If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:7:43
AP.BIO:
ENE‑2 (EU)
,
ENE‑2.D (LO)
,
ENE‑2.D.1 (EK)
,
ENE‑2.D.2 (EK)

Video transcript

we've talked a lot about cells in general but what I thought I would do in this video is focus on plant cells and in particular focus on the cell walls of plant cells so this right over here this is a drawing of a plant cell and the thing that might jump out at you immediately is instead of drawing it as just kind of a roundish shape like that the way i've drawn a lot of other cells i've drawn this it's kind of a cubic structure or rectangular prism and that's because plant cells can have a structure like that and so the next question is well what gives them that shape what's allows them to to form that kind of cubic rectangular prism shape and the answer is it's the cell wall so let's say cell cell wall so let's make sure we can orient ourselves properly in this picture so clearly if I was if I didn't have this cutout all I would be seeing is the outside all I would be seeing is the cell wall but we've cut it out and we can see the different layers we have the cell wall on the outside right below that right below that we have the cellular membrane or the plasma membrane so this is a cell cellular cellular membrane cellular membrane right under that and then under that the cellular membrane is control is is containing the cytoplasm and inside of the cytoplasm we have all sorts of things this big thing that is taking up a lot of the volume inside of this plant cell that's a vacuole which we have described another in other in other videos vacuole as a combination of this internal pressure things like the vacuole and just frankly the pressure from from all of the fluid inside the cell pushing outwards plus the cell wall kind of holding it all in that's what gives plants their structure that's why a plant is able to grow and be a plant is able to grow and be upright so that's my drawing of a plant actually have a plant in my room that I'm looking at right now and it's able to grow and be upright and so you have the cell wall you have the cellular membrane you have the other organelles I have some chloroplasts here key for photosynthesis have my our good friends mitochondria we have our nuclear membrane or I should say this yellow thing is the inner nuclear membrane has the DNA inside then you have the endoplasmic reticulum kind of containing that the rough ER containing the ribose or having the ribosomes on the membrane the smooth ER not having the ribosomes Golgi apparatus so that's a little bit of a review but our focus here is on the cell wall so let's go back to that so if we zoom in on this if we zoom in on the cell wall right over here we can look at we can look at this diagram and over here it might be a little bit surprising to you because what I've always imagined a wall a cell wall I imagine something like a brick wall something that's impenetrable but this drawing shows us something different and just to be clear what's going on here so this is our cellular membrane so I already wrote cellular membrane so if right over here I have my lipid bilayer and then right on top of that I have the cell wall but you see it isn't just a thick like a brick wall something that's impenetrable you see you have all of these polysaccharide fibers running across it so you have things like cellulose which we saw as a polymer of glucose arranged in a certain way Hemi cellulose cellulose which has different types of monomers associated with it we have pectin which is another polysaccharide and all of these things you've actually probably eaten if not today probably in the last week when we talk about fiber in your diet you're talking about things like the cellulose and the pectin things that your body can't digest when you eat a plant you're getting it because you're eating their cell walls and it does cool things like slows the absorption of glucose in your intestines it absorbs water so I guess you could say things pass a little bit easier but just to see but the the key thing here is this isn't a wall this actually allows or it is a wall it's officially the cell wall but it's not a thick it's not an impenetrable wall like you might associate the wall of the room that you're in you can see that it has space for small molecules to flow and it's really more like a mesh or like a fabric and so the cellular membrane actually has access to this this to the fluid and to the molecules that are between the cells and so just to be clear what we're looking at this layer editor that's a cellular membrane that's the lipid bilayer this right over here this is the cell wall than a different color that is the that is the cell wall and then right above the cell wall that's the space between the cells which we call the middle lamella so the space between the cells we call the middle lamella so this also is this right over here is also the middle lamella so all of that is interesting but you might say okay well how hard is the cell I get that it's a mesh but you know clearly the cells are able to or the plants are able to stand upright is that because the cell wall provides all of that rigidity and the answer is kind of the cell wall is like this mesh it helps these cells have their shape but if you stop watering a plant you're going to see it you're going to see it kind of wilt over and that's because part of that that it's ability to stand up is from the internal pressure of the cells but also part of its shape is the actual cell wall now some of you might say well I've seen plants that are much much more rigid than this plant you've just drawn what about things like trees what about wood wood seems very rigid in fact so rigid that we can build actual walls out of wood and the answer there is these more mature plants actually once the the cell has stopped growing and you have your cell wall more layers of cellulose and other molecules can be built to form what's called a secondary cell wall so this could be viewed as a primary cell well then a thicker secondary cell wall could be built which gives much much much more rigidity and so when you look at wood what gives wood its structure even if you were to take out all of the water I mean if you were to dehydrate the wood it's still going to have its rigidity because the cellulose layers and the other molecules are so thick that's able to have its rigid form now the last thing I want to talk about we've already seen that the cellular membrane has access to the the molecules floating around between between cells but there's actually also direct tunnels between adjacent plant cells and those direct tunnels I've drawn here on this outside of the cell wall as these little yellow circles these are plasmodesmata plasma these are plasmodesmata and to forget about understanding what they're like imagine this is one cell so I'll write mirror cell one and let's say this is cell to cell two right over your never crow section you see the plasma you see the plasmodesmata are these tunnels that form between not just the membrane and the cell wall and the plasma deck it forms between the two cells and so you can actually have a flow of cytosol and small molecules directly directly between these two cells if you want to get a little bit more involved in the structure you have this kind of smooth endoplasmic reticulum type going through it but I want to make it very clear because a lot of times when you study biology it's all explained it seems all neat and clean and textbook but people are still studying exactly why do we have these things why are they why why are they necessary what gets transported across these things and how are they able to transport it under what conditions are they so all of these areas when you were to kind of dig one layer deeper than frankly I'm talking about you're getting into an area of active research so anyway hopefully this whole thing gives you a little bit more appreciation for the wood around you the plants the house plants around you and even the salad that you might have for lunch
Biology is brought to you with support from the Amgen Foundation