1 28 15 Lecture 7 Membrane Transport Diffusion through lipid bilayer o With gradient ex high to low no energy cost for the cell o Small nonpolar ex Oxygen carbon dioxide o What problem does this cause There is no storage of oxygen in the cell because it can just keep diffusing through the membrane Getting the gradient in the correct direction Try to create a high of oxygen on the outside so oxygen will go o Solution into the cell o Alternative solution deceive the gradient Once oxygen is inside the cell we can attach it to something else so it takes on the whole characteristics of that compound When it is attached to something it is no longer a part of its original gradient Now the compound is too large to cross the membrane Diffusion through protein channel Ion Channels o Integral transmembrane proteins o Creates a way for polar components to pass through a channel with a polar interior o Movement with the gradient no energy cost for the cell o Gating We want diffusion to happen sometimes but not all the time so we need to be able to control if the channel is open or closed Channel is normally closed and we do something to get it open True for more channels not all There are different ways to open this channel 1 ligand gated o ligand binding to channel o Whenever we bind something to a protein we will change its shape reason the channel opens o Analogous to using your key to open your car o Ex sodium potassium pumps 2 Voltage gated o change in electrical distribution across the membrane 1 28 15 o Analogous to using the clicker to open your car because it uses electrical activity 3 Mechanically gated membrane stretched o The membrane is mobile so we can move things around o Think about your bladder stretching when it is full o Analogous to breaking into the car and physically opening the door o Special Case Aquaporins water channels Not ion channels because ions don t move in Moves via osmosis NOT diffusion o Osmosis net diffusion of H O across a membrane Movement from high to low concentration of water Movement from low to high concentration of solutes Water moves to higher Osmolarity High osmolarity lots of things not a lot of water Measurement used is osmolarity It is the total solute concentration in a solution o X of x volume o Math on this exam Ex If we have a solution of 3 mol Mg 2 mol of glucose in 1 L of water the osmolarity is 5 Osm 3 mol MgCl 2 mol of glucose in 1 L 11 Osm Ionic compounds dissociate glucose does not A higher osmolarity means there is a higher solute water concentration Lots of solutes not a lot of water Water moves towards places of high osmolarity in attempt to make the osmolatirites equal The higher the osmolarity difference the higher the osmotic pressure will end up being after the water has finished moving Size of cells impacted by water movement Isotonic solution same osmolarity level o Same concentration of non penetrating solutes outside cell o Water does not move since the osmolarity outside and inside the cell are equal o Volume of the cell stays the same mobile membrane Osmotic Pressure OP 1 28 15 Hypertonic solution excess of non penetrating outside the cell o Water moves out of the cell so the volume of the cell decreases cell shrinks Hypotonic Solution lower concentration of non penetrating solutes outside the cell o Water moves into the cell and it swells o The way our immune cell kills cells that are not working properly Most cells do not have osmotic pressure because they have 1 If membrane is permeable to both o Water and solutes can move o There is no osmotic pressure because the gradients are removed and the concentrations are balanced via diffusion 2 If permeable to water but not solute non penetrating solutes o Figure 4 18 o Assume the membrane can slide o Water can move so osmosis occurs o There is no osmotic pressure o We end up with the same osmolarity on both sides because Osm solute volume We changed the volume but not the solute concentration 3 Only permeable to water AND the membrane if immobile it doesn t slide around o Water diffusion causes pressure o PV nRT We increases the number of items n and therefore increase the pressure o Force applied to prevent diffusion Osmotic pressure Aim is to make the water concentration equal in membranes that are ONLY water permeable 1 28 15 o Water moves from low solute to high solute so one water level drops and the other one drops The osmotic pressure is higher in the area with the higher water level To have Osmotic pressure the membrane must be o Semipermeable only permeable to water o Immobile Osmotic difference results in osmotic pressure Carrier Mediated Transport o We are transporting things across the membrane o Membrane protein acts as a shuttle o Binding of transportee regulates transfer by causing the shape change of the transporter o Analogous to a revolving door or those horizontal rotating gate thingys at sporting events amusement parks We are interacting with the door the protein the cause a change o 1 Facilitated Diffusion With the gradient no energy cost How is this different to ligand gate o Response Ligand gated can move more than one molecule Based only on conformation shape changes Only one thing can move at a time Problem can become saturated overloaded Ex glucose We have a backup and the ligands need to wait their turn Ex too many people trying to get into a concert Remember glucose concentration is higher outside cell Important so glucose will go through transporters and go into the cell via facilitated diffusion
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