welcome biologists today we're going to be looking at osmosis and this is taken from the biological membranes topic so we need to know first of all our definition for osmosis which is slightly different from a gcse definition we're still looking at the net movement of molecules really important you say the word net but this time we're looking at it from a region of high water potential to low water potential what's potential is the ability of water to move depending upon solutes really important as well we say through a partially purple membrane it's not a semi-permeable we don't get marks for that but we say partially permeable now water potential is represented by this little strange sign here at the bottom you can see here like a devil's fork and if we have pure water which contains no solute at all there's no sugar no salts dissolved into that pure water has the highest water potential at zero therefore if i start dissolving solutes into pure water the water potential will start to become more and more negative now if i take these two cells for example you get certain marks here for talking about osmosis and how it and where water and how water flows so as you can see here my cell b which very unlikely for cells have this water potential but it's just to get an example across in this particular cell here it has a high water potential of zero so it's really important as well you you're using comparable term terminology here so you're saying higher or lower than otherwise you don't get the marks so here what we need to say to get the marks is that cell b has a higher water potential than cell a therefore water will move from cell b into cell a down the water potential gradient by osmosis so always get marks for saying where the water potential is high where it's low that water's moving by osmosis and down the water potential gradient you may also get more marks for talking and applying it to the actual question which we'll get on to talking about in a second if i apply now cell c to this and i just compare cell b and c so ignore a for now if i'll just look at b and c b still has a higher water potential than c so therefore water would move from cell b to c down the water potential gradient by osmosis if i looked at and compared cell a and c and ignored b for now cell c has the higher water potential of those two cells therefore cell the water would move from cell c to cell a down the water potential gradient by osmosis i hoping you're getting this the pattern there to get your marks a couple of questions there if you want to pause it and have a go but there are a couple of solutions that we need to know about so there are three kind of solutions you'll get asked about you don't really need to know the words here in red never seen them they're not very common on the master scheme but the key thing here is to notice they can either be put into pure water which has a higher water than the er higher water potentials surrounding the cell or it can be put into a solution with a lower water potential than the contents inside the cell or it can be put into an isotonic solution where the surrounding solution is the same water potential as inside the cell so my red onion cells are a plant cell in a hypotonic solution this is where water has entered into the cells by osmosis so this is where it's been put into for example a pure water a higher water potential so there's a higher water potential outside of the cell therefore water has moved in by osmosis down the water potential gradient now in plant cells you get do get mass for explaining that that water goes into the vacuole that the vacuole um pushes out and puts turgor pressure on and put pressure onto the cell wall to make those cells turgid now if i put that same plant cell into a lower water potential this means that the water inside the vacuole will leave the plant cells by osmosis down the water potential gradient into the surrounding solution and what will happen here is my cells become fasted and eventually plasmalized and plasmalized as you can see in this image here is where the cell membrane is actually pulled away from the cell wall now the last kind of questions you'll get here to do with animal cells in these different solutions so as you can see here my water potential surrounding my red blood cells here was lower than inside the cells therefore water has left those red blood cells down the water potential gradient by osmosis if i look at this one over here that my cells here have been put into pure water so what happened what happened what's happened here water's moved into the cells by osmosis down the water potential gradient causing these cells to burst now these burst because there is no plant cell wall because it's an animal cell