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People and plants

Learn more about photosynthesis and cellular respiration through a classic story: Jack and the Beanstalk. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

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Video transcript

There's a classic story out there and it has to do with a character named Jack. And you may have heard this story, but I'm sure that there is parts of that story that you have not heard. And so I'm actually going to just try to fill in those parts that you get a complete idea of what happened. Now Jack came across, a long time ago, a famous, now famous beanstalk. So this beanstalk was growing and growing and had these huge leaves. And actually Jack used these leaves to make his way up this beanstalk. And so, this is how this beanstalk became very famous. Because it basically allowed Jack to use it like a ladder. Now the part that we don't hear about is what was going on between Jack and the beanstalk. He was exercising, right? So he was actually making a lot of carbon dioxide. He was making a lot of this gas, this carbon dioxide gas, as kind of a waste product as he was running, scampering up the beanstalk. And the beanstalk was helping him physically, but also was actually providing him with very precious oxygen. In fact, if the beanstalk didn't do that, he may not have even made it. And we also, we don't know for sure, but we think that perhaps some of this story may have taken place during the day. And in fact, we know that sunlight is quite important for this process. And we think that this process, the name that we give it for the beanstalk anyway, is photosynthesis. And so what is really happening-- we're actually going to kind of write it out here-- between Jack and the beanstalk, and really between all plants and animals? What is this process between them? We know that on the one hand, you have beanstalks doing photosynthesis, and on the other hand, you have folks like Jack doing cellular respiration. And there's this really kind of interesting symbiosis. And by that I just mean that the two are kind of relying on each other to really work. So you kind of need both of them to work well. And so let's actually take a moment to write out these processes that are happening between Jack and the beanstalk. So let's start with the process of photosynthesis, the beanstalk. So on the one hand, you've got what? You've got water because, of course, the beanstalk needs water, and you've got carbon dioxide. And I'm going to do carbon oxide in orange. So it's taking in water and carbon dioxide. And it's going to put out, it's going to actually take these ingredients if you want to think of it as kind of cooking, it's going to take these ingredients and it's going to put out. It's going to put out what? Oxygen and glucose. So I'll put glucose up top and oxygen down below. So these are the inputs and outputs of photosynthesis. And on the other side, you've got something very similar. You've got inputs. You've got glucose and oxygen going in. You're going to start seeing some serious similarities here. You've got glucose and oxygen going in. So Jack is taking in those two things. And he is again, of course, processing them. And he's putting out water and carbon dioxide. So this looks really, really nice, right? Looks perfect actually. Because everything is nice and balanced. And you can see how it makes perfect sense that, not only did Jack need the beanstalk, but actually it sounds like the beanstalk needed Jack, based on how I've drawn it. Now remember, none of this would even happen if there was no sunlight. So we actually need light energy. In fact, that's the whole purpose of this, right? Getting energy. So you have to have some light energy. I'm going to put a big plus sign, and I might even circle it because it's so important. I don't want you to lose track of it. And on the other side, of course, Jack is getting something as well. He's getting chemical energy. In fact, he's using the chemical energy to help him climb the beanstalk. And so the chemical energy comes in the form of what we call ATP, which is just a molecule of high energy. And so Jack and the beanstalk are basically going from light energy to chemical energy using these two equations. Now here's the part that people don't always appreciate. And I'm actually going to take just a moment to show you that this isn't the full story. There's actually something else going on as well. And that is that there's actually some cellular respiration happening on the plant's side. So remember, not only does the human, or the Jack, need energy, but so does the plant. The plant needs energy as well. And in fact, if it takes in light energy right here, it needs to find a way to actually, eventually get some chemical energy itself, so that it can do all the things it needs to do. It doesn't need to run because plants don't move in that sense, but it might need to make new roots, and may need to make a flower, and all these things take energy. So actually, photosynthesis is happening during the day, but at all times plants are also capable of doing cellular respiration, just like humans are. So humans and plants have actually more in common than you might think. So, this brings up an obvious question. Why in the world would a plant send its glucose and oxygen this way, when it needs it itself? You know, why would it actually get rid of it? Well, the truth is, that the glucose ends up oftentimes in fruits and vegetables that we eat. But as far as the oxygen goes, it makes an excess of oxygen. So there is actually enough oxygen to go both to us, or to Jack and to be used by itself. So it actually has an excess of oxygen that it's making. So this is actually kind of interesting and good to know. Now if you think about it, if I was to, let's say sketch out a planet. Let's draw a little planet over here, and ask you the question. If this was your planet Earth, and you've got thousands, instead of just one Jack, let's say now you have thousands of Jacks and thousands of beanstalks, in fact, not even thousands. Let's say billions, because really, that's what we have. We have a planet full of humans, and full of other animals, and full of plants. What would the atmosphere look like? This is the atmosphere. What would the atmosphere look like? Well you'd guess that the atmosphere is gas. And so what would those gases be? Well, the way I've drawn things again, it looks like I've got lots of oxygen and lots of carbon dioxide. So I would say well, I guess there must be, I don't know, maybe 50-50 carbon dioxide and oxygen based on what we know so far. And the truth is, that's actually not true. That if you actually look at air, if you actually break down the atmosphere or air-- I'm just going to write "air" here-- if you actually break it down, turns out that the ratios are actually a little different. So for example, oxygen makes up about 21% of our air. This is our air breakdown. And carbon dioxide makes up about less than 1%. So, that leaves you wondering, what the heck is making up all that other parts of air? What is it made of? And in truth, it's about 78% nitrogen. Now you know, you've got nitrogen in your proteins, we've got nitrogen in our DNA. So nitrogen is part of us and is part of many, many living things. But nitrogen gas, specifically, is actually N2. And N2, this nitrogen gas, really is not too reactive. It kind of just hangs out by itself, does not like to react with things. So, looking at our little atmosphere graph, if you want to now think about it, knowing that we've got very little carbon dioxide and about 21% oxygen-- you could think of oxygen being, let's say, something like that-- well then relative to that nitrogen would be much more. You have much more nitrogen hanging out. And so this is really what our atmosphere looks like. It looks more filled with nitrogen than anything else. And in terms of carbon dioxide, it's just got a little smidge of carbon dioxide. Maybe right there. That could be carbon dioxide, maybe even less than that. So this is really what our atmosphere looks like, visually. And the nitrogen again, it's making up the majority. And if you actually wonder where all that nitrogen is coming from, because I didn't mention it in any of the equations, most of that nitrogen has been around, scientists think, since the beginning of when earth even had an atmosphere. And that nitrogen, we're just kind of carrying with us at all times. And that's why it just remains around 78%, and will probably remain there for many, many years to come.