BMB 462 1st Edition Lecture 5 Outline of Last Lecture I Review of Hydropathy Plots II Fluid Mosaic Model a Additional proteins b Membrane dynamics III Membrane Fusion Outline of Current Lecture I Selectively permeable membranes II Membrane Energetics a Activation barrier b Electrochemical potential c Passive and Active transport III Means of Classifying Membrane Transporters a Energetics b Transport Properties c Solute and Direction of Movement IV GLUT family transporters V Primary Active Transport Current Lecture Beginning of the unit on Membrane Transport Concepts to remembers from previous courses lectures Entropy G G Gdouble dagger Concentration Gradients and electrochemical gradients Half saturations km and kt Read in the book Structure and classification of protein transporters so that you know mechanism I Selectively permeable membranes some molecules can cross the membrane with or without aid and others cannot a Small hydrophobic molecules can go across the membrane unaided b Need proteins in the membrane to get other molecules across i The type of protein determines what type of molecule can cross These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute c Different cells in different tissues will have different transporters depending on what molecules are needed d There are probably over 1000 membrane transporters in the human genome about 4 5 of proteins in the genome are for transporters i Aka it s really important for cells to move molecules across membranes in the right way e Complications with membrane transport i An ABC transporter that moves chloride across lung epithelial from the cells into the airways This movement also helps move water into the lungs and the extra water loosens mucus in the airways 1 Absence or mutations with this transporter cause Cystic Fibrosis mucus in the airways thickens and results in many bacterial infections Individuals with C F typically die by age 30 from complications II Membrane Energetics a Important coefficients and variables i Vmax is the maximum saturation of a solute the transporter can support ii Kt is analogous to Km which equals the concentration at which a transporter is half saturated iii Gdouble dagger is a transporter s activation energy and determines the rate of transport G is standard conditions at 1 mol of everything at equilibrium so there is no net movement of solutes across the membrane G is transport at cellular conditions b Activation barrier i Activation energy determines the rate at which molecules move ii Transporters lower activation energy to make movement of molecules more favourable 1 i e moving polar molecules directly across the membrane is thermodynamically unfavourable but if you get a polar channel in the membrane it is much more favourable to move the molecules a The rate of transfer increases c Electrochemical potential i When moving molecules without charge only concentration matters for transport ii In charged molecules concentration and electropotential are import 1 Solutes naturally move from high concentration to low concentration 2 Ions move to the opposite charge i e a positively charged molecule moves towards a negative charge d Passive and Active transport i If G is favourable negative in a direction the molecule will move in that direction using passive transport 1 No energy is required ii A cell uses active transport for unfavourable movement 1 The molecule moves against the gradient and needs aid from either a direct or indirect use of ATP energy III Means of Classifying Membrane Transporters a 2 examples of classification methods include whether the transporter uses active or passive diffusion and based on structure i A structure classified family of transporters are the Major Facilitator Superfamily MFS transporters 1 They can do passive active anti symport and uniport transportation they have many functions 2 But structurally all have 12 14 transmembrane alpha helices b Energetics read more details on these transporters in the book i Passive diffusion No energy input is required solutes move down the gradient 1 Simple diffusion does not require a transporter the solutes can diffuse directly through the membrane Not many molecules transport through simple diffusion 2 Facilitated diffusion Involves a protein transporter 3 Ion Channels offer a polar environment for charged molecules to move through Because the channel is open it doesn t require energy to move molecules through 4 Ionophores small molecules that bind to ions and move the ions across the membrane note we won t focus on these in BMB462 ii Active transport This requires energy and is used to move molecules against their gradient 1 Primary transport uses energy directly from ATP creating ADP Pi a The 3 types of primary transport include P type F type and ABC transporters 2 Secondary transport 2 solutes move through the transport a 1 solute moves with its gradient at a G which causes the release of free energy b The release of free energy causes a second solute to move against gradient c Frequently primary active transport creates the gradient needed for secondary transport c Transport Properties i Channels i e ion channels form pores in the membrane that can open or close When opened transport reaches the approximate rate of unhindered diffusion 1 Channels are not very stereospecific and do not interact with every molecule that passes through ii Carriers have enzyme like kinetics they saturate well below the rate of diffusion 1 Carriers are very stereospecific and interact with every molecule 2 They open on 1 side of the membrane a molecule binds conformational change of the protein occurs and the carrier now opens on the other side of the membrane where the molecule is released d Solute and Direction of Movement i Uniport 1 solute molecule moves through the transport at a time ii Cotransport 2 molecules move through the transporter 1 Symport Solutes 1 and 2 move in the same direction across the membrane 2 Antiport Solute 1 moves in the opposite direction of solute 2 IV GLUT family transporters a Function Move glucose and sometimes other types of sugar are very stereospecific so they can only move D isomer sugars not L isomers b Structure Major Facilitator Superfamily i 12 helices some of them are amphipathic partially polar and nonpolar 1 If you assemble the helices with all the polar sides facing in you get a channel c
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