Photosynthesis
Photosynthesis: Light Reactions and Photophosphorylation More detail on the light reactions and photophorylation
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- I want to review a little bit of what we
- did in the last video.
- And maybe draw a larger, more spread-out diagram.
- Because I think in the last video I started to cram things
- on the right-hand side here.
- And this is a very important concept, so I want to do it
- nice and spread-out in a way that we can breathe.
- And maybe in the process I can fill in some more blanks.
- So let's go back and draw the membrane of a thylakoid that's
- sitting inside of a chloroplast. I'm going to draw
- this same membrane here.
- So let me draw it nice and spread out.
- So let me draw a nice big membrane like that.
- That's the inside of the membrane.
- So you can imagine that this loops around and that would
- form the thylakoid.
- On this side of the membrane we have the lumen.
- And on the outside of the membrane we have the stroma,
- where all the fluid that fills up the choloroplast. So this
- is the stroma right there.
- And this is just a kind of standard membrane that we see
- in a lot of organelles.
- But this is actually a membrane within an organelle.
- And then maybe there will be a phospho-bilipid layer.
- And I just say that, or I'm pointing that out because I
- want to think a little bit about, in this video, how
- protons can actually get across this thing.
- How do we use the energy from these electrons going to lower
- energy states to actually pump protons across this membrane.
- So you know when you have these bilipid layers, your
- outside is hydrophilic.
- And of course, it's hydrophilic because it
- operates well in a polar environment.
- And then the insides are non-polar or they're
- hydrophobic and you have these tales.
- So I could draw the whole membrane like that, but I
- won't do that.
- It will take me forever.
- But let me draw some of the components that I did in the
- last video.
- So we have these complexes that
- span across this membrane.
- And the place we started off with was the
- photosystem II complex.
- And then later on we have the photosystem I complex.
- And let me draw the ATP synthase right here.
- So ATP synthase also spans across it.
- Then it has little motor part of it.
- And the hydrogens go through and it spins the motor and it
- crams the phosphate groups into the ADP to make ATP.
- I'll talk about that in a second.
- But the first thing I want to point out is, as I said in the
- last video, the first place where the electrons get
- excited is in the chlorophyll and photosystem II.
- And then it gets less and less and less excited, it gets
- headed off from one complex to another complex.
- And eventually ends up in photosystem I.
- It gets excited again.
- Then it gets handed off, handed off, the whole time
- that energy is being used to transfer hydrogen protons from
- the stroma into the lumen .
- But the first question that I would ask is, why is this
- called photosystem II, while this is called photosystem I
- when we're starting over here?
- And the reason is, this was discovered first. Even though
- in the light reaction it actually comes into use, or it
- comes into play, second.
- This was discovered first. That's why they call it
- photosystem I.
- But the reality is photosystem II is where
- everything starts from.
- Now in some textbooks you'll also see this written as P680.
- And you'll see photosystem I written as P700.
- And these numbers come from the wavelength of light that
- is best absorbed by the chlorophyll in these
- photosystems. So 680 corresponds to 680 nanometers.
- That's the wavelength of light that this absorbs the best.
- 700 corresponds to 700 nanometers.
- That's the wavelength of light it absorbs best.
- Now what I want to do here is draw a little diagram below
- here to kind of talk more about the
- electron energy states.
- I just kind of handwaved it a little bit in the last video.
- So I'm going to draw a little diagram here.
- And over here I'm going to write the different things
- that the electron can be a part of.
- So right now the electrons can be part of H2O.
- It could be part of chlorophyll A.
- It could be a part of-- I'll talk more about this in a
- little bit-- pheophytin And then you have all of the
- molecules or the complexes it can become a part of.
- I'll actually write them down here.
- So let me write.
- I don't want to take up too much space.
- Plastoquinone and then there's a cytochrome B6F complex.
- I'll just write B6F.
- Then you have plastocyanin .
- I'll just write as PC.
- You don't have to memorize these.
- You'll forget them in a week if you do.
- But unless you're studying photosynthesis, then it might
- make sense to memorize them.
- And this is in photosystem II.
- Then you have chlorophyll in photosystem I.
- And then you have some other, you know you have ferredoxin
- I'll just write FD for ferredoxin Some other
- molecules, you keep going and then you have your eventual
- electron acceptor NADP plus.
- Which, once it accepts the electron, becomes NADPH.
- Now, electrons are very-- so this is, if we go up that's a
- high energy state, down it's a low energy state.
- So electrons are already very comfortable.
- in water.
- And in chlorophyll A they're even more comfortable.
- At least this is how I view it.
- But left to its own devices, this electron will never leave
- chlorophyll A.
- But we know what happens.
- A photon comes in from 93 million miles away.
- You could imagine photons as little light packets or you
- could view it as a light wave. Either way.
- And it excites-- not necessarily directly the
- chlorophyll A.
- It might excite other antenna chlorophyll or
- other pigment molecules.
- And then through resonance energy, you can imagine them
- vibrating, and it eventually will excite the
- photophorylation A directly.
- Or excite the electrons in chlorophyll A directly.
- And this dude right here gets excited.
- Let me do that in a brighter color.
- So it goes to a high energy state.
- So the electron here is in a high energy state.
- Ignore that lumen right there.
- It has nothing to do with this electron.
- And then it goes-- and actually when it goes to the
- high energy state, maybe I should draw it like this, it
- actually shows up in pheophytin.
- That is the primary electron acceptor.
- And it's actually a chlorophyll A molecule.
- Actually, let me show you what a chlorophyll A
- molecule looks like.
- This is what a chlorophyll A molecule looks like.
- In general, it has a hydrocarbon tail.
- You see that right here.
- And it has a porphyrin ring.
- Or porphyrin head, I guess you could call it.
- This little group right here is called a porphyrin.
- And right in the center of it, you have a magnesium.
- That green right there, that's a magnesium ion.
- And when the photon comes in or when the resonance energy
- comes in from some of the antenna molecules, electrons
- in the double bond sitting here in the
- porphyrin head get excited.
- Those are the electrons that we're talking about.
- And they get excited.
- And the first electron acceptor is this pheophytin
- that I just talked about.
- Pheophytin.
- It actually looks just like a chlorophyll, but it has no
- magnesium ion in the middle.
- And maybe I'm getting a little bit into the weeds a little
- bit too much.
- But the pheophytin, you actually see in this diagram
- right here.
- It's part of this photosystem complex.
- So the electron, you can imagine, jumping from the
- chlorophyll to the pheophytin that does not have that
- magnesium in the center.
- And when it sits in the pheophytin it's at a very,
- very, very, very, high energy state.
- And then it keeps being transferred from the
- pheophytin.
- It goes to the plastoquinone So maybe we go to a slightly
- lower energy state here.
- We keep using the electron in green.
- Then it keeps going to a slightly lower energy state in
- the cytochrome B6F complex.
- And then you have the plastocyanin complex, lower
- energy state.
- And then eventually it goes into photosystem I at an even
- lower energy state.
- Maybe slightly higher than the energy state that it was
- originally in the chlorophyll A molecule in photosystem II.
- Another photon or another set of photons comes and hits
- photosystem I.
- Maybe its antenna molecules, through resonance energy, that
- excites the electron.
- It might directly hit the chlorophyll in photosystem in
- its reaction center.
- And then this gets excited again.
- And so once again we have an electron with a high potential
- that can keep going to, from one molecule to another as it
- gets more and more comfortable.
- And this releases energy that can drive the proton pump.
- And it eventually ends up in the NADPH.
- At a fairly high level of energy still.
- This electron can still be transferred to other things
- and release energy.
- And we'll see that when we talk about the light
- independent reactions.
- Now the whole point of me showing you this is, I wanted
- to kind of depict graphically that the electron is starting
- off at a pretty low energy state.
- And the only way this happens is by energy from light.
- This would not happen on its own.
- Going from a low energy state to a higher energy state.
- And I touched on in the last video, you have this electron
- going here and it gets transferred from one molecule
- to another.
- Gets excited again, then keeps going all the way, eventually
- being accepted by the NAD plus to become NADPH.
- And you're like, where did that H come from?
- You could say, well that H is a proton.
- It gets that electron and then they merge together and you
- have NADPH.
- But either way.
- But the question is, what replaces this electron?
- And that's where that amazing thing that I talked about in
- the last video happens.
- Water gets oxidized.
- Oxidizing is losing electrons.
- OIL RIG.
- So water gets oxidized by the water oxidation
- on photosystem II.
- And that electron ends up and replaces the electron in the
- chlorophyll.
- So once again, that's an amazing idea, that you're
- oxidizing oxygen.
- So the net effect of what happens is, is you're using
- energy, using this photon energy right here, to
- essentially strip electrons off of water.
- And as you know, when it's on water it's spending most of
- its time on the oxygen.
- So it essentially strips electrons off of oxygen and
- put them in a higher energy state and have
- them end up on NADPH.
- And in the process, it had gone to an even
- higher energy state.
- And then as it goes down to NADPH, you are pumping protons
- across the membrane.
- We learned in the last video, through chemiosmosis,
- eventually goes through the ATP synthase channel, turns
- around this part of this protein complex or enzyme
- complex and actually generates ATP.
- ATP from ADP and phosphate groups.
- And in the electron transport chain video, when I talk about
- cellular respiration, I give a visual concept of how this
- actually might happen.
- How you could, as these go through, you actually can jam
- together the ATP and the ADP.
- So that's another question in my head is, we talk about
- these electrons going from one molecule to another.
- But how does that actually pump hydrogen through?
- And I'm just going to do a very gross oversimplification.
- I'm sure it's much more complicated in
- actual plant cells.
- But you could imagine that we have our pheophytin right here
- that has that electron on it right there.
- Maybe it has its electron right there.
- This is a gross oversimplification.
- And then you have your plastoquinone right here.
- That's the next acceptor.
- Now maybe on this protein complex right there, the point
- that wants to accept the electron is right there.
- And let's say that there's another point on it that can
- accept a hydrogen.
- Maybe it accepts a hydrogen proton there.
- So you can imagine when it's on this side of the membrane,
- a hydrogen can become attached right there.
- And this guy will want to be attracted to that.
- So he'll rotate around.
- So you can imagine this-- if this is kind of a wheel-- this
- attraction.
- Because the electron wants to go into a lower energy state
- right here.
- It'll rotate around.
- That'll allow, essentially, this hydrogen as it rotates.
- As this molecule, as this protein rotates around this
- hydrogen will be able to cross the barrier.
- And then once this guy and that guy meet, then the
- hydrogen will be on the other side.
- And so it can freely go away again.
- So that's, at least in my head, how I imagine the
- electrons going from a high energy state to a lower energy
- state, how that can actually drive a reaction.
- Remember, the electrons want to do this.
- So they'll attract the different parts of the
- molecules together.
- And as those molecules turn and rotate and move, that can
- help facilitate hydrogen going from the stroma, the outside
- of the thylakoid, to the inside of the thylakoid.
- That will drive the chemiosmosis later on.
- Now there's one other point I want to touch on here.
- Everything I've described so far, we started with an
- electron in water.
- And obviously when water loses its hydrogens, it loses both
- the hydrogen protons and electrons associated with it.
- You end up with just water.
- So you start off with hydrogens and then you end up
- with just O2.
- And then the hydrogen protons-- the electron got
- taken up by the chlorophyll.
- When you start off with that, we've seen already that you
- end up with the electron sitting in NADPH.
- The electron sitting out here in NADPH.
- At some point you have NAD as the final acceptor.
- Let me do it in the right color.
- You have NAD plus as the final acceptor.
- And it becomes NADPH.
- You can imagine it accepts maybe a hydrogen
- proton from out here.
- It accepts the electron from this electron transport chain
- in photosynthesis.
- And it becomes NADPH and that travels in the stroma, which
- is where the dark reactions occur that actually produce
- the carbohydrate.
- But this idea of an electron going from water to NADPH,
- this is called non-cyclic photophosphorylation And it's
- called non-cyclic because you're not reusing the same
- electrons over and over again.
- The electrons start off, and depending how you view it, in
- the chlorophyll or the water.
- And they end up in the NADPH.
- Now there's another type of photophosphorylation and you
- might guess what it's called.
- It's called cyclic.
- Cyclic photophosphorylation We'll see when we study the
- dark cycles or the Calvin cycle or the dark reactions or
- the light independent reactions, that it
- uses a lot of ATP.
- Actually ATP in disproportion to the
- amount of NADPH it uses.
- It uses both, but it uses a ton of ATP.
- So cyclic phosphorylation only produces ATP and actually does
- not oxidize water.
- So what happens in that situation is this electron,
- after it gets activated or after it gets excited in
- photosystem I, it's the electron, it eventually ends
- up-- instead of at NADPH, it ends up at photosystem II.
- So instead of this guy having to be replaced by electrons
- from water, this guy, in cyclic photophosphorylation
- ends up-- well, maybe I should do it from here-- ends up
- getting replaced by the original electrons.
- It gets excited here.
- It goes from molecule to molecule, lower energy states,
- hydrogen gets pumped into the lumen.
- Gets excited again in photosystem I and it enters
- lower and lower energy states.
- But then ends up again in photosystem II.
- That is cyclic photophosphorylation.
- So you can imagine in this situation, since the electron
- never ends up at NAD plus, you don't end up producing NADPH.
- And since you're replacing this electron from the
- photosynthesis or from the electron transport chain
- directly, you don't have to strip the
- electrons off of the water.
- So you're not going to produce your oxygen.
- So, in this situation-- so this non-cyclic
- phosphorylation, which is kind of what most
- photophosphorylation, is what most people associate with
- photophosphorylation, this produces O2 and NADPH.
- And of course it produces ATP.
- While cyclic photophosphorylation, because
- it doesn't have to strip electrons off of water and the
- electrons don't end up at NADPH, only produces ATP.
- So I think we now have a very good understanding, hopefully,
- of the light reactions in photosynthesis.
- We're now ready to take the products of this.
- Now let's remember what the products were.
- Well, the oxygen just gets eliminated.
- We don't need the oxygen anymore.
- But that goes into the atmosphere and you and I can
- breathe that and we can use that for cellular respiration.
- But in the photosynthesis context, we've now generated a
- bunch of ATP.
- And now we have a bunch of NADPH.
- And we can use that in conjunction with carbon
- dioxide to produce actual carbohydrates in the stroma.
- Outside the thylakoids, but we're still inside of the
- chloroplast. And I'll cover that in the next video.
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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