Photorespiration and C 4 Plants Adapted from

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1 Photorespiration and C 4 Plants Adapted from All plants carry on photosynthesis by adding carbon dioxide (CO 2 ) to a phosphorylated 5-carbon sugar called ribulose bisphosphate. This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase oxygenase (RUBISCO) The resulting 6-carbon compound breaks down into two molecules of 3-phosphoglycerate (PGA) which is then reduced to glyceraldehyde-3-phosphate (G3P) These 3-carbon molecules are the starting material for the synthesis of glucose and other molecules. The process is called the Calvin cycle and the pathway is called the C 3 pathway. Photorespiration As its name suggests, RUBISCO catalyzes two different reactions: adding CO 2 to ribulose bisphosphate the carboxylase activity adding O 2 to ribulose bisphosphate the oxygenase activity. Which one predominates depends on the relative concentrations of O 2 and CO 2 with high CO 2, low O 2 favoring the carboxylase action, high O 2, low CO 2 favoring the oxygenase action. The light reactions of photosynthesis liberate oxygen through photolysis at photosystem II. This creates a higher local concentration of O 2 in plant cells. The solubility of CO 2 decreases faster than O 2 at high temperatures. This means that the proportion of oxygen dissolved in the cytosol is higher relative to carbon dioxide at higher temperatures. Therefore, high light intensities and high temperatures (above ~ 30 C) favor the oxygenase reaction. The Details of Photorespiration The uptake of O 2 by RUBISCO forms: o the 3-carbon molecule 3-phosphoglycerate just as in the Calvin cycle o the 2-carbon molecule phosphoglycolate. phosphoglycolate is converted to glycolate and enters the peroxisomes. There it uses O 2 to form intermediates that enter the mitochondria where they are broken down to produce CO 2. Ultimately this process uses O 2 and liberates CO 2 as cellular respiration does, which is why it is called photorespiration. It undoes the good anabolic work of photosynthesis, reducing the net productivity of the plant. For this reason, much effort so far largely unsuccessful has gone into attempts to alter crop plants so that they carry on less photorespiration. The problem may solve itself. If atmospheric CO 2 concentrations continue to rise, perhaps this will enhance the net productivity of the world's crops by reducing losses to photorespiration.

2 C 4 Plants Over 8000 species of angiosperms (flowering plants) have adaptations which minimize the losses to photorespiration. They all use a supplementary method of CO 2 uptake which forms a 4-carbon molecule instead of the two 3-carbon molecules of the Calvin cycle. Hence these plants are called C 4 plants. (Plants that have only the Calvin cycle are thus C 3 plants.) Some C 4 plants called CAM plants separate their C 3 and C 4 cycles by time. Other C 4 plants have structural changes in their leaf anatomy so that their C 4 and C 3 pathways are separated in different parts of the leaf with RUBISCO sequestered where the CO 2 level is high and the O 2 level is low. The details of the C 4 cycle After entering through the stomata, CO 2 diffuses into a mesophyll cell. o Being close to the leaf surface, these cells are exposed to high levels of O 2, but have no RUBISCO so cannot start photorespiration (nor the dark reactions of the Calvin cycle). Instead the CO 2 is added to the 3-carbon compound (C 3 ) phosphoenolpyruvate (PEP) forming the 4-carbon compound oxaloacetate (C 4 ). This is accomplished by a DIFFERENT ENZYME. Carbon dioxide is fixed to oxaloacetate by PEP carboxylase. This new enzyme ONLY picks up CO 2 and eliminates the problem of photorespiration. Oxaloacetate is converted into malate or aspartate (both have 4 carbons), which is transported into a bundle sheath cell. Bundle sheath cells o are deep in the leaf surrounding the vascular bundles, so atmospheric oxygen cannot diffuse easily to them; o often have thylakoids with reduced photosystem II complexes (the one that produces O 2 ). o Both of these features keep oxygen levels low. Here the 4-carbon compound is broken down into o carbon dioxide, which enters the Calvin cycle to form sugars and starch. o pyruvate (C 3 ), which is transported back to a mesophyll cell where it is converted back into PEP. These C 4 plants are well adapted to (and likely to be found in) habitats with: high daytime temperatures intense sunlight. Some examples: crabgrass corn (maize) sugarcane

3 Although they comprise only about 3% of the angiosperm species on Earth, C 4 plants are responsible for about 25% of all the photosynthesis on land! The figure to the left shows the separation of carbon fixation by RUBISCO from the Calvin cycle in two different cell layers using the C 4 pathway. Name the enzyme used for carbon fixation in the mesophyll Name the enzyme used for carbon fixation in the bundle sheath cell CAM Plants These are also C 4 plants but instead of segregating the C 4 and C 3 pathways in different parts of the leaf, they separate them in time instead. (CAM stands for crassulacean acid metabolism because it was first studied in members of the plant family Crassulaceae.) At night, CAM plants take in CO 2 through their open stomata (they tend to have reduced numbers of them). PEP carboxylase joins CO 2 with PEP to form the 4-carbon oxaloacetic acid. This is converted to 4-carbon malic acid that accumulates during the night in the central vacuole of the cells. In the morning, the stomata close (thus conserving moisture as well as reducing the inward diffusion of oxygen). The accumulated malic acid leaves the vacuole and is broken down to release CO 2. The CO 2 is taken up into the Calvin (C 3 ) cycle.

4 These adaptations also enable their owners to thrive in conditions of: high daytime temperatures intense sunlight low soil moisture. Some examples of CAM plants: cacti the pineapple and all epiphytic bromeliads the "ice plant" that grows in sandy parts of the scrub forest biome C 4 cells in C 3 plants The ability to use the C 4 pathway has evolved repeatedly in different families of angiosperms a remarkable example of convergent evolution. Perhaps the potential is in all angiosperms. A report in the 24 January 2002 issue of Nature (by Julian M. Hibbard and W. Paul Quick) describes the discovery that tobacco, a C 3 plant, has cells capable of fixing carbon dioxide by the C 4 path. These cells are clustered around the veins (containing xylem and phloem) of the stems and also in the petioles of the leaves. In this location, they are far removed from the stomata that could provide atmospheric CO 2. Instead, they get their CO 2 and/or the 4-carbon malic acid in the sap that has been brought up in the xylem from the roots. If this turns out to be true of many C 3 plants, it would explain why it has been so easy for C 4 plants to evolve from C 3 ancestors. Website content last updated on December 29 th, 2010

5 Practice! 1) Draw a simplified overview diagram that shows the main differences between the Calvin Cycle and Photorespiration. 2) Explain the problem of photorespiration 3) Under what conditions does photorespiration present the greatest problem and why? (i.e. which environments most favour a reduction to photorespiration) 4) State the main difference between RUBISCO and PEP Carboxylase. 5) Explain the C 4 pathway where carbon fixation is separated from RUBISCO by location. In your answer explain why RUBISCO is only found in the bundle sheath cells. 6) How is the CAM pathway different from the conventional C 4 pathway? Which environments favour the CAM strategy and why? 7) Only a relatively small proportion of plants use the C 4 pathway, but important crops like corn and sugar cane do. These crops were selected by humans long before the C 4 pathway was discovered. Provide an explanation for why this may have been more than just coincidence? Think of at least two clear reasons.