If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Photosynthesis

Photosynthesis is essential for life on Earth. Photosynthesis involves two stages: the light-dependent reactions, which require sunlight and water to produce oxygen, ATP, and NADPH, and the light-independent reactions (or "dark reactions"), which use the products of the light-dependent reactions along with carbon dioxide to produce carbohydrates. Created by Sal Khan.

Want to join the conversation?

  • leaf blue style avatar for user Mark Jackson
    so how is there life in the deep ocean if plants don't have light?
    (235 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user FalconVIII
      Very good question! These deep sea plants would be relying on chemical energy, which can be created by Chemoautotrophs (Chemosynthesis) bacteria living in there area to convert inorganic matter in the area into organic compounds. Chemosynthesis is effectively the process of using CO2 as an carbon source, and by oxidizing inorganic compounds to make energy. There are also a few other types of Bactria with can use pretty much anything near by and turn it in to usable organic compounds for plant life to use as energy, or just 'tap-into'. So in other words these bacteria 'replace' the job of sunlight. Hope this helps!
      (326 votes)
  • leaf green style avatar for user Parker
    In the photosynthesis equation, what does 'n' mean?
    (15 votes)
    Default Khan Academy avatar avatar for user
    • leaf orange style avatar for user John C. Fisher III
      It is a coefficient which can be moles, molecules, dozens, or anything else you want. Just any number, even fractions or decimals, it's used in stoichiometry and shows equivalency in the reaction (eg, n molecules of O2 = n molecules of H2O).

      When he says (CH2O)n, the n stands for a number equivalent to the coefficients, which produces O
      H C - (multiplied by n)
      H
      For instance:
      (CH2O)6 = C6 H12 O6 , or a six carbon sugar (usually glucose).
      (CH2O)5 = C5 H10 O5 , or a five carbon sugar. Ex:
      O OH OH O OH
      H - C - C - C - C - C - H
      H H H H H
      (34 votes)
  • male robot hal style avatar for user Muhammad Shaheer Amir
    Which colour(s) is/are least absorbed by plants?
    (7 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user FalconVIII
      Least absorbed....that would have to be Green, that's why they are green, as it reflects the green light completely making the plants 'appear' green :) , yellow, and orange are also partially reflected. Plants absorbs most of the blue, and red wave lengths. Hope this helps! :)
      (34 votes)
  • aqualine ultimate style avatar for user sailikhith03
    Can the light reaction be done with an artificial light?
    (11 votes)
    Default Khan Academy avatar avatar for user
  • piceratops tree style avatar for user Sami
    A bit of an open question here, but photosynthesis is the direct opposite of aerobic respiration. Aerobic respiration= glucose+oxygen-->carbon dioxide and water; photosynthesis=carbon dioxide + water--> glucose (carbohydrate) + oxygen.
    My question is: is there any way, scientifically, that photosynthesis is similar to anaerobic respiration? Sorry if this sounds a tad stupid :/
    (7 votes)
    Default Khan Academy avatar avatar for user
    • leaf green style avatar for user Alan
      You've made an excellent observation in noting how photosynthesis is the opposite of aerobic respiration where the inputs and product of the first are the product and input of the latter. However, respiration is the extraction of energy while synthesis is the storage of energy, and cannot be considered similar. Furthermore, most anaerobic respiration processes do not use sugars as a source of energy but rather other compounds, and so have no link to photosynthesis.
      (11 votes)
  • primosaur seedling style avatar for user branch
    I know that mitochondria is called the "power house" of the cell, but what exactly is a power house itself in real life?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user FalconVIII
      The term 'Power-house' is referring to Power stations (i.e Thermal power stations, Nuclear power stations, 'Hydro's' ,etc). They supply the 'energy' (i.e power) needed for 'modern' life, i.e lighting, trains, traffic lights, phones, tv's, computers, laptops, etc.... So it is kind of what the Mitochondria do in the cell, i.e proved's the cell with 'power'/ energy. :)
      (8 votes)
  • blobby green style avatar for user Jan Kendrick
    what are photosystems I and II is it the same as light reaction and dark reaction
    (5 votes)
    Default Khan Academy avatar avatar for user
  • leaf yellow style avatar for user Mohd. Nomaan
    how photosynthesis is important for humans?
    (1 vote)
    Default Khan Academy avatar avatar for user
  • area 52 purple style avatar for user MadisonB
    Can the light reaction be done with an artificial light?
    (4 votes)
    Default Khan Academy avatar avatar for user
  • leaf blue style avatar for user Manan Mehta
    Why is ATP called the Energy currency of the cell?? Pls explain well.
    (3 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user Flying Emu
      Well consider that whenever we use energy for active transport, move, growth, even to think, we break apart the bonds of ATP to create ADP +Pi.

      When we break the bonds of ATP, energy is released. So its the ATP which is producing us with the energy we require.
      (6 votes)

Video transcript

Let's talk about one of the most important biological processes. Frankly, if this process didn't occur, we probably wouldn't have life on Earth, and I wouldn't be making this video for you, because there'd be no place for me to actually get food. And the process is called photosynthesis. And you're probably reasonably familiar with the idea. The whole idea is plants, and actually bacteria and algae and other things, but we normally associate it with plants. Let me make it in very simple terms. So we normally associate it with plants. And it's a process that plants use, and we might have learned this when we were very young. It's the process that plants use to take carbon dioxide plus some water plus some sunlight and turn it into some sugars or some maybe carbohydrates. Carbohydrates or sugars plus oxygen. Obviously, this has two very profound pieces to it for us as a living species. One, we need carbohydrates or we need sugars in order to fuel our bodies. You saw that in the cellular respiration videos. We generate all of our ATP by performing cellular respiration on glucose, which is essentially a byproduct, or a broken down carbohydrate. It's the simplest one for us to process in cellular respiration. And the second hugely important part is getting the oxygen. Once again, we need to breathe oxygen in order for us to break down glucose, in order to respire, in order to perform cellar respiration. So these two things are key for life, especially for life that breathes oxygen. So this process, other than the fact that it's interesting, that there are organisms around us, mostly plants, that are able to harness actual sunlight. You have these fusion reactions in the sun 93 million miles away, and it's releasing these photons, and some small subset of those photons reach the surface of Earth. They make their way through clouds and whatever else. And then these plants and bacteria and algae are able to harness that somehow and turn them into sugars that we can then eat or maybe the cow eats them and we eat the cow if we're not vegetarians, and we can then use that for energy. Not that the cow is all carbohydrates, but this is essentially what is used as the fuel or the energy for all of the other important compounds that we eat. This is where we get all of our fuel. So this is fuel for animals. Or you know, if you eat a potato directly, you are directly getting your carbohydrates. But anyway, this is a very simple notion of photosynthesis, but it's not incorrect. I mean, if you had to know one thing about photosynthesis, this would be it. But let's delve a little bit deeper and try to get into the guts of it and see if we can understand a little bit better how this actually happens. I find it amazing that somehow photons of sunlight are used to create these sugar molecules or these carbohydrates. So let's delve a little bit deeper. So we can write the general equation for photosynthesis. Well, I've almost written it here. But I'll write it a little bit more scientifically specific. You start off with some carbon dioxide. You add to that some water, and you add to that-- instead of sunlight, I'm going to say photons because these are what really do excite the electrons in the chlorophyll that go down, and you'll see this process probably in this video, and we'll go in more detail in the next few videos. But that excited electron goes to a high energy state, and as it goes to a lower energy state, we're able to harness that energy to produce ATPs, and you'll see NADPHs, and those are used to produce carbohydrates. But we'll see that in a little bit. But the overview of photosynthesis, you start off with these constituents, And then you end up with a carbohydrate. And a carbohydrate could be glucose, doesn't have to be glucose. So the general way we can write a carbohydrates is CH2O. And we'll put an n over here, that we could have n multiples of these, and normally, n will be at least three. In the case of glucose, n is 6. You have 6 carbons, 12 hydrogens and 6 oxygens. So this is a general term for carbohydrates, but you could have many multiples of that. You could have these long-chained carbohydrates, so you end up with a carbohydrate and then you end up with some oxygen. So this right here isn't so different than what I wrote up here in my first overview of how we always imagined photosynthesis in our heads. In order to make this equation balance-- let's see, I have n carbons so I need n carbons there. Let's see, I have two n hydrogens here. Two hydrogens and I have n there, so I need two n hydrogens here. So I'll put an n out there. And lets see how many oxygens. I have two n oxygens, plus another n, so I have three n oxygens. So let's see, I have one n, and you put an n here, and then I have two n, and I think this equation balances out. So this is a 30,000-foot view of what's going on in photosynthesis. But when you dig a little deeper, you'll see that this doesn't happen directly, that this happens through a bunch of steps that eventually gets us to the carbohydrate. So in general, we can break down photosynthesis. I'll rewrite the word. We can break down photosynthesis-- and we'll delve deeper into future videos, but I want to get you the overview first-- into two stages. We can call one the light reactions. Or sometimes they are called the light-dependent reactions, and that actually would probably be a better way to write it. Let me write it like that. Light dependent means that they need light to occur. Light-dependent reactions. And then you have something called the dark reactions, and that's actually a bad name, because it also occurs in the light. Dark reactions, I wrote in a slightly darker color. And the reason why I said it's a bad name is because it still occurs in the light. But the reason why they probably called it the dark reaction is that you don't need light, or that part of photosynthesis isn't dependent on photons to occur. So a better term for it would have been light-independent reaction. So just to be clear, the light reactions actually need sunlight. They actually need photons for them to proceed. The dark reactions do not need photons for them to happen, although they do occur when the sun is out. They don't need those photons, but they need the byproducts from the light reaction to occur, so that's why it's called the light-independent reaction. They occur while the sun is out, but they don't need the sun. This needs the sun, so let me make it very clear. So this requires sunlight. This requires photons. And let me just make a very brief overview of this. This'll maybe let us start building a scaffold from which we can dig deeper. So the light reactions need photons, and then it needs water. So water goes into the light reactions and out of the other side of the light reactions. We end up with some molecular oxygen. So that's what happens in the light reactions, and I'm going to go much deeper into what actually occurs. And what the light the actions produce is ATP, which we know is the cellular or the biological currency of energy. It produces ATP and it produces NADPH. Now, when we studied cellular respiration, we saw the molecule NADH. NADPH is very similar. You just have this P there. You just have this phosphate group there, but they really perform similar mechanisms. That this agent right here, this molecule right here, is able to give away-- now let's think about what this means-- it's able to give away this hydrogen and the electron associated with this hydrogen. So if you give away an electron to someone else or someone else gains an electron, that something else is being reduced. Let me write that down. This is a good reminder. OIL RIG. Oxidation is losing an electron. Reduction is gaining an electron. Your charge is reduced when you gain an electron. It has a negative charge. So this is a reducing agent. It gets oxidized by losing the hydrogen and the electron with it. I have a whole discussion on the biological versus chemistry view of oxidation, but it's the same idea. When I lose a hydrogen, I also lose the ability to hog that hydrogen's electron. So this right here, when it reacts with other things, it's a reducing agent. It gives away this hydrogen and the electron associated with it, and so the other thing gets reduced. So this thing is a reducing agent. And what's useful about it is when this hydrogen, and especially the electron associated with that hydrogen, goes from the NADPH to, say, another molecule and goes to a lower energy state, that energy can also be used in the dark reactions. And we saw in cellular respiration the very similar molecule, NADH, that through the Kreb Cycle, or actually more importantly, that through the electron transport chain, was able to help produce ATP as it gave away its electrons and they went to lower energy states. But I don't want to confuse you too much. So the light reactions, you take in photons, you take in water, it spits out oxygen, and it spits out ATP and NADPH that can then be used in the dark reactions. And the dark reactions, for most plants we talk about, it's called the Calvin Cycle. And I'll go into a lot more detail of what actually occurs in the Calvin Cycle, but it takes in the ATP, the NADPH, and it produces-- it doesn't directly produce glucose. It produces-- oh, you probably saw this. You could call it PGAL. You could call it G3P. These all stand for-- let me write these down-- this is phosphoglyceraldehyde. My handwriting broke down. Or you could call it glyceraldehyde 3-phosphate. Same exact molecule. You can almost imagine it as-- this is a very gross oversimplification-- as three carbons with a phosphate group attached to it. But this can then be used to produce other carbohydrates, including glucose. If you have two of these, you can use those two to produce glucose. So let's just take a quick overview again because this is super important. I'm going to make videos on the light reactions and the dark reactions. Those will be the next two videos I make. So photosynthesis, you start with photons. All of these occur when the sun is out, but only the light reactions actually need the photons. The light reactions take photons-- we're going to go into more detail about what actually occurs-- and it takes in water. Oxygen gets spit out. ATP and NADPH get spit out, which are then used by the dark reaction, or the Calvin Cycle, or the light-independent reaction, because these still occur in the light. They just don't need photons. So they're the light-independent reaction. And it uses that in conjunction-- and we'll talk about other molecules that are used in conjunction. Oh, and I forgot a very important constituent of the dark reaction. It needs carbon dioxide. That's where you get your carbons to keep producing these phosphoglyceraldehydes, or glyceraldehyde 3-phosphate. So that's super important. It takes in the carbon dioxide, the products from the light reactions, and then uses that in the Calvin Cycle to produce this very simple building block of other carbohydrates. And if you remember from glycolysis, you might remember that this PGAL molecule, or this G3P-- same thing-- this was actually the first product when we split glucose in two when we performed the glycolysis. So now we're going the other way. We're building glucose so that we can split it later for energy. So this is an overview of photosynthesis, and in the next couple of videos, I'm actually going to delve a little bit deeper and tell you about the light reactions and the dark reactions and how they actually occur.