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
Current time:0:00Total duration:4:48

Breaking down photosynthesis stages

AP.BIO:
ENE‑1 (EU)
,
ENE‑1.I (LO)
,
ENE‑1.I.1 (EK)
,
ENE‑1.I.1.i (EK)
,
ENE‑1.I.2 (EK)
,
ENE‑1.J (LO)
,
ENE‑1.J.5 (EK)

Video transcript

so I'm going to give a another quick overview of photosynthesis and this time we're going to break it down into two big stages so as you are probably familiar just looking at the word the photosynthesis essentially has two parts his 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 what in particular what it's going to synthesize as we will 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 but 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 is used for it's actually a source of electrons to do this to to go to make use of that light energy frankly and we're also going to need we're also going to need some carbon dioxide really as a source of carbons as because there's a lot of carbon in those sugars we're essentially going to fix the carbon we're going to 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 byproduct of this process is molecular oxygen once you strip the you strip a couple of electrons from the water and the hydrogen ions are stripped away from it as well all you're left it with is molecular is is oxygen and you do that twice then you have Oh two and you have a molecular oxygen and this is a byproduct 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 byproduct of photosynthesis now what I want to do now is break this out into two stages and these two stages we can call the light dependent reactions light dependent dependent reactions and then the second stage I will call the Calvin cycle Galvan Calvin cycle and as the name implies the light dependent reactions are dependent on light so what's happening here is we're going to take light energy light energy plus we're going to 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 going to produce ATP which is a store of energy and we're also going to reduce nad p+ into n a d pH which has energy as a 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 going to do is we're going to take these products of the light dependent reactions so we're going to take our ATP and our n a d pH 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 byproducts 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 and light along with electrons from the water to produce to store energy as ATP and ADP NADPH and it has oxygen molecular oxygen as a byproduct in order for to get one molecular oxygen you're going to have to need two of these water molecules and then as we go into the calvin we can take these the ATP and the NADPH along with some carbon dioxide and we can use that to actually store our energy as actual sugar and as we'll do in future videos we'll go into more depth and what exactly happens in these light dependent reactions and what exactly happens in the calvin cycle
Biology is brought to you with support from the Amgen Foundation