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Breaking down photosynthesis stages

Photosynthesis, a process vital for life, involves two main stages: light-dependent reactions and the light-independent reactions (also called the Calvin cycle). Light-dependent reactions use light energy and water to produce ATP, NADPH, and oxygen. Light-independent reactions 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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  • orange juice squid orange style avatar for user hope179
    I don't understand why a plant would need the dark reactions to make ATP, CO2, and NADPH into carbohydrates to undergo cellular respiration if they can just get ATP from the light reactions. Why take the extra step to make carbohydrates into ATP again?
    (10 votes)
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    • purple pi teal style avatar for user Erin Abraham
      You only get very few ATP from the light reactions. If I remember right, 2 or 3. On the other hand through cellular respiration one can make around 30-34 ATP. Which is a lot. 2 - 3 ATP is nothing for a multicellular organism like a plant. If you have learned about anaerobic respiration, which also produces around 2 - 3 ATP, you'll know that this only works for unicellular/extremely simple organisms, like yeast. In fact, humans undergo anaerobic respiration if unable to get oxygen. As I said before anaerobic respiration provides around 2 - 3 ATP. Hold your head underwater and you'll know how long that much ATP will last you :D :D

      Hope this helps!
      (54 votes)
  • blobby green style avatar for user Amogh Agrawal
    why can't plants directly transfer ATPs from one part to another.
    why do they need to form carbohydrates which have to be broken down by cellular respiration
    (10 votes)
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    • purple pi purple style avatar for user Jay
      ATP is a large molecule and can be hard to transport, it is also very unstable. Carbohydrates are much more stable which makes them a lot easier to move around. Imagine that the phosphates of the ATP are three golf balls stacked on top of each other, and then imagine that the Carbohydrates are blocks that have been glued together. Which do you think would be easier to move?
      (25 votes)
  • duskpin ultimate style avatar for user Cindy
    Is glucose the only sugar that can possibly be produced at the end of photosynthesis?
    (9 votes)
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  • starky seed style avatar for user Yaman Kumar
    At , what is NADPH ?
    (7 votes)
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  • marcimus pink style avatar for user Tanzim
    I want to more about of ATP and NADPH
    (4 votes)
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  • starky seedling style avatar for user Elizabeth
    Where did the Calvin Cycle get it's name?
    (6 votes)
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  • duskpin sapling style avatar for user Iltimass Gouazar
    Isn't it true that plants can also breathe by taking O2 and giving away CO2?
    So what's the point then from turning CO2 in air to O2 if it's going to do the opposite too? Or does it take more CO2 than it gives? Can you explain please?
    (5 votes)
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    • purple pi teal style avatar for user Erin Abraham
      It's true because plants also do cellular respiration - taking in oxygen and giving out CO2. Cellular respiration occurs in the mitochondria. Now since photosynthesis creates glucose you may be wondering why do we need mitochondria? Well if you remember, the biological "currency" is ATP. So we need to convert the glucose into ATP. This is done in the mitochondria and is called cellular respiration. We humans only do cellular respiration since we are heterotrophs - we don't make our own glucose.
      (7 votes)
  • spunky sam blue style avatar for user Angel Xu
    What's NADPH? Is it ATP or something?
    (4 votes)
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    • leaf grey style avatar for user ILVKA
      Nicotinamide adenine dinucleotide phosphate abbreviated NADP+, or in older notation ,TPN, is a cofactor used in anabolic reactions ,such as the Calvin cycle and lipid and nucleic syntheses which require NADPH as a reducing agent. It is used by all forms in cellular life.NADPH is the reduced form of NADP+.

      Hopefully that helped 👍.
      (8 votes)
  • mr pink red style avatar for user K
    where do the NAD+ and ADP (that are then converted to NADPH & ATP in the light dependent stage) come from in the process?
    (5 votes)
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  • duskpin ultimate style avatar for user Boboa
    What is Nadph? Like what does it do?
    (3 votes)
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    • hopper cool style avatar for user [name]
      NADPH is produced by the first stage of synthesis. It carries a high-energy electron that received its energy when it was hit by a beam of light during the light reaction of photosynthesis. Then, it carries that electron into the Stroma (the liquid inside of the chloroplast), where the high-energy electron is used to form Glucose and other carbohydrates from CO2.
      Hope this helps!
      (6 votes)

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.