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Lecture 2

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Jane Mitchell

LECTURE 2 MEMBRANE TRANSPORT OF SMALL MOLECULES Slide 2: - Some molecules - Some molecuels are able to diffuse across the membrane and for those who can’t there are specific types of transport proteins carry out passive transport (no nrg) and some carry out active protein (require energy) Slide 3: lipid bilayer is permeable to: - Hydrophobic molecules can diffuse across membranes easily because they can easily interact with the lipid tails within the bilayer o Ex: oxygen, nitrogen, carbon dioxide and benzene - Small UNCHARGED polar molecules are also allowed to pass through the bilayer but they move through at a lower rate. - It’s the concentration gradient that determines whether or not the molecule will move through the cell - Hydrophobic and the nonpolar the molecule is the faster it moves through the bilayer that’s b/c the hydrophobic ones can interact well with the lipid tails whereas the polar hydrophilic ones can’t Slide 4: lipid bilayer is impermeable to: - Larger polar molecules even uncharged one’s like glucose and sucrose ( and does at extremely extremely slow rate) - Also doesn’t allow the movement of ions across through simple diffusion - All of these types of molecules require membrane proteins for transport - Molecules like glucose need to be transported in a regulated manner because if they could just pass through by simple diffusion it would cause nutrients to leak Slide 5: membrane transport proteins: - Are always multipass transmembrane proteins = cross the bilayer a multiple times and that’s because they need to create a pore through which the molecules can travel so with one transmembrane protein they wouldn’t be able to create any sort of pore for molecules to pass through - They transport polar and charged molecules often o Ex: ions, sugars, amino acids, nucleotides etc. - All different types of molecules that need to enter the cell in a regulated manner and also need to be kept in different organelles inside the cells so cells can perform their functions that are required in different organelles. - Each transport protein is selective Slide 6: passive transport/active transport - Simple diffusion is passive (don’t need energy) and it occurs down a concentration gradient - Transport proteins that need a transport protein to facililate the movement >> called facilitative diffusion. - Channel mediated: gated channels different process.. when gates are open multiple molecules can pass through - Transporters: undergo conformational changes>> molecules will bind on one side of the membrane and the protein will undergo a transformational change and that will transfer a molecule across the membrane - both don’t need energy and occurs down the concentration gradient which gives it the motive force to move it across - active transport: energy is needed to allow the transfer proteins to occur and it moves AGAINST the concentration gradient. Slide 7: resting mem potential: electrical potential - most cells under resting conditions have a positive charge on the outside of the mem and –‘ve on inside of the mem. - Clicker ques: assuming all 3 proteins can travel through the membrane using channel proteins which molecule has the greatest motive force across the membrane? o B: membrane potential will favour the entry of positively charged molecules into the cell, that’s b/c in addition to the concentration gradient which is driving all of these molecules into the cell, the positive one has more pull on the –‘vely charged inside of the cell.  when you think of charged molecules you can’t just think of their concentration gradient but also think of the electrochemical gradient o For a charged molecule, the concentration gradient plus the membrane potential or the electrical gradient together gives the electrochemical gradient which determines which direction the molecules moves through facilitative diffusion. o For a charged molecule it’s motive force not only depends on its own concentration but on the concentration of all other molecules on the extracellular and intracellular components and together they make up the resting membrane potential across the membrane. o Transporters can be also called carrier transport Slide 11: types of active transport: - Some proteins are involved in passive transport/active transport - Active transport always involves the movement of atleast one molecule moving against it’s concentration gradient so it requires energy - Coupled transporters or cotransporters: o Move one molecule down the concentration gradient  provides the motive force to move the other molecule against it’s concentration gradient  red molecule is the cotransported molecule which provides the motive force to move the other one across the gradient - ATP driven pumps  use energy stored in ATP .. hydrolyze ATP and they move then against gradients. - Light driven pumps: light provides energy to move molecules across concentration gradient:  Ex: bacteria redopsin  uses light to pump proton against gradients in bacterial cells Slide 13: - All transporter proteins bind to a specific solute and undergoes conformational change and that change transforms the molecule - Uniporter: move one molecule by passive transport down the chemical gradient and the direction is reversible (in or out of the cell) o Ex: glute uniporter o The protein is still inserted in the same orientation but the protein works by binding to molecules on the side of the membrane where the concentration is higher and transfers it to the side where the concentration is lower (it can work in either direction and it doesn’t need any orientation to do that and it’s just the concentration difference that drives its function so it’s the facilitating the movement across the membrane and not putting any energy into the process). Slide 17: - glute uniporter: glucose doesn’t move across membranes very well but it’s a very essanetial nutrient that drives energy production in cells so this uniporter helps that. Glucose is high in the blood stream and that high conc creates a gradient that drives it into the cells that line the blood vessels.. but it needs a carrier protein (glute uniporter)  binds to the extraceuller space which causes a conformational change which transfers the glucose  dan move glucose in both direction depending on the conc gradient (in or out of cell)  Uniporters carry out passive transport  no energy Slide 14/15: coupled transport: - Symporters: move in the same direction as the transported molecule o The cotransported ion, if it has a gradient that favours the entry of the molecule into the cell then the transporter molecule will be moved in the same direction but it’s moved against the concentration gradient (so conc or electrochemical gradient would actually be the opposite) - Antiporters: cotransporter moves in the opp direction of the transported molecule o Cotransported ion is moved out of the cell down it’s electrochemical gradient and the transporter molecule is moved in the opp direction into the cell and b/c it’s moving against it’s concentration gradient then the electrochemical gradient for it would be in the same direction as the contransported ion. - Both carry out active transport but they move molecules in different ways - free enrgy from the cotransported ion moving down from the electrochemical gradient is used to transport the 2 molecule against the electrochemical gradient and this is often called secondary active transport  needs energy but the energy that’s being used is the enrgy that was stored in the concentration gradient and it’s called 2dary active transport b/c there are other active transport processes that build up these electrochemical gradients that are harnessed by these type of moelcules. Slide 21: example of a symporter transporter: - Na+/glucose  allows for the movement of glucose against a concentration gradient… when you want to want to move glucose into the cell when the concentration of glucose in the cell is already high, the uniporter wouldn’t work and it would work in reverse and move glucose out of the cell  in this case you need active transport molecule that wsould still move glucose against it’s concentration gradient - Sodium ions move down the gradient and that provides energy to move glucose agains it’s conc gradient - Cooperative binding of glucose and Na+ leads to the conformational change in the protein So there are 2 type of molecules that need to bind before you get that conformational change. - The Na+ electrochemical gradient is shown where there’s more Na outside of the cell compared to inside of the cell and that’s the normal situation. Normally Na+ is high in the extracellular enviroenment so the high concentration of Na drives the binding of Na onto the Na-glucose symporter. - After Na+ binds it increases the affinity of the transporter for glucose and now glucose can bind even if the concentration is relatively low in the extracellular environment. - Once all of those molecules are bound there’s a conformational change in the proteins that closes the pore on the extracellular side of the membrane and opens it on the intracellular side - That conformational change facilitates the release of glucose and Na inside the cell. Now glucose inside the cell the concentration is very high but when Na dissociates from the transporter the transporter loses some of its affinity to glucose and glucose is then able to diffuse out of the pore. - So the process relies on Na to drive glucose into cell Slide 22: regulating cytosolic pH - Most proteins require specific pH’s to function and diferent cellular compartments can have different pH’s - Cytosol is mostly neutral - Cytosolic proteins and most active in a neutral pH - Enzymes in lysosols require an acidic environment, they require excess hydrogen ions to function properly. - However cells need to maintain pH’s the neutral and the acidic ph’s - Excess hydrogen can leak into the cell and can be formed from acid forming reactions in the cell so cells need to be able to remove excess Hydrogens from the cytosol to maintain a neutral pH. - There are Na driven antiporters that maintain that cytosolic pH - These are called Na-H exchanger which is an antiporter  molecules now move in the opposite direction - Na concentration is higher in the extracellular space than the intracellular space and that electrochemical gradient drive Na into the cell using this antiporter and in the process the antiporter moves hydrogen ions out of the cell into the extracellular environment and does that against a proton gradient o Ex: this transporter can also respond to cytosolic pH so it has to be able to regulate pH.. if the pH drops and the cytosol acidifies then transporter activity will increase to pump excess H+ out of the cell - These transporters use the energy stored in the Na electrochemical gradient to move other molecules against their electrochemical gradient - Even if you have a very high electrochemical gradient for Na, continued action of action of these 2 symporters would start to equalize the gradient so the more you move Na into the cell the more you would equalize the gradient and eventually you will get to an equilibrium stage and then these 2 proteins would no longer function b/c they don’t have that energy source to drive that function - So how would the Na electrochemical gradient maintained? o Maintained by the Na-k+ pump and that’s to transport ATPase o That protein uses the energy stored in ATP to move Na out of the cell to build/maintain that Na gradient and as it does that it pumps K+ into the cell o This pump is directly using ATP but it’s using ATP to build an electrochemical gradient that is then harnessed by many other transport proteins  why these are called secondary transport b/c their not directly using ATP they’re using a gradient but their relying on the action of this other protein that does use ATP Slide 26: ATPases - Use energy stored in ATP - P-type ATPases: o Called P type ATPases b/c their phosphorylated during their pumping cycle o When ATP is hydrolyzed the hydrolysis of ATP generates an inorganic phosphate and ADP and in this case the protein itself the protein is phosphorylated when ATP is hydrolyzed that`s why their classified as P type ATPases o They move both Na and potassium against their electrochemical gradient o Ex:K+ pump o Na gradient is important for several processes:  Transport of nutrients into the cell  Maintanence of pH  Maintain cell volume: osmotic balance o K high on the intracellular compartment and Na high on the extracellular compartment o Pump works to move Na into the cell and K out of the cell to maintain the gradient Slide 28: pumping cycle of the K+ Na pump - This is a cycle and when one cycle is completed then 3 Na’s would be pumped outside of the cell and 2 K+ would be transported into the cell and at the same time one ATP molecule would have been hydrolysed - Covalent modification to the protein causes conformational changes in protein and through this whole process phosphorylation is removed and you can start the cycle again: o 1) 3 Na’s bind to the channel and this binding along with the addition and hydrolysis of ATP and the phosphorylation of the pump causes a conformational change in the protein and this conformational change opens up the the channel to the extracellular environment and the 3 Na’s leave the extracellular pore o 2) two K+ bind fro the extracellular environment and the binding of K+ causes another conformational change releasing the phosphate from the protein and then that release causes a conformational change that opens up the intercellular compartner and the 2 K+ can be transported into the cell - Process also relies on changes in affinity of the pore as the conformational changes occur - Initially Na binds in the intercellular environment where conc of Na is initially low and as the conformational change occurs Na+ needs to leave the transporter protein in the extracellular environment where the concentration is very high and how that occurs is that the conformational change in the protein reduces the affinity of the pore for Na+. so throughout the cycle there’s changes in affinity of the binding cites of Na and K that allow those ions to be released into the different environments o Allows Na to be released into the extracellular environment where Na is high o Allows K to be released into the intercellular environment where K or ion concentrations are high Slide 30: transport proteins work together to transfer glucose from the intestine to the blood stream - 3 different proteins that move glucose and Na: o The uniporter o Na-glucose symporter o K+ pump  In certain situations these 3 proteins can function together and rely on eachother to perform a specific function  Ex: transcellular transport of glucose across the intestinal epithelium  this is an example when the glucose concentration is very high inside cells and it’s lower in the intestinal lumen and glucose needs to be moved into the cells that line the intestine and out of the cells in the other side and eventually into the blood stream from this extracellular fluid  can’t do this using passive transport (b/c have to move glucose against its conc gradient)  different proteins used in this process is restricted to different parts of the cell o on the apical domain the Na-glucose symporter proteins are localized upto that apical domain and glucose and Na binds and glucose is transported into the cell against its concentration gradient  active transport process (harnesses Na gradient) o on the basal domain the glucose uniporter can transport glucose out of the cell down its concentration gradient (not active b/w glucose goes from high conc to low conc) o Na-K pump maintains a low concentration of Na inside the cell by pumping Na out of the cell and K in and that low conc of Na on the inside of the cell allows the Na-glucose symporter to function on the apical domain  So these 3 proteins work together in the transporting of glucose and all depend on eachother acting in the right way for this process to occur Slide 31: - Polarized cell  cell that doesn’t look the same from all sides  has different functions from different sides of the cell - The apical side of membrane and the basal part of membrane are different  have different compliments of proteins and perform different functions - Basal membrane is much flatter, and attached to an extracellular matrix called basla laniar. - Some cells have a round shape  but polarized cells have distinct functions and proteins on ether side. - Apical domain has different morphology  has vili that increase the surface area and allow more nutrients to be extracted from the intestine - Compared to apical domain the basal domain is much flatter and it’s attached to an extracellular matrix called a laminar - Recall that membrane proteins can move freely in the bilayer so how does this happen? How do you end up having the Na-glucose symporter up in the apical domain and the glucose uniporter and
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