Photorespiration & Alternate Pathways

Objectives:

By the time you finish with this page (and the accompanying reading in Campbell)

  • How photorespiration arises from the behavior of Rubisco.
  • How CAM & C4 photosynthesis pathways reduce photorespiration.

Gas exchange causes water loss

Just like animals, plants need to do gas exchange. For animals and plants that live in the air (as opposed to the water), there is a fundamental challenge: to perform gas exchange without losing too much water to evaporation. This challenge is particularly acute for the leaves of plants, because they need to take up CO2 for photosynthesis, but CO2 is rare in the atmosphere. From the macroscopic to the microscopic, plants have an amazing diversity of water-conserving adaptations.

You may have seen one kind of water-conserving adaptation in the structure of plant leaves. The interior (mesophyll) of a typical leaf is structured like a set of lungs: an air-filled interior space with high surface area, allowing for gas exchange by diffusion, with small openings (stomata) that allow for controlled bulk airflow in and out. Leaves can close their stomata to reduce water loss, or open them to maximize gas exchange and the rate of photosynthesis.

Plants must be able to adjust their balance of water loss and evaporation to suit an ever-changing environment. Excessive evaporation can easily kill a plant, but some water loss is necessary to drive transpirational pull for transporting nutrients throughout the plant body.

When a plant is facing severe dehydration, it must close the stomata, reducing water loss from the leaves. As photosynthesis continues in the closed leaves, CO2 concentration decreases and O2 concentration increases in the air spaces surrounding the mesophyll cells.

Photorespiration consumes organic carbon

On the level of photosynthesis, the gas exchange problem directly affects the Calvin cycle. When a plant is facing severe dehydration, it must close its stomata, reducing water loss from the leaves. As photosynthesis continues in the closed leaves, CO2 concentration decreases and O2 concentration increases in the air spaces surrounding the mesophyll cells. This interferes with photosynthesis.

In good conditions, the enzyme Rubisco catalyzes the addition of CO2 to RUBP, forming new organic carbon. This is carbon fixation.

Carbon fixation vs oxygenation by Rubisco.

However, Rubisco sometimes adds O2 instead of CO2 for this reaction. Instead of carboxylation, it performs an oxygenation reaction. Instead of producing organic carbon, it oxidizes existing organic carbon. The two-carbon product produced in the oxygenation reaction shown above (2-phosphoglycolate) can't be used directly in the Calvin cycle. It can be converted to other useful products, including a Calvin cycle intermediate, in a long series of reactions called photorespiration. Photorespiration requires both ATP and reducing power, and releases CO2 waste. Thus, photorespiration is not an efficient way to produce G3P, the normal output of the Calvin cycle.

The problem is that the active site of Rubisco, while it is fairly specific for CO2, begins to bind to O2 instead when the ratio of [O2 ] to [CO2] is extremely high. Keep in mind that the concentration of O2 in the air is normally much higher than the concentration of CO2, and the difference becomes more extreme when a plant's stomata are closed on a dry, hot sunny day. Under these conditions, Rubisco pushes the plant toward photorespiration.

Photorespiration: mistake or adaptation?

Rubisco may well be the most abundant protein on earth. It catalyzes the rate-limiting step in photosynthesis, and every photosynthetic cell (plants, algae, or cyanobacteria) has numerous copies of it. Is it possible that this enzyme, which is responsible for every atom of organic carbon that you've ever consumed, simply isn't very good at its job?

Rubisco, or ribulose bisphosphate carboxylase, is sometimes referred to as ribulose bisphosphate carboxylase-oxygenase in recognition of the fact that it can catalyze either reaction. Why isn't it able to perform carboxylation exclusively? It's true that when this type of photosynthesis originally evolved, the earth's atmosphere was much higher in CO2 and lower in O2. The job of binding CO2 has become more difficult since then. On the other hand, that was around 2.5 billion years ago. Since then, earth has seen the origin of eukaryotes, life on land, and the entire evolutionary history of animals. In all that time, Rubisco couldn't impove its ability to recognize CO2? This is one possible explanation of photorespiration: it's an inefficient and wasteful process, and it's an evolutionary relic of a time when earth's atmosphere was different. However, some researchers suggest that the photorespiration pathway may have advantages for plants, and those advantages explain the behavior of Rubisco.

One of the apparent downsides of photorespiration is that it seems to waste ATP and reducing power (NADH). After all, plants don't get that kind of energy for free, do they? Well, actually, they do. If a plant's rate of photosynthesis is limited by its CO2 supply, but it continues to harvest solar enegy with its photosystems, all that excess energy can be a problem. Just like in cellular respiration, when too many high-energy electrons are fed into the electron transport chain, reactive oxygen species (ROS) will be produced. In respiration or photosynthesis, one solution to that problem is to allow protons to flow down their electrochemical gradient. When photorespiration consumes excess energy, it may be protecting the cell against oxidative damage. In addition, the photorespiration pathway also leads to other products that help protect plant cells against oxidative damage.

Regardless of whether photorespiration is seen as an evolutionary misstep or an advantageous pathway, some plants have extra biochemical pathways that work in conjunction with the Calvin cycle and reduce the amount of photorespiration in hot, dry conditions. These pathways work by providing extra CO2 for Rubisco.

CAM photosynthesis reduces photorespiration

Regardless of whether photorespiration is seen as an evolutionary misstep or an advantageous pathway, some plants have extra biochemical pathways that work in conjunction with the Calvin cycle and reduce the amount of photorespiration in hot, dry conditions. These pathways work by providing extra CO2 for Rubisco. One example is the CAM (Crassulacean Acid Metabolism) pathway, which is only found in succulent plants such as cactus or agaves.

CAM plants have a simple trick that reduces the amount of water lost during gas exchange: they only open their stomata at night. Since the air is normally cooler and more humid at night, these plants can take in CO2 while losing relatively little water. However, there is one problem that prevents most plants from taking advantage of this approach: CO2 isn't very abundant in air, and it's not very water-soluble. Simply opening the stomata at night won't let most plants take in enough CO2 to meet their daily needs. Plants can't perform photosynthesis at night because it's dark, so Rubisco can't make use of the CO2 until the sun shines. The CAM pathway circumvents this problem by taking in CO2 at night and converting it into an organic acid molecule that's highly water-soluble. Since these plants store large amounts of water (especially in their central vacuoles), they can store the organic carbon at night and then convert it back to CO2 in the daytime for use in the Calvin cycle. This provides extra CO2 for Rubisco, reducing photorespiration.

CAM photosynthesis pathway.

There are two carbon fixation steps in this pathway, catalyzed by two different carboxylases. Step 1 is catalyzed by PEP carboxylase. This enzyme has an extremely high affinity for CO2, and adds the carboxyl group to PEP (phosphoenolpyruvate), a high-energy substrate. During the day, when the light reactions can power photosynthesis, the stored organic acid can be decarboxylated, feeding CO2 to Rubisco.

The extra carbon fixation step costs energy. Just as the carbon fixation step in the Calvin cycle costs energy for the synthesis of the CO2 acceptor RuBP, the CAM pathway requires ATP energy to fix CO2 in the form of a 4-carbon acid (which gets converted to malate). The entire pathway is considerably more complex than I've shown here; this diagram only emphasizes the CO2 part of the pathway.

Overall, CAM photosynthesis avoids photorespiration by increasing the amount of CO2 available in the plant for photosynthesis, making it easier for Rubisco to do the job of carboxylation.

C4 photosynthesis reduces photorespiration

The C4 photosynthesis pathway is similar to the CAM pathway in many respects. Compared to the standard C3 pathway, the CAM and C4 pathways both add an additional carbon fixation step before the Calvin cycle. In both cases, the extra step is catalyzed by PEP carboxylase, which has a very high specificity for CO2 (and not O2), and produces a four-carbon molecule as its product. In contrast, standard C3 photosynthesis uses only Rubisco as a carboxylase, and produces 3-carbon products in the carboxylation reaction. In both the CAM and C4 pathways, the initial 4-carbon organic molecule ends up being decarboxylated, releasing CO2 for Rubisco to use in the Calvin cycle. Thus, they reduce photorespiration.

C4 photosynthesis pathway.

The fundamental difference between the CAM and C4 pathways is in how the extra carbon fixation step is separated from Rubisco and the Calvin cycle. In CAM, the two cycles are separated in time: initial carbon fixation happens at night, while the Calvin cycle happens during the day. In C4 photosynthesis, both cycles happen during the day, but in different cells This requires leaves with specialized anatomy:

C4 leaf anatomy, with bundle sheath and mesophyll cells.

The leaves of C4 plants perform the Calvin cycle (with Rubisco) in bundle sheath cells. They're called that because they form a sheath around the vascular bundles. As you may recall from Bio 6A, the phloem cells in the vascular bundles will eventually receive the sugars produced by photosynthesis. The bundle sheath cells are somewhat isolated from the air space in the leaves. In contrast, the mesophyll cells surrounding the bundle sheath are in contact with the air inside the leaf.

On a hot, dry day, when the leaf is forced to close its stomata, photosynthesis depletes the CO2 in the leaf, while increasing the O2 concentration. C4 photosynthesis can't change that fact, but it helps the plant adapt. The water solubility of O2 is low, and it decreases with increasing temperature. Thus, it tends to accumulate in the air spaces, not the bundle sheath cells, where it is produced. Meanwhile, even though the CO2 concentration is increasing throughout the day, PEP carboxylase can continue to find CO2 molecules and use them for carboxylation. The O2 is then released into the bundle sheath cells by decarboxylation. Rubisco, protected from increasing O2 levels and given an extra supply of CO2, can continue to function with little photorespiration.

Alternate pathways evolved multiple times

The CAM and C4 pathways are found in a wide range of plants worldwide. If the weather is hot and water is limited, it's likely that CAM and C4 plants will dominate the landscape. These pathways aren't limited to particular taxonomic groups. Apparently, each of these pathways has evolved independently multiple times in different groups of plants. This might seem surprising: how can a complex pathway evolve the same way multiple times?

The key to understanding the evolution of these alternate photosynthetic pathways is this: they don't require any unique enzymes or genes. Instead, these pathways have evolved by changing the expression of existing genes, allowing existing enzymes to be produced at different times and places and act in new pathways. In fact, most of the genetic changes involved in the origin of these pathways aren't changes in protein-coding regions; instead, they are mutations in regulatory regions such as promoters and enhancers. The ways that these kinds of regulatory regions help control gene expression in eukaryotes will be featured in the next unit of this class.

Review

Terms & concepts

  • C4 photosynthesis
  • CAM photosynthesis
  • PEP carboxylase
  • Photorespiration
  • Rubisco

Review questions

  1. What happens to the concentrations of O2 and CO2 in the air spaces in the leaf if the stomata are closed and photosynthesis is happening?
  2. What do carboxylases do in photosynthesis?
  3. What is photorespiration? What are some advantages and disadvantages of this pathway?
  4. How do the CAM and C4 pathways reduce photosynthesis?
  5. If CAM and C4 have advantages, why don't all plants use these pathways?

References & further reading

Photorespiration

Photorespiration (Khan Academy). Includes videos on photorespiration, C4, and CAM.

Deeper reading

Photosynthesis, Reorganized. West-Eberhard et al., 2011, Science Magazine. This Perspective summarizes research on the evolutionary origins of CAM and C4.

Photorespiration: metabolic pathwaysand their role in stress protection. Wingler et al., 2000. A review article from a scientific journal. Detailed, but readable.

Photorespiration. A detailed scientific review article (actually a book chapter) focused on research in the widely studied plant Arabidopsis. Research on this plant has led to a much deeper understanding of photorespiration, and led biologists to realize that photorespiration is less an accident than essential part of photosynthesis.

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