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Breaking down photosynthesis stages

Photosynthesis, a process vital for life, involves two main stages: light-dependent reactions and the Calvin cycle. Light-dependent reactions use light energy and water to produce ATP, NADPH, and oxygen. The Calvin cycle then uses ATP, NADPH, and carbon dioxide to create sugar. This process transforms light energy into a usable form, supporting life on Earth. Created by Sal Khan.

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Video transcript

- [Voiceover] So, I'm gonna give another quick overview of photosynthesis. And this time I'm gonna break it down into two big stages. So, as you are probably familiar, just looking at the word, photosynthesis. It essentially has two parts, it has photo and it has synthesis. The photo is referring to that it's going to use light somehow. And what's it going to do with that light energy? Well, it's going to synthesize something. And, in particular, what it's going to synthesize, as we'll see, is sugar. So, we are going to go from energy in light, let me just write light, light energy, and we're going to use that light energy to synthesize, to synthesize, sugar, very broadly speaking. Obviously this is a very, very high-level overview. The light energy isn't the only input here. We're also going to need some water and as we go into future videos, we'll see what that water's used for. It's actually a source of electrons. To do this, to make use of that light energy, frankly. And we're also going to need some carbon dioxide, really as a source of carbons, because there's a lot of carbon in those sugars. We're essentially going to fix the carbon. We're gonna take it from this carbon dioxide gas, and we're going to incorporate it into organic molecules and eventually into the sugar. And sugar isn't the only output. Another bi-product of this process is molecular oxygen. Once you strip a couple of electrons from the water, and the hydrogen ions are stripped away from it as well, all you're left with is oxygen. And you do that twice, then you have o two and you have molecular oxygen. And this is a bi-product of photosynthesis, but you can imagine this is very important to life on earth as we know it, in particular for us. We would have trouble breathing if this was not a bi-product of photosynthesis. Now what I'm gonna do now is break this out into two stages. And these two stages, we can call the light-dependent reactions. Light-dependent reactions, and then the second stage, I will call the Calvin cycle. Calvin, Calvin, cycle. And as the name implies, the light-dependent reactions are dependent on light. So, what's happening here is, we're gonna take light energy. Light energy. Plus we're gonna take the water as a source of electrons, and we're going to use these two things. We're going to use these two things to produce, to produce, let me write this in another color, to produce ATP from ADP, so we're gonna produce ATP, which is a store of energy, and we're also going to reduce NADP plus into NADPH, which has energy as a strong reducing agent. So this is what is happening, broadly speaking, in the light reactions. And then in the Calvin cycle, what we're gonna do is we're gonna take these products of the light-dependent reactions, so we're gonna take our ATP and our NADPH, and we can use their energy in conjunction with some carbon dioxide, with some carbon dioxide, in order to produce, in order to produce, sugar. In order to produce sugar. And, let me see, have I got everything here? Oh, of course, I'm missing one of the bi-products of the light-dependent reactions. A very important one. I'm missing the molecular, the molecular oxygen. So, once again, this is what makes up photosynthesis, but you can break it up into these two segments. Light-dependent reaction is using the energy from photons in light along with electrons from the water to produce, to store energy, as ATP and NADPH, and has oxygen, molecular oxygen, as a bi-product. In order for it to get one molecular oxygen, you're gonna have to need two of these water molecules. And then, as we go into the Calvin cycle, we can take these, the ATP and the NADPH, along with some carbon dioxide, and we can use that to actually store energy as actual sugar. And as we'll do in future videos, we'll go into more depth and see what exactly happens in these light-dependent reactions, and what exactly happens in the Calvin cycle.