hi everyone and welcome to learn a level biology for free with miss estrich in this video i'm going to be going through transport across membranes so it follows on from the video on the cell surface membrane and organelle membranes which i'll link here for you to watch if you haven't already and just a recap from that one in that lesson i went through all the components that you find in a plasma membrane and this is then used to explain why lipid solid molecules can simply diffuse through that phospholipid bilayer as well as very small molecules like carbon dioxide oxygen and water however because of the properties of that phospholipid water soluble or polar or ion substances cannot simply diffuse through the phospholipid bilayer or if molecules are too large like glucose so within this video we'll be looking at simple diffusion but also the other mechanisms for how molecules can transport from one side of a membrane to another whether that is on the outside of a cell into the center of the cell or from the cytoplasm into an organelle so the first type of diffusion is simple diffusion and this is the net movement of molecules from an area of higher concentration to an area of lower concentration and that will continue until equilibrium is reached or you have the same concentration on both sides of the membrane and this process doesn't require any atp so we can see here we've got a high concentration on one side of the membrane compared to the other and those molecules are starting to diffuse through that membrane and that continues until equilibrium is reached a few other points about diffusion for the molecules to move they do have to have energy because they are moving but that energy is just kinetic energy that they possess already which enables them to flow so diffusion simple diffusion only occurs in liquids and gases and that's because they possess kinetic energy to enable them to flow so solids cannot undergo diffusion for molecules to diffuse across the membrane as we said in the slide earlier they have to be small and they have to be lipid soluble so the second type of diffusion is facilitated diffusion it's still a passive process and by that we mean it doesn't require any additional atp energy but it's different from simple diffusion because it does use proteins embedded within the membrane and those are used to transport the molecules from one side of the membrane to the other and this is to enable ions so molecules that are polar and therefore they are not lipid soluble or any molecules are too big to simply diffuse through the phospholipid bilayer they will move by facilitated diffusion so it's still the movement from a high concentration to a low concentration the only difference is it's through either protein channels or carriers to enable these types of molecules to pass so a protein channel this is a protein embedded all the way through the phospholipid bilayer and the hollow tube that you can see here fills with water and because it's filled with water that enables water-soluble ions or molecules to pass they dissolve and then they pass through the channel whereas they are unable to dissolve through that phospholipid bilayer so it is selective because the channel proteins are only open in the presence of certain ions when they bind to that protein alternatively we have these carrier proteins which these final three are carrier proteins and in this case the molecule has to be complementary in shape to the carrier protein it will bind and that will then cause the protein to slightly change shape and that change in shape causes it to release the molecule on the other side of the membrane so this doesn't require energy it's just to do with the binding of shapes and that causes the protein to slightly change shape in itself and that is the way that glucose would be transported by facilitated diffusion next then is osmosis and this is only the movement of water and it's the movement of water from an area of higher water potential to an area of a lower water potential and it's through a partially permeable membrane so that is a three mark definition the two points that are in bold but also the fact that it's the movement of water so a bit more about what water potential means then that is the pressure created by water molecules so that's why the unit for water potential is kilopascals which is a units of pressure and the symbol used to represent water potential is this here so water potential is used instead of concentration of solution which you would have said at gcse and to put into context what this means pure water has a water potential of zero so you cannot get a positive value for water potential zero is the highest value you'll get so when you then dissolve solutes in the water the water potential will become negative so the more negative the value for the water potential that is an indication that you have more solutes dissolved in that water so it's a more concentrated solution so for example here we've got minus 3.1 kilopascals compared to minus 0.5 kilopascals so this side has got a more negative water potential which you can see with the diagram does show there are more solutes dissolved in that particular solution so that means the water is going to move across this partially permeable membrane from an area of a higher water potential to an area of a lower water potential both of which are still negative water potential so so following on from that is what we mean by these three terms isotonic hypotonic and hypertonic so iso is when the water potential on both sides of the membrane are the same hypotonic is when the water potential of a solution is more positive compared to the cell and when we say more positive it does not mean it's a positive value it just means it's a negative value that is getting closer to zero hypertonic is when the water potential of a solution is more negative compared to the cell so it'd be a more concentrated solution compared to the cell and this is just indicating here with these diagrams what would happen to animal cells for example red blood cells if they were placed in these types of solutions so if you put an animal cell in an isotonic solution there's not going to be any net gain of water because it's already at equilibrium there's the same water potential inside and outside however in animal cells if they are placed in a hypotonic solution for example distilled water that means that water will move into the cells by osmosis because the outside of the cell is more positive compared to the inside of the cell which is more negative so water will move in and eventually so much water could move in that the cell bursts or lysis occurs and that's because animal cells do not have a cell wall to strengthen them plant cells in comparison their cells wouldn't burst in hypotonic solutions instead because they do have a strength in cell wall they'll become turgid both animal and plant cells though if they're placed in a hypertonic solution because the solution outside of the cell is more negative the water will leave the cells by osmosis and that will cause the cell to shrivel up so the last type of transport is active transport now i have got an entire video on active transport in more detail as well as co-transport which i'll link just here for you to have a look at if you want more details but a basic summary of active transport is it's the movement of molecules and ions from an area of a lower concentration to a higher concentration so this is the only one which is going against the concentration gradient and because it's going against that gradient it does require atp and it also requires carrier proteins the carrier proteins they act as a pump to move substances from one side to the other side of the membrane this is very selective as only certain molecules which are complementary in shape to the protein will be able to bind and that's what we can see here in this diagram so we've got sodium ions are complementary and they're able to bind but because it's going against the concentration gradient atp is needed to attach to that protein to enable it to have enough energy to change shape and release those sodium ions on the other side of the membrane so certain molecules can bind to the receptor site on carrier proteins atp as i said that will then bind to the protein on the inside of the membrane and it's hydrolyzed into adp and pi and when it is hydrolyzed that releases a small amount of energy which is used to change the shape of the protein so it can then open on the other side and release the molecules that will then cause those molecules to be released the pi which is the inorganic phosphate molecule is then released from that carrier protein and this results in the protein reverting back to its original shape so the process can continue to happen as long as there is a supply of atp available so that is it for transport across membranes i hope you found that 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