We can't give photoreceptors 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 auxin. This is a super important plant hormone, technically its chemical name is indoleacetic acid. I'm going to call it auxin. You can see auxin 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 coleoptiles, which you might recall are going to be those coverings on the cotyledons in monocots. These coverings, as the plant grows, will release auxin and allow seedlings to bend toward light. Now actually, the hypothesis for how this works is known as the Colin D. Wendt hypothesis, named after the scientists who helped develop it. It essentially says that auxin produced at the tip is going to move from the light side to the shade side of the plant, essentially, from the side getting the light to the side opposite the light source. So, as you can see in this figure, we have auxin, which probably can't read this little text, but these little pink dots in here are supposed to be auxin molecules. That auxin is produced at the tip in response to the light, and as the light source, if the light source is off-center from the plant, we'll actually get an asymmetric auxin distribution. You can see that happening here, where the auxin has actually become concentrated on the side opposite the light, so you could call it the shade side. This is the shade side, this would be the light side. So the auxin concentrates on the shade side and causes that side to grow more than the light side. So essentially, you get an asymmetric auxin distribution, and that results in asymmetric growth. And as we can see here, if you envision these little green boxes separated by the black lines on the outside of this plant diagram, as the plant cells, what's going to happen is the cells on the shade side, in response to auxin, are going to grow and be longer, they're going to elongate more 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 to 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 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 to more 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, which, remember, is the polysaccharide 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 these hydrogen bonds, the strands actually group together really tightly, so tightly that water is unable to get in. Water can't get into the cell wall, it is considered insoluble. Now, what's going to happen is these proton pumps, so this is going to be our proton pump, these proton pumps are going to pump hydrogens out of the cell. So what's going to happen is we're going to wind up with a high concentration of protons in the cell wall. Now there are these proteins in the cell wall called expansins, and their job is to loosen these hydrogen bonds in cellulose and that is going to allow water to get through. Normally, cellulose is watertight, the expansins, 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 into 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, we're actually also going to be decreasing our concentration inside the cell. Hopefully you see this coming. What we have here, folks, is an electrochemical gradient, ever important in biology, and this electrochemical gradient 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 gradients, osmotic gradients. So those potassiums entering the cell, right, 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, aquaporins. I'll squeeze that in here. Aquaporins.

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 through 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 auxin. Now that's not the only role auxin plays. Auxin has an important function in 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 take the plant, flip it upside down, so that gravity is going the opposite way, but it, you know, so the shoots are on the bottom, the roots are on top, the plant's still going to transport oxen from the shoots to the roots. We call that polar transport because it's unidirectional. Now, oxen is going to 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 oxen plays a role in a wide variety of things, that the theme that ties it all together is light. So auxin plays a role in pattern formation, you know, the forms that develop in a developing plant. Also, phyllotaxy, which is the arrangement of leaves on a stem. It also has a role in something we'll talk about more in a 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, the 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 apical dominance, which is basically that the central plant stem is over the lateral stems and controls the growth of the plant. So auxin has a 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? Ranging your leaves towards the light, growing towards the light, whatever it is.

Alright. With that, let's flip the page.