hello and welcome to this learn video in the series of a level biology for free lessons and today we're going to be covering transpiration and the cohesion tension Theory make sure you have some paper as you go through and there's a few questions and quizzes that you might want to have a go at so first of all transpiration now this is a concept you would have learned at GCSE it's the loss of water vapour from the stomata and that is by evaporation and this here is just demonstrating two guard cells which it surge it and therefore bent creating this stoma or pore and that's where the water vapor can evaporate out of and you mainly find those on the leaf in particular the lower side of the leaf so next thing is looking at the four key conditions which affect the rates the for our lice intensity temperature humidity and wind so pause the video at this stage to see if you can describe the effects that each of those conditions would have on the rate of transpiration but also explain why they have that effect so in light intensity is a positive correlation so the higher the lice intensity the faster the rate of transpiration the explanation is that as there's a highlights intensity that causes more stomata to open if you have more stomata open you have a larger surface area in which water vapor can evaporate out of temperature the higher versus the faster the rate of transpiration this time it's because if you add Heat then the water molecules will be gaining kinetic energy and therefore they'll be moving around faster and that will increase the rate of evaporation which is transpiration humidity has a negative correlation so the more humid that air is surrounding the leaf the lower or the slower the rates of transpiration and the explanation for this is if you have more water vapor in the air that will make the water potential in that air more positive compared to the inside of the leaf and therefore you're reducing the water potential gradient and the ability for the water to evaporate out when does the opposite idea and this is why I have a positive correlation the more windy it is the faster the rate and that's because wind or air movements will carry away the water vapor that is in the air surrounding the leaf and if you don't have that saturated air anymore you've maintained the water potential gradients and therefore more water can evaporate out so that's transpiration but the next idea is the cohesion tension theory and this starts to go into how it's possible for water to evaporate out of these leaves but then a whole column of water to move up to replace it against gravity and for some plants such as large trees that could mean water is moving against gravity for several meters so there must be some quite strong forces in play to enable that to happen and this is what the cohesion tension theory explains and it's a combination of cohesion adhesion or capillarity and re pressure so I'm going to go through each of those starting with cohesion I would need to think back to biological molecules when you learns about the structure of water so water is dipolar and that means we have a slight negative which is on the oxygen and a slight positive region which is on the two hydrogens and because we've got these two different charge regions the water molecules are able to form hydrogen bonds with each other and those occur between the hydrogen and oxygen of different water molecules and that's what these dashed lines are representing the effect that has then is the water molecules stick together and that we mean by cohesion cohesion is the sticking together of water molecules the impact that has is that water will travel up the xylem as one continuous column so instead of it moving up droplets at a time it's one continuous column so if you pull this water column from the top the entire column will move up with it capillarity so this is the idea of adhesion adhesion is when the water molecules can stick to the walls of the xylem and the impact that has I'm going to demonstrate with this analogy of drinking out of a straw so you've got three options here of the diameter of straw we've got the widest through the narrowest and when you put straw into water the liquid does move up it doesn't just stop here where the straw starts it does move up and that's because when you put the straw into the water adhesion is happening the water molecules are sticking to the walls of the straw and this one over on the right the most narrow tube the water is in more water is in contact with the walls of the straw so therefore more of it can stick and actually move further up the straw without you even sucking or drinking from the other end so this will be happening in the xylem as well without even having any pull action from the top the water will actually move up slightly because of adhesion and the narrower the xylem is the bigger the impact of capillarity so a narrow xylem will help move water up the last idea that we can add to this is route pressure so as water moves into the roots by osmosis you then have a larger volume of liquid but in the same space and this increases the pressure inside of the roots and as a result we get what's called positive pressure which gives this push action so as water moves into the roots because it's now a higher pressure it pushes any water above it further up so you get this pushing upwards from the bottom of the water column so those three ideas collectively cohesion sticking together with the water molecules adhesion the water molecules stick into the water the xylem and the positive root pressure so they're pushing up from the bottom are all parts of explaining the cohesion tension theory but we'll put those all together now and link it to the original question how can we move water up a tree several meters against gravity so step one we said that water evaporates out of the stomata which is what transpiration is so here we have our water vapor evaporating and that then means that there's a loss of water volume creating a lower pressure in the space where the water evaporated from as a result water from the xylem moves up to take the place and this is negative pressure so because the pressure had decreased we get this pulling action on the water column behind it the next idea is this explanation of the water column so this cohesion occurs between water molecules so the water molecules stick together and that's because of the hydrogen bonds between them so as the water is being pulled from the top it all moves up as one continuous water column because all the molecules are stuck together and not only that we say that the water molecules also stick to the walls of the xylem and the reason that helps is if you're pulling up that column and they can also stick to the walls along the way it means they're less likely to fall back down due to the force of gravity the last part then is this tension part of cohesion tension theory and the tension is created by the pull from the top so as that wall is moving up or being pulled up to replace the water that's evaporated that pull on the water column which is attached to the water the xylem draws in or pulls in the walls of the xylem so you actually end up with a narrower xylem cheap and therefore increase in the impact of capillarity even further so in summary transpiration is the loss of water vapor from the stomata this is the evaporation and it mainly occurs on the leaves which is where the smarterer temperature light intensity humidity and air movement all affect the rate cohesion we said that is between water molecules and it creates a continuous water column in the xylem adhesion is the water molecules sticking to the wall of the xylem and that creases creates the capillarity and lasts these idea of tension so as the water vapor evaporates it causes an upwards pull on the water column this creates tension pulls the xylem walls in making their diameter narrower and thus increasing the action of Kappa Kappa rarity even further so collectively that is the cohesion tension theory so hope this has helped you understand transpiration and the cohesion tension theory further you might want to go on to one of the next learned videos on mass transports and so you could look at how sucrose is transported on mass in a plant as well and don't forget to subscribe to keep up to date and all the latest videos that are added