Photorespiration - Online Tutor, Practice Problems & Exam Prep

Photorespiration is a process in plants that occurs under hot conditions when stomata close, preventing carbon dioxide (CO₂) from entering and causing oxygen (O₂) to accumulate. This leads to the enzyme Rubisco binding O₂ instead of CO₂, resulting in the production of CO₂ and wasting ATP and NADPH, making photosynthesis inefficient. Understanding stomata's role in gas exchange is crucial, as their closure during high temperatures protects plants from dehydration but hinders photosynthesis, highlighting the balance between water conservation and efficient energy production.

Photorespiration

In this video, we're going to introduce photorespiration, which kinda sounds like photosynthesis, but not really and they're actually really different from each other. And so what we need to recall from our previous lesson videos is that photosynthesis is going to use or consume carbon dioxide gas or CO₂ from the atmosphere in order to build sugars like glucose. But photorespiration is actually a process that causes plants to make carbon dioxide gas or CO₂ rather than consume it like what photosynthesis normally does. And so in a way, photorespiration is kind of the opposite of photosynthesis. Since again photosynthesis will use or consume carbon dioxide and decrease carbon dioxide levels. Whereas photorespiration is going to make carbon dioxide rather than consume it, which is going to increase carbon dioxide levels. And so because photorespiration is kind of the opposite of photosynthesis in a way, that is going to make photosynthesis really, really inefficient. Photorespiration makes photosynthesis really, really inefficient because it kind of does the opposite of photosynthesis. It makes carbon dioxide instead of consuming it.

Now when you break down the roots in the word photorespiration, what you'll see is that it has the root photo, which we know is a root that means that it's going to occur with light. And we also know that photosynthesis has this root photo, so both photosynthesis and photorespiration occur with light. And so that means that they are going to kind of compete with each other in a way. But of course, photorespiration also has this respiration term in here, which is really referring to the ability for it to produce or release carbon dioxide gas, just like we already indicated here. And so if you put it together, photorespiration means that it's going to occur with light and actually produce or release carbon dioxide gas.

Now, in order to better understand photorespiration, we also need to recall from our previous lesson videos what the stomata of a plant are. And so the stomata of a plant are not to be confused with the stroma of the chloroplast. And so recall from our previous lesson videos that the stomata are these openings or pores or holes that are found in the leaves themselves that can open and close and control gas exchange between the leaf and the environment of the leaf. And so the stomata can either be in an open position or again they could be in a closed position, and if the stomata are in the open position then that means that gas exchange is able to occur. And that means that carbon dioxide is able to come into the leaf and oxygen is able to leave the leaf.

And so if we take a look at our image down below over here on the left-hand side, notice what we're showing you is that in cooler temperatures, when the temperatures are relatively cool, then the stomata of the plant are going to be in an open position. And so if you take a look at this region of our image here, notice that we're zooming into a leaf here, and down below, this represents the mesophyll tissue of the leaf. And what you'll notice is that this structure right here represents the stomata. And notice that in cooler temperatures, the stomata are going to be in an open position just like what we see right here. And so with this gap here between, the stomata here, the open position of the stomata, when it's open, it allows for gas exchange. So it allows for carbon dioxide gas to diffuse into the plant, and it allows for oxygen gas that's being produced to diffuse out of the plant, but it also allows for water molecules to diffuse out of the plant as well through evaporation. And so normally, the stomata of a plant are going to be in the open position during photosynthesis so that the carbon dioxide gas is able to diffuse into the plant, and the oxygen gas that's made in photosynthesis is able to diffuse out of the plant.

However, in order to understand photorespiration, what we need to understand is what happens in a very specific scenario. And that very specific scenario is the scenario of a very hot environment, such as a desert for instance. And so in really, really hot environments, if the stomata are in the open position like what they are over here, then the plant is going to allow for gas exchange. So, carbon dioxide will be able to come in and oxygen will be able to leave, but also in hot environments, water is going to be more likely to leave the plant via evaporation. And so in really, really hot environments plants are susceptible to dehydration by losing water molecules by evaporation. And so in really, really hot environments, plants are actually able to prevent dehydration in hot environments by closing their stomata. And so instead of leaving their stomata in an open position where they could lose water by evaporation and dehydrate and then wilt and unfortunately die, the plants prevent dehydration and prevent themselves from dying by closing their stomata.

However, when the stomata are closed, they are going to prevent dehydration, but they also prevent gas exchange. And so when the stomata are closed, if you take a look at this image down below over here on the right-hand side where we have increasing temperature and hot temperatures, plants are going to close their stomata. And so notice here we have the stomata in the closed position. And yes, it is going to prevent dehydration because water will not be able to leave the plant. However, it also prevents gas exchange. And so carbon dioxide gas is no longer going to be able to diffuse into the plant like it was over here when the stomata are open. And oxygen gas is not gonna be able to diffuse out of the plant like it was able to over here when the stomata were open. When the stomata are closed, oxygen gas cannot diffuse out.

And so ultimately, when the stomata are closed preventing gas exchange, this is gonna lead to decreased carbon dioxide levels inside of the leaf because, again, the carbon dioxide is not able to enter the plant, instead the carbon dioxide is going to be blocked from entering the plant. So that means that inside of the plant, carbon dioxide levels are gonna decrease. And also because oxygen is not able to leave the plant, when the stomata are closed, oxygen is going to remain and build up inside of the plant.

And so when oxygen builds up inside of the plant, this is going to lead to increased oxygen levels inside of the plant when the stomata are closed. And if the oxygen gas concentration, remember, the brackets here represent the concentration of, if the concentration of oxygen gas is too high when it's increased, what this means is that the oxygen gas is going to compete with the carbon dioxide gas for binding to the enzyme, Rubisco, which recalls from our previous lessons of the Calvin cycle is the enzyme that performs carbon fixation of the carbon, Calvin cycle. And so if oxygen is too high, then oxygen is gonna compete for binding to Rubisco, and Rubisco is going to actually add oxygen to the first molecule of the Calvin cycle, RuBP, instead of adding carbon dioxide to RuBP like what normally happens during photosynthesis. And so during photorespiration, photorespiration is more likely to occur in hot environments when the stomata are closed, and these conditions exist, decreased carbon dioxide, increased oxygen. And during photorespiration when oxygen levels are so high, RuBisCO adds oxygen to RuBP instead of adding carbon dioxide. And ultimately, this is going to waste the ATP and the NADPH that were made by the light reactions.

And so, during photorespiration, ATP and NADPH are wasted, and it also ends up making carbon dioxide instead of making glucose. And so if we take a look at our image here in the middle, notice what we're showing you is a zoom-in of one of the chloroplast of the mesophyll cells. And so this is a zoom-in of the chloroplast, and over here on the left, notice what we're showing you is actually the normal conditions when the stomata are open, and this over here represents normal photosynthesis, which includes the normal Calvin cycle. But over here on the right-hand side, what we're showing you is the process of photorespiration, and how photorespiration occurs in hotter environments when the stomata are closed.

So, if we were to go over this, notice that by covering the left-hand side, in cooler temperatures, the leaves and the plant are gonna have open stomata. And when the stomata are open in these cooler temperatures, it allows for gas exchange. It allows for carbon dioxide to diffuse in and oxygen to diffuse out. And so carbon dioxide levels inside of the leaf are gonna build up, and so there's gonna be high carbon dioxide levels, high CO₂ levels. And the high CO₂ levels allows carbon dioxide or CO₂ to interact with Rubisco and be bound to RuBP, as it normally proceeds in the Calvin cycle. And so under these conditions and cooler temperatures when the stomata are open, the normal Calvin cycle occurs, and glucose is going to be produced as normal. And so you can see the glucose over here being produced as normal. But over here on the right-hand side, what we're showing you is what happens in hot temperatures. The stomata are going to close. And so in hot temperatures, as this says, the leaves and the plants are going to close their stomata, there's gonna be closed stomata. And closed stomata are gonna prevent gas exchange. Yes, it it's going to prevent dehydration, but it also prevents gas exchange. And so carbon dioxide is no longer gonna be able to enter the plant. So carbon dioxide levels are gonna get really low inside of the plant. And oxygen levels are no longer oxygen is no longer able to leave the plant, and so oxygen is going to start to build up inside of the plant under these conditions. And so there's gonna be quite high levels of oxygen here under photorespiration conditions. And so when oxygen levels are really, really high, oxygen will compete with carbon dioxide for binding to rubisco and binding to RuBP. And so ultimately, when oxygen binds to rubisco and is bound to RuBP instead of carbon dioxide, this leads to photorespiration. And ultimately, photorespiration, when oxygen interacts with RuBisCO, it's going to waste the ATP and NADPH that would have normally been used during the Calvin cycle. Instead, this ATP and NADPH is gonna be used for photorespiration, and photorespiration ends up producing carbon dioxide. Whereas recall the Calvin cycle over here is going to consume carbon dioxide.

Ultimately, what we can see here is that photorespiration is going to waste ATP and NADPH, and it's going to make photosynthesis really, really inefficient. And so photorespiration is something that the plant would want to try to avoid. And so notice down here what we have is a little boxing match occurring here between these, this person with the blue gloves and this person with the red gloves, and this is really just supposed to represent how carbon dioxide over here and the Calvin cycle is going to compete with oxygen and photorespiration. And so the whole idea here is gonna all depend on whether the stomata are in an open position or if the stomata are in a closed position. And so again photorespiration occurs when the stomata are in a closed position as we've indicated here. And photosynthesis, as we've discussed it previously in our previous lesson videos, occurs when the stom ata are open, and this one occurs when the stomata are closed. And so this here concludes our introduction to photorespiration, and we'll be able to get some practice applying the concept that we've learned here as we move forward in our course. So I'll see you all in our next video.

Plants are more likely to use Photorespiration instead of the Calvin Cycle when:

• A. Stomata remain open and CO₂ concentrations within the plant are high
• B. Stomata remain closed and O₂ concentrations within the plant are high
• C. Glucose concentrations within the plant are low
• D. CO₂ binds to Rubisco

Here’s what students ask on this topic:

What is photorespiration and how does it differ from photosynthesis?

Photorespiration is a process in plants that occurs under hot conditions when stomata close, preventing carbon dioxide (CO₂) from entering and causing oxygen (O₂) to accumulate. This leads to the enzyme Rubisco binding O₂ to RuBP instead of CO₂, resulting in the production of CO₂ and wasting ATP and NADPH. In contrast, photosynthesis uses CO₂ to produce glucose, reducing CO₂ levels. Therefore, photorespiration makes photosynthesis inefficient by counteracting the carbon fixation process essential for glucose production.

Why does photorespiration occur more frequently in hot environments?

Photorespiration occurs more frequently in hot environments because plants close their stomata to prevent water loss through evaporation. When stomata are closed, CO₂ cannot enter the leaf, and O₂ accumulates inside. High O₂ levels lead to Rubisco binding O₂ to RuBP instead of CO₂, initiating photorespiration. This process wastes ATP and NADPH and produces CO₂ instead of glucose, making photosynthesis inefficient. Thus, hot conditions favor photorespiration due to the need for plants to conserve water.

How do stomata function in regulating photorespiration?

Stomata are openings on the leaf surface that regulate gas exchange. In cooler temperatures, stomata remain open, allowing CO₂ to enter and O₂ to exit, facilitating photosynthesis. In hot environments, stomata close to prevent water loss, which also prevents CO₂ from entering and causes O₂ to accumulate. This leads to photorespiration, where Rubisco binds O₂ to RuBP instead of CO₂, wasting ATP and NADPH and producing CO₂. Therefore, stomatal behavior directly influences the occurrence of photorespiration.

What role does the enzyme Rubisco play in photorespiration?

Rubisco is the enzyme responsible for carbon fixation in the Calvin cycle during photosynthesis. However, in photorespiration, Rubisco binds to O₂ instead of CO₂ when O₂ levels are high and CO₂ levels are low, such as when stomata are closed in hot environments. This binding leads to the production of CO₂ and the waste of ATP and NADPH, making photosynthesis inefficient. Thus, Rubisco's dual affinity for O₂ and CO₂ is central to the process of photorespiration.

How does photorespiration affect the efficiency of photosynthesis?

Photorespiration significantly reduces the efficiency of photosynthesis. During photorespiration, Rubisco binds O₂ to RuBP instead of CO₂, leading to the production of CO₂ and the waste of ATP and NADPH. This process counteracts the carbon fixation in the Calvin cycle, which is essential for glucose production. As a result, the energy and resources that would normally be used to produce glucose are instead used in a process that produces CO₂, making photosynthesis less efficient.