Calvin Cycle: Study with Video Lessons, Practice Problems & Examples

The Calvin Cycle, a crucial part of photosynthesis, occurs in the stroma of chloroplasts and utilizes ATP and NADPH from light reactions to convert carbon dioxide (CO₂) into glucose. It consists of three phases: carbon fixation, where the enzyme RuBisCO attaches CO₂ to ribulose bisphosphate (RuBP); G3P synthesis, producing glyceraldehyde 3-phosphate (G3P) as a glucose precursor; and RuBP regeneration, which restores RuBP for the cycle to continue. Overall, six CO₂ molecules are required to synthesize one glucose molecule, emphasizing the cycle's role in carbon fixation and energy transformation.

Calvin Cycle

In this video, we're going to begin our introduction to the Calvin Cycle. And so we know already that the Calvin Cycle is the second stage of photosynthesis following the light reactions. And so the Calvin Cycle as the second stage of photosynthesis is actually going to use the NADPH and the ATP that was generated from the light reactions, the first stage of photosynthesis. And so ultimately the NADPH and the ATP made from the light reactions is going to be used to power the Calvin Cycle. And ultimately, this energy from the light reaction is going to be utilized by the Calvin Cycle to make organic molecules, for example, glucose, a sugar that is widely prevalent and abundant. Now the Calvin Cycle specifically occurs in the stroma of the chloroplast. And so this is contrary to the light reactions which occurs in the thylakoids of the chloroplast. And so the light reactions occur in the thylakoids of the chloroplast, but the Calvin Cycle does not occur in the thylakoids. It occurs in the stroma of the chloroplast. And again, stroma is not to be confused with stomata. Stomata are the openings that can open and close within leaves themselves, but stroma is referring to the fluid-filled space within the chloroplast. And so it's going to occur in the stroma and it's also going to consume carbon dioxide gas or CO₂ gas from the atmosphere and ultimately the CO₂ gas from the atmosphere is going to be utilized to build glucose. And so let's take a look at our image down below, which notice, the light reactions region over here is all grayed out and it's much smaller and deemphasized because we already covered the light reactions over here in our previous lesson videos. And so we already know from our previous lesson videos that the light reactions occur in the thylakoids, specifically these green pancake structures that are within the chloroplast and of course this, grayed out structure in the background represents the chloroplast itself. And so we know from our previous lesson videos that the light reactions use photons of light from our sun and water molecules and ultimately, it converts these reactants into the products of oxygen gas and ATP and NADPH. And ultimately, the ATP and NADPH coming from the light reactions are going to be utilized to power the Calvin Cycle, which is really the main focus of this video. And the Calvin Cycle occurs again in the stroma of the chloroplasts, which is this fluid-filled space that we see here within the chloroplast itself. And the Calvin Cycle is going to be powered by the ATP and NADPH provided by the light reactions, and it's also going to take in carbon dioxide gas from the atmosphere and ultimately use the ATP and NADPH and carbon dioxide gas to generate organic molecules such as glucose, which we can see down here. And so we're going to talk more about the Calvin Cycle as we move forward in our course. So, this here is just the introduction and I'll see you all in our next video when we'll talk more about the Calvin Cycle.

Where in a plant cell does the Calvin cycle take place?

• A. Stroma
• B. Thylakoid space
• C. Thylakoid membrane
• D. Chloroplast inner membrane

3 Phases of the Calvin Cycle (C3 Pathway)

In this video, we're going to introduce the 3 phases of the Calvin Cycle, which we know the Calvin Cycle is the second stage of photosynthesis following the light reactions. And we're going to talk about the three phases of the Calvin cycle and what's known as the C3 pathway, which we'll get to define what that means here very shortly. Now notice that the three phases of the Calvin cycle, we have numbered down below as number 1, number 2, and number 3. And notice that we have each of these phases color coordinated so that they color coordinate to the image that we have down below for each of these three phases of the Calvin Cycle and so that's important to keep in mind. Now, the first phase of the Calvin Cycle is what's known as carbon fixation. And the fixation part here actually has nothing to do with fixing something that's broken. Instead, this term fixation is derived from the term to affix something. And the term affix is really just a term that means to fasten something to something else, to attach something to something else. And really that's what's happening here in carbon fixation. And so carbon fixation is going to utilize the enzyme that's known as rubisco. And rubisco is a very important enzyme because rubisco has this amazing ability to affix or to add carbon dioxide gas from the atmosphere to a 5 carbon sugar molecule called ribulose bisphosphate or RuBP. And so RuBisCO is the enzyme that takes carbon dioxide and adds it to RuBP. And so, because this is adding carbon dioxide, it's called carbon, and because it's being affixed to RuBP, it's called carbon fixation. Now it turns out that the very first stable molecule that's produced in this carbon fixation phase is a 3 carbon molecule and because the very first stable molecule that's produced is a 3 carbon molecule or a C3 molecule, this pathway is called the C3 pathway. Now, later in our course when we're talking about different types of photosynthesis we'll talk about other pathways that do not form a stable 3 carbon molecule. Instead, they end up forming a 4 carbon molecule. But we'll talk about those other types of photosynthesis including C4 and CAM pathways later in our course. For now, this pathway, the C3 pathway, is the standard pathway and it forms a stable 3 carbon molecule and that stable 3 carbon molecule is called phosphoglyceraldehyde. And so phosphoglyceraldehyde is also known as PGA. Now, if we take a look at our image down below at the first phase of the Calvin cycle, just focusing on this left-hand side, Notice that the first phase of the Calvin cycle is right here in green called carbon fixation. And what it does is it uses this enzyme rubisco, and the enzyme rubisco has this amazing ability to take carbon dioxide which we are showing you up here, and it can take the carbon dioxide and affix it to the RuBP molecule that we're showing you here. And ultimately through a series of reactions, the very first stable molecule that's produced is a 3 carbon molecule called PGA. And again, each of these black circles that you see throughout here represent carbon atoms. And so that's really it for the carbon fixation stage. RuBisCO is going to take carbon dioxide and attach it to RuBP and ultimately through a series of reactions is gonna generate, PGA, a 3 carbon molecule. Now moving on to the 2nd phase of the Calvin cycle, what we have is G3P synthesis. And in this G3P synthesis phase, what's going to happen is the PGA is going to be used to synthesize Glyceraldehyde 3 Phosphate or G3P. And G3P is a precursor molecule that's needed to build glucose. And so the cell is going to use specifically 2 G3P molecules to synthesize a single glucose molecule. And so if we take a look at our image down below at the G3P synthesis phase, notice that it starts with PGA and it converts this PGA into another 3 carbon molecule called G3P. And G3P is going to be the precursor that's needed to build glucose, and so ultimately there are going to need to be 2 G3P molecules needed to build 1 glucose. And also in this G3P synthesis phase, it's actually going to require the use of energy, and this energy is going to come in the form of NADPH and ATP. And this NADPH and ATP that's needed as energy for G3P synthesis, this NADPH and ATP are coming from the light reactions which is the first phase of photosynthesis. And so, the light reactions produce the energy in the form of NADPH and ATP that's needed to power the Calvin Cycle, specifically needed to power the second phase of the Calvin Cycle and G3P synthesis. And so ultimately, what we have here are some G3Ps, and it takes 2 G3P molecules again to make just one single glucose. But ultimately, there's going to be some leftover G3P molecules that are not going to be utilized to build glucose. Instead, these leftover G3Ps are going to be used in the 3rd and final phase of the Calvin cycle. And so in the 3rd and final phase of the Calvin cycle, what we have is RuBP regeneration. And recall that RuBP is ribulose bisphosphate. It's the very first molecule that was generated, the very first molecule that was reacting here in the Calvin cycle. And so in order for the Calvin cycle to be a cycle, in order for it to start and end in the same place, it needs to have a phase that's dedicated to regeneration essentially, getting our RuBP back to its original state. And so here in the 3rd phase, what we have is RuBP regeneration which is gonna use the remaining over G3P that's not utilized to build glucose and it's gonna rearrange this G3P in a series of enzymatic reactions that's driven by ATP to regenerate RuBP and then once RuBP has been regenerated it's ready to go for a second round of the Calvin cycle here. And so, when we take a look at RuBP regeneration phase down below, it's over here, and notice that it takes the remainder of the G3P and uses it to rebuild to regenerate RuBP here. And it's going to take energy in the form of ATP in order for this regeneration to occur. And so ultimately that is the end of the Calvin cycle, those three phases. And at the end of the Calvin Cycle, again, notice that carbon dioxide is ultimately being used as a reactant, and also NADPH and ATP are being used as reactants as well. And in terms of the products, what we get is a glucose molecule. And so in order to remember the reactants and the products of the Calvin Cycle, Over here on the right what we have is a memory tool to help you guys remember the reactants and the products of the Calvin cycle. And so when you think about the Calvin cycle you really want to think about Calvin's can of sugar, which is, really just this soda can right here. And so here we have Calvin and you can see he's got even a little name tag that says, hi, my name is Calvin. And so Calvin has his can of sugar here, this soda can here. And so when you're thinking about the Calvin cycle just think about Calvin's can of sugar and when you think about that again that'll help you remember the reactants of the Calvin cycle and the products of the Calvin cycle. And so the reactants of the Calvin cycle are gonna come from this can over here where the C in the can represents the C and CO2 carbon dioxide. The A in the can represents the A in ATP, and the N in the can represents the N in NADPH. And so CO2, ATP, and NADPH are the reactants needed for the Calvin cycle. And of course, Calvin's can of sugar, the sugar over here, is going to represent the product which is glucose. And so by remembering Calvin's can of sugar, once again you'll be able to remember the reactants and the products of the Calvin cycle. Now in terms of this image over here and what you're supposed to memorize, that is going to be up to your professor. Your professors are going to vary in exactly the amount of detail that they want you to know about the Calvin cycle. But, one thing that is pretty consistent about what you should know is that it's going to take a total of 6 Carbon Dioxide Molecules entering the Calvin Cycle to create Glucose. And one way to help you remember that is that, within 6 carbon dioxide, there are 6 carbon atoms. And glucose also has 6 carbon atoms. And so ultimately, these 6 carbon atoms of the carbon dioxide end up as the 6 carbon atoms that are in a glucose molecule. And also, you should know that it takes 2 G3P molecules to make a glucose. So basically, the interactive portions here are the portions that you really want to focus on the most. And so this here concludes our introduction to the three phases of the Calvin Cycle in the C3 Pathway, and we'll be able to get some practice applying these concepts as we move forward in our course.

The enzyme rubisco combines RuBP with a carbon atom from:

• A. Glucose
• B. ATP
• C. Carbon monoxide
• D. Organic compounds
• E. Carbon dioxide
• F. NADPH

Which of the following processes occurs during the Calvin cycle?

• A. Reduction of NADPH
• B. Release of oxygen
• C. Regeneration of RuBP
• D. Production of ATP

The function of the light reactions is to ___________, while the function of the Calvin Cycle is to __________.

• A. Convert light energy into chemical energy; Store chemical energy in the form of organic molecules.
• B. Use light energy to produce ATP; Use chemical energy to produce ATP.
• C. Store light energy; Use light energy to produce carbon.
• D. Transfer heat captured from light to electrons; Use electrons to generate organic molecules.

Here’s what students ask on this topic:

What is the Calvin Cycle and where does it occur?

The Calvin Cycle is the second stage of photosynthesis, following the light reactions. It occurs in the stroma of the chloroplasts, which is the fluid-filled space within the chloroplast. Unlike the light reactions that take place in the thylakoids, the Calvin Cycle utilizes ATP and NADPH produced during the light reactions to convert carbon dioxide (CO₂) from the atmosphere into glucose, a type of sugar. This process is crucial for carbon fixation and energy transformation in plants.

What are the three phases of the Calvin Cycle?

The Calvin Cycle consists of three phases: (1) Carbon Fixation, where the enzyme RuBisCO attaches CO₂ to ribulose bisphosphate (RuBP); (2) G3P Synthesis, which produces glyceraldehyde 3-phosphate (G3P) as a precursor to glucose; and (3) RuBP Regeneration, which restores RuBP to allow the cycle to continue. These phases collectively convert CO₂ into glucose using the energy from ATP and NADPH generated in the light reactions.

What role does RuBisCO play in the Calvin Cycle?

RuBisCO is a crucial enzyme in the Calvin Cycle, specifically in the carbon fixation phase. It catalyzes the attachment of carbon dioxide (CO₂) to ribulose bisphosphate (RuBP), forming a 3-carbon molecule called phosphoglycerate (PGA). This step is essential for incorporating atmospheric CO₂ into organic molecules, ultimately leading to the production of glucose.

How many CO2 molecules are required to produce one glucose molecule in the Calvin Cycle?

To produce one glucose molecule, the Calvin Cycle requires six carbon dioxide (CO₂) molecules. Each CO₂ molecule contributes one carbon atom, and since glucose (C₆H₁₂O₆) contains six carbon atoms, six CO₂ molecules are needed. This process also involves multiple rounds of the cycle to generate the necessary intermediates and energy molecules.

What is the significance of G3P in the Calvin Cycle?

Glyceraldehyde 3-phosphate (G3P) is a significant intermediate in the Calvin Cycle. During the G3P synthesis phase, PGA is converted into G3P using ATP and NADPH from the light reactions. G3P serves as a precursor for glucose synthesis. Two G3P molecules are required to form one glucose molecule. Additionally, some G3P molecules are used in the RuBP regeneration phase to ensure the cycle can continue.