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Current time:0:00Total duration:6:55

Video transcript

you've probably seen plants either in your house or if you go for a walk you've seen parts of the plants twist and turn in all sorts of directions and if you observe closely you'll see that oftentimes it looks like the plant is twisting or turning towards the light and what we're going to do in this video is study that phenomenon a little bit more so in general whenever we're talking about an organism twisting or turning or changing direction because of some type of external stimulus we call that tropism tropism and in the case of turning or switching directions because of because the external stimulus is light we would call that photo tropism photo tropism and phototropism is the general term for any type of organism turning direction or switching direction or or moving in a certain direction because of light and it could be moving towards light or it could be moving away from the light now most of what we observe if you were to go see a forest you'll see that there might be a a patch in the camera or there might be an opening in the canopy where the light is coming and you will often see trees and plants moving towards that light growing towards that light so they are turning towards it this would be called positive phototropism if for some reason there was some type of a plant that was moving away from it that would be negative phototropism which is a little bit less or salaat less usual especially for the stem of a plant but we might talk about that in future videos but what you typically see is positive phototropism like this and so the question is is how does the plant do this what what causes it to turn in the direction of the light in the case of positive phototropism well the key actor here is a molecule called oxen it's oxen and oxen is a phyto hormone so this is a Fido Fido hormone and it's a fancy word for this is just a molecule involve called in the actual plant growth and so what happens let me zoom in here on this plant let me zoom here on this plan and to be clear we know ox and ax but all of the mechanisms by which the distribution of oxen changes or how it gets activated or deactivated isn't completely understood and this is still an area of active research but what we know happens so let's say the light is coming from that direction over here is for for various reasons or for some reasons you have an increased concentration of active oxen on the side of the plant away from the light so you're going to have more oxen in this case since the light is coming from the left you're going to have more oxen on the right then you're going to have on the left and what the oxen on the right is causing to do it causes the cells on the right side of this stem right here to elongate so if we were to zoom in so let me zoom in over here so let me take that part of the plant and if I were to zoom in if I were to zoom in and if the cells I'm just kind of a two-dimensional view here let's say on this side these are the cells and on this side these are the other cells now if we have more oxen more of this vital hormone on the right hand side right over here that we have on the Left what we know happens is it causes the cell walls where there's more oxen to break down a little bit more and so it allows those cells to stretch or elongate and so what's going to happen let me see if I can draw this is the cells on the right hand side are going to stretch are going to stretch or elongate because of the auxin because they have more oxen in them than the cells on the left-hand side and since these have a higher concentration of actor active oxen they stretch and if the right-hand side gets longer than the left-hand side what's going to happen well the plant is going to bend to to the left so the plant is going to bend like that because now the right-hand side is longer than the left-hand side so this is what we know about oxen it's a Fido hormone that whatever it is in higher concentrations and it's active the cells there are going to elongate which will cause this bending now the things that are still being studied is well what caused there to be a higher oxygen concentration on the right hand side and all of the exact mechanisms by which the oxen is actually acting we know things like it creates a more acidic environment which helps break down the cell walls but how does that you how do you have a higher concentration here on the right well in many plants studied it's actually the blue light that causes the sensitivity so I drew this as yellow but really I should draw this as blue light that causes the sensitivity although it's not always blue line it could be red light it could be other frequencies of light and the various theories and it might be some combination of it is when you have the light on one side of the plant that it causes one possibility is maybe it causes the aux-in to migrate from the side with the light to the side that has less light another possibility is if the auxin is migrating down from the top of the stem that it might cause more to migrate where there is less light then when there is more light there's other possibilities that somehow the light deactivates the aux-in on the left or maybe allows less oxen to be produced then what would happen on the right so these are all possibilities and this is what's interesting about science is that there's always more for us to understand but what we do know is that this type of positive phototropism which you will often see in the forest or you leave you'll even see on a house plant if you put it next to a window it'll Bend towards the window it will often bend towards the window it's caused though the key actor here is oxen and the distribution of oxen changing in response to the light and then the oxen and that the oxen distribution increases on the side away from the light when you have positive phototropism now this picture here it looks like this might be phototropism but this is actually a mutant plant that actually is lacking oxytocin so it's growing in all sorts of random directions but it's just interesting to look at
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