Biology1107 Chapter 7 Notes Membrane Structure and Function Overview 1 The plasma membrane is the boundary that separates the living cell from its surroundings 2 The plasma membrane exhibits selective permeability allowing some substances to cross more easily than others 7 1 Cellular membranes are fluid mosaics of lipids and proteins Lipids and proteins are the main ingredients of membranes carbs are also important 1 2 Phospholipids are the most abundant lipids in the plasma membrane 3 A phospholipid is an amphipathic molecules meaning it has both a hydrophilic region and a hydrophobic region most proteins within membranes have both hydrophobic and hydrophilic regions as well a Hydrophobic region b Hydrophilic region 4 How are phospholipids and proteins arranged in the membranes of cells a Fluid mosaic model states that a membrane is a fluid structure with a mosaic of various proteins embedded in it or attached to a double layer bilayer of phospholipids Membrane Models Scientific Inquiry 1 Membranes have been chemically analyzed and found to be made of proteins and lipids 2 Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer a Such a double layer of molecules could exist as a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water 3 Hugh Davson and JAMES Daniello proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins a Had problems 1 Inspections of a variety of membranes revealed that membranes with different functions differ in structure and chemical compositions 2 Membrane proteins are not very soluble in water because they are amphipathic the placement of membrane proteins which have hydrophobic and hydrophilic tails 4 Singer and Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer with only the hydrophilic regions exposed to water 5 A method for preparing cells for electron microscopy called freeze fracture demonstrated visually that proteins are indeed embedded in phospholipid bilayer of the membrane It splits a membrane along the middle of the bilayer pulling apart a pb j sandwich 6 Fluidity of Membranes 1 Membrane is held together primarily by hydrophobic interactions Weaker than covalent bonds 2 Phospholipids in the plasma membrane can move within the bilayer 3 Most of the lipids and some proteins drift laterally rapid movement switch positions about 10 7 times per second 4 Proteins are larger than lipids and move slowly but some membrane proteins do drift 5 Rarely does a molecule flip flop transversely across the membrane switching from one phospholipid layer to another 6 As temp cools membranes switch from a fluid state to a solid state 7 Temp at which a membrane solidifies depends on the types of lipids 8 Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids 9 Membranes must be fluid to work properly usually as fluid as salad oil 10 Steroid cholesterol is wedged between phospholipids molecules in the plasma membrane of animal cells has different effects on membrane fluidity at diff temps 11 Warm temp 37 C cholesterol restrains movement of phospholipids 12 Cool temp maintains fluidity by preventing tight packing 13 Permeability changes when a membrane solidifies enzymatic proteins in the membrane become inactive if their activity requires them to be able to move within the membrane too fluid can t support protein function either Evolution of Differences in Membrane Lipid Composition 1 Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions fish that live in extreme cold have membranes with high proportion of unsaturated hydrocarbon tails enabling their membranes to remain fluid 2 Bacteria thrive at temperatures greater than 90 C in hot springs their membranes include unusual lipids that may prevent excessive fluidity at such high temperatures 3 Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary a Winter wheat percentage of unsaturated phospholipids increases in autumn keeps membranes from solidifying in the winter b Bacteria and archea change proportion of unsaturated phospholipids in their cell membrane too depending on temp at which they are growing Membrane Proteins and their functions matrix of the lipid bilayers membrane s functions 1 Membrane is a collage of different proteins often grouped together and embedded in the fluid 2 Phospholipids from the main fabric of the membrane but protein determine most of the 3 Diff types of cells contain diff sets of membrane proteins and the various membranes within a cell each have a unique collection of proteins 4 2 major populations of membrane proteins a Integral proteins penetrate the hydrophobic core majority span the membrane and are called transmembrane proteins other extend only partway into the hydrophobic interior i ii i Hydrophobic regions of an integral protein consist of one or more stretches of non polar amino acids often coiled into alpha helices Some proteins also have a hydrophilic channel through their center that allows passage of hydrophilic substances b Peripheral proteins loosely bound to the surface of the membrane Not embedded in the lipid bilayer at all often to exposed parts of integral proteins 5 Cytoplasmic side of the plasma membrane some membrane proteins are held in place by 6 Extracellular side certain membrane proteins are attached to fibers of the extracellular matrix 7 These combine to give animal cells a stronger framework than plasma membrane could alone attachment to cytoskeleton provide 8 6 MAJOR FUNCTIONS PERFORMED BY PROTEINS OF THE PLASMA MEMBRANE PAGE 129 1 Transport 2 Enzymatic activity 3 Signal transduction 4 Cell cell recognition Intercellular joining 5 6 Attachment to the cytoskeleton and extracellular matrix ECM The Role of membrane carbohydrates in cell cell recognition 1 Cell cell recognition Cell s ability to distinguish one type of neighboring cell from another 2 Cells recognize each other by binding to surface molecules often containing carbs on the extra crucial to the functioning of an organism cellular surface of the plasma membrane 3 Membrane carbs are usually short branched chains of fewer than 15 sugar units 4