CHAPTER 7 Membrane Structure and Function CH 7 Learning Objectives 1 Explain the fluid mosaic model describing the components of the membrane 2 Discuss how membrane structure results in selective permeability 3 Use examples to demonstrate the processes of diffusion osmosis and facilitated diffusion 4 Describe the process of active transport 5 Identify the mechanisms a cell uses to transport materials across the membrane in bulk I would suggest completing the crossword puzzle to help you understand the terminology and correlate how the terms relate to topics covered in this chapter How does the plasma membrane regulate inbound and outbound traffic Some small molecules move across the cell membrane using passive transport no input of energy and may require transport protein Some small molecules use active transport which requires both energy and transport protein s Large molecules move in and out using bulk transport exocytosis or endocytosis Figure 7 1b CONCEPT 7 1 Cellular membranes are fluid mosaics of lipids and proteins Lipids and proteins are the main components of membranes but carbohydrates are also important Membranes are composed mainly of phospholipids Phospholipids are amphipathic molecules containing hydrophobic water fearing and hydrophilic water loving regions Phospholipids form a bilayer with hydrophobic tails inside the membrane and hydrophilic heads exposed to water on either side Hydrophilic regions are oriented toward the cytosol and extracellular fluid inside outside the membrane Hydrophobic regions are embedded in the biilayer Figure 7 2 The fluid mosaic model of membrane structure depicts the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids Proteins are not randomly distributed in the membrane they often form groups that carry out common functions The Fluidity of Membranes Membranes are held together mainly by weak hydrophobic interactions Most of the lipids and some proteins can move sideways within the membrane Rarely a lipid may flip flop across the membrane one phospolipiid layer to the next As temperatures cool membranes switch from a flid to solid sstate The temperature at which a membrane solidifies depends on the types of lipids Membranes rich in unsaturated fatty acids are more fluid than those rich saturated fatty acids Membranes must be fluid to work properly Cholesterol is a membrane component in animal cells that has variable effects on membrane fluidity at different temperatures At warm temperatures such as 37 C cholesterol restrains movement of phospholipids At cool temperatures it maintains fluidity by preventing Plants use different but related steroid lipids to buffer tight packing membrane fluiditty Membranes must be fluid to work properly fluidity affects both permeability and movement of transport proteins Membranes that are too fluid cannot support protein function Organisms living in extreme temperatures have adaptive differences in membrane lipid compossitions Evolution of Differences in Membrane Lipid Composition The lipid composition of the cell membrane appears to be adapted to environmental conditions in many species EX cell membranes have a high proportion of unsaturated hydrocarbon tails in fish that live in extreme cold Organisms living in variable temperature conditions are able to change lipid composition in response to changing temps EX in winter wheat the percentage of unsaturated phospholipids increases in autumn to prevent membrane solidification in the winter Membrane Proteins and Their Functions A membrane is a collage of different proteins often clustered in groups embedded in the fluid matrix of the lipid bilayer This structure resembles a tiile mosiac Phospholipids form the main fabric of the membrane but proteins determine most of the membranes functions The protein composition of membranes varies among cells within an organism and among intracellular membranes within a cell There are two major types of membrane proteins Peripheral proteins are bound to the membrane surface Integral proteins penetrate the hydrophobic core Transmembrane proteins are integral proteins that span the membrane Hydrophobic regions of an integral protein consist of nonpolar amino acids often coiled into alpha helices Figure 7 6 Some membrane proteins are held in place by attachment to the cytoskeleton inside the cell Other proteins attach to the outside EX proteins called integrins attach to fibers of the extracellular matrix Cell surface membrane proteins can carry out several functions a b c d e f Attachment to the cytoskeleton and extracellular transport enzymatic activiitty sigmal tansducttion cell to cell recognition intercellular joining matrix Figure 7 7 Cell surface proteins are important in medicine EX HIV enters immune cells by binding to cell surface protein CD4 and a co receptor CCR5 Individuals lacking CCR5 are immune to HIV Drugs are in development to mask CCR5 and block HIV entrance in non immune individuals The Role of Membrane Carbohydrates in Cell Cell Recognition Cells recognize each other by binding to molecules on the surface of the membrane Many of these surface molecules are bonded to short branched chains of carbohydratess Glycolipids are carbohydrates bonded to lipids Glycoproteins are carbohydrates bonded to proteins give red blood cells their types or idenities ABO blood groups MOODLE SURVEY ON BLOOD TYPES The diversity of surface carbohydrates enables them to function as Synthesis and Sidedness of Membranes Membranes have distinct inside and outside faces The composition and distribution of proteins lipids and associated carbohydrates is asymmetrical across the membrane CONCEPT 7 2 Membrane structure results in selective permeability The plasma membrane controls the exchange of materials between the cell and Membranes exhibit selective permeability some substances cross more The fluid mosaic model explains how membranes regulate molecular The Permeability of the Lipid Bilayer Hydrophobic nonpolar molecules dissolve in the lipid bilayer and pass EX hydrocarbons CO2 and O2 pass The hydrophobic interior of the membrane impedes the passage of hydrophilic EX sugars water and ions pass Transport Proteins Hydrophilic substances cross membranes more quickly by passing Channel proteins have a hydrophilic channel that certain molecules or ions Carrier proteins bind to molecules and change shape to shuttle Channel proteins called aquaporins greatly increase the rate of They are
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