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Let's explore the photosynthesis process. Created by Mahesh Shenoy.

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Video transcript

- [Instructor] Photosynthesis is pretty awesome. It's where the plants and the trees and all the green stuff that you find around yourself take in water and carbon dioxide from their surrounding, and then use the energy of the sun to manufacture their own carbohydrate molecules. And why is it called photosynthesis? Because the plants use photo, which means light energy, to synthesize, meaning to create, carbohydrates from the raw materials. And why do they do that? Because carbohydrates is food for them. But guess what? Carbohydrates are food for us as well. Even we need these carbs for our energy. This means photosynthesis feeds almost all living things on this planet. And on top of that, there is a bonus. During this process, the plants kick out a particular molecule into the atmosphere. Guess which one? Oxygen. This means photosynthesis gives me my carbohydrates, my food, and it gives me oxygen to breathe. That's why I love photosynthesis. But here are my questions. Why can plants and trees synthesize their own carbohydrates but we animals can't? What do they have that we don't? And what exactly happens during this process? How exactly do they produce carbohydrates from the raw materials? These are the questions that we're gonna try and answer in this video. So let's start with the first one. What do plants and trees have that we don't? Well, it turns out that they have something called chlorophyll. What's that? Well, to answer that question, let's clear this up a little bit. And then we have to look at one of these leaves very carefully. So if we zoom in, we will see a leaf, which is not surprising, but if we zoom in even further maybe using a microscope, now we see something very interesting. So these compartments that you are seeing, they are the cells of that leaf. And inside these cells you can see all these green things, right? These green things are called chloroplasts. And guess what? Photosynthesis happens inside these chloroplasts, okay? There are a lot of complex molecules inside working together to carry out this reaction. Now one of the most important molecule inside is a pigment called chlorophyll. So chlorophyll is a pigment found in these green stuff, and chlorophyll is the reason why they are green. Okay, chlorophyll gives them their green color. It makes all the plants and trees green. But another important job of chlorophyll is to absorb the energy from the sunlight. Without chlorophyll, energy wouldn't be absorbed and photosynthesis wouldn't happen. So here's how I like to think about it. Imagine we are concentrating on one of the chloroplasts, as the sunlight comes and hits it, the chlorophyll absorbs that energy and makes it available for all the molecules. Now they have the energy to take in carbon dioxide and water and then carry out the photosynthesis reaction which we'll look at in detail in a little while. And then it spits out this juicy carbohydrate molecules. And then it also produces oxygen, and the oxygen gets thrown out. Now before we dive into this chloroplast to see exactly how this chemical reaction happens, let me tell you a couple of things that I used to get really confused about, okay. First is because we see plants throwing out oxygen during this process, I thought they don't need oxygen, I thought they need carbon dioxide but they don't need oxygen. It turns out that's wrong. Even plants need oxygen just like how we need it, okay, and we'll talk more about this in videos of respiration, but plants and trees, they also need oxygen for the same reasons why we need it. Then why do they throw it out? Well, it turns out that during photosynthesis, they create a lot of oxygen, okay, so they keep whatever they want and the rest they throw out. So they don't throw out all the oxygen, okay, only the ones they don't want they throw out, and we are breathing that right now. Secondly I used to think this process only happens in the leaves. But no, they can also happen in the stem and the bulbs and sometimes in the fruits as well. Okay, photosynthesis can happen in any part of the tree which is green, because any part which is green has chloroplast, and wherever chloroplast is present, photosynthesis can happen. So for example you may have seen these green bananas and apples, right? Why are they green? Because of chloroplasts and chlorophyll. They can also carry out photosynthesis, okay? So not just leaves, but leaves carry out most of the photosynthesis. And lastly what about this houseplant which is in my balcony which has non-green leaves? Can they carry out photosynthesis? I used to think no, because they are not green so they don't have chlorophyll so they can't photosynthesize. But it turns out that's wrong. It turns out that even these plants do have chlorophyll and so they can photosynthesize. Then why aren't they appearing green? Well, because plants can have other pigments as well. And in most cases where green plants are there, chlorophyll is dominating, and that's why we see them to be green. But in this plant for example, the other pigments like maybe the red color pigments, they are dominating, and that's why when we look at them, it looks a little purplish reddish. It doesn't mean chlorophyll is not there. Chlorophyll is there, chloroplasts are there, they can also photosynthesize. Alright, so now that that's clear, let's dive into this chloroplast and see exactly what chemical reaction is taking place. So let's begin by writing a chemical reaction for this process. How do we do that? Well, whenever I forget what's going on, I start by thinking about the products. Now I know that in photosynthesis I get carbohydrate, okay. This helps me remember the reaction. Here's how. See, carbohydrate has two words, it has carbo and hydrate. Carbo means carbon. So to manufacture carbohydrate, I need carbon, plants need carbon. Where do they get that from? Well, they get it from carbon dioxide. That's how I remember carbon dioxide. Then they also need hydrate. Hydrate is water, right? Just like how we say dehydrated means loss of water. So they also need water. Oh, so plus H20. So these are the two reactants needed. But of course remember to carry out this reaction, they also need energy from the sun. Photosynthesis, photo part is over here, okay. So these are the three things needed. Now what do we get? Well, let's see. Now whenever we write this arrow mark, we put a chlorophyll on top of that to signify that energy is absorbed by the chlorophyll. Without chlorophyll, this cannot happen, right? Okay, anyways, what do we get? We get carbohydrate, right, that's the major product, but how do we write the formula for carbohydrate? Again I look at this. See, carbo means carbon so there's a C. Hydrate means water so H20. This is carbohydrate, okay. But wait, carbohydrates need not have just one atom of carbon and one molecule of water, they can have three and three or six and six, or any number they can have, alright. So there are many carbohydrates. So in photosynthesis, which carbohydrate do we get? Well, we get the carbohydrate in which there are six atoms of carbon and six molecules of water. That's also something we need to remember, okay. So the one we get is where there are six of these, alright. And this molecule, this carbohydrate is given a name, it's called glucose. So photosynthesis gives us glucose, alright. And what else? We get one more product, which is that? A byproduct which they throw out, which is that? Plus oxygen. And you will see in a second why I have colored oxygen this way. And here is our chemical reaction. But if you look carefully, you see that it's not balanced. For example on the right, we have six carbon atoms, on the left we just have one. Okay, so we need to first balance it. And so can you try balancing it first? Great idea to pause the video and see if you can balance this whole equation yourself first. Okay, hopefully you tried. So I'll just quickly go ahead and balance this. Since there are six carbon over here, we multiply this by six. There are six water molecules over here, so I multiply this by six as well. Now let's look at the oxygen. The oxygen here is balanced, six and six. But over here we have 12 oxygens, so this also will balance by multiplying by six, we get 12. Now everything is balanced. Now I know this was fast but we've talked a lot about balancing in great detail in chemistry videos. So if you need more clarity on how to balance equations, great idea to go back and watch chemistry videos. Anyways, now let's think about what's going on in this reaction. I'll start with what I thought again. So when I learned this, I thought that here carbon dioxide is being broken into carbon and oxygen. So I thought that plants take in carbon dioxide, break it, and throw away oxygen. That's how we get our oxygen. And then they take that carbon and attach it to water to give us carbohydrate. That makes sense, right? Easy, right? Unfortunately that's now how it works. Why not? Well, the main reason for that is because carbohydrates are not really carbon attached to water. So glucose is not six carbons attached to six water. I'll show you the structure of glucose. There are many structures but one of the structures of glucose looks like this. Now I know this looks all scary, and don't worry, we don't have to remember any of this. But what I'm going to show you is do you see carbon attached to water anywhere? No, right? It's not like that. It's a little bit complicated, right? So even though the name is stuck, carbohydrate, it's not really a good name actually. The name is stuck but it's not really carbon attached to water. So you can't attach carbon to water and expect to get a carbohydrate, okay. So let me get rid of this. Carbohydrates are not really carbon and water. But it's useful to remember the formula. I use it to remember the formula, okay. Let's get rid of this. So if that's not what's going on, what's really happening in this reaction? So here's what's happening. The chlorophyll absorbs the energy from the sun and then uses it to break open water into hydrogen and oxygen. And by the way it's not chlorophyll alone, there are many molecules working together, but some of their energy is used to break open hydrogen and oxygen, and that oxygen is thrown out. So the oxygen in photosynthesis comes from water, not from carbon dioxide. And this process is pretty important, at least historically, because that's how oxygen came into our atmosphere. So almost all the oxygen that we are breathing today was once upon a time locked in water. Pretty cool, right? So we need to change the color of this. This came from water. And then what about this hydrogen? Well, then the hydrogen gets attached to carbon dioxide in a series of steps, it's not a one-step reaction, there are many more molecules involved over here, but in a series of steps to finally give us glucose. And so the oxygen over here is the oxygen from the carbon dioxide. So again we need to color that properly. So again what happens? Chlorophyll absorbs the energy from the sun and uses some of it to break open water into hydrogen and oxygen. Oxygen is thrown out and then the hydrogen is attached to carbon dioxide in a series of steps to finally give us glucose. And I can stare at this forever because this is literally what's keeping me and you alive, but we need to end this video so let's quickly go ahead and summarize. What did we see in this video? We saw that photosynthesis takes place inside this green things called as chloroplasts. They are green because of a pigment called chlorophyll, and chlorophyll helps in absorbing the energy from the sun. And what does it do after absorbing the energy? It uses it to split open water into hydrogen and oxygen. The oxygen gets thrown out, and then the hydrogen is attached to carbon dioxide in a series of steps to give us glucose. This is what happens inside the chloroplast. This is photosynthesis. This is how we get our food and our oxygen.