Photosynthesis
Photorespiration More detail on the Calvin Cycle and Photorespiration
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- Let's review the Calvin cycle a little bit.
- Maybe I'm going to go into a little bit more detail than
- the last video.
- And by doing that, we'll be able to appreciate an
- inefficiency, or what we think is an inefficiency that occurs
- in many photosynthetic plants.
- So just to review the Calvin cycle, you start
- off with some CO2.
- In the last video, I had six molecules of CO2 and six
- molecules of ribulose bisphosphate.
- But in this one, just to show you that I could have
- literally just divided that last cycle by two, I'm going
- to start with three and three.
- Three molecules of carbon dioxide.
- Three molecules of ribulose bisphosphate.
- If you don't remember what ribulose bisphosphate was, it
- was just a 5-carbon molecule.
- It had two phosphates on it.
- I can draw the phosphates.
- So you have a phosphate there.
- And you have a phosphate there.
- And we saw on the last video-- and here I'm going to do a
- little bit more detail.
- Of course carbon dioxide, there's only one carbon.
- I can just draw a carbon like that.
- I mean, I don't want to draw the oxygens right now.
- I just want to focus on just the carbons and maybe the
- phosphates.
- But in the last video we saw that these two merged.
- Or these two reacted.
- They reacted in the presence of ribulose bisphosphate
- carbocylase, which is this enzyme.
- It was that big protein that I showed you in the last video.
- And it's called RuBisCo for short.
- So I'm going to write RuBisCo in the middle
- of this whole cycle.
- Because all of this is going on with the RuBisCo enzyme.
- These molecules are joining on to the RuBisCo enzyme and then
- the ATP and the NADHs are reacting on different parts.
- And that's essentially what's driving the whole reaction.
- So you have all of this is happening with that big
- RuBisCo protein, or enzyme, or whatever you want to call it.
- And this just stands for ribulose bisphosphate
- carboxylate.
- So it essentially telling you that it merges a carbon onto
- ribulose bisphosphate.
- And the last video, I just did a very hand-wavey, I didn't
- show you all of the intermediates.
- But I showed you, look, if in this situation you end up
- using six ATPs and six NADHs, you'll end up with
- glyceraldehyde 3-phosphate.
- Or G3P.
- Or that's also called phosphoglyceraldehyde so you
- can also write it as PGAL.
- But a very easy way to visualize it as just a
- 3-carbon chain with a phosphate.
- With a phosphate group.
- And the last video when I started with six carbon
- dioxides and six ribulose bisphosphates.
- And I ended up with 12 of these.
- Now I have three and three.
- Now I'm going to end up with six of these.
- So I'm going to end up with six G3Ps.
- And the math works out, right?
- I have three carbons plus three times five carbons.
- That's a total of 18 carbons.
- And here I have six times three carbons.
- Or 18 carbons.
- And that's what I showed you in the last video, but there
- are actually some intermediary steps.
- So the very first thing that's actually formed is
- 3-phosphoglycerate.
- And the whole reason why I'm showing you this detail, and
- maybe you're taking a biology class where they actually go
- into this level of details, is I want to show that this
- compound also gets produced in that inefficient cycle that
- we're going to explore in this video.
- So you get 3-phosphoglycerate, which is
- also a 3-carbon chain.
- But it's a different one than this.
- And this isn't ready to produce glucose just yet.
- But you could also imagine that as a 3-carbon chain with
- a phosphate group.
- And then, and course you're going to produce six of these.
- Just to make sure all the carbon gets accounted for.
- You have 18 carbons here, you have 18 carbons here.
- So you're going to have six of these
- 3-phosphoglycerates get produced.
- And then each of those gets a phosphate from ATP.
- So you're going to have six ATPs come in.
- Remember these ATPs got produced in the light
- dependent reactions that occur in the membrane of our
- thylakoids.
- Anyway, we have these ATPs.
- You have six ATPs.
- And then they become six ADPs.
- So essentially they have given away their phosphate groups.
- So they're going to give one phosphate group to each of
- these six 3-phosphoglycerates.
- And then you're going to end up with this, right here.
- I'm running out of space.
- Maybe I didn't do it as neatly as I should have.
- But you're going to end up with this three chain carbon,
- or 3-carbon compound.
- It's going to have two phosphates then.
- And this, you could know the name.
- This is 1 3-bisphosphoglycerate.
- This was just 3-phosphoglycerate, which
- means that the phosphate is on the third carbon.
- Now we have 1 3-bisphophoglycerate.
- We have a phosphate on the 1-carbon and the 3-carbon.
- We have two of them.
- So it's bisphosphoglycerate.
- So that's to get us right there.
- And then to get from the 1 3-biposphoglycerate, and of
- course we have six of these as well.
- Six of these turn into six of these.
- And to turn these six 1 3-phosphoglycerate into our
- three glyceraldehyde 3 phosphates, or
- phosphoglyceraldehyde the names are very daunting.
- This is where we're actually going to oxidize our NADPH.
- Remember oxidizing is losing electrons or losing hydrogens
- with their electrons, either way.
- So here, on this step right here, two things are going to
- happen, actually.
- You're going to have six NADPHs become six NAD pluses.
- So they're losing that hydrogen and the electrons.
- And also one phosphate is going to be lost from each of
- these molecules.
- And then you're going to also have plus six of
- the phosphate groups.
- The phosphate groups don't get lost here.
- They get added onto these molecules right there.
- And then we end up with our sixth glyceraldehyde
- 3-phosphates, which we learned in the last video, we can then
- use to produce fuel or glucose or other
- carbohydrates in the cell.
- But we also learned that this is called the Calvin cycle.
- So all of it doesn't get used to produce actual glucose or
- other carbohydrates.
- Most of it, out of the six, five of them are going to be
- used to produce our three ribulose bisphosphates.
- So let me do it like this.
- So you're going to have five G3Ps go in that direction.
- So for every six G3Ps you produce, five go back into the
- cycle and then one exits the cycle.
- We saw this in the last video.
- In the last video I had 12 here, so 10 went in this
- direction and two in this direction.
- I'm just dividing everything by two in this video.
- And the whole reason why I multiplied by two in the last
- video is so that we end up with two going into this
- direction because two is enough to produce
- at least one glucose.
- But we don't have to have two.
- We could just say we do this whole cycle twice to get two.
- But anyway, I have one glyceraldehyde 3-phosphate
- coming out of this end.
- And then these five glyceraldehyde 3-phosphates
- that stay in the cycle.
- Remember those are just 3-carbons
- with a phosphate group.
- I drew it over here already.
- They will then use another three ATP.
- So we end up with three ADPs.
- And the phosphate groups actually end up on the
- ribulose bisphosphate and they get essentially recycled into
- ribulose bisphosphate for the whole cycle to happen again.
- Remember when all this is happening you can kind of
- imagine in or on the surface of this big enzyme.
- This ribulose bisphosphate carboxylase.
- And actually, just so you know a piece of terminology, this
- type of photosynthesis is called C3 photosynthesis.
- And the whole reason why it's called C3 photosynthesis is
- because the very first product that you get when you react
- with carbon dioxide, when the first time you fix your carbon
- dioxide-- remember carbon dioxide fixation is when you
- take it in its gaseous form and you actually
- put it into a molecule.
- The very first carbon molecule you get once the carbon
- dioxide gets fixed is a 3-carbon molecule.
- It's phosphoglycerate.
- That's why I showed it to you there.
- Because that's where the three in the C3
- photosynthesis comes from.
- Now, everything I've just done is a review of what we did in
- the last video.
- Maybe with a little bit more detail.
- And I showed you that you don't have to start
- with six and six.
- You could start with three and three and just have five go
- down here and one go down here instead of doubling
- everything.
- But what I want to show you now is an inefficiency that
- goes on in plants.
- This RuBisCo doesn't necessarily
- just fix carbon dioxide.
- It can also react with oxygen.
- And actually, its name is ribulose bisphosphate
- carboxylase oxygenase, which means it can react
- with carbon or oxygen.
- And this mechanism-- so let's say you have your ribulose
- bisphosphate hanging around.
- You know we're in the stroma of our chloroplast.
- So let's say we have our five ribulose bisphosphates.
- And instead of reacting with carbon it can actually react
- with oxygen.
- So instead of having carbon dioxide here, I can have O2
- coming in here.
- We have oxygen coming in here.
- And all of this, once again, is occurring on the surface or
- with the assistance of the RuBisCo.
- The exact same, the RuBisCo enzyme.
- Ribulose bisphosphate carboxylase oxygenase.
- That's why.
- Because it can also fix oxygen.
- And when these two things react, you don't get useful
- things that can be used for fuel and all of that.
- You end up with-- and my drawing is a little messy--
- you end up with-- well when these two guys react-- you'll
- end up with five molecules of this guy right here.
- Five molecules of this
- 3-phosphoglycerate right there.
- Remember here we ended up with six of them.
- But now we're only going to end up with five
- phosphoglycerates.
- I know the names are all very confusing.
- But the basic idea is to remember that what's happening
- here is if carbon dioxide isn't getting fixed, oxygen
- can get fixed.
- And you end up with five phosphoglycerates.
- And you actually also end up with five-- this is called
- phosphoglycolate.
- I know these are very daunting names.
- Phosphoglycolate, which is a 2-carbon molecule.
- Which makes sense because we only have five carbons to
- start with.
- This thing right here is a 3-carbon molecule.
- That's a 3-carbon molecule and this right here
- is a 2-carbon molecule.
- So this right here is going to be a 2-carbon molecule and it
- has a phosphate group.
- So as you can imagine, in this situation we're not going to
- be able to keep going forward and produce our glyceraldehyde
- 3-phosphate, which we can then use to make up carbohydrates.
- We're stuck.
- We just have these five phosphoglycerates.
- These can go on and some percentage of them, but the
- ratios are all getting messed up.
- But everything doesn't necessarily happen this
- cleanly in the cell.
- These things can go on, but we have one less being produced.
- And this thing right here, it's actually using up some of
- our carbon from our ribulose bisphosphate.
- And if this thing kept going up, all of our ribulose
- bisphosphate is going to get eaten up.
- We're not going to be able to continue on into this cycle
- over and over again.
- And actually this is kind of a waste product.
- Right here this is a waste product.
- Or we think it's a waste product.
- And this actually has to exit your chloroplast and actually
- get processed by other organelles in the plant cells.
- And these are called, these waste processing organelles in
- your cells, these are peroxisomes I know there's a
- lot of complicated terminology here.
- But the important thing to remember, when RuBisCo fixes
- oxygen, this is actually called photorespiration.
- That's an important word to know.
- Photorespiration.
- All of a sudden, instead of being able to carry forward
- with your Calvin cycle and produce a lot of sugar,
- instead you are depleting your RuBisCo.
- So this is a very bad process.
- This is going to get in the way of your Calvin cycle.
- And remember how important each of these G3Ps are.
- Because for every turn of the Calvin cycle, or at least the
- way I did it, we only produce one G3P that actually gets
- used for something useful.
- The other five G3Ps have to go back to produce ribulose
- bisphosphate.
- RuBisCo of course is the enzyme.
- So in this situation we only have five phosphoglycerates to
- begin with.
- Maybe if we have our ATP or an NADH, we can convert these
- five-- and it doesn't have to go in this direction-- but
- maybe we can convert these five phosphoglycerates into
- five glyceraldehyde 3-phosphates.
- G3Ps.
- Then all of these are going to be used back to produce our
- ribulose bisphosphate.
- So to go from here to here, we actually had to
- use up ATP and NADH.
- And then to go from here to here, we have to use more ATP.
- But we went through this whole cycle and we didn't produce
- anything, in terms of useful things that can be used to--
- essentially sugars or carbohydrates that can be used
- to fuel or provide structure for the plant in any way.
- So this is a completely-- or we think-- this
- is a wasteful process.
- So people wonder, when you look a lot of biological
- systems you don't see wasteful processes all the time.
- You would say, well wouldn't natural selection have
- selected against this?
- And some people believe that this is just a remnant from
- our evolutionary past, or our plants' evolutionary past,
- where there wasn't a lot of oxygen in the atmosphere.
- And if there wasn't a lot of oxygen in the atmosphere, this
- was not that likely of an occurrence.
- But there's some people who actually believe that no, this
- might actually have been selected
- for in natural selection.
- Because if there is a lot of oxygen hanging around in the
- cell, more than you need, that oxygen might actually react
- with your ATP and create other harmful compounds in the cell.
- And this might be a way of sopping up the harmful oxygen
- that's actually hanging around the cell.
- So who knows?
- But it's interesting idea.
- That you have this one enzyme that can react with ribulose
- bisphosphate and carbon dioxide.
- And if that happens, we just get our regular Calvin cycle.
- Or it can react with ribulose bisphosphate and oxygen and
- actually fix oxygen into these two molecules.
- Especially the phosphoglycolate which we
- think is a waste compound.
- And if you just went on and on in the Calvin cycle in this
- way, you will not produce any useful sugars.
- So in the next video, what I want to do is study some
- plants that have been able to get by this
- photorespiration problem.
- And you could imagine, photorespiration could be a
- really big deal or it could be very harmful in situations
- where, one, it's very important for a plant to be
- very productive of sugars.
- And it can also, we'll see, it can be a problem-- it's
- definitely a problem if there's a huge amount of
- oxygen content.
- But in the next video I'm going to show you plants that
- have gotten around this problem by, instead of
- performing C-3 photosynthesis, which is the classic Calvin
- cycle that I just showed you, they perform C-4
- photosynthesis.
- And I'll show you what that means in the next video.
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