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Ch. 11 - Membrane Transport of Small Molecules - Part 1 Summary of chapter and lecture on membrane transport of small molecules and electrical properties of membranes. Includes illustrations and graphics.

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BIOL 2021
Julie Clark

Biol 2021- March 17 2009- lecture Chapter 11 CHAPTER 11- Membrane transport of small molecules and electrical properties of membranes  The lipid bilayer of cell membranes prevents the passage of most polar molecules  This allows cell to maintain concentrations of solutes in its cytosol that differ from those in the extracellular fluid and in each of the inter cellular membrane enclosed compartments (domains).  Cells use specialized transmembrane proteins to transport inorganic ions and small water soluble organic molecules across the bilayer. Transmembrane transport proteins make up usually 15-30% of all membrane proteins in a cell  Cell membrane= permeability barrier  Small hydrophobic molecules and small uncharged polar molecules (slow rate) are able to diffuse through the membrane  Large uncharged polar molecules and ions cant diffuse through the membrane and require transport *membrane is highly impermeable to charged molecules (ions) no matter how small they are * In general the smaller the molecule and the more hydrophobic or non polar it is, the more rapidly it will diffuse across the membrane bilayer. Size of cargo Transport Mechanism - Small molecules: - Transport proteins  Ions, sugar - Marcomolecules: - Protein translocaters and  Proteins, RNA nuclear pores (chap 12) - Large particules - Endocytosis or phagocytosis  Other cells  Cell needs transport to: i. Ingest nutrients (glucose) ii. Excrete waste (urea) iii. Ion regulation - Plasma membrane (PM), internal membrane (IM)  Membrane transport proteins: transfer such solutes across cell membranes. > Each transport protein only transports specific type of molecule Two major classes of transport proteins: 1) Transporters: bind the specific solute to be transported and undergo a series of conformational changes to transfer the bound solute across the membrane.  CAN BE ACTIVE OR PASSIVE TRANSPORT 2) Channels: interact with the solute to be transported much more weakly. They form aqueous pores that extend across the lipid bilayer; when open, these pores allow specific solutes (usually inorganic ions of appropriate size and charge) to pass through them and thereby cross the membrane. Faster.  ALWAYS PASSIVE TRANSPORT AND ENERGY (panel 2-7) - Free energy: the total change in free energy during a set of reactions determines whether or not the entire reaction sequence can occur.  Enclosed system: collection of molecules that does not exchange matter with the rest of the universe. Any such system will contain molecules with a total energy E.  First law of thermodynamics: in any process, the total energy of the universe remains constant.  Second law of thermodynamics: the universe constantly changes so as to become more disordered. -h= ∆H= ∆E Entropy (∆S): randomness of molecules ∆S= h/T Gibbs Free Energy (G): Free energy change is a direct measure of the entropy change of the universe - Free energy of universe= free energy of cell + free energy of environment - G= H-TS - At constant temperature : -∆G= -∆H + T∆S  At equilibrium ∆G= 0 (only dead cells)  We are burning up energy and changing environment or universe by being alive ION CONCENTRATIONS - Table 11-1 is important - Ion gradients across membrane creates membrane potential  Voltage across membrane (-20mV to -150 mV)  - means negative inside cell Electrochemical gradient - Electrical= charge difference - Chemical = concentration difference Electrical (how much energy does it take to push ion across membrane) ∆G= Zfv z= charge on ion F= faradays constant V= voltage ~ membrane potential Chemical ∆G= -RTln (concentration outside/concentration inside) R: gas constant T: temp in cells is constant Electrical + chemical = amount of energy required for ion to go across membrane PASSIVE TRANSPORT (FACILITATED DIFFUSION)  Transporters that allow solutes to cross the membrane only passively  For a neutral molecule, passive transport occurs down a concentration gradient (high concentration to low concentration)- the concentration gradient will determine the direction of transport.  For a charged molecules, electrochemical gradient and concentration gradient influence transport (electrochemical gradient- combination of both gradients)  Inside of cell is usually negative  Mediated channels are always passive ACTIVE TRANSPORT  Proteins actively pump certain solutes across the membrane against their electrochemical gradients; is mediated by transporters, which are called pumps.  Coupled with an energy source ie. ATP, light, ion gradient  Coupling to an energy source creates a gradient MEMBRANE TRANSPORT PROTEINS - Transport proteins do not modify the solute it transports - Transport proteins have multiple binding sites for its solute - The binding site can be blocked by competitive inhibitors or non competitive inhibitors (bind elsewhere and specifically alter the structure of the transporter)  General properties of transport proteins: 1) Multipass transmembrane: goes many times through membrane 2) Specific for certain molecule (like enzyme specificity) 3) Role of transport saturates at increasing concentration  Similar to enzyme saturation kinetics  Diffusion through membrane: no saturation; increase concentration= increased rate  Vmax: max rate  Km: binding constant 4) Can be inhinited by small molecule inhibitors - Transporters undergo reversible conformational change during transport- expose solute binding site first on one side of the membrane and then on the other - Channel (firm pore): doesn’t undergo conformational change; uses selectivity filter  In transporter (figure 11-5): affinity for substrate binding is higher on outside of cell and lower on inside  Flipping randomly for transport  For passive transport: down concentration gradient; probability  Movement doesn’t stop when ion concentrations become equal  For active transport energy sources are needed  For passive transport, electrochemical gradient is the energy source Cells carry out active transport in three main ways: I. Coupled transporters: couple the uphill transport of one so
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