hydrophobic tails are all shielded from the water as they lie next to one another in the interior Molecules in the bilayer rearrange to eliminate tears that water can seep through Amphipathic molecules such as phospholipids assemble into self sealing containers that define closed compartments b The lipid bilayer is a two dimensional fluid The aqueous environment outside and inside the cell prevents the membrane lipids from escaping the bilayer but they move and change places within the bilayer The flexibility of the membrane is crucial to its membrane function and integrity c The fluidity of a lipid bilayer depends on its composition the closer and more regular the packing of the tails the more viscous and less fluid the bilayer will be Two major properties of hydrocarbon tails affect how tightly they pack in the bilayer their length and the number of double bonds they contain The shorter the chain length the more fluid the bilayer is Chains with double bonds are unsaturated and chains with no double bonds are saturated Each double bond in an unsaturated tail creates a small kink in the hydrocarbon tail making it more difficult for the tails to pack against one another Lipid bilayers that contain a large portion of unsaturated hydrocarbon tails are more fluid Fluidity is important for function because it allows for the diffusion of lipids and proteins once they re synthesized it allows for membranes to fuse with one another and it ensures membrane molecules are distributed evenly between daughter cells during division i Bacterial yeast cells have to adapt to varying temperatures At higher temperatures the cell makes membrane lipids with tails that are longer with fewer double bonds so that they are more solidified think of margarine take out the double bonds so they are solid at room temp not liquid ii Animal cells membrane fluidity is modulated by the inclusion of cholesterol Because cholesterol molecules are short and rigid they fill the spaces between neighboring phospholipid molecules left by the kinks in their unsaturated hydrocarbon tails They make the bilayer more rigid and less permeable d The lipid bilayer is asymmetrical The animal cells use selective flippases to produce asymmetry in the lipid bilayers III e Lipid asymmetry is preserved during membrane transport Nearly all new membrane synthesis occurs in the membrane of the ER and it s exported to the other membranes of the cell via vesicles The preservation and orientation of the bilayer allows for all cell membranes to have distinct cytosolic and noncytosolic faces Glycolipids are located mainly in the plasma membrane and they are only found in the noncytosolic half of the bilayer allowing their sugar groups to form a protective coat of carbohydrate that surrounds most animal cells When a glycolipid molecule is delivered to the plasma membrane it faces away from the cytosol and displays its sugar on the exterior of the cell Membrane Proteins carry out most membrane functions They transport nutrients metabolites and ions across the lipid bilayer act as receptors that detect chemical signals and act as enzymes to catalyze specific reactions a Membrane proteins associate with the lipid bilayer in various ways Integral proteins that attach directly to the lipid bilayer can be removed only by disrupting it with detergents Peripheral membrane proteins can be released from the membrane by more gentle extraction procedures i Transmembrane proteins extend through the bilayer with part of their mass on either side Hydrophobic regions lie in the interior of the bilayer and their hydrophilic regions are exposed to the aqueous environment on either side of the membrane ii Other membrane proteins are located entirely in the cytosol iii Some proteins are entirely outside the bilayer covalently attached to lipid groups iv Other proteins are bound indirectly to one or the other face of the membrane held together by their interactions with other membrane proteins b A polypeptide chain usually crosses the bilayer as an helix The peptide bonds that join the successive amino acids in a protein are normally polar making the polypeptide backbone hydrophilic Hydrogen bonding is maximized if the polypeptide chain forms a regular alpha helix where the hydrophobic side chains are exposed on the outside of the helix and the polypeptide backbones form hydrogen bonds with each other When the polypeptide chain in transmembrane proteins only crosses the membrane once they are usually receptors for extracellular signals Other transmembrane proteins form aqueous pores that allow water soluble molecules to cross and are usually more complicated in structure possessing a series of helices that cross the bilayer a number of times Some transmembrane proteins have polypeptide chains that cross the lipid bilayer as a sheet that is curved into a cylinder forming a barrel The hydrophilic amino acids in this structure face the inside and line the aqueous channel where the hydrophobic amino acids face the outside of the barrel This structure can be seen with porin proteins barrels can only form wide channels and are therefore less versatile than a collection of helices c Membrane proteins can be solubilized in detergents and purified It is important to separate proteins from all others in order to study it in detail This can be achieved by solubilizing the membrane with agents


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FSU PCB 3134 - CELL STRUCTURE AND FUNCTION

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