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The C 4 and CAM pathways for fixing CO 2 are two adaptations that improve the efficiency of photosynthesis, by ensuring that Rubisco encounters high CO 2 concentrations and thus reduces photorespiration. These two photosynthetic adaptations for fixing CO 2 have evolved independently a number of times in species that evolved from wet and dry, but typically warm climates. Why have these mechanisms evolved independently so many times? Plants that minimize photorespiration may have a significant competitive advantage, because a considerable amount of energy (in the form of ATP) is lost in plants during photorespiration. In many environments, plants that use solar energy more efficiently should out-compete those which are less efficient.
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 and the Calvin cycle occurring in special cells that surround the veins in the leaves. These cells are called bundle-sheath cells. How does this work? Atmospheric CO 2 is fixed in the mesophyll cells as a simple 4-carbon organic acid (malate) by an enzyme that has no affinity for O 2 . Malate is then transported to the bundle-sheath cells. Inside the bundle sheath, malate is oxidized to a 3-C organic acid, and in the process, 1 molecule of CO 2 is produced from every malate molecule ( [link] ). The CO 2 is then fixed by Rubisco into sugars, via the Calvin cycle, exactly as in C 3 photosynthesis. There is an additional cost of two ATPs associated with moving the three-carbon “ferry” molecule from the bundle sheath cell back to the mesophyll to pick up another molecule of atmospheric CO 2 . Since the spatial separation in bundle-sheath cells minimizes O 2 concentrations in the locations where Rubisco is located, photorespiration is minimized ( [link] ). This arrangement of cells reduces photorespiration and increases the efficiency of photosynthesis for C 4 plants. In addition, C 4 plants require about half as much water as a C 3 plant. The reason C 4 plants require less water is due to the fact that the physical shape of the stomata and leaf structure of C 4 plants helps reduce water loss by developing a large CO 2 concentration gradient between the outside of the leaf (400 ppm) and the mesophyll cells (10 ppm). The large CO 2 concentration gradient reduces water loss via transpiration through the stomata.
The C 4 pathway is used in about 3% 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, because the enzyme that fixes the CO 2 in the mesophyll is less efficient at lower temperature. One hypothesis for the abundance of C 4 plants in hot habitats is that the benefits of reduced photorespiration and water loss exceeds the ATP cost of moving the the CO 2 from the mesophyll cell to bundle-sheath cell.
Many plants such as cacti and pineapples, which are adapted to arid environments, use a different energy and water saving pathway called crassulacean acid metabolism (CAM). This name comes from the family of plants (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 spatially, CAM plants separate these processes temporally ( [link] ). At night, CAM plants open their stomata, and an enzyme in the mesophyll cells fix the CO 2 as an organic acid and store the organic acid in vacuoles until morning. During the day the light-dependent reactions supply the ATP and NADPH necessary for the Calvin cycle to function, and the CO 2 is released from those organic acids and used to make sugars. Plant species using CAM photosynthesis are the most water efficient of all; the stomata are only open at night when humidity is typically higher and the temperatures are much cooler (which serves to lower the diffusive gradient driving water loss from leaves). The CAM pathway is primarily an adaptation to water-limited environments; the fact that this pathway also stops photorespiration is an added benefit.
Overall, C 3 , C 4 and CAM plants all use the Calvin cycle to make sugars from CO 2 . However, the various ways in which plants fix CO 2 varies with the advantages and disadvantages associated with the mechanism and the habitats where plants can be found (Table 1).
As humans continue to burn fossil fuels, CO 2 levels in the atmosphere will continue to increase. This human alteration of the environment has sparked the development of a number of interesting questions. What influence will increasing CO 2 have on the distributions of C 3 , C 4 and CAM plants? What influence will increasing CO 2 have on agricultural production? Is it possible that an increase in agricultural production by additional CO 2 in the atmosphere could offset or mitigate the decrease in agricultural production caused by climate change?
| C 3 plant | C 4 plant | CAM Plant | |
| Cost | Photorespiration | ATP cost associated with fixing carbon twice. Carbon fixation is less efficient under cold conditions. | Reduced amount of fixed carbon, stomata only open at night |
| Benefits | Carbon fixation without using ATP | Reduced photorespiration and ability to fix Carbon under high temperatures and reduced water loss | Reduced photorespiration and reduced water loss |
| Separation of light-dependent reactions and carbon fixation | None, all of these reactions occur in the same cells | Spatial, these two sets of reactions occur in different cells | Temporal, these two sets of reactions occur at different times of day |
| Habitat | Cool and moist | Hot, not in cold environments (see cost.) | Hot and dry, large temperature differential between night and day |
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