Light Independent Reactions: Videos & Practice Problems

The Calvin cycle, or light-independent reactions, is essential for carbon fixation in plants, primarily C3 plants. It utilizes ATP and NADPH from light-dependent reactions to convert CO₂ into sugars, releasing oxygen as a byproduct. The enzyme RuBisCO plays a crucial role in this process, though it operates slowly and can lead to photorespiration under high oxygen conditions. C4 and CAM plants have adapted to hot environments by separating carbon fixation processes or timing them to optimize CO₂ uptake, thus enhancing efficiency in sugar production.

Calvin Cycle

Hi. In this video, we're going to be talking about light-independent reactions, also known as the Calvin cycle. So the Calvin Cycle is best described as carbon fixation. This uses ATP and NADPH, which were created in the light-dependent reactions to fix carbon from CO₂. It takes CO₂ and uses that carbon to create sugar. The process that we're going to talk about occurs in what's known as C3 plants, which are the majority of plants. You may remember these from your Bio 101 class.

So, how this happens is CO₂, the carbon from it, is attached to a sugar called ribulose 1,5-bisphosphate, which is a 5-carbon sugar. So, when that one carbon comes in and attaches to this 5-carbon sugar, it creates the possibility of 6 carbons, which is then converted into two 3-carbon sugars, and that sugar is called phosphoglycerate, and that's going to be step 1. In step 2, that is going to be reduced, and when it is reduced, it releases oxygen as a waste product. Carbon fixation is the process of taking that carbon from CO₂ and using it and attaching it to sugars, and in that process, oxygen is released.

The really important part of carbon fixation is the enzyme responsible for it, and this enzyme is called RuBisCO. RuBisCO is the enzyme that catalyzes carbon fixation, the removal of carbon from carbon dioxide and adding it to a sugar. RuBisCO is such an interesting enzyme because it works really slowly. Even though enzymes tend to work very fast, RuBisCO is actually fairly slow. Thus, because it's a critical component that allows plants to create sugar from light, they need a lot of it. It is thought to be the most abundant protein on earth because plants need so much of it.

In the third step, the ATP and NADPH, which I mentioned were needed before, come from the light-dependent reactions. They are used to generate more of this 5-carbon sugar. In the end, three carbon dioxides end up making one of these sugars, which consumes 9 ATP and 6 NADPH, which is a lot of energy to create just one sugar. Here's an overview of the Calvin cycle. In this overview, we focus on how the ATP and NADPH, obtained in the light-dependent reactions, are used to produce sugars, which is the last step. And so, this is the overview of the Calvin cycle. So with that, let's now move on.

C4 and CAM Plants

So now, we are going to talk about carbon fixation in other types of plants, and these are smaller classes of plants called C4 and CAM plants. So, the carbon fixation that we talked about before occurs in what's known as C3 plants, which is most plants. But the problem with it is that it's not really efficient in a hot environment. So, things like deserts or really hot summers, and this is because there are these regions of plants called stroma, which are essentially just pores in plant leaves, and they close when it's hot to prevent the loss of water. But the problem with stroma closing means that CO₂, which is super important for carbon fixation, which is, I mean, essentially it's required for carbon fixation, can't get into the leaves if those are closed. Stroma when they are closed prevents gas exchange and so what happens is carbon fixation is still going, I mean, it's still pumping along, but the CO₂ it needs are decreasing rapidly, because they are not getting any more from the environment. But what is increasing is oxygen, because, remember, carbon fixation is going to produce oxygen. So, you get this super big buildup of oxygen in plants, which it doesn't need, and it needs more carbon dioxide but isn't getting it because it's hot, so the plants close their leaves.

Now, this is where we get back to RuBisCO. So RuBisCO, like I said, is this really kind of inefficient enzyme. We have a lot of it. It works slowly, but it also works, to bind CO₂ or O₂. Whether it works with CO₂ or O₂, it will actually produce something different. So like I said before, if it is RuBisCO working to fix carbon, it's going to produce sugar, which is what we know as photosynthesis. But if it works on O₂, what happens? And this is just kind of this big waste of energy. It doesn't need to do this. It's not efficient. The plant's just literally just wasting all of its energy doing this when it wants to be doing this. So, if the plant closes their pores, and there's this buildup of O₂, Rubisco then is going to follow this pathway and do photorespiration instead.

C4 plants, which are things like corn and certain types of grasses in grasslands, handle this buildup of oxygen in a specific way. They actually just separate carbon fixation. They have two different cell types that handle different things. Mesophyll cells handle the light-dependent reactions and fixation of CO₂. When they do this, they form this molecule called malate. Next to them, it has bundle sheath cells. What happens is that CO₂, which was originally formed in this malate molecule, can be released, and then that CO₂ can be used in the Calvin cycle pathway. RuBisCO, here, is actually only found in the bundle sheath cells, so it prevents this accumulation of O₂, and prevents wasting energy.

Let me back up and I can show you what this is like. Here we have the C4 photosynthesis pathway. We have our two types of cells: the bundle sheath, which is this weird color, and we have the mesophyll. And they're really close to each other. So what happens is CO₂ comes in, it goes through all its different processes, and what happens is it gets turned and fixed into this chemical called malate. Then, that gets transferred to the bundle sheath cells, and malate is then released, and then it undergoes the C3 pathway.

Other types of plants are called CAM plants, and these are mostly just cacti, and they handle this oxygen buildup very differently. What they do is they actually control the amount of CO₂ differently at different points of the day. So at night, it's cool outside, so the stroma is open, and then CO₂ is fixed normally. It's fixed using RuBisCO in that normal pathway that we talked about. But in the daytime, it's super hot, so the stroma is closed. CO₂ is fixed to this malate that was talked about before and stored in the cell.

So we're not even going to deal with it; we're just going to store it here. And then, when the stroma opens, what happens is the CO₂ which is attached to this malate, that carbon, can be released and fixed normally by RuBisCO. Where the C4 plants actually just separated the processes in different cells, the CAM plants say, okay, well, we're not going to do this during the day. We're going to do this at night. So, what you can see here is that, at night, these pores are open. It can get through, and it can be processed by RuBisCO. Then what happens during the day, it closes. The CO₂ can't get through. It will store it as malate. You don't necessarily need to understand or follow all these chemicals and formulas, but know that it gets stored as malate and then, whenever it undergoes processing again, that malate is released and can be used by the cell, normally, and fixed by RuBisCO.

So, again, it's kind of a mouthful, but these are all the different ways that carbon is fixed to sugars in plants, and hopefully, that was clear about the differences between the C3, C4, and CAM plants. With that, let's now move on.

What is the function of RUBISCO in carbon fixation?

• A. Catalyzes carbon fixation
• B. Convert carbon into starch
• C. Promoting gas exchange
• D. Opening and closing the stroma

Answer: (A)

Why does the Calvin cycle require light?

• A. Because light forms ATP through the light dependent reactions
• B. Because light forms NADPH through the light dependent reactions
• C. Because light forms CO₂ through the light dependent reactions
• D. Because light forms O₂ through the light dependent reactions

Answer: (B)

How many molecules of CO₂ are required to create one glyceraldehyde 3-phosphate?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (C)

True or False:RUBISCO only binds to CO₂.

• A. True
• B. False

Answer: (B)

Which of the following plants handles O₂ buildup by physically separating RUBISCO from O₂?

• A. C3 Plants
• B. C4 Plants
• C. CAM Plants

Answer: (B)

Here’s what students ask on this topic:

What is the Calvin cycle and how does it work?

The Calvin cycle, also known as the light-independent reactions, is a crucial part of photosynthesis that occurs in the stroma of chloroplasts. It uses ATP and NADPH produced in the light-dependent reactions to convert CO₂ into glucose. The cycle involves three main stages: carbon fixation, reduction, and regeneration. In carbon fixation, CO₂ is attached to a 5-carbon sugar called ribulose-1,5-bisphosphate (RuBP) by the enzyme RuBisCO, forming a 6-carbon compound that splits into two 3-carbon molecules of 3-phosphoglycerate (3-PGA). In the reduction phase, ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P). Finally, in the regeneration phase, some G3P molecules are used to regenerate RuBP, allowing the cycle to continue.

What role does RuBisCO play in the Calvin cycle?

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is a key enzyme in the Calvin cycle, responsible for catalyzing the first major step of carbon fixation. It attaches CO₂ to ribulose-1,5-bisphosphate (RuBP), forming a 6-carbon compound that quickly splits into two molecules of 3-phosphoglycerate (3-PGA). Despite its crucial role, RuBisCO is relatively slow and inefficient, necessitating large quantities in plants. Additionally, RuBisCO can also bind to O₂ instead of CO₂, leading to a wasteful process called photorespiration, especially under high oxygen conditions.

How do C4 and CAM plants differ in their carbon fixation strategies?

C4 and CAM plants have evolved different strategies to optimize carbon fixation in hot environments. C4 plants, like corn, separate carbon fixation into two different cell types: mesophyll cells and bundle sheath cells. CO₂ is initially fixed into a 4-carbon compound (malate) in mesophyll cells, which is then transported to bundle sheath cells where CO₂ is released and enters the Calvin cycle. CAM plants, such as cacti, fix CO₂ at night when their stomata are open, storing it as malate. During the day, when stomata are closed to conserve water, the stored CO₂ is released for use in the Calvin cycle. These adaptations help minimize water loss and photorespiration.

Why is RuBisCO considered an inefficient enzyme?

RuBisCO is considered inefficient for several reasons. Firstly, it has a slow catalytic rate, meaning it processes CO₂ at a slower pace compared to other enzymes. Secondly, RuBisCO can bind to both CO₂ and O₂. When it binds to O₂, it initiates a process called photorespiration, which consumes energy and releases CO₂ without producing sugars, thus wasting resources. This inefficiency is particularly problematic in hot environments where stomata close to conserve water, leading to higher internal O₂ levels and increased photorespiration.

What is photorespiration and why is it considered wasteful?

Photorespiration is a process that occurs when the enzyme RuBisCO binds to O₂ instead of CO₂. This leads to the production of a 2-carbon compound that must be recycled through a series of reactions involving multiple organelles, consuming ATP and releasing CO₂ without producing sugars. It is considered wasteful because it uses energy and resources that could otherwise be used for photosynthesis, reducing the overall efficiency of the plant's energy production, especially under conditions where CO₂ is limited and O₂ is abundant.