All right, now let's go ahead and move on to the Calvin cycle. So this is sort of summarizing what we mentioned last time. So we know from our light reactions that we get ATP and NADPH. So we get water going in, we have NADP plus and ADP going in.
And from our light reactions, we get these two, which feed directly into the Calvin cycle plus CO2. And here's where we're going to get to sugar. So if some of you are thinking like, where the heck is sugar?
We're going to get to sugar in the Calvin cycle. So the Calvin cycle, unlike the light-dependent reactions, is cyclic electron flow. And it's going to use ATP and NAD, whoops, I should say NADPH, my apologies, NADPH, to reduce carbon dioxide to sugar. And the sugar is in the form G3P.
Okay, so it's going to use ATP and NADPH. which again, remember, came from the light reactions to reduce our sugar or to reduce carbon dioxide to sugar. For the net synthesis of one G3P molecule, the cycle has to take place three times total, three times total.
Okay, now we're going to go into the steps of the Calvin cycle. Kind of like with the light dependent reactions, you don't need to memorize the steps, but We're going to go through them so that you can understand why this process is happening and how each step actually goes into the production of sugar. So we're going to do a very general overview.
You don't need to know the specifics and you don't need to memorize the steps. You just need to know a general overview of what's happening. Okay, so like I said, there's three steps or three phases.
So firstly we have carbon fixation, then we have reduction, And then we have the regeneration of RUBP. So again, we're going to go through very generally what happens in carbon fixation, what happens in reduction, and then what happens in regeneration. Now, in the packet, you'll see a whole overview of the cycle, and you'll get to see every single step. And I know some of you might really be interested in that because it's pretty awesome. And especially for those of you interested in biochemistry, in biochemistry, you get to go through, you know, the nitty-gritty.
pretty details of all of these processes. So for those of you interested, it is provided so that you can see step by step what happens. But again, together, we're just going to go through this quite generally.
So let's start with phase one, carbon fixation. So CO2 here is incorporated into the Calvin cycle one at a time. And again, remember, we have to go through this three times to produce our sugar, but it's incorporated one at a time.
Each carbon dioxide that's going to go into the Calvin cycle is going to attach to a molecule of RUBP. This is going to be catalyzed by the enzyme rubisco. Rubisco plays a really important role because it's how we fix carbon. It's how we actually get carbon to attach to RUBP. This is going to form what's known as 3-phosphoglycerin.
So we have each CO2 coming in one at a time. It's going to attach to a molecule. RUBP, and then it'll be catalyzed by the enzyme rubisco.
Okay. And then I'll form three phosphoglycerate. All right.
RUBP, just so you know, have some background is a five carbon sugar. 5-carbon sugar. Now this 3-phosphoglycerate that's formed is unstable, so we're going to see that this is going to change, and we're going to get lots of intermediates here. All right, now let's go ahead and move on to reduction.
So each molecule of 3-phosphoglycerate that we got is going to be phosphorylated by ATP, and it uses a total of 6. So it needs 6 ATP in order to change, or I'm sorry, phosphorylate 3-phosphoglycerate. Now every time it does this, it's going to become 1,3-bisphosphoglycerate. At this point, 6 NADPH molecules donate electrons to the 1,3-bisphospho. That's right.
So this is why. So why are we why are we saying this? Why are we even talking about this?
Right. Because we're not really going any more in depth on this. Well, we're mentioning this because here we're using our ATP and here we're using our NADPH. And remember, this is what we got from the light dependent reactions. So guess what happens when this ATP phosphorylates this?
What do we get? We're going to get ADP plus our endocrine phosphate. When our NADPH molecules donate electrons to 1,3-bisphosphoglycerate, guess what we're going to get? We're going to get NADP+. And these are going to then go back to the light-dependent reactions.
And so that's why we're talking about this, so that you can understand where these things are coming from, right? How do we get these? And every year I have students ask, where are these molecules? Where are they coming from? And this is where they're coming from.
So you don't have to memorize any of these names, but when you look at your image, going through this text is going to help you as you go step by step through the images that are provided in your textbook and also in your packet. Now this will reduce to G3P when this happens. And so here's what we've been waiting for, right?
We've been waiting for this G3P molecule to come up because this is going to be your, basically what will give us our sugar. So six molecules of G3P are formed, but only one is counted as a net gain, which kind of seems like a waste, right? So I'm sure some of you are thinking, wait, we get six, but we only use one as an actual gain for the plant? And yes, because in phase three, we're going to use those other five molecules of G3P to regenerate RUBP, which remember, was used in the very beginning of the cycle which attached to CO2. So RUBP, remember, attached to CO2 and that's how we fixed it.
And so without RUBP we wouldn't have a cycle happen. So yes, we're using a lot of that net or a lot of that G3P that we got, but it's going to regenerating this cycle. So yes, it's a lot, but it regenerates the cycle. So we only get one G3P net.
All right, so now let's go ahead and talk about that regeneration. So we have five G3P molecules used to regenerate the three molecules of RUBP. It's going to use 3 ATP for regeneration, which means at this point in time, we're going to get more ADP plus our inorganic phosphate, which can go back to the light dependent reactions. And now the cycle is ready to take in more carbon dioxide. So now we've completed this cycle and it can take in more CO2 and start over.
again. And remember, while this is happening, the plant is also getting the components that's needed from the light reactions to fuel other parts here of the reaction too. It needs ATP and the NADPH from the light reactions to have this happen as well.
Okay, so let's go ahead and put it all together with our inputs and outputs. So in terms of our inputs, we have carbon dioxide, and that's what gets fixed. We have ATP, and then we have our NADPH. from the light-dependent reactions. And then in terms of our outputs, we get our one net G3P, we get nine ADP and six NADP plus.
And again, the ADP and NADP plus go to the light-dependent reactions. Also, sometimes I just write ADP just as a shorter form, but remember it's the same as with the inorganic phosphate. Sometimes we just don't write the inorganic phosphate, but it's always there. So now let's put it into words. So the Calvin cycle itself uses NADPH, ATP, and CO2 to produce a three carbon sugar known as G3P.
The three phases were carbon fixation, reduction, and regeneration of RUBP. One thing I want to point out too, students sometimes just think that G3P is the only thing produced. The process of photosynthesis actually produces many more organic molecules too that are necessary for the plant. That also produces some amino acids as well.
So even though this is our primary focus, the G3P, please note that photosynthesis is important for many other things. Many other organic molecules necessary for the function and survival of plants are being produced throughout the world. this process.
All right, let's transition to a problem for our plants, a problem. And the main problem that plants experience is known as photorespiration. So on really hot days, plants close their stomata to stop water loss, which makes sense because they need water for the light-dependent reactions, remember that was split water. But on really hot days, they want to close their stomata to prevent excessive.
water loss. But why is that an issue? Well, how do they get CO2 for the photosynthesis, which is used in the Calvin cycle? Well, they get it from their stomata.
So if they close their stomata, they're going to get less carbon dioxide and more oxygen. Why? Because remember, oxygen is released as a byproduct.
But if they close their stomata, oxygen becomes trapped inside of our cells. So they can't get CO2 in and then oxygen becomes trapped. And this is a problem because Rubisco, which we talked about in the Calvin cycle, can bind to oxygen instead of carbon dioxide.
So when we go through the Calvin cycle, if we're not taking in carbon dioxide, we're not getting anything. So Rubisco, excuse me, can actually bind to oxygen on accident. And to make it worse, it's going to use up a lot of ATP to do so.
And then the process will produce CO2, but no sugar is produced here. No sugar. So you produce CO2 and then you don't get any G3P. So it's a huge waste of time for the cell because it's using energy.
And then we're not getting the necessary molecules like G3P that we need. So photorespiration is a huge problem for plants in very hot. weather, bad for the planet.
We don't want this to happen. We don't want to waste our energy when we're in a really hot environment. So there's been some adaptations that we've seen over time evolve in plants.
So plants that live in really hot, dry climates show signs of evolution and they have alternative mechanisms of carbon fixation. So we're going to go through two examples of adaptations. Now the first adaptation is seen in what are called C4 plants.
And remember all other plants are called C3 plants because we form a three carbon sugar, G3P. That's why they're called C3 plants. C4 plants show a spatial separation of steps.
So what will happen is their stomata partially closed to conserve water in very hot heat or in very high heat areas. And then their mesophyll cells will fix CO2 into a... four carbon molecule.
That's why they're called C4 plants. These four carbon molecules will be transferred to the bundle shape cells inside of the plant. Then they'll be released or they'll release, excuse me, CO2 to be used in the Calvin cycle.
Plants that go through this are corn, grasses, sugar cane. So we see a spatial separation. So instead of all sort of happening in the same place, like in C3 plants, remember C3 plants, we have a light dependent reactions in the thylakoid membrane, Calvin cycle in the stroma.
In C4 plants, we see a spatial separation. So mesophyll cells will fix carbon dioxide to a four carbon molecule, and then it's transferred over. So that's the spatial separations and transfer to the bundle sheath cells and all release CO2 to be used in the Calvin cycle there.
So a spatial separation. So this allows the plant to conserve water by partially closing the stomata and separating where the steps occur. So that's the first one. The second adaptation are can plants. So can plants actually open their stomata at night and then they close them during the day to conserve water.
Carbon dioxide is going to be incorporated into organic acids and stored in vacuoles. So CO2, think of it as almost like a storage method. It's going to be storing them in organic acids in vacuoles.
Now, C4 plants was a spatial separation. Think of this as a temporal separation, a separation of time, because now we're focusing on somata being open at night and then closed during the day. So during the day the light reactions occur and carbon dioxide is released from the organic acids and then it's incorporated into the Calvin cycle. So again we have a temporal separation of steps. Examples of plants that go through this are things like pineapples, cacti, succulents, jades, right?
All these plants are ones that we know live in very hot environments. So C4 plants produce 4-carbon sugar. We had a spatial separation of steps and then CAM plants we have a temporal separation of steps. Okay let's go ahead and look at an FRQ.
So it says a student wanted to identify the effect of varying colors of light on the growth of plants. The student exposed tomato plants to three different wavelengths of light for the course of one week and measured the overall height of each plant each day. All other conditions were kept consistent between the plants and the results are below.
So A, identify the wavelength that caused the least amount of growth. And then B, explain why the wavelength identified in part A was ineffective. Okay, go ahead.
Take about five to seven minutes. Work on this one. Okay, so A, identify the wavelength that caused the least amount of growth.
Well, the wavelength that caused the least amount of growth is going to be the 550 nanometers or the green. And we can see that. decrease in height, right? So if we look at the height, it had the lowest height.
Why was that? Well, part B explained why the wavelength identified in part A was ineffective. Chlorophyll is green.
Chlorophyll A is our primary pigment. If chlorophyll A is green, that means it reflects green light. So it cannot absorb green to excite electrons.
So A, it's going to be the 550 nanometer one. B. Chlorophyll A is green, which means it absorbs, or excuse me, reflects green light. It cannot absorb green light.
So electrons are never excited to be powered up and travel in the linear electron flow in the light-dependent reactions because a green light does not excite the electrons. All right, let's look at our next one. Rubisco can bind to carbon dioxide or oxygen in the Calvin cycle. After learning this, a student was interested in photorespiration. After doing research, she claimed that it is simply a metabolic relic, meaning early atmospheric conditions did not have oxygen, therefore never posed a problem early on.
Would you support or reject this claim? Justify your response. Go ahead, take a few minutes, write up a response.
All right, well I would say that this does indeed support her claim because we know that photosynthesis evolved very early on in cyanobacteria. And we know that that was responsible for what oxygenated the earth because oxygen is released as a byproduct through our photosynthesis reactions. So oxygen was never a worry of plants because Rubisco never had the option to bind to oxygen in early earth.
But now that earth is so abundant with oxygen, We know that during photorespiration that can happen. So yes, her claim could be supported in that Rubisco never had the option to bind to oxygen on early Earth. Therefore, now that it does have the option, we do see that happening. And so it's basically an evolutionary, like she said, an evolutionary relic that has been accidentally, or I don't want to say accidentally.
But it's been conserved over time because it was never an issue on early Earth. All right, that's going to be it for topic five. Next topics to be covered are cellular respiration and fitness.