hi there and welcome to a level biology with miss estrich in this video i'm going to be going through gas exchange implants and i'll go through it at a level standard however there is a lot of overlap with gcse if you are new here then click subscribe so you don't miss out on any of the latest videos so the first thing you need to be aware of is the structure of the leaf of a dicotyledonous leaf and this is the same in gcse as it is for a level so i've got a diagram here compared to an image from under the microscope so you can see the difference and you need to know these three key layers for gas exchange so on the lower side of the leaf is where you find most stomata and stomata is the plural stoma is singular and a stoma is the gap and it's the gap that is formed by these two guard cells so we have two guard cells and those will create a hole or a pore and that is what the stoma is and if you're talking about multiple that is stomata now that is the exact site of gas exchange and the gases as carbon dioxide diffuses in and go into the spongy mesophyll where you can see there's lots of space and that is space for the gases to diffuse in and it helps to maintain a concentration because as the carbon dioxide diffuses into this space can then diffuse up to the next layer the panacea mesophyll which is where most photosynthesis occurs because it's closer to the top surface and therefore will receive more direct sunlight so those are three structures you can actually also see the vascular bundle and that is where the xylem and phloem occur to transport the water and dissolve mineral ions in the xylem and the sucrose in the phloem and if we have a look at the micrograph we can see here is our vascular bundle where the xylem and phloem tissue is you can clearly see here as well lots of gaps between the cells so that whole layer which they've labeled e is the spongy mesophyll the palisade mesophyll you have much narrower longer oblong shaped cells and that's so you can pack lots of those cells in containing lots of chloroplasts and therefore chlorophyll to maximize the light absorption for photosynthesis in particular the light dependent reaction which i'll link up here so you can have a look at the stomata is a bit harder to see it on this image but as i said they're mainly on the lower side and you can see one here it appears a bit darker we've got another one here and another one just there so stomata are the holes the guard cells are the cells which create that pore so gas exchange occurs at the stomata and it's actually oxygen that diffuses out and carbon dioxide diffuses in and that's because carbon dioxide is required for photosynthesis and therefore it's constantly being used by the cells within the leaf particularly in the palisade mesophyll and that maintains this concentration gradient that you'll have a lower concentration in the spongy mesophyll compared to the atmosphere and that's why carbon dioxide diffuses in now oxygen is a useful gas it will be used within the plant for respiration however because oxygen is also a product of photosynthesis there will be high concentrations of oxygen within that spongy mesophyll compared to the atmosphere and that's why oxygen will diffuse out of the stomata now there is this compromise this balance between having the stomata open to get carbon dioxide diffusing in for photosynthesis and reducing the amount of water that will be evaporating out of that open pore so for that reason photosynthesis mainly occurs in the daytime when there's light and therefore at night time there's not an advantage of having the stoma open so the stomata actually close at night as the guard cells become less bent there are other ways to reduce water loss as well and plants in particular like serophites really do have to battle this compromise of requiring the carbon dioxide for photosynthesis but not losing that essential water for evaporation so xerophytic plants are plants that can survive really harsh conditions which are a lack of water and they have lots of structural features to enable them to still be able to exchange gases through their stomata whilst also limiting water loss so we're gonna have a look at some of those i'm just showing you down here the micrograph of a cross section through the leaf of marine grass and this is what marine grass looks like so you find that sand dunes and the reason why there's not very much water here is um the sand is really porous so it drains away but also it's very very salty because it's near the sea so let's have a look at some of these adaptations the first thing you might notice is the leaf is actually curled up and the reason that's an advantage is any water that does evaporate tends to get trapped in this area and therefore it becomes very very humid and it reduces that water potential gradient from the inside of the plant to the outside and that should reduce any further evaporation second adaptation is all of these pink hair like structures that you can see sticking out all spiky and what they do is trap the water that is being evaporated out from the leaf and again because it's another way of trapping that moisture in the air it makes it really really humid to reduce the water potential gradients and therefore less evaporation or transpiration next feature we have is the fact that the stomata are actually sunken in so you can see we've got lots of folds here and within each fold there's a stomata sunken in deeper than you would have on other plants and it's exactly the same reason why it helps attract moisture more humid less evaporation now there are other features and you can't see all of them on this particular image but they'll also have a thicker cuticle and that is to again prevent evaporation and they'll have a longer network of roots so that it can reach water at further distances so that is it for gas exchange implants i hope you found it helpful if you have please give this video a thumbs up you