Why aren’t plants black? 🌿

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[MUSIC PLAYING] What color do you think the sun is? The sun's white because it's a white dwarf star. So it would be all the colors. Fire reddish-orange. Orange-yellow because of its heat. Yellow because that's the way it looks up in the sky. What about above the atmosphere? What color do you think the sun would look? Probably not yellow anymore because there's an atmosphere that we're looking through. So I'm going to give you the answer. The color of the sun is actually white. It's a bit of a misleading question because you can't really take like one of the colors between red, orange, yellow, green, blue and say that the sun is only that color. Because the sun has all the colors. Knowing that the sun is white, which of the colors of the rainbow do you think the sun emits the most of? I guess orange. Orange. Be likely, yeah. Because it looks– Because it– yeah. pretty orange. Sure.

I'll go with yeah. I would say just a clear white light. You want to know what color it emits the most? It's actually green. Really? OK. I don't know why it would be green. Last question. Why do you think that plants are green? Because the concentration of chlorophyll. The fiber probably is clear and so you can just see that colors through the plant. Well, because if the sun is emitting mostly green light, then that would be absorbing the most light that's available from the sun. That's what I thought. Yes. I set up the question so you would think that the spectrum of the sun was related to green plants. You're welcome. But I thought that they were. My train of thought was something like this. Leaves are green because chlorophyll is green. And chlorophyll is the molecule that takes light from the sun and turns it into usable energy in the plant, photosynthesis. Now, if the sun's spectrum peaks in green and even the light that makes it all the way down through our atmosphere peaks in green, then plants are green to absorb the most green light.

Wait, hold on. If leaves are green, that means they are reflecting green light. They're shooing away the most common color of light from the sun. What? Why? That doesn't work then. I don't know why it doesn't work. But also, I did ask a bit of a leading question telling you that the sun emits the most green and then asking about green plants. You did sort of get me thinking about green. You cheated. I did cheat. If plants wanted to absorb the most energy, wouldn't they absorb all the colors? Wouldn't they be black? So black pigmented plants exist but they're rare, like helium. So maybe this whole getting rid of green thing has its benefits. I mean, it's kind of a global thing, right? Like all plants on Earth are pretty much green. If they were black, then the Earth would look like this. So why are plants green? Seems like something scientists should know. But the truth is, they don't know why plants evolved to be green. Hmm.

Here are some super interesting theories, though. Plants absorb sunlight to make food. Like every good farmer's daughter knows, more sunlight equals more food. But there is a point called saturation, where shining more light on a leaf doesn't lead to a higher output. So full sunlight on, say, a spinach leaf is 3 to 5 times the saturation level. It's like being served 3 to 5 hamburgers when you can really only fit one in your stomach. So reflecting away some green light isn't necessarily bad if plants always got full sunlight, but they don't. One theory goes that chlorophyll being bad at absorbing green light means that plants can actually absorb more light overall. What? Here's how that works. Chlorophyll does absorb and use some green light.

But because not all green light gets absorbed at the surface of the leaf, some of it can then penetrate deeper into the leaf, like burrowing worms. And even if the top of the leaf gets saturated, and get this, 90% of blue and red light is absorbed in the upper one-fifth of a spinach leaf. So that means that chlorophyll deeper down is there to absorb the green light that made it through. Genius. And as an added bonus, this prevents your leaf from overheating. It's like reheating your burger in an oven versus in a fire. You don't want a frozen patty with a burnt bun. Plus, lower leaves of the plant get some of the green light that is reflected and scattered, which is good for them. Because you know, if you ate all 3 or 5 hamburgers, then your family would starve.

Why might chlorophyll be evolutionarily awesome? Maybe it's a molecule that's easier for evolution to come up with. Maybe it's more efficient at turning sunlight to energy than other potential photosynthesizing black or purple pigments that absorb green better. But neither of these theories is in the leaf, nor this next one. Early light guzzling life on Earth included an archaea called halobacteria that absorbs green and reflects red and blue so it looks kind of purple. It's what gives this lake its purple color. Covered in this stuff, Earth might have looked like this, except the plates would have been in different places 3 billion years ago, but I digress. Now, the theory goes like this. If halobacteria were absorbing all the green light, then another, say, green algae that were layers deeper in the ocean would have to make do without green light. It's like if someone else ate the hamburger patty and lettuce and threw you the bun. You'd have to get good at surviving off of carbs. Which brings me to the last theory, which is that maybe evolution is not a perfect engineering process.

Maybe chlorophyll is just good enough. I know, I'm not super happy with that answer either. But it's possible that chlorophyll was the accident we got during the complex process of evolution on Earth. Maybe with slightly different conditions, it would go differently if life on Earth went through evolution again. So what color is the sun? Well, I'd say white but kind of greenish. And I said, I'd say because color is kind of a perception thing more than a physics thing. But that's a topic for a whole other video. So if you have any questions about the world around you or really anything that can get me out of the studio and filming back outside, let me know in the comments. And hey, happy physicsing. [MUSIC PLAYING] .

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