we can't get photo receptors all the credit. Their job is to detect the light. But that signal for a plant to actually grow or move towards the light is carried by hormones and specifically the hormone oxen. This is a super important plant hormone. Uh, technically, it's chemical name is in dole acetic acid. I'm gonna call it oxen. You can see oxen right here. This lovely little molecule and this hormone again is going to be responsible for plant growth towards light. Now, it has been found that Kohli op tiles, which you might recall, are gonna be those coverings on the Kotil. A dons in Monaco cots These coverings, as the plant grows, will release oxen and allow seedlings to bend toward light. Now, I actually, uh, the hypothesis for how this works is referred is known as the colony went hypothesis named after the scientists who helped develop it. And it essentially says that oxen produced at the tip is going to move from the light side to the shade side of the plant. Essentially, uh, from the side, getting the light to the side opposite the light source. So, as you can see in this figure we have oxen, which probably can't read this little text. But these little pink dots in here are supposed to be oxen molecules that oxen is produced The tip in response to the light and as the light source, uh, if the light source is off center from the plant will actually get an asymmetric oxen distribution. You can see that happening here, where the oxen is actually become concentrated on the side, uh, the side opposite the light so you could call it the shade side. This is the shade side. This would be the light side. It's the oxen, concentrates on the shade side and causes that side to grow mawr than light side. So essentially, you get asymmetric oxen distribution, and that results in asymmetric growth. And as we can see here, if you envision, um, these little green boxes separated by the black lines on the outside of this plant diagram as the plant cells. What's gonna happen is the cells on the shade side In response, toe oxen are going to grow and be longer. They're gonna elongate mawr than those on the side with light. Now, if you have the cells on one side getting longer than the cells on the other side that's going thio bend the plant away from the side where the cells are getting longer. The result for this is going to be that the actual tip of the plant grows towards the source of light. Now, how does this actually happen? How do these cells Uhh! Expand like this? Well, the leading hypothesis is known as the acid growth hypothesis. Basically, you have proton pumps that will concentrate protons in the cell wall, and this will eventually lead Teoh Mawr water getting in the cell. Now, before we get ahead of ourselves, let's set up our diagram here. So here we have our membrane. That's the membrane. This is our cell wall and this is cellulose. Remembers the poly Sacha ride that is going to make up plant cell walls and the strands of cellulose will bind together with hydrogen bonds. And due to the structure of cellulose and the these hydrogen bonds, the strands actually grouped together really tightly so tightly that water is unable to get in water. Can't get into the cell wall. It is, um, it is considered insoluble. Now what's gonna happen is these proton pumps. So this is gonna be our proton pump. Thes proton pumps are gonna pump hydrogen zones out of the cell. So what's gonna happen is we're gonna wind up with a high concentration, big concentration of protons in the cell wall. Now, there are these proteins in the cell wall called expansions, and their job is to loosen these hydrogen bonds in cellulose and that is going to allow water to get through. Right. Normally, cellulose is watertight. The expansions in response to this high concentration of protons are going to loosen those hydrogen bonds and allow water. I'm sorry. Allow water to get in, uh, to the cell wall. Now, the other thing that happens, right? If we're pumping protons into the cell wall and increasing our concentration in the cell wall were actually also going to be decreasing our concentration inside the cell. Hopefully see this coming. What we have here, folks, is an electrochemical radiant ever important in biology. And this electrochemical Grady int is going to bring potassium into the cell so potassium ions are going to enter the cell. And as we've learned, water follows ions, right? Water moves based on osmotic radiance. Automatic Grady INTs. So those potassium entering the cell great is the inside the outside. As the potassium ions enter the cell, water is going to follow. Actually, I shouldn't draw it this way because water is going to move through different channels. Right? Called. Hopefully you remember Aqua por ins. I'll squeeze that in here, aqua por ins. So just to quickly summarize that the acid growth hypothesis is essentially that by pumping protons into the cell wall you will allow. Or the plant cells will allow water into the cell wall and that water will get pumped inside the cell, causing them to swell up. And that is how the plant cells can swell and elongate rapidly in response to oxen. Now that's not the Onley role Oxen plays. Oxen has a important function in many different plant, uh, many different plant behaviors and functions. Now it's transported in a polar manner, from the shoots to the roots, right? That's the direction it moves in. And it actually does this regardless of gravity. You could, uh, you know, take the plant, flip it upside down so that gravity is going the opposite way. But you know, so the shoots around the bottom. The roots are on top plants still gonna transport oxen from the shoots to the roots. We call that polar transport because it's unit directional. Now oxen is going thio play a role in a bunch of other functions, as we said, and a lot of these functions are actually also going to be related to light. So you know, even though even though Oxen plays a role in a wide variety of things, that the theme that ties it all together is light, so oxen plays a role in pattern formation, you know, the the the forms that develop in a in a developing plant also file a taxi, which is the arrangement of leaves on a stem. It also has a role in something. We'll talk about MAWR in later lesson called obsession, which is going to be the shedding of leaves and fruits as well. But hopefully you can see this theme of light right, the arrangement of leaves to absorb that light thes shedding of leaves because they're not getting the light now. It also has a role in an idea mentioned in a previous lesson called a Pickle dominance, which is basically that the central plant stem is over the lateral stems and controls the growth of the plant. So oxen has, you know, wide variety of functions that you know help. Hopefully, it helps you remember what they are by thinking of that theme of getting light right, arranging your leaves towards the light, growing towards the light, whatever it is. All right with that, let's flip the page.