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CHAPTER 6 - Biology 1A03

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Lovaye Kajiura

LIPIDS, MEMBRANES, AND THE FIRST CELLS Cell membrane encloses the cell – it is a layer of molecules surrounding the cell, separating it from the external environment. Results in:  Chemical reactions necessary for life to occur more efficiently – b/c reactants collide more  Selective barrier forms – keeping molecules that could cause damage out The most abundant molecule in the cell membrane is the “oily” and “fatty” compound – lipids LIPIDS  Two types have been produced in spark-discharge origin of life experiments, indicating existence in prebiotic soup.  When shaken, they enclose around water spontaneously forming compartments of water. o This could explain how they enclosed around first self-replicating molecule. LIPIDS  Carbon containing compounds; largely nonpolar (hydrophobic) because C-H bonds are nonpolar  Lipid building blocks: o isoprene – five carbon compound; links end to end to form a hydrocarbon chain o fatty acid – carboxyl group head and hydrocarbon chain LIPIDS FOUND IN CELLS  Lipids are defined by the physical property of solubility; their structures vary widely.  Three most important lipids in cells: fats, steroids, phospholipids FATS  Three fatty acids linked to a three carbon molecule, called glycerol  Also called “triacylglycerol” or “triglyceride”  Forms from dehydration reaction of hydroxyl group of glycerol and carboxyl group of fatty acids – creating an ester linkage STEROIDS  Distinguished by their four ring structure – differ amongst each other by the functional groups attached  I.e. cholesterol: o Hydrocarbon chain of isoprene o Aids in synthesis of signal molecules (hormones) in mammals like estrogen, testosterone and progesterone o Polar hydroxyl group at top PHOSPHOLIPIDS  Polar heads (hydrophilic) consisting of glycerol, phosphate group and sometimes choline  Nonpolar tail: two hydrocarbon tails o Hydrocarbon tail of isoprene: domain archae o Hydrocarbon tail of fatty acids: bacteria and eukarya  Important in plasma membrane MEMBRANE LIPIDS  Important lipid function is role in plasma membrane; only 2 lipids are found in the plasma membrane  Membrane-forming lipids are amphipathic – they have a polar - hydrophilic - region and a non-polar – hydrophobic – region PHOSPHOLIPID BILAYER  In water, phospholipids form either: micelles or lipid bilayer  Micelles: droplet, spherical formations where the heads face the water and tails are forced together on the inside o Shorter tailed molecules typically  Phospholipid bilayers, or lipid bilayers: 2 sheets of phospholipid molecules aligned such that heads face a surrounding solution and tails are inside – interacting and facing each another o Longer tailed molecules  These structures form spontaneously; they increase stability and reduce potential energy ARTIFICIAL MEMBRANES  Researchers produce artificial membranes for research regarding permeability  2 artificial membranes: o Liposomes spherical membrane bound vesicles o Planar bilayers – membranes constructed across a hole in a plastic or glass wall that separates two aqueous solutions SELECTIVE PERMEABILITY  Lipid bilayers have selective permeability – some substances cross easier than others  Permeability speed: 1. Small, non-polar molecules – quickest 2. Small, polar or charged molecules 3. Large, non-polar molecules 4. Large, polar or charged molecules – slowest, if at all  Probably because large polar molecules and charged compounds can’t pass the nonpolar hydrophobic tails of the bilayers o Charged ions are more stable in water, where they can form hydrogen bonds with the molecules vs. the electrically neutral interior environment LIPID STRUCTURE AND PERMEABILITY Aspects of hydrocarbon chains affect the way the chain behaves in a lipid bilayer: 1. Number of double bonds contained  Double bonded carbon atoms lock in place, do not rotate, and form kinks o Kinks produce space, reduce strength of hydrophobic interactions, reduce tightness, and increase permeability  Unsaturated: double bonded hydrocarbon chain – such as unsaturated fats  Saturated: hydrocarbon chain without double bond - such as saturated fats; C-H bonds have greater free energy than C=C bonds  Polysaturated: multiple double bonds; very healthy 2. Length  Increase in length increases/strengthens hydrophobic interactions, increases stiffness, and reduces permeability; Degree of interactions along tail – caused by length and double bonds – dictates permeability; phospholipids with longer, saturated tails form membranes that have low permeability Observations: o Unsaturated lipids – such as oil – are liquid at room temperature; o Highly saturated fats – i.e. butter – are solid at room temperature o Long saturated lipids like wax are stiff solids at room temperature 3. Addition of cholesterol – steroid rings in cholesterol are bulky, so they fill spaces between the bilayer (increase density) and create a lower permeability TEMPERATURES EFFECT ON PERMEABILITY  Decrease in temperature causes less fluidity and a lower permeability  The individual molecules move slower, and the tails within the membrane pack tighter together o At very low temperatures, the bilayer solidifies o Otherwise, at room temperature, phospholipid molecules are constantly moving amongst each other in the layer at 2 micrometers per second SUMMARY Overall, the speed at which molecules go through the bilayer is a funct
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