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BIO241H Lecture 2.docx

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Jennifer Harris

Thursday, January 8, 2008 Slide 1 - What is covered in the exam is what’s covered in the lectures. - For the next lectures, there will be fill-ins. - Tutorials will start the following Monday (not next Monday) – schedule is posted on Blackboard. Slide 2 Membrane transport of small molecules st Slide 3 - In the 1 lecture we looked at the membrane that surrounds the cell & surrounds the organelles of the cell. Went through the features of this membrane & 1 of the features of these membranes is that they isolate those membranes from many other parts of the cell. - Now we’re going to cover how small molecules are transported across these membranes b/c these organelles & the cell itself need many of the molecules that the membranes are impermeable to so there needs to be transport of small molecules across these membranes in order for the cell to survive. So we’re going to cover how small molecules are transported across the membranes. Slide 4 Slide 5 - The lipid bilayer is not permeable to all molecules. So the lipid bilayer is permeable to hydrophobic molecules such as O, carbon dioxide, N & benzene & the movement of these molecules across the membranes will be by simple diffusion across the membrane & these molecules will move across the membranes from a high concentration to a low concentration & this is referred to as going down the concentration gradient so hydrophobic molecules will do this readily. - Small uncharged polar molecules will also cross the membrane so the membrane is permeable to these molecules such as water, urea & glycerol but to a lesser degree than hydrophobic molecules. - Can conclude that more hydrophobic or nonpolar molecules will diffuse faster across the lipid bilayer, down its concentration gradient. Slide 6 - The lipid bilayer is relatively impermeable to large uncharged polar molecules like glucose & sucrose – there is a conundrum there right away as the cell needs this glucose in order to survive – so there has to be an active mechanism present to be able to transport this glucose across the membrane since the membrane itself is relatively impermeable to glucose which is a large uncharged polar molecule. - Also ions which are charged molecules & the membrane is impermeable, almost completely impermeable, to ions so the membrane requires proteins in order to transport ions & large uncharged polar molecules like glucose across the membranes. Slide 7 - Membrane transport proteins are what transport these molecules across the membrane. These are multipass transmembrane proteins – that means they transverse the membrane multiple times. - There’s different cell membranes & the different cell membranes such as the membranes that will surround an organelle or the plasma membrane will each have a different set of transport proteins which are selective for specific molecules. So you’ll have transport proteins that can transport glucose, for example, & then you’ll have other transport proteins that can transport ions, such as Ca or Na ions across the membrane. Na transport proteins can’t transport a glucose molecule. Slide 8 - There are 2 different types of transport that can occur across the membrane by transport proteins: 1) Passive transport – the transport of molecules down their concentration gradient – this is a process that is also called facilitated diffusion so diffusion will also occur down the concentration gradient. Transport proteins that partake in passive transport will transport molecules that are normally impermeable in the plasma membrane & they’ll transport these molecules down their concentration gradients also. 2) Active transport – the transport of molecules against the concentration gradient – so it will transport the molecule from a low concentration to a high concentration – since this is against the concentration gradient this active transport requires an input of energy. - Membranes have an electrical potential difference across them – this means that there is an uneven distribution of charges on each side of the membrane. Look at the plasma membrane: the outside of the plasma membrane is positively charged relative to the inside of the plasma membrane. So the inside is negatively charged & the outside is positively charged. Slide 9 - So for a charged molecule, the driving force for the transport of a small charged molecule will be a combination of both the concentration gradient & the membrane potential & this adds up to produce the driving force which is called the electrochemical gradient – so this is for a charged molecule. The movement of a charged molecule is influenced by both its concentration gradient & its membrane potential. For an uncharged molecule, it’s only the concentration gradient that will influence its transport. - Left diagram: This is a charged molecule with a high concentration outside & a low concentration inside – there is no membrane potential here so then the movement will only be influenced by the concentration gradient. - Middle diagram: If we add a membrane potential which is positive outside relative to inside there will be additive effect onto the concentration gradient b/c the positive charges tend to be repelled by the positive charges on this side of the membrane & so the positive charges tend to move towards negative charge on the inside of membrane – that additive effect will be the electrical gradient & the driving force of the 2 combined is stronger than in this situation where only the concentration gradient will be driving the movement of these molecules. In this situation, both the membrane potential & concentration gradient are driving the molecule in the same direction, having an additive effect. - Right diagram: in this example, the charges are reversed so you have a positive charge on the inside of the membrane & a negative charge on the outside of membrane – so the positive charges inside will tend to repel the positive charges as they are going in & the negative charges will tend to attract them & will want to maintain them on this side – so in this case, the concentration gradient & membrane potential are working against each other to present this electrochemical gradient. - For a charged molecule, it’s the both of these that will contribute to this electrochemical gradient which is the driving force that will influence the movement of these molecules across the membrane. Slide 10 - Passive transport: will transport a charged molecule down the electrochemical gradient. Remember for an uncharged molecule, it will transport it down its concentration gradient. - And for a charged molecule, active transport will be against the electrochemical gradient & again this requires energy. Slide 11 - Active transport is against concentration gradient & it requires energy since you’re driving a molecule against the gradient, you need energy. This energy can come from coupled transport – this means that the energy of driving 1 molecule down its concentration gradient is used to move a 2 nd molecule against its gradient so this is energetically favourable so you would be moving 1 molecule down its electrochemical gradient in this case & the nd energy that this would release would be used to move the 2 molecule against its electrochemical gradient – these would be 2 different molecules butndne transport protein would be carrying out both of these actions. - 2 type of transport is driven by ATP: hydrolysis of ATP is used to produce energy to drive the movement of molecules against their electrochemical gradient. - Some bacteria can harness light energy & use the energy from light in order to move molecules against their concentration gradient also – not going to go through any detailed examples of this kind, only focus on the previous 2. Slide 12 - What is passive transport? – Because it’s “electrochemical gradient”, we can assume it is a charged molecule. If it was an uncharged molecule, could also say it was down the concentration gradient of that solute. - What is active transport? – Again, if it was uncharged molecule, it would be against the concentration gradient of that solute. Slide 13 - This animation shows the movement of molecules by simple diffusion, different types of passive transport, channel-mediated, carrier-mediated passive transport & also active transport against the electrochemical gradient of a molecule. - Simple diffusion: there is no animation – just the movement of molecules from a high concentration to a low concentration for an uncharged molecule & down the electrochemical gradient for a charged molecule. - Channel-mediated passive transport – this is down the electrochemical gradient – the yellow molecules are moving down their electrochemical gradient to eventually lead to an equal concentration if this is an uncharged molecule. - Carrier-mediated passive transport – this is actually a protein that is involved in mediating passive transport across the membrane so again, it goes down the electrochemical gradient so you can see that the molecule is actually transporting the molecule across the membrane but this does not require any source of energy since the molecules are moving down the electrochemical gradient. Can see in this case something that is rather important is that the protein, once it binds to the molecule undergoes a conformational change. - Active transport – carried out by transport proteins is against the electrochemical gradient so we can see that this one would be at a low concentration at the inside of the membrane & this carrier would be actively transporting out using the hydrolysis of ATP as a source of energy to move this molecule against its electrochemical gradient. Slide 14 Slide 15 - They bind to a specific molecule. Ex: A glucose transporter will bind to glucose but not to ions. They undergo a conformational change once they bind to their molecule of interest & then what happens they will transport this molecule across the lipid bilayer. Slide 16 - A uniporter means that it is transporting one molecule at a time. Slide 17 - The direction of transport is reversible – so depending on the direction of the electrochemical gradient, if the electrochemical gradient was going down, it would transport this molecule in this direction (as indicated in the diagram). If the electrochemical gradient was going up, it could reverse its direction & move this molecule down its concentration gradient – depending on the direction of the electrochemical/concentration gradient, the direction of transport by uniporters can be reversed. - Animation: In the process known as facilitated diffusion, a special carrier protein with a central channel acts as a selective corridor which helps molecules move across the membrane. These special carrier molecules that form the protein channel bind only to a specific molecule, such as a particular sugar or amino acid. Once the molecule binds to the carrier protein, this protein helps or facilitates the diffusion process by changing shape & moving the molecule down its concentration gradient, through the membrane into the cell where it is released. Facilitated diffusion is similar to simple diffusion in that both involve movement of molecules down their concentration gradient & this movement is carried out without any input of energy. However, in facilitated diffusion, the movement of molecules will only take place if it is facilitated or helped by a special protein carrier in the membrane. Facilitated diffusion can occur in either direction depending on the concentration gradient. If there is a higher concentration of the particular molecule inside the cell, the same carrier protein would then transport the molecules out of the cell. - You can see how the uniporters can transport the molecules in either direction but again, the important part to recognize is that the molecules will only move across if the proteins are there to transport them across. In that way, these transporters can be thought of as enzymes – the reaction that they’re carrying out is the transport of the molecule across the membrane & their substrate is that small molecule Slide 18 - Like enzymes, they have characteristic Michalis-Menten kinetics where the molecule or uniporter will have a maximum velocity where the carrier is saturated & this is the maximum speed at which it can carry that small molecule across the membrane. At this concentration, that carrier is saturated. - It also has a Michalis constant which is half of v so if we look at the max maximum velocity on this curve, half of that maximum velocity is the solute concentration – this Michalis constan
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