How the C4 and CAM pathways help minimize photorespiration.

Key points:

  • Photorespiration is a wasteful pathway that occurs when the Calvin cycle enzyme rubisco acts on oxygen rather than carbon dioxide.
  • The majority of plants are C, start subscript, 3, end subscript plants, which have no special features to combat photorespiration.
  • C, start subscript, 4, end subscript plants minimize photorespiration by separating initial C, O, start subscript, 2, end subscript fixation and the Calvin cycle in space, performing these steps in different cell types.
  • Crassulacean acid metabolism (CAM) plants minimize photorespiration and save water by separating these steps in time, between night and day.

Introduction

High crop yields are pretty important—for keeping people fed, and also for keeping economies running. If you heard there was a single factor that reduced the yield of wheat by 20, percent and the yield of soybeans by 36, percent in the United States, for instance, you might be curious to know what it wasstart superscript, 1, end superscript.
As it turns out, the factor behind those (real-life) numbers is photorespiration. This wasteful metabolic pathway begins when rubisco, the carbon-fixing enzyme of the Calvin cycle, grabs O, start subscript, 2, end subscript rather than C, O, start subscript, 2, end subscript. It uses up fixed carbon, wastes energy, and tends to happens when plants close their stomata (leaf pores) to reduce water loss. High temperatures make it even worse.
Some plants, unlike wheat and soybean, can escape the worst effects of photorespiration. The C, start subscript, 4, end subscript and CAM pathways are two adaptations—beneficial features arising by natural selection—that allow certain species to minimize photorespiration. These pathways work by ensuring that Rubisco always encounters high concentrations of C, O, start subscript, 2, end subscript, making it unlikely to bind to O, start subscript, 2, end subscript.
In the rest of this article, we'll take a closer look at the C, start subscript, 4, end subscript and CAM pathways and see how they reduce photorespiration.

C, start subscript, 3, end subscript plants

A "normal" plant—one that doesn't have photosynthetic adaptations to reduce photorespiration—is called a C, start subscript, 3, end subscript plant. The first step of the Calvin cycle is the fixation of carbon dioxide by rubisco, and plants that use only this "standard" mechanism of carbon fixation are called C, start subscript, 3, end subscript plants, for the three-carbon compound (3-PGA) the reaction producesstart superscript, 2, end superscript. About 85, percent of the plant species on the planet are C, start subscript, 3, end subscript plants, including rice, wheat, soybeans and all trees.
Image of the C3 pathway. Carbon dioxide enters a mesophyll cell and is fixed immediately by rubisco, leading to the formation of 3-PGA molecules, which contain three carbons.

C, start subscript, 4, end subscript plants

In C, start subscript, 4, end subscript plants, the light-dependent reactions and the Calvin cycle are physically separated, with the light-dependent reactions occurring in the mesophyll cells (spongy tissue in the middle of the leaf) and the Calvin cycle occurring in special cells around the leaf veins. These cells are called bundle-sheath cells.
To see how this division helps, let's look at an example of C, start subscript, 4, end subscript photosynthesis in action. First, atmospheric C, O, start subscript, 2, end subscript is fixed in the mesophyll cells to form a simple, 4-carbon organic acid (oxaloacetate). This step is carried out by a non-rubisco enzyme, PEP carboxylase, that has no tendency to bind O, start subscript, 2, end subscript. Oxaloacetate is then converted to a similar molecule, malate, that can be transported in to the bundle-sheath cells. Inside the bundle sheath, malate breaks down, releasing a molecule of C, O, start subscript, 2, end subscript. The C, O, start subscript, 2, end subscript is then fixed by rubisco and made into sugars via the Calvin cycle, exactly as in C, start subscript, 3, end subscript photosynthesis.
In the C4 pathway, initial carbon fixation takes place in mesophyll cells and the Calvin cycle takes place in bundle-sheath cells. PEP carboxylase attaches an incoming carbon dioxide molecul to the three-carbon molecule PEP, producing oxaloacetate (a four-carbon molecule). The oxaloacetate is converted to malate, which travels out of the mesophyll cell and into a neighboring bundle-sheath. Inside the bundle sheath cell, malate is broken down to release COstart subscript, 2, end subscript, which then enters the Calvin cycle. Pyruvate is also produced in this step and moves back into the mesophyll cell, where it is converted into PEP (a reaction that converts ATP and Pi into AMP and PPi).
This process isn't without its energetic price: ATP must be expended to return the three-carbon “ferry” molecule from the bundle sheath cell and get it ready to pick up another molecule of atmospheric C, O, start subscript, 2, end subscript. However, because the mesophyll cells constantly pump C, O, start subscript, 2, end subscript into neighboring bundle-sheath cells in the form of malate, there’s always a high concentration of C, O, start subscript, 2, end subscript relative to O, start subscript, 2, end subscript right around rubisco. This strategy minimizes photorespiration.
The C, start subscript, 4, end subscript pathway is used in about 3, percent of all vascular plants; some examples are crabgrass, sugarcane and corn. C, start subscript, 4, end subscript plants are common in habitats that are hot, but are less abundant in areas that are cooler. In hot conditions, the benefits of reduced photorespiration likely exceed the ATP cost of moving C, O, start subscript, 2, end subscript from the mesophyll cell to the bundle-sheath cell.

CAM plants

Some plants that are adapted to dry environments, such as cacti and pineapples, use the crassulacean acid metabolism (CAM) pathway to minimize photorespiration. This name comes from the family of plants, the Crassulaceae, in which scientists first discovered the pathway.
Image of a succulent.
Image credit: "Crassulaceae," by Guyon Morée (CC BY 2.0).
Instead of separating the light-dependent reactions and the use of C, O, start subscript, 2, end subscript in the Calvin cycle in space, CAM plants separate these processes in time. At night, CAM plants open their stomata, allowing C, O, start subscript, 2, end subscript to diffuse into the leaves. This C, O, start subscript, 2, end subscript is fixed into oxaloacetate by PEP carboxylase (the same step used by C, start subscript, 4, end subscript plants), then converted to malate or another type of organic acidstart superscript, 3, end superscript.
The organic acid is stored inside vacuoles until the next day. In the daylight, the CAM plants do not open their stomata, but they can still photosynthesis. That's because the organic acids are transported out of the vacuole and broken down to release C, O, start subscript, 2, end subscript, which enters the Calvin cycle. This controlled release maintains a high concentration of C, O, start subscript, 2, end subscript around rubiscostart superscript, 4, end superscript.
CAM plants temporally separate carbon fixation and the Calvin cycle. Carbon dioxide diffuses into leaves during the night (when stomata are open) and is fixed into oxaloacetate by PEP carboxylase, which attaches the carbon dioxide to the three-carbon molecule PEP. The oxaloacetate is converted to another organic acid, such as malate. The organic acid is stored until the next day and is then broken down, releasing carbon dioxide that can be fixed by rubisco and enter the Calvin cycle to make sugars.
The CAM pathway requires ATP at multiple steps (not shown above), so like C, start subscript, 4, end subscript photosynthesis, it is not an energetic "freebie." start superscript, 3, end superscript However, plant species that use CAM photosynthesis not only avoid photorespiration, but are also very water-efficient. Their stomata only open at night, when humidity tends to be higher and temperatures are cooler, both factors that reduce water loss from leaves. CAM plants are typically dominant in very hot, dry areas, like deserts.

Comparisons of C, start subscript, 3, end subscript, C, start subscript, 4, end subscript, and CAM plants

C, start subscript, 3, end subscript, C, start subscript, 4, end subscript and CAM plants all use the Calvin cycle to make sugars from C, O, start subscript, 2, end subscript. These pathways for fixing C, O, start subscript, 2, end subscript have different advantages and disadvantages and make plants suited for different habitats. The C, start subscript, 3, end subscript mechanism works well in cool environments, while C, start subscript, 4, end subscript and CAM plants are adapted to hot, dry areas.
Both the C, start subscript, 4, end subscript and CAM pathways have evolved independently over two dozen times, which suggests they may give plant species in hot climates a significant evolutionary advantagestart superscript, 5, end superscript.
TypeSeparation of initial C, O, start subscript, 2, end subscript fixation and Calvin cycleStomata openBest adapted to
C, start subscript, 3, end subscriptNo separationDayCool, wet environments
C, start subscript, 4, end subscriptBetween mesophyll and bundle-sheath cells (in space)DayHot, sunny environments
CAMBetween night and day (in time)NightVery hot, dry environments
This article is licensed under a CC BY-NC-SA 4.0 license

Attribution:

This article is a modified derivative of "Photosynthetic pathways," by Robert Bear and David Rintoul, OpenStax CNX, CC BY 4.0. Download the original article for free at http://cnx.org/contents/d0b6df3d-22b7-411f-8f28-5eeed0e1c82d@9.
The modified article is licensed under a CC BY-NC-SA 4.0 license.

Works cited:

  1. Walker, Berkeley J., VanLoocke, Andy, Bernacchi, Carl J., and Ort, Donald R. (2016). The cost of photorespiration to food production now and in the future. Annual Review of Plant Biology 67, 107. ://dx.doi.org/10.1146/annurev-arplant-043015-111709.
  2. Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). Alternative mechanisms of carbon fixation have evolved in hot, arid climates. In Campbell biology (10th ed.) San Francisco, CA: Pearson, 201.
  3. Crassulacean acid metabolism. (2016, May 29). Retrieved July 22, 2016 from Wikipedia: https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism#Biochemistry.
  4. Raven, Peter H., Johnson, George B., Losos, Mason, Kenneth A., Losos, Jonathan B., and Singer, Susan R. (2014). Photorespiration. In Biology (10th ed., AP ed.). New York, NY: McGraw-Hill, 165.
5.Guralnick, Lonnie J., Amanda Cline, Monica Smith, and Rowan F. Sage. (2008). Evolutionary physiology: the extent of C4 and CAM photosynthesis in the genera Anacampseros and Grahamia of the Portulacaceae. Journal of Experimental Botany, 59(7), 1735-1742. http://dx.doi.org/10.1093/jxb/ern081.

References:

Bareja, B. (2015). Plant types: II. In C4 plants, examples, and C4 families. Retrieved from http://www.cropsreview.com/c4-plants.html.
Berg, J. M., Tymoczko, J. L., and Stryer, L. (2002). The Calvin cycle synthesizes hexoses from carbon dioxide and water. In Biochemistry (5th ed., section 20.1). New York, NY: W. H. Freeman. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK22344/.
Bowsher, C., Steer, M., and Tobin, A. (2008). Photosynthetic carbon assimilation. In Plant biochemistry (pp. 93-141). New York, NY: Garland Science.
C4 carbon fixation. (2015, September 26). Retrieved October 26, 2015 from Wikipedia: https://en.wikipedia.org/wiki/C4_carbon_fixation.
Crassulacean acid metabolism. (2015, September 16). Retrieved October 26, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism.
De, D. (2000). Crassulacean acid metabolism. In Plant cell vacuoles: an introduction (pp. 186-187). Collingwood, VIC: CSIRO Publishing.
Guralnick, Lonnie J., Amanda Cline, Monica Smith, and Rowan F. Sage. (2008). Evolutionary physiology: the extent of C4 and CAM photosynthesis in the genera Anacampseros and Grahamia of the Portulacaceae. Journal of Experimental Botany, 59(7), 1735-1742. http://dx.doi.org/10.1093/jxb/ern081.
Koning, R. E. (1994). Photorespiration. In Plant physiology information website. Retrieved from http://plantphys.info/plant_physiology/photoresp.shtml.
Photorespiration. (2015, September 4). Retrieved October 26, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Photorespiration.
Photosynthesis - an overview. (n.d.) In Biomes. Retrieved fromhttp://w3.marietta.edu/~biol/biomes/photosynthesis.htm.
Purves, W.K., Sadava, D., Orians, G.H., and Heller, H.C. (2003). Photosynthesis: energy from the sun. In Life: the science of biology (7th ed., pp. 145-162). Sunderland, MA: Sinauer Associates, Inc.
Raven, Peter H., Johnson, George B., Losos, Mason, Kenneth A., Losos, Jonathan B., and Singer, Susan R. (2014). Photorespiration. In Biology (10th ed., AP ed., pp. 163-165). New York, NY: McGraw-Hill.
Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). Alternative mechanisms of carbon fixation have evolved in hot, arid climates. In Campbell biology (10th ed., pp. 201-204). San Francisco, CA: Pearson.
RuBisCO. (n.d.) In Combining algal and plant photosynthesis. Retrieved from https://cambridgecapp.wordpress.com/improving-photosynthesis/rubisco/.
Trueman, Shanon. (n.d.). CAM plants: survival in the desert. In History of botany. Retrieved from http://botany.about.com/od/HistoryBotany/a/Cam-Plants-Survival-In-The-Desert.htm.
Walker, Berkeley J., VanLoocke, Andy, Bernacchi, Carl J., and Ort, Donald R. (2016). The cost of photorespiration to food production now and in the future. Annual Review of Plant Biology 67, 107-129. http://dx.doi.org/10.1146/annurev-arplant-043015-111709. Vascular bundle. (2015, October 19). Retrieved October 26, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Vascular_bundle.