Stumpf and E. Conn, eds. Hatch and N. Boardman, eds. Gardestrom, P. New Series. Douce and D. Day, eds. Ogren, W. David T. Dennis 1 1. Personalised recommendations. These pathways work by ensuring that Rubisco always encounters high concentrations of CO 2 making it unlikely to bind to O 2. About 85 percent of the plant species on the planet are C 3 plants, including rice, wheat, soybeans and all trees.
In C 4 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. First, atmospheric CO 2 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 2. 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 CO 2. The CO 2 is then fixed by rubisco and made into sugars via the Calvin cycle, exactly as in C 3 photosynthesis. This strategy minimizes photorespiration. The C 4 pathway is used in about 3 percent of all vascular plants; some examples are crabgrass, sugarcane and corn.
C 4 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 CO 2 from the mesophyll cell to the bundle-sheath cell.
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.
Instead of separating the light-dependent reactions and the use of CO 2 in the Calvin cycle in space, CAM plants separate these processes in time. This CO 2 is fixed into oxaloacetate by PEP carboxylase the same step used by C 4 plants , then converted to malate or another type of organic acid [3]. The organic acid is stored inside vacuoles until the next day. This process is called photorespiration — an awfully misleading name for students, because it has nothing to do with respiration and yields no ATP.
All Biol students need to remember about photorespiration is that it reduces photosynthetic efficiency, and that it occurs when Rubisco oxygenates RuBP instead of carboxylating RuBP. Rubisco evolved even before oxygenic photosynthesis, when there was no oxygen in the atmosphere or in the ocean waters, so there was no selection against oxygenase activity. Nevertheless, in over 2 billion years, neither nature nor human genetic engineering has been able to eliminate or even significantly reduce the oxygenase activity of Rubisco without also affecting the carboxylase activity.
In order for plants to take in CO2, they have to open structures called stomata on their leaves, which are pores that allow gas exchange.
Plants also lose water vapor through their stomata, which means that they can die from dehydration in dry conditions as they keep their stomata open for photosynthesis. In response, plants close their stomata to prevent dehydration. The rising O2 levels increase the rate of photorespiration reaction of rubisco with oxygen instead of carbon dioxide , when then drastically reduces the efficiency of rubisco, which is already a very slow-working enzyme.
So this means plants in dry conditions are at risk of dehydration if they open their stomata to promote gas exchange, or inability to produce sugar if they keep their stomata closed to minimize dehydration. PEP carboxylase is located in the mesophyll cells, on the leaf exterior near the stomata. There is no rubisco in the mesophyll cells.
The malate is then transported deeper into the leaf tissue to the bundle sheath cells, which are both far away from the stomata and thus far away from oxygen and contain rubisco. Once inside the bundle sheath cells, malate is decarboxylated to release pyruvate and CO2; the CO2 is then fixed by rubisco as part of the Calvin cycle, just like in C3 plants.
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