Phototropism

animals get a lot of credit for their ability to sense and respond thio things going on in their environments. But plants don't get the credit they deserve. So in this lesson, we're going to look at plants ability to sense and respond to what's going on in their environments. But before we get to the specifics of plants, I wanna talk generally about cell signaling. And if you want a bigger refresher than this, I recommend you go and check out our videos on cell signaling that will cover these ideas in greater detail. Now signal transaction, which is going thio. Results from molecular signals leading to some change in metabolism. Gene expression or something of like can be broken down into three steps. The first is reception. The signal is received, and this happens when a ligand, a signaling molecule, binds to a receptor and you can see we have to lie guns being released by ourselves. Here we have blue dot and red square, these air very important biological molecules. I'm kidding, by the way, so these Legans will bind to their appropriate receptors and notice how the receptors on these cells are the specific shape of these lions, right? This cell here has these receptors that will fit those red squares, and this one has receptors that will fit the blue dots. And guess what? That's because receptors and Legans are specific to each other, That is Liggins. Will Onley bind to specific receptors and receptors? Will Onley bind specific Legans? So our second step is trans deduction. The signal is carried through the cell and you can see that happening here These little molecules interacting with each other trying to represent here a cascade of molecular interactions that air carrying the signal from these receptors. Lastly, we have some response in the cell and this response is determined by the receptors present on the cell and the signal transaction pathways available to the cells. So you can see here that ourselves air kind of growing outward towards each other. That is their response from those molecular signals they received. Now there are many types of signaling molecules, but the one that I want to specifically are the type I want to specifically talk about is, or our hormones. Hormones are signaling molecules that will affect gene expression, cell division and growth. So these air super important signaling molecules, as we said about Legans, LaGon Zehr specific receptors, hormones or no exception there. Ah, hormone structure means that it will Onley bind to certain receptors that are meant to bind that hormone. So one way that a cell can mediate, or rather, one way that an organism can mediate its cells. Response is with the presence or absence of specific receptors for a hormone. For example, if I release blue dot hormone and some of my cells don't have any receptors for the blue dot hormone, then they won't receive the signal. So by including or not, including the necessary receptor, you can mediate whether or not a cell will respond to that signal. Now another really cool thing about hormones is their ability to our the ability of their signals to be amplified. So signal amplification will result when ah, few signaling molecules have a huge effect. So in our little diagram, we have our hormone here, and this hormone will influence this protein too, you know, do something to this molecule, and this molecule will have an effect on two molecules, and each of those blue molecules will have an effect on two pink molecules and so on and so forth. So this is a huge oversimplification. But hopefully you get the general idea that a single hormone can lead to on effect on many molecules downstream. That's what we mean by signal amplification that you can amplify the signal as you carry it. Now, often the a signal trans duck shin pathways will involve what are called phosphor relation cascades, which is when you basically have a Siris of proteins that activate and deactivate each other through the transfer of phosphate groups. And you can see sort of, ah, model of what that might look like here, where you know, the ligand binding at this receptor leads to the activation of this protein. And that protein activates this protein, and that protein activates this protein and so on and so forth. And you have a cascade of activation and deactivation and the whole time they're transferring phosphate groups to essentially turn each other on and off. Now, you can also have what are called second messengers, and these are intracellular signaling molecules. So they signal within the cell and they're gonna be involved in various signal transaction pathways. We have a little model for one. Right here. Uh, this is the, you know, a new interest cellular signal being carried through these various molecules. So this would be our second messenger. And that's going to activate a syriza signaling molecules and elicit some response in cell. Let's turn the page.

it should hopefully come as no surprise that one of the most important things for plants to be able to sense and respond Teoh is light as that is pretty much the basis for their survival. They need that sunlight to carry out photosynthesis, and they need to carry out photosynthesis in order to produce sugars to eat and survive. So Italy elation is going to be the general term for plant responses to the absence of sunlight. And these responses include growing towards the sun. Right. So if you detect an absence of sunlight, your response logically is a plant would be to grow towards the sun. Uh, one way they might accomplish this is by having longer inter nodes. Remember, that is the portion of the stem between leaves. So if you increase that, uh, that length the length of the inter notes, you're gonna have a lower density of leaves. But that doesn't matter because you're not getting enough sunlight anyways, right? These air responses to an absence of sunlight. So by having longer Internet nodes, you're going to have mawr vertical growth, right, and that's going to allow you to reach the sun now one of the last symptoms, if you wanna call it that is Clore ASUs. And this is a lack of chlorophyll. And you can see an example of that in this very sad looking plant right here. Now, the reason plants will experience sclerosis as a response to an absence of sunlight is actually kind of an energy conservation mechanism. Why would you wanna waste energy producing chloroplasts and chlorophyll if you're not going to be catching sunlight there and so not performing photosynthesis anyways? Now D Italy Elation is going to be the opposite of of Italy elation. It's responses to sunlight, and these were going to be in part regulated by some photo receptors called Fighter Chrome's. And we're gonna talk about fighter. Chrome's in just a little bit now just because I'm piling on the terminology right here. Photo more for Genesis is going to be a big focus in, uh in this lesson, and that is essentially plant growth in response to different spectrums of light. Hopefully, you recall from the section on photosynthesis that there are different types of light and plants selectively absorb specific wavelengths of that light, meaning that they're going to have responses. Two different types of light. So depending on the type of light they're able to receive, they will output different responses. And growth patterns based on these different spectrums of light is again called photo More Food Genesis and that's photo for light Morpho for form and Genesis for origin. So it's basically the origin of the form do toe light. You could think of it that way. Now, trope is, um trope ISMs are big category of plant responses and these air just movements of plants in response to something in the environment. Here we're focusing on photo trumpism, right growth toward or away from light. So essentially plants responding to light, however, in other lessons, will be looking at different types of trope ISMs. Now, photo trope is, um is going thio be, uh controlled or is going thio require, I should say, photo receptors thes air Gonna be proteins that respond to stimulation from certain wavelengths of light. Right. So if you're going to grow toward or away from light, you're going to need something to detect. That light and photo receptors is what plants are going to use Now. One type of photo receptor is or one class of photo receptor, I should say is photo trope ins. These are blue light photo receptors. Blue light, if you recall, is one of the main main lights that plants are going to absorb for photosynthesis. In addition, they also will absorb red light. And hopefully you also recall that blue light is what we consider higher energy light. That is to say, it's light of a higher frequency. Uh, this is kind of getting into physics. So, you know, don't worry about understanding. Uh, you know why Blue Light is higher energy and how frequency factors into that and all the mathematics just know that blue light is going to be higher Energy and red light is going to be lower energy light, And so blue light is very important for photosynthesis. And it's also important for photo trope is, um uh, additionally, it's actually involved in stem mata opening and closing, which will get back Thio. So here you can see a nice example of photo trope is, um so we have this plant here that is bending towards the light source it has here, which is lamp here. Obviously could be the son to now just as it's important for plants to be able to detect, uh, detect light they need for photosynthesis. They also need to detect when they're not getting great light for photosynthesis. So plants use red light and blue light for photosynthesis, as I've said, and, uh, red light is the red light, they preferably uses roughly in the range of 62 700 nanometers. Don't worry about memorizing these numbers. This is just, ah, detail for the sake of detail, in case you wanna look this up and blue light roughly around 4. 32 470 nanometers. So those were gonna be the main bands of light that plants want for photosynthesis. So it hopefully should seem logical that they are very sensitive to those particular wavelengths. They're gonna respond. Uh, they're going to have, you know, strong responses to those wavelengths. Now they also can detect what is called far red light. This is the range that plants want for photosynthesis, right light past that range further down the red spectrum is what we call far red light. You know, usually it's thought of his light of wavelength greater than 710 nanometers. This light is not absorbed by photosynthetic pigments, meaning it's not going to help photosynthesis. It actually passes through leaves, and it helps indicate shade. So what that means is high up leaves that far red light will actually pass through them and we'll hit light are sorry. Hit parts of the plant that are underneath those top leaves getting the direct sunlight. So it's a way for plants that are not in direct sunlight to detect that they are in shade, right, because the leaves above are gonna absorb those photosynthetic wavelengths. But that far red lights gonna make it through. So it's going to allow the plant Thio say, Oh, hey, I'm not I'm not getting the good son right now. I got a Hey, you get moving here. I got to do some photo Trope is, um Now plants will actually use that far red light for a really nifty thing called the Red Far Red Switch. This is kind of a hypothetical idea, and it's or I should say, it's a theoretical idea that Red Light will promote. See germination and far red light will inhibit it. And it's based on this particular type of photoreceptors mentioned earlier. Fight a chrome, which is a photo reversible photo receptor, meaning that it actually is a molecule that has two different forms. It has to alternate forms and each of those forms reacts to a different wavelength of light. And when it reacts to one of those wavelengths of light that actually changes its confirmation. So fighter chrome's are these photo reversible photo receptors that are sensitive to both red and far red wavelengths of light. So the way this works is basically light stimulation. If this fight a chrome, the red fighter chrome absorbs sunlight, it will change its confirmation and turn into the far red fighter chrome. And I mean red fighter chrome is infighter chrome that absorbs red light far red fighter chrome as in fighter chrome that absorbs far red light. So when this far red fighter chrome absorbs far red light that will switch it back Thio the fighter chrome red confirmation And you can see this is a nifty mechanism to detect light and shade or dark. However you want to think of it Now get my head out of the way here and behind my head you can see this, uh, nifty little graph showing you the wavelengths absorbed by the the red fighter chrome and the far red fighter chrome confirmations. So the red fighter chrome you can see has a strong response to light in groups light in this particular band right here. And while far Red Thief are red fighter, chrome confirmation can also absorb that particular band. It doesn't absorb it nearly a strongly and also but much more importantly, the far red fighter Chrome is able to absorb light past this particular wavelength and into this region here, right, the far red region now thes light stimulations are going thio cause phosphor relations and defrost for relations that will induce thes confirmation. All changes don't need to worry too much about the biochemistry. Just know that when the fight a chrome in the red confirmation absorbs red light, it switches to the far red confirmation. And when that far red confirmation absorbs far red light, it flips into the red absorbing confirmation. Now all of this is part of a behavior no nas shade avoidance, where far red light will actually cause plants to lengthen their stems or induce branching in an attempt to grow into direct light. So lengthening the stems is great if you just need to get up to the light, right, if the problem is your verticality. But it can also induce branching meaning, like making the plant get bushier. So it has a greater area of absorption, which is sometimes, uh, the behavior needed in order to absorb more light. And you can see that these are responses thio, far red, light or shade basically, right with that, let's flip the page.

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.

plants like animals, experience circadian rhythms, these air daily cycles that will do things like fluctuate the concentration of a particular hormone based on the time of day. Now these cycles are maintained and generated internally by what you could think of as a biological clock. However, they can be influenced by the external environment, and plants can be can have their circadian rhythms influenced by what these things called crypto chrome's. Now these are going to be photo receptors that detect blue light and can have an influence on, for example, the turgid ity of the plant in response to daylight. Now this plant over on the left, as you can see, is perked up. Its leaves air open, and it's ready to absorb sunlight. This is going to be it's it's daytime condition. At night, the plant will lose targeted E and get droopy. Its leaves will close up, and it will not be in a prime position to absorb sunlight. However, it will give the plant certain other advantages. So this fluctuation over the day night cycle will allow the plant to maximize its photosynthesis and also, you know, do things like help protect it from environmental conditions by folding in it night. Now, some plants actually bloom in response thio seasonal changes and they detect this by sensing the lengths of the day night cycle. Now we call these physiological responses Photo period is, um and some plants are considered long day plants. These are gonna be plants that bloom when the days air longest, which is going to be during the summer. Some plants are called short day plants because they bloom when the days are shorter and the days air gonna be shorter during the spring and late summer fall. Now, obviously the days they're gonna be shortest in winter but hopefully realize why. It's not really a great time for plants to be blooming. However, that cold does have an effect on plants ability to bloom some plants, that is, and we call this vernal ization. It's essentially a pre treatment with cold that is necessary for the photo period blooming response to take place. Essentially, these plants are still going to be detecting lengths of those day night cycles, right, examining the relative length of day tonight to detect seasonal changes. However, they require a period of cold Before that photo period, response can kick in. And essentially, this is a way to ensure that they have passed through winter and are going to, for example, bloom in the spring time. And we call this vernal ization from the Latin Virna for spring. Now it should be noted that some plants bloom independently of day length, and we call these day neutral plants now. The day night cycle is going to have a NIF effect on when the plant blooms, but it's thought that it's actually ah, hormonal signal that causes flowering. Believe it or not, this hormone is yet to be discovered, and we simply called the hypothetical hormone Florida gin. So essentially florid gin is the hormone yet to be discovered that, uh, induces flowering. However, there's good evidence to suggest that such a hormone exists. It could just be trickier than you might realize toe. Actually identify these things when you know you don't know exactly what toe look for ID, like searching for a needle in a haystack. And before we leave, I just want to point you to this figure, because, honestly, it's just a really nice display of information, and this will actually show you day length based on your latitude on the earth and, uh, month, as you can see here. So very nice chart showing you when the days are longest and shortest. All right, that's all I have for this video. I'll see you guys next time.