Calvin Cycle - Online Tutor, Practice Problems & Exam Prep

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), producing a 3-carbon molecule (PGA); G3P synthesis, converting PGA into glyceraldehyde 3-phosphate (G3P) using energy; and RuBP regeneration, which reconstitutes RuBP for the cycle to continue. Ultimately, six CO₂ molecules yield 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. As the second stage of photosynthesis, it is actually going to use the NADPH and the ATP that was generated from the light reactions, the first stage of photosynthesis. Ultimately, the NADPH and the ATP made from the light reactions are going to be used to power the Calvin cycle. 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. This is contrary to the light reactions, which occur in the thylakoids of the chloroplast. The light reactions occur in the thylakoids of the chloroplast, but the Calvin cycle does not. It occurs in the stroma of the chloroplast. Again, stroma is not to be confused with stomata. Stomata are the openings that can open and close within leaves themselves, but stroma refers to the fluid-filled space within the chloroplast. The cycle is going to occur in the stroma and is 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.

Let's take a look at our image below, noticing the light reactions region over here is all grayed out and much smaller and deemphasized because we already covered the light reactions over here in our previous lesson videos. We 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. From our previous lesson videos, we know that the light reactions use photons of light from our sun and water molecules and ultimately convert these reactants into the products of oxygen gas and ATP and NADPH. Ultimately, the ATP and NADPH coming from the light reaction are going to be utilized to power the Calvin cycle, which is the main focus of this video. 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.

The Calvin Cycle is going to be powered by the ATP and NADPH provided by the light reactions and it is 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. 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 3 phases of the Calvin Cycle in 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 oken. 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 ribulose-1,5-bisphosphate carboxylase/oxygenase or 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, because, this is adding carbon dioxide, it's called carbon, and because it's being affixed, to RuBP, it's called carbon fixation. 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, it's going to generate PGA, a 3 carbon molecule.

Now moving on to the second 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 2G3P 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 2G3P 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 is 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 need to power the Calvin cycle, specifically needed to power the second phase of the Calvin cycle in G3P synthesis. And so ultimately, what we have here are some G3Ps, and it takes 2G3P 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 G3P molecules 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 RuBP, 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 RuBP back to its original state. And so here in the 3rd phase, what we have is RuBP regeneration, which is going to use the remaining leftover G3P that's not utilized to build glucose, and it's going to 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 Belgium

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. The cycle uses ATP and NADPH produced during the light reactions to convert carbon dioxide (CO₂) from the atmosphere into glucose, a type of sugar. The Calvin Cycle is essential 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), producing a 3-carbon molecule called PGA; (2) G3P Synthesis, where PGA is converted into glyceraldehyde 3-phosphate (G3P) using energy from ATP and NADPH; and (3) RuBP Regeneration, where leftover G3P molecules are used to regenerate RuBP, allowing the cycle to continue. These phases collectively transform CO₂ into glucose.

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 PGA. This process is essential for incorporating atmospheric CO₂ into organic molecules, ultimately leading to the production of glucose. RuBisCO's ability to fix carbon makes it one of the most important enzymes in photosynthesis.

How is ATP and NADPH used in the Calvin Cycle?

ATP and NADPH, produced during the light reactions of photosynthesis, provide the energy and reducing power needed for the Calvin Cycle. In the G3P Synthesis phase, ATP and NADPH are used to convert PGA into glyceraldehyde 3-phosphate (G3P). ATP is also required in the RuBP Regeneration phase to rearrange G3P molecules back into RuBP, ensuring the cycle can continue. Thus, ATP and NADPH are essential for driving the chemical reactions that produce glucose.

Why is the Calvin Cycle also known as the C3 pathway?

The Calvin Cycle is referred to as the C3 pathway because the first stable molecule produced during the Carbon Fixation phase is a 3-carbon compound called phosphoglycerate (PGA). This distinguishes it from other photosynthetic pathways, such as the C4 and CAM pathways, which produce 4-carbon compounds as their first stable molecules. The C3 pathway is the most common form of photosynthesis in plants.