Phototropism | Plant Biology | Khan Academy

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. If you observe closely, you'll see that oftentimes it looks like the plant is twisting or turning towards the light. 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. In the case of turning or switching directions because the external stimulus is light, we would call that phototropism. Phototropism is the general term for any type of organism turning direction or switching direction or moving in a certain direction because of light. It could be moving towards the 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 patch in the canopy or there might be an opening in the canopy where the light is coming. You'll 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 it's a lot less usual, especially for the stem of a plant. But we might talk about that in future videos.

What you typically see is positive phototropism. So the question is, how does the plant do this? 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 auxin. It's auxin, and auxin is a phytohormone. This is a phyto, phytohormone, and it's a fancy word for this is just a molecule involved in the actual plant growth.

What happens, let me zoom in here on this plant. To be clear, we know auxin acts, but all of the mechanisms by which the distribution of auxin changes or how it gets activated or deactivated isn't completely understood. This is still an area of active research. But what we know happens is, let's say the light is coming from that direction. For various reasons, you have an increased concentration of active auxin on the side of the plant away from the light.

So you're going to have more auxin in this case. Since the light is coming from the left, you're going to have more auxin on the right than you are going to have on the left. What the auxin on the right is causing to do is it causes the cells on the right side of this stem right here to elongate. If we were to zoom in, let me zoom in over here. 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 going to have 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 auxin, more of this phytohormone on the right-hand side right over here than we have on the left, what we know happens is it causes the cell walls where there's more auxin to break down a little bit more. It allows those cells to stretch or elongate.

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 or elongate because of the auxin, because they have more auxin in them than the cells on the left-hand side. Since these have a higher concentration of active auxin, they stretch. 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 the left. So the plant is going to bend like that because now the right-hand side is longer than the left-hand side.

This is what we know about auxin. It's a phytohormone that, whenever 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 are well, what causes there to be a higher auxin concentration on the right-hand side and all of the exact mechanisms by which the auxin is actually acting. We know things like it creates a more acidic environment, which helps break down the cell walls.

But how do you have a higher concentration here on the right? Well, in many plants studied, it's actually 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 light, it could be red light, it could be other frequencies of light. The various theories might be some combination of it.

When you have the light on one side of the plant, that causes one possibility. Maybe it causes the auxin 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 than where there is more light. There are other possibilities that somehow the light deactivates the auxin on the left or maybe allows less auxin to be produced than what would happen on the right.

These are all possibilities, and this is what's interesting about science is that there's always more for us to understand. What we do know is that this type of positive phototropism, which you will often see in the forest or you'll even see it on a house plant, if you put it next to a window, it'll bend towards the window. It'll often bend towards the window. It's caused—the key actor here is auxin and the distribution of auxin changing in response to the light.

Then the auxin that the auxin 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 is lacking auxin, so it's growing in all sorts of random directions. But it's just interesting to look at.