hi and Welcome to My OC a level biology revision session with me Christine so today's lesson I want to look at the xylem and the flow and tissue so let's start off with the xylem then now it's important to note that when I talk about the xylem I'm talking about a tissue so we have cells that are grouped together to form a tissue so the xylm tissue is composed of dead cells so these cells are not living they are dead therefore they do not contain a nucleus they do not have the genetic material that allows them to perform some of the metabolic processes necessary like protein synthesis respiration any of that so therefore we call this dead cells and they're arranged in end to end now what that means is they form this really Long Hollow tube so a bit like a straw if you think about a straw a straw is just a Long Hollow chip and therefore only thing that is present is the outside of the straw well the zylin vessels the same what we have is that all these cells have been connected to form this long Hol tube and they are perforated by an end plate now that end plate allows for the movement of water and mineral ions to make their way up through the zylon V vessel before the cells actually died what the cell needed to do to ensure that the zylon vessel was strengthened is they had to thicken their cell walls so remember plant cells have a cell wall which is made up of cellul that is all cell walls but what the plant actually does in the xylon vessel is it thickens this wall with a polysaccharide called lignin now that polysaccharide is going to to ensure that that xylon vessel stays open and does not collapse so it ensures that the flow of water can continue up through the plant now alongside the xylem vessels what we also have are these xylm parenchima so xylm parenchima are little cells that are surrounding the zylon vessel which are there to store food and also store tanin deposits so when you look at plant responses this is a bitter tasting chemical which is going to prevent or deter the herbivores from feeding on the plant so it's a protective mechanism and also the Zin parenchima are going to need to store the fidge remember these are cells these are cells that are going to require the storage of macroo molecules so that they can perform respiration to allow for the metabolic processes so the xylon vessel itself is a dead cell it's tissue that's made up of dead cells whereas the xylon parenchima have not yet got to the point where they have matured to become the xylon tissue so the other thing to note is not only does the xylon parenchima store the food and the Tannon deposits it needs a way of getting that substance to it and therefore the xylem is actually bordered with these pits now what that allows is that allows for lateral water movement our cells are made up of cytoplasm cytoplasm is a liquid and therefore the water needs to move across to the cells providing the mineral ions that may be necessary for the cell to perform its function so how do we know what the xylem vessel looks like how can we observe it so one of the practical skills that you are expected to know is that if you were to take a plant that has a flower or some leaves above so you can do this with celery you can do this as you see here I did it with daffodils what you do is you place the plant in water contain containing these dyes now I used red and I used a blue colored dye and what I then did is I left it now I had to leave it a good 2 to three hours before the dye was able to move up now normally you would leave it for about 24 hours and what it means is that when you actually take either a transverse or longitudinal section so when you use a very sharp blade and you cut through the stem of the plant that you're observing what you should see is you should see that as the water has been pulled up through the xylin vessel it has taken the dye with it and because of that the dye will be deposited and it will stain the section that you're looking for so it's important to note that when you do this type of practical that you have to think about the precautions you're going to need to use a sharp blade so therefore you need to be careful that you do not cut yourself you really want to have very thin sections that you are cutting so that you are able to observe them that the light can penetrate now I was using a microscope which was looking from above which was connected to a computer so I could only zoom in so much whereas if you were going to be doing this your section would want to be very very thin to allow the light to penetrate through so that you could observe the different sections so if we move on then and we start to look at the flum so the vascular bundle is always made up of the flum and the xylm and there are some similarities but there's also some differences so one of the key things that we have here is with the flum the Flo is the vessel that is going to be transporting the food molecules for the plant so things like the sugars the sucros and the amino acids so if you haven't checked out my trans location video then please do check that one out but what we want to understand is that our cells are actually connected together but they are perforated by the Civ plate so this cell wall has been perforated and we call that the Civ plates but the column of the specialized Civ chup cells is actually called the Civ chup element so that's the name you want to give when you are talking about the flum so the flum is made up of these Civ chbe elements these Columns of specialized cells because we're talking about a tissue we therefore need to ensure that we're talking about Civ tube cells now the difference between the flum to the xylm is that the flum is living It's Not Dead Like The zylon Vessel was however it is is living it has reduced its cytoplasm it does not have a nucleus or organel so what is it that makes it living well the fact that it's living is because it has these companion cells associated with it so all around the flin vessel you have these companion cells now those companion cells have got the nucleus therefore they have the genetic information that is necessary for the metabolic functions they have the mitochondria necessary for ATP synthesis so because the flum has these companion cells which are connected by these plaso desata it means that the flu and vessel is class as being living so the fluin vessel is living whereas the xylin vessel is dead The fluin Vessel has got these Civ plates these perforated cell walls whereas The xylm Vessel has got these perforated end walls so it's important that you note the similarities but also the differences because that's what they'll be looking for in these exam questions and if I were you I would be writing it in a table or with connective sentences this one has this whereas that one has that so the xylin vessel has the bordered pits whereas The fluen Vessel does not the flu FL vessel has the Civ plates whereas The zylin Vessel does not the fluin vessel is living whereas the zylon vessel is made of dead cells so what does it actually look like in different places this vascular bundle within the plant now they love throwing an electron micrograph at you and they expect you to know how to label which one's the xylm and which one's the flum so the way I look at it and how it helps me is I always think of inside the root it the xylem looks like an X so therefore if I look for the area inside the middle that kind of gives me an X shape that's my zylon vessel also I can also note where is the larger Holo tube so if I've got a larger Holo tube a large hole section there then I can use that to identify my xylem so in my picture I can see that my m is labeling my xylon vessel then around the xylon vessel very close you have what's known as your flm so the flm is always very close to the xylm vessel and in between the flu and the zylon vessel what you actually have are merry stem cells the cambium and what the cambium can do is they are undifferentiated cells that have the capacity to go through mitotic Division and then differentiate into either the flum or the xylm so as we move away from the root and as we move into the stem what you start to see is the location of the vascular bundle is no longer in the center as it was in the root it's now towards the outside however the xylm is always located in my vascular bundle in the inside of the stem so therefore towards the middle so when we look at my xylm vessel in my vascular bundle I can see that it is always located on the inside and the flu and vessel is always located on the outside closest to the edge of the stem now in between that xylm and the flum is where you find your cambium and that cambium is filled with those merry stem cells so I've tried to identify that with my red line going all the way around to distinguish between my zylon vessel which is the blue colored section my Flor and vessel which is the Lilac section and then the cambium is in between again if you're looking at an electro micrograph then do make sure you look for the one that's got the bigger hole that is the xylem the smaller ones underneath that would be the flum and then you can look at the leaf and if we're looking at the Lea what we tend to find again look for the size of the holes but the xylon vessel is always at the top section of the vascular bundle in the leaf that's because that xylon vessel needs to be closest to the palisade misail layer and that therefore means because the palisade misail layer is the place where the most photosynthesis occurs that's providing the necessary mineral and water molecules for photosynthesis to occur so that's how I remember that the xylem is at the top because we want photosynthesis happening mainly at the top the water is needed for photosynthesis so therefore that is located at the top and the Flor is below and again look for the size of the hole to help you to identify that so I hope you like this video if you 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