So if we want to compare, now, between plants and human photoreceptors. We know that humans have four photoreceptors. And that they can see between the blue to the red spectrum. Plants actually are much more complex than we are, when it comes to light signaling. Plants have upwards of 13 photoreceptors. They have one that's called UVR8, which we didn't go into details of. Which allows the plant to detect UV light. Light that we're blind to. . Plants can see this and respond. Plants have upwards of five photo receptors that detect blue light. They have two, what we call phototropins. These are now the name of the photo receptors in blue light. A photo receptor for flowering in blue light. And another photoreceptor for letting a plant know about seeding development called cryptochrome. Plants also can detect green light. And they have multiple photoreceptors for red and for far red light. Actually Arabidopsis has five different types of phytochromes. So that we can see that, from a plant's perspective, humans actually may be somewhat visually dysfunctional. 'because plants can see much more. Now, I have to point out here, that, while both animals and plants have photoreceptors. While both animals and plants see blue and red light. The photoreceptors that detect and respond to the light are completely different. Animals don't have phytochrome. They have something called erythrolabe, which allows them to see the red light. And they don't have phototropin. They have another of the blue cones, which allow them to see blue light. These are not the same photoreceptors. There's no evolutionary connection between them. But the ability to see light, and respond to it, is conserved among all organisms. So why would a plant need to be so sensitive to light. Well this goes back to the first lecture where we talked about plants being unmovable. So what is light for a plant? Light is its ability to eat. Plants use light, sunlight, to power photosynthesis. A plant needs to know where the light is, so that it could do photosynthesis and make its own food. So a plant needs to know where the direction of its food is, what the intensity of its food is, and when it should be flowering in order to make its next generation. So because of the plant can't move, it's developed the ability to detect both the direction, and the intensity, and the type of light, which is much more complex than what animals have. I want to end with one other thing here. I was, I talked about how, plant signaling, or light signaling Is not evolescent, evolutionarily connected between plants and animals. And that's correct, except for one molecule, or one example. And that's this molecule here called cryptochrome. Cryptochrome is the blue light receptor, that allows a plant seedling to know how to develop in the light. Now the name cryptochrome is actually quite an oxymoron. So while plant scientists had known that there should be a blue light receptor, they didn't know what it was. And Johnny Gressel, Professor Johnny Gressel at the Weizmann Institute named it cryptochrome. Because they didn't know what the receptor was. Today we've cloned that gene, and we know where this blue light receptor is, what it looks like but, the name is, has remained the same. Cryptochrome. Now you also have chryptochrome in your bodies. So what was the connection between cryptochrome and plants? And cryptohchrome and animals? Well there is one way that we are influenced by light. Which is the same way that plants are influenced by light. And that's our biological clocks. If you've ever gone through jet lag, you know that being out in the sun helps reset your clock. And actually through evolution, all organisms have biological clocks. And all organisms have to have the ability to have their clocks set by the light environment. And this cryptochrome sets a plants clock, and also sets an animals clock, so at least at this level, blue light signaling is conserved between plants and animals. To summarize sight, what sight is for a plant than what it is for a person. We respond to about 400-700 nm Whereas plants respond to a spectrum of 300 to about 750 nanometers, a wider spectrum of vision. For us, light signals are sensed in the retina, whereas in plants, it's sensed in all cells. Humans have about five photoreceptors. Plants have 12 or 13 photoreceptors. Actually, the number is dependent on which species we're talking about. But in all cases, it's more than in humans. For us, it enables primarily identifying details. Details that we see in pictures. Plants don't need to see the details, but for them it enables them to find food. Actually, if you think about it, the details also enable us to find food. Light for us synchronizes the biological clock. And also for plants it synchronizes the biolgical clock; it enables a plant to know what season of the year it is.