CHAPTER 5 Membrane Structure and Function 5 1 Plasma Membrane The plasma membrane is an amphipathic molecule with a phospholipid bi layer that includes integral wholly embedded and peripheral partially embedded cytoplasm proteins Cholesterol is a lipid found in the animal plasma membrane to modify the fluidity When temperature is high cholesterol stiffens membrane less fluid When temperature is low cholesterol helps prevent membrane from freezing Only animal cells have an extracellular matrix which contains protein fibers and carbohydrates The fluid mosaic model describes the plasma membrane Fluidity lipid component the greater the concentration of unsatu rated fatty acids the more fluid is the bilayer cells pliable Mosaic protein content bond to ECM and cytoskeleton number and kinds vary Glycolipids and glycoproteins phospholipids and proteins with attached carbohydrate chains Glycocalyx in animal cells is a sugar coat that protects the cell facilitates adhesion between cells reception of signaling and cell to cell recognition Carbohydrate chains basis for the A B and O blood groups Proteins channel passage of molecules ATP production carrier passage of molecules combine with a substance and help it move across nerve con duction cell recognition glycoproteins help body recognize foreign sub stances generates immune response pathogen deletion receptor binding of messenger molecule cell responds coordination of body s organs enzy matic metabolic reactions junction signaling discussed later An aquaporin is a channel protein for water allows water to cross a mem brane quickly Passage of molecules Diffusion high low concentration gradient lipid sol uble molecules and gases CO2 O2 facilitated transport high low chan nels carrier and concentration gradient some sugars and amino acids ac tive transport low high carrier energy sugars amino acids and ions bulk transport outside or inside vesicle utilization macromolecules or virus 5 2 Passive Transport Temperature pressure electrical current and molecular size all affect diffu sion Osmotic pressure is the pressure that develops in a system due to osmosis diffusion is more likely to occur A 0 9 NaCl solution is isotonic to red blood cells Hypotonic solutions LOW solute HIGH water cause cells to swell and burst cytolysis hemolysis red blood cells turgor pressure is the swelling of a plant cell in hypotonic solution Many protozoans have contractile vacuoles that rid the body of excess water Freshwater fishes would not be able to survive in distilled water or a marine environment Hypertonic solutions HIGH solute LOW water cause cells to shrink and shrivel crenation red blood cells plasmolysis shrinking of the cytoplasm due to osmosis plant cells Carrier proteins both facilitated and active transport Facilitated transport passage of water channel and glucose carrier 5 3 Active Transport low high uses energy Sodium potassium pump pumps Na out and K in Bulk Transport exocytosis vesicle produced by Golgi body fuses with plasma membrane secretion endocytosis plasma membrane invaginates to form vesicle Types of endocytosis phagocytosis large particles food virus ex White blood cells pinocytosis small particles liquid macromolecule ex Blood and plant root cells receptor mediated endocytosis form of PINOCYTO SIS with a receptor protein coated pit selective and more efficient 5 4 Modification of Cell Surfaces The ECM and cell junctions both contribute to cell communication and tissue formation ANIMALS Extracellular matrix meshwork of proteins and polysaccharides Collagen resists stretching and elastin fibers resilience are two well known structural proteins in the ECM Fibronectin binding to integrin allows ECM to influence activities in the cy toskeleton shape of cell Proteoglycans amino sugar protein resists compression of ECM regu late passage of molecules through the ECM to the plasma membrane cell mi gration ECM differs in consistency flexible connective tissue semiflexible carti lage solid bone Adhesion junctions mechanically attach adjacent cells MOST COMMON IN SKIN CELLS Desmosomes internal cytoplasmic plaques joined by intercellular fila ments present in tissues that stretch such as heart stomach and bladder Tight junctions plasma membrane proteins attach to each other zipper like present in barriers such as in the intestine and kidneys Gap junctions allows cells to communicate two identical plasma mem brane channels join present in heart muscle and smooth muscle provide flow of ions for contraction Adhesion chains Tight rope zipper Gap nails PLANTS cell wall Pectin abundant in the middle lamella layer of adhesive substances that holds cell together allows cell wall to stretch non cellulose polysaccharides harden the wall lignin strength in secondary cell walls which has a greater quantity of cellulose fibrils than the primary cell wall Plasmodesmata membrane lined channels that pass through the cell wall and connect cytoplasm of living cells only allow water and small solutes to pass from cell to cell