hi everyone and welcome to learn a level biology for free with Miss Esther ik in this video I'm gonna be going through how pedometers can be used to measure the uptake of water as a way to measure the rate of transpiration so I've got a whole video on transpiration cohesion tension which I've it's linked here so you can watch that first so just a brief recap what is transpiration so that is when water vapour is evaporating out of open stomata which are mainly found on the leaves and we can see here that when that water vapor does evaporate out of the leaves that will then draw up this continuous water column through the xylem of the plant and you get this continuous flow of water from the soil where it's absorbed bars from bars maysa and then up the xylem and out by transpiration and there are four key factors which affect the rate of transpiration temperature humidity air movement or wind and light intensity so in this session we're going through how you can actually measure the rate of uptake of water to be a representation of the rate of transpiration so it's using this piece of equipment which is called a potometer and what potometer z' measure is how much water a particular piece of plant is taking up in a period of time now the reason that it's measuring uptake of water rather than transpiration is because it's near impossible to measure the rate of transpiration and that's because it's water vapor that is coming out sits evaporation it's very difficult to measure that so instead because the amount of water that is taken up is almost the same as the amount of water which evaporates by transpiration we take it to be that however much water was taken up is proportional to the rate of transpiration so that's what it's measuring the uses are you could just use it to see the rate of transpiration for one particular plant and compare that to a different plant species or it could be the same plant but looking at the effect of those four variables that we said have an impact so light intensity air movement humidity and temperature so looking at how it's set up so the first thing is you need to get your sample or plant into the potometer and you tend to just cut off a small section which you can see in this diagram from the main plants and when you cut it it has to be cut underwater and the reason for that is these xylem has negative pressure meaning it's constantly pulling up the water from lower down so if you were to cut the plant in the air it's going to be pulling air into the xylem and that will then break the continuous column of water and you won't have transpiration working or at least it won't work very efficiently so we cut it under water to make sure only water is being drawn into the xylem the next step is the potometer equipment all needs to be filled with water and again this is done completely submerged in water and that's to make sure all air bubbles are removed we then get that's leafy part of the plant is put into the potometer through a rubber seal like a rubber bung and all of the sections where there is a joint gets covered in petroleum jelly to make the equipment completely airtight and that's to make sure no air bubbles can get in which would prevents the flow of water but also to make sure no water can leak out and therefore affect the accuracy of the measure of the uptake of water the final thing in terms of the setup is one single bubble does have to deliberately be introduced and the way that is introduced is at this end here now this capillary tube will be lifted out of the water probably for about five ten seconds and then place back into the water and that five ten-second gap will be enough time for some air to be drawn in to create that one air bubble and the point of that is when that air bubble reaches zero on the scale on your capillary tube you can then start your stop clock and then you can see how far that air bubble travels in a certain period of time so these measurements and are used for you to see how much water has taken up and the way you're getting that idea is you're looking at how far that air bubble moves so this water is continuously moving towards the plant because as that plant is transpiring more water is drawn up into the plant so to convert this into an estimate transpiration the rate the distance of that bubble has moved and can be used to work out the volume of water so you would need to know the volume of a cylinder to be able to work out the volume of water has moved and then you retire as I divide that by the time it took for that water to travel and you can reset the apparatus and that is the purpose of this reservoir of water and this tap is closed when you're doing the experiment but if you open this tap that allows water from this reservoir to move into the capillary tube and it then pushes that air bubble and you can leave it flowing so it pushes the air bubble all the way out or just back to the starts then close the tap and restart the experiment so you can get multiple repeats so that's how you would use a pedometer just to check the understanding then I've got four exam questions which are quite common linked to the use of potometer x' so we'll go through them one at a time if you want to have a go at the first just pause the video now have a go at those four questions and then press play and continue at that point so the first one was explain why the apparatus must be set up and the plant shoot must be cut underwater so this is what I was saying it's due to that cohesion tension theory it creates a negative pressure in the xylem so it's constantly pulling up the water so if it was that I should say it was there so if it was cut in the air it would draw air into the xylem instead and I'd break that continuous water column and prevent transpiration so instead you have to cut it underwater to make sure no air bubbles are introduced into the xylem the next one and why must all the joints in the apparatus be covered in petroleum jelly first thing it's just pointing out petroleum jelly is waterproof so by covering all of these joints in this waterproof substance it makes sure that no air can be introduced into the apparatus but also make sure that no water can leak out which would impact your accuracy of your estimate of water taken up so the common maths question is to work out the volume of water taken up and to represent transpiration and to work that out because it's a rate you need to know the volume of water that has been taken up by the plants and that would be divided by the time it took for that quantity of water to be taken up so here are the figures that were given in this example the air bubble has moved 15 point 28 millimeters in one minute so they left it for one minute and that air bubble has moved 15 point to 8 millimeters along that scale the next piece of information is the radius of that capillary tube is not 0.5 millimeters so using that information we can work out the volume and because it's a rate it would be the volume divided by time taken it was only one minute so only be divided by one so essentially we just need to work out the volume so to work out the volume of a cylinder it's PI squared times L which is the length or the distance that the air bubble moved now it's going to assume that pi is 3.14 - in this example so we can then put in the data so we've got PI not 0.5 squared and the length of the distance was 15 point 2 8 multiply all of those and it comes to 12 so that's 12 millimeters per minute sometimes you might be asked to do a conversion they might ask you to convert that to the different units it may be centimeters instead of millimeters and per minute perhaps they might ask you to convert it to seconds or hours so the last question is what variables would have to be controlled if you had to perform this experiment on two different plant species so you're examining is there a difference between the rate of transpiration on those two different plant species and the key variable that has to be controlled is the surface area of the leaves so you need to make sure there's the same number and the same size leaves so that's a fair comparison and that is it for the potometer measuring the rate of transpiration if you found it helpful please give it a thumbs up [Music]