hi everybody and welcome back to miss angler's biology class i am miss angler and in today's video we are going to be taking a look at transpiration which is the water loss that we see through the stomata in plants now if you don't know enough about plant tissues you're going to want to click the link above now to watch my video on plant tissues because it's going to facilitate your learning and make it much easier to understand this topic now as i mentioned now transpiration is the plant's way of essentially losing water through their stomata which we can see here in the diagram as these small openings that we find on the underside of the leaf some plants have stomata on the stems as well but for our explanations we're going to focus on the stomata being on the leaf and how water moves from the roots up the stem and then through and out of the leaves i'm also going to walk you through how you should explain that and what kind of questions and investigations you might have to do in class and in exams now in order to have a really good understanding of transpiration you need to be very well versed in water moving through plants and you need to be able to understand the structures that facilitate this now on the left hand side what we see is the route that water takes and that means you actually need quite a bit of knowledge about how water moves into the roots up a stem and then eventually out of the leaves via the stomata which brings me to the second picture that we see here it is a diagram of stomata which are the poor life openings on the underside of leaves which is where transpiration occurs now in exams and tests you may be asked to draw or label a stomata and i just want to run through some of the key things about its structure first things first the most identifiable thing about stomata is their guard cells which are these sort of like jelly bean shaped cells that we see over here and what you will notice in the two pictures is that one of them is open and one of them is closed now the opening that it creates is called the stoma please don't confuse it with the stro ma that's s t r o m a that's uh the filling of a chloroplast so let's not confuse those two things the stoma is the opening and um essentially that is affected by the vacuole and whether the vacuole is inflated or deflated will open and close these guard cells now the guard cells are also really unique because they have chloroplasts in them and they have chloroplasts in them because it needs to provide the stomata with energy in order to open and close the next important thing that you will also notice is that they are surrounded by these other epidermal cells and these epidermal cells are thin and they have no chloroplasts in them because they don't need to now that we have a good understanding of the root that water is taking and the structure they are moving through now let's actually look at what actually is happening and how to actually explain transpiration so let's get into the route that water is taking when it is going through transpiration in the leaf in particular now at this point you should be pretty well versed in plant tissues and cells because if you don't know their structure and their function it's going to be really difficult to explain this now in this picture here we have a cross section through a leaf and at this point i also expect you to know what all of these structures do we have the cuticle which is a waxy layer that prevents water loss the epidermis which is a protective outer thin one cell layer thick we have the palisade mesophyll which is where the majority of photosynthesis takes place we have the vein which is made up of xylem and phloem very important for what we're going to talk about now we have the lower dermis which is just the mirror image of the epidermis also for protection we have the spongy mesophyll which is very important for transpiration it's the packaging tissue in the leaf it also has lots of intercellular air spaces and lastly we have the stomata which is of course the gateway for water and gases to leave the plant now with being said of all of these structures i'm going to walk you through how water actually diffuses out of the xylem through the plant and then out through the stomata so let me give you just the basics and then i'm going to break it down into how you should actually be explaining this so in the vein of a leaf water is going to move from the xylem and it's going to move into the spongy mesophyll from the spongy mesophyll it is going to move into these intercellular air spaces that we see here from there it is going to then move out of the stomata into the atmosphere so that is the route it takes but now what i want to do is explain to you how it actually does this so first things first we need to start off at the source of water that is entering the leaf which is in the xylem so let us draw two little xylem vessels next to each other and then sitting up against the xylem vessels i'm going to draw some spongy mesophyll cells and i'm going to leave a big area or an intercellular airspace that you can see here now what's important to understand is that water is consistently moving through the plant via transpiration and transpiration pool but the mechanism that is driving that is concentration gradients and so what's happening is you are always moving from a high concentration of water inside the xylem to a low concentration inside the spongy mesophyll and that is moving through a process which we know as diffusion right but more specifically it's not just diffusion it's osmosis because osmosis is the movement of water specifically diffusion can be anything else it can also be gases it can be nutrients so make sure you use the word osmosis when you talk about water moving from a high to a low now as the water leaves the xylem and makes its way into the spongy mesophyll cells it starts to turn that area inside of our spongy mesophyll also into a higher area higher in relevancy in other words in relation to what it was before it's now become a little bit higher now as that happens water starts to accumulate on the surface of these cells now that means that the outside of the cell is becoming a higher concentration but these intercellular air spaces these little pockets are lower now this is where it gets interesting the water cannot just move through air the only way that water can just move through air is if it evaporates and that is why we need the little air pockets because water will only evaporate if it is exposed to air and of course a temperature so what happens is the water on the surface of these cells starts to evaporate into these intercellular air spaces it becomes water vapor and now what you end up having is water vapor collecting inside of these empty spaces and now that in turn creates a higher pressure on the inside so now we're not a high pressure i make a part in a higher concentration and now that high concentration of water is going to need to leave because if i were to draw at the bottom here an opening which would represent our stomata now what you have is a higher concentration of water vapor inside these spaces and a lower concentration outside of the stomata outside of the plant in the atmosphere so now what you have is water evaporating and then moving out into the atmosphere where it moved to the lower concentration gradient and that's essentially how transpiration works it is a process of diffusion via osmosis as well as a process of evaporation which is when of course water is evaporating turning from a liquid into a gas moving from a high concentration to a low concentration now when it comes to measuring transpiration we also need to know how this works and what device we use now the most classic device that you may see in your textbook or you've seen explained before is called a pitometer now a potometer is a very basic setup and essentially there are some precautions that you need to take just to make sure that you set it up correctly the first thing is this little leafy twig that we're going to use to test for transpiration needs to be fresh and also when you cut it and you want to place it into the potometer you need to make sure that number one you cut it under water that's because you don't want any air getting into the stem you just want a continuous stream of water and the other thing that you want to do is if this represents the stem of our leafy twig we want to make sure we cut it at an angle this increases the surface area and it makes this experiment work a whole lot better another little precaution that we always take is wherever this rubber stopper is and anywhere else like where the tap is you want to put some vaseline because vaseline is water and airtight and so basically it makes sure that nothing leaks in and affects the validity of your experiment now how does the petometer work i need you to imagine that this entire setup is filled with water as you can see and there is no air in here other than this little air bubble over here now that air bubble is an indicator of whether there's been any movement of water now if transpiration is happening correctly and this is working properly essentially what should happen is the water that is in the beaker should move up through the tube past the markers up and then into the leafy twig through its stem and then out through its leaves that's how it should work now if that is working and that is the case then this little air bubble that we see over here should be moving over like that to the left and often what you see in exams or tests is they also include like a ruler and they'll be like measurements or increments that are on it and it will tell you something like it moved this many millimeters over this many hours and that can give you a rate at which transpiration is occurring because it's giving you a speed distance and time so you can calculate the speed if you have distance and you have time and you can calculate the rate of transpiration that's happening in a leafy twig now the final thing that we need to look at is the factors that affect the rate of transpiration so effectively how quickly transpiration is occurring and we're going to start off by looking at wind now wind is a really interesting one because as wind increases the rate of transpiration increases and this is how it works if this represents a stomata opening the regular movement of water is always going to be a higher concentration on the inside of the leaf moving outwards to a lower concentration on the outside now generally what will happen is water vapor will accumulate just outside the stomata and slowly but surely that can then turn this into a high concentration just outside and i would slow it down however if wind comes along it blows away all of these water vapor molecules that have accumulated on the outside and it maintains a low pressure sometimes it even makes it even lower than what it was before and so because of this blowing the water vapor away all the time it leads to a very steep concentration gradient now we move on to humidity now what's interesting about humidity is it is the opposite of what happens in wind and this is the reason why again if we have our stomatal opening the regular movement is from a high to a low but as we know humidity means that there is going to be a high concentration of water in the air on the outside of the leaf now that means if there is a lot of water vapor on the inside of the leaf and a lot of water vapor on the outside of the leaf it's going to reduce the concentration gradient and it's going to slow it down and so humidity if you don't know is how much water is in the air and that ultimately is what decreases transpiration as humidity increases the next factor affecting transpiration is temperature now temperature is a little bit of an easier one to understand because you've got to think of it again like a concentration gradient we have our stomata and we have our high concentration of water on the inside of the leaf and a low concentration on the outside now as water makes its way out of the stomatal opening when it is really really hot all of the water vapor that is collecting on the outside of the leaf starts to evaporate now if it evaporates that means you maintain the concentration gradient you maintain a high to a low and so as temperature increases the rate of transpiration also increases now the final one light intensity is a little bit abstract but if you understand the fact that light is needed for photosynthesis as light intensity increases transpiration will increase and there's two reasons why number one it does have to do with the temperature as we spoke about earlier and we saw in this diagram over here in the yellow if there is a lot of light there is generally a higher temperature so more water evaporates but also if there is more light there is going to be more photosynthesis and photosynthesis needs water so that means more water is going to be pulled up to the leaves so that they can photosynthesize and that water is ultimately lost through transpiration now as always i like to finish off my lessons with a terminology recap you can use all of these words on flash cards mind map out your ideas it makes studying so much easier first of all we spoke about the xylem tissue and its importance in transporting water and how it gets from the roots to the stem and into the leaves we also spoke about diffusion which is really really important to understand in order to explain transpiration and how substances move from a high to a low concentration specifically we need to know about osmosis which is the movement of water and there comes in that concentration gradient where we move from a high to a low and that is the way in which water goes from the xylem to the spongy mesophyll and then into the intercellular airspaces and out through the stomata the spongy mesophyll is the filling of the cell which is where the majority of this action and transpiration takes place and that water that moves into the spongy mesophyll is then evaporated into the intercellular air spaces it then exits through the stomata which are the openings or the pores in the undersides of leaves and to measure all of this we use a petometer now if you like this video don't forget to give it a thumbs up and make sure you are subscribed and i will see you all again soon bye