CHAPTER 6 LIPIDS MEMBRANES AND THE FIRST CELLS cell plasma membrane separates life from nonlife acts as a selective barrier that sequesters appropriate chemicals to increase frequency of reactant collisions facilitate reactions o amphipathic nature of phospholipids responsible for plasma membrane o water keeps molecules oriented in membrane o permeability affected by amount of cholesterol in membrane saturation of hydrocarbon tales in membrane 2 9 2014 phospholipids and temperature fats DNA proteins all have carbonyl groups C O lipid C containing compounds nonpolar hydrophobic o characterized by insolubility in water see 5c compound isoprene large hydrocarbon component o high proportion of nonpolar C C and C H bonds relative to polar functional groups o dissolves in nonpolar organic compounds o independent lipids unstable in water o store chemical energy act as pigments act as messengers for signals btwn cells waterproof coatings on leaves vitamins used in cellular processes primary role is in cell membranes only lipids that can form membranes have a polar region hydrophobic region compounds that have both hydrophilic hydrophobic elements are amphipathic dual sympathy hydrocarbon molecules that contain only C and H nonpolar hydrophobic o C C and C H bonds can be oxidized releasing much energy o type of C C bond used in hydrocarbon chains key factor in lipid structure C C bonds cannot rotate freely and make a kink whereas C C bonds can kinks reduce strength of van der Waals interactions and weaken barrier to solutes o saturated hydrocarbon chains only single bonds between C s long hydrocarbon tails i e waxes stiff solids at room temperature denser membranes bc stronger van der Waals interactions o unsaturated hydrocarbon chains 1 double bonds between C s C H bond removed to form C C bond liquid room temperature hydrogenation removes double bonds so tails can straighten saturates solidifies stiffens fats aka triacylglycerols triglycerides nonpolar molecules made of 3 fatty acids o form when dehydration rxn occurs btwn hydroxyl group of glycerol 3 carbon molecule carboxyl group of fatty acid joined by an ester linkage o fat synthesis glycerol fatty acids fats water o vary according to C atoms and double bonds in tails o need the help of phospholipids to travel in water o fatty acids simple lipids made of hydrocarbon chain bonded to carboxyl COOH group o oils nonpolar liquid triacylglycerols steroids family of lipids distinguished by bulky four ring structure o differ by functional side groups attached to different carbons in hydrophobic rings o polar hydrophilic head OH and nonpolar hydrophobic tail isoprenoid o certain steroids i e cholesterol are amphipathic cholesterol s bulky steroid rings create a denser membrane interior cholesterol helps to stabilize the structure of the plasma membrane o do not contain glycerol phospholipids family of lipids that consist of a glycerol linked to a phosphate group glycerol fatty acids 2 9 2014 o most amphipathic b c of hydroxyl functional group attached to rings PO43 group bonded to a charged polar organic molecule some tails have double bonds o ionize o contain ester linkages o made w fatty acids bacteria eukarya o made w isoprenoid chains archaea o crucial component of plasma membrane o ions and hydrophilic molecules i e water glucose cannot pass directly though phospholipid membrane must use membrane transport proteins amphipathic lipids assume the structure of micelles or lipid bilayers form spontaneously energetically stable o micelles tiny droplets created when hydrophilic heads of lipids form hydrogen bonds w water hydrophilic tails face away from water form fatty acids simple amphipathic hydrocarbon chains o phospholipid bilayer made when 2 sheets of lipid molecules align hydrophilic heads face solution hydrophobic tails face one another within bilayer form from phospholipids that contain 2 hydrocarbon tails highly selective small nonpolar molecules move across quickly selective permeability lipids can pass readily through lipid bilayer osmosis net movement of water across semipermeable membrane from lower solute concentration to higher solute concentration attempt to reach equilibrium o water flows from lower concentration to higher so solutes with higher concentration always increase in vol o to prevent crenation hemolysis solution must be placed into an isotonic solution 9 m v NaCl or 5 0 m v glucose isotonic solution behaves as if they have same solute concentration as cell hypertonic solution behaves as if it has higher solute concentration than cell o water flows out of cell shrinkage crenation decrease in volume hypotonic solution behaves as if it has lower solute concentration than cell o water flows into cell swelling may burst hemolysis increase in volume 2 9 2014 o occurs if cell is placed into distilled water passive transport no energy required to move molecules across cell membrane o facilitated diffusion use of transport protein doesn t require energy from cell carrier channel proteins both protect polar charged solutes from contacting hydrophobic interior of lipid bilayer both specific for solutes they transport most solutes requiring facilitated diffusion are more hydrophilic than hydrophobic o channel proteins protein lined pores across membrane may be open at all times or gated opens only under specific conditions transport inorganic ions or water o carrier proteins poreless binding of transported solute to the carrier protein on one side of the membrane causes a change in the protein to expose the solute binding site to the opposite side of the membrane where solute is released transport small polar solutes i e sugars amino acids active transport energy from ATP is required to move molecules across cell membrane o cotransport use of cotransporter protein one moves towards concentration gradient one moves against glucose sodium transporter Na moves into cell down electrochemical gradient providing energy for glucose to move into the cell against its concentration gradient o exocytosis materials exported in vesicles that fuse with the plasma membrane and release contents outside cell o endocytosis cell membrane pinches in forms a vesicle that contains material from outside cell o sodium potassium pump for each ATP that is broken down via ATP hydrolysis 3 Na ions transported outside the cell and 2 K ions transported inside cell due to phosphorylation l formation of concentration gradient diffusion of Na ions into
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