CHAPTER 12.1 MEMBRANE TRANSPORTTransport Across the Plasma Membrane-Membranes are selective barriers-Diffusion of molecules across a synthetic bilayer is dependent on size-Small nonpolar molecules: Will pass right through the membrane (the membrane doesn’t pose a barrier to this group)-Small uncharged polar molecules: Membrane poses a barrier; they can go through the membrane, but just very slowly, so sometimes they need help getting through in sufficient quantities-Large uncharged polar molecules: Cannot go through; amino acids cannot go through because in a cell, they are charged; as the size increases, as long as they are not charged, the likelihood of getting through decreases-Anything charged (ions): are basically repelled; no matter the size, they cannot get throughMembrane transport proteins-Are responsible for selective transfer water-soluble molecules (solutes) across the membrane-Selectively shows that only specific molecules are able to get through-Protons won’t get through the membrane without a specific channel on the lysosome for themto go through-There’s no selective transfer of non-water-soluble molecules (such as gasses) -Selective transport can lead to differential distribution of solutes inside and outside the cell (and also between the cytosol and organelles)-Ion concentrations:-Na+ (cation):-Intracellular concentration (mM): 5-15-Extracellular concentration (mM): 145-K+ (cation):-Intracellular concentration (mM): 140-Extracellular concentration (mM): 5-Ca 2+ (cation):-Intracellular concentration (mM): 10^-4-Extracellular concentration (mM): 1-2-Cl- (anion): -Intracellular concentration (mM): 5-15-Extracellular concentration (mM): 110Simple Diffusion-Movement of molecules from high to low concentration- “Downhill movement of molecules”-Diffusion is energetically favorable (Negative Delta G)-Delta G = RT ln [solute]inside / [solute]outside-It is energetically favorable to cross into the cell as a negative Delta G-The concentration of K+ inside the cell is much greater than outside-It takes energy to ger K+ inside the cell-Would have a positive delta G-Always have to have proteins for positive Delta GClassification of Membrane Transport Proteins-Based on:-Mechanism of solute discrimination:-Carrier proteins (transporters): -A few carrier proteins will allow a few ions to get through the membrane, but everything else will get through the membrane because of carrier proteins-Have a binding site for whatever the molecule is, and when they bind they change conformation so that they can be released on the other side of the membrane-Channels: -Will only allow ions to get through the membrane-Don’t have binding sites, but what they essentially have is a “molecular screen” that will allow one, or a few, through at a time, but won’t let them through if they’re too big or not right-Don’t physically bind to the substrate that they are going to move acrossthe membrane-Don’t let all molecules through, only certain ones-Transport Energetics:-Passive-Carriers/Transporters may be active or passive-Channels use only passive transport (facilitated diffusion)-Active-Means that the molecules are being transported AGAINST their concentration gradient-Meaning it will have a positive Delta G, because it is using energy to go from an area of low concentration to high concentration-Membrane transporters increase the rate of diffusion-As you change the concentration of the solute that is going across the membrane, eventually it is not going to want to transport because it is the same as the concentration on the other side-Eventually facilitated diffusion will plateau because the molecules will have nowhere to go because there is no concentration gradient driving it-Carrier proteins/Transporters: Passive Transport  uncharged molecules-For an uncharged molecule like glucose, the direction of transport is dependent soley on concentration-When the concentration is the same, it will be in equilibrium and the reaction will be reversible -How are charged molecules transported across the membrane?-There is a new driving force, besides concentration, dictating whether or not it gets across the membrane, called electrochemical gradient-Sum of the concentration and electrical forces-Cell membranes have a voltage across them called the membrane potential (E or Em) that exerts a force on charged molecules-If you are trying to transport a positively charged through the membrane that is negatively charged inside, it is more likely to go through-If you are trying to transport a positively charged through the membrane that is positively charged inside, it is less likely to go throughMembrane Potential-Animal cells have a negative charge on the inside and a positive charge on the outside-Produces a membrane potential of ~ -60 mV-Typical membrane potential is always a negative numberActive Transport-Transport of solutes AGAINST their electrochemical gradient - “Uphill transport”-3 mechanisms to get molecules through the membrane-Coupled transporter  coupled pump-Bringing a molecule down its concentration gradient, and the energy concentrated from brining it down its concentration gradient (negative Delta G) is more than you need to bring it up its concentration gradient -Going from concentrated to diluted (down the gradient): negative Delta G-Going from diluted to concentrated (up the gradient): positive Delta G-ATP-Driven Pump: Driving molecule up its concentration gradient-Driving molecule up its concentration gradient-Light-Driven Pump:-Driving molecule up its concentration gradientNa+ -K+ Pump (Active Transport)-Energy from ATP hydrolysis drives Na+ out and brings K+ in-Sodium and potassium are going against their electrochemical gradient-Sodium binds to pump and fills the binding sites, which attracts ATP-ATP phosphorylates itself into ADP and the phosphate is in high-energy linkage-As soon as phosphorylation is triggered, there are conformational changes and Na+ is ejected-Conformational changes are what make proteins actually do what they are going to do-K+ then binds, and the pump has a dephosphorylation change -The dephosphorylation reverses the process and returns to its original conformation, creating a low affinity for K+-K+ is then ejected from the pumpActive Transport: Coupled Transporters-Transported molecule:-Uniport: -Facilitated diffusion-Goes down the concentration gradient through the membrane-Symport: -Coupled pump-Goes down the concentration