C4 Photosynthesis

1. C4 Photosynthesis

2. C4 carbon fixation or the Hatch–Slack pathway is a photosynthetic process in some plants. It is the first step in extracting carbon from carbon dioxide to be able to use it in sugar and other biomolecules production. It is one of three known processes for carbon fixation. "C4" refers to the four-carbon molecule that is the first product of this type of carbon fixation. Examples of C4 species are the economically important crops corn or maize (Zea mays), sugarcane (Saccharum officinarum), sorghum (Sorghum bicolor), and millets, as well as the switchgrass (Panicum virganum) which has been utilized as a source of biofuel.

3. Review: C3 Photosynthesis • During “regular” photosynthesis, CO2 is trapped into a 3-carbon compound by Rubisco  C3 Photosynthesis • This 3 carbon compound then goes through the calvin cycle to produce glucose (eventually)

4. C4 Photosynthesis • Certain plants go through a slightly modified photosynthesis process (C4 Photosynthesis) • C4 Photosynthesis is an adaptation that evolved due to the environment these plants are in. • The C4 pathway is in effect a turbocharger for the more conventional C3 pathway. Just as a turbocharger improves performance of an engine by forcing more air into the manifold, C4 improves photosynthetic performance by forcing CO2 into the standard C3 photosynthetic apparatus. The added efficiency of this mechanism is obvious at a global level. Only about 3% of flowering plant species use the C4 pathway, but this relative handful of species account for 23% of the carbon fixed (primary productivity) in the world

5. Rubisco • Rubisco can react with CO2 (Carboxylase Reaction) – good for glucose output  • Rubisco usually reacts with CO2, but it can also react with O2 – 2 competing reactions. • Rubisco can also react with O2 (Oxygenase Reaction) not good for glucose output, even though CO2 is eventually regenerated, it wastes time and energy (occupies Rubisco)

6. Photorespiration • When Rubisco reacts with O2 instead of CO2 • Occurs under the following conditions: • High O2 concentrations • High heat • Photorespiration is estimated to reduce photosynthetic efficiency by 25%

7. Why high heat? • When it is hot, plants close their stomata to conserve water • They continue to do photosynthesis  use up CO2 and produce O2  creates high O2 concentrations inside the plant  photorespiration occurs

8. C4 Photosynthesis • Certain plants have developed ways to limit the amount of photorespiration • C4 Pathway* • CAM Pathway* * Both convert CO2 into a 4 carbon intermediate  C4 Photosynthesis

9. Mesophyll cells Bundle sheath cells (no photosynthesis Leaf Anatomy • In C3 plants (those that do C3 photosynthesis), all photosynthesis processes occur in the mesophyll cells. Image taken without permission from http://bcs.whfreeman.com/thelifewire|

10. C4 Pathway • In C4 pathway plants photosynthesis occurs in both the mesophyll and the bundle sheath cells. • Light reactions in mesophyll • Calvin Cycle in Bundle sheath • The leaves of most C4 plants possess a Kranz‐type anatomy consisting of bundle sheath and mesophyll cells. A more rare form of this pathway, called single‐cell C4, uses partitioning of dimorphic chloroplasts to separate different sets of reactions within a single leaf cell type. Image taken without permission from http://bcs.whfreeman.com/thelifewire|

11. C4 Pathway • CO2 is fixed into a 4-carbon intermediate first • Has an extra enzyme– PEP Carboxylase that initially traps CO2 instead of Rubisco– makes a 4 carbon intermediate

12. C3 Pathway C4 Pathway • The 4 carbon intermediate “smuggles” CO2 into the bundle sheath cell • The bundle sheath cell is not very permeable to CO2 • CO2 is released from the 4C molecule  goes through the Calvin Cycle

13. How does the C4 Pathway limit photorespiration? • Bundle sheath cells are far from the surface– less O2 access • PEP Carboxylase doesn’t have an affinity for O2  allows plant to collect a lot of CO2 and concentrate it in the bundle sheath cells (where Rubisco is present)

14. These additional steps, however, require more energy in the form of ATP. Using this extra energy, C4 plants are able to more efficiently fix carbon in drought, high temperatures, and limitations of nitrogen or CO2. Since the more common C3 pathway does not require this extra energy, it is more efficient in the other conditions. Carbonic anhydrase (CA: carbonate hydratase, EC 4.2.1.1), which catalyses the first step in C4 photosynthesis, is located in the mesophyll cell cytoplasm, where it equilibrates the aerial CO2 to HCO;.

15. The first step in the pathway is the conversion • ofpyruvate to phosphoenolpyruvate (PEP), • by the enzyme pyruvate orthophosphate dikinase. • This reaction requires inorganic phosphate and ATP •  plus pyruvate, producing phosphoenolpyruvate,  • AMP, and inorganic pyrophosphate (PPi). The • next step is the fixation of CO2into oxaloacetate by the enzyme  PEP carboxylase. Both of these steps occur in the mesophyll cells: • pyruvate + Pi + ATP → PEP + AMP + PPi • PEP + CO2 → oxaloacetate • PEP carboxylase has a lower Km for HCO3and, hence, higher affinity than RuBisCO. Furthermore, O2 is a very poor substrate for this enzyme. Thus, at relatively low concentrations of CO2, most CO2 will be fixed by this pathway.

16. The product is usually converted to malate, a simple organic compound, which is transported to the bundle-sheath cells surrounding a nearby vein. Here, it is decarboxylated to produce CO2 and pyruvate. The CO2 now enters the Calvin cycle and the pyruvate is transported back to the mesophyll cell.

17. Since every CO2 molecule has to be fixed twice, first by four-carbon organic acid and second by RuBisCO, the C4 pathway uses more energy than the C3 pathway. The C3 pathway requires 18 molecules of ATP for the synthesis of one molecule of glucose, whereas the C4 pathway requires 30 molecules of ATP. C4 plants represent only about 3% of the world's flora (C3 are 87%; CAM are 10%), suggesting that not may plants use the C4 path. C4 plants need an additional 2 ATP--one to decarboxylate and one to regenerate PEP. This energy debt is more than paid for by avoiding losing more than half of photosynthetic carbon in photorespiration as occurs in some tropical plants, making it an adaptive mechanism for minimizing the loss.

18. There are several variants of this pathway: The four-carbon acid transported from mesophyll cells may be malate, as above, or aspartate. The three-carbon acid transported back from bundle-sheath cells may be pyruvate, as above, or alanine. The enzyme that catalyses decarboxylation in bundle-sheath cells differs. In maize and sugarcane, the enzyme is NADP-malic enzyme; in millet, it is NAD-malic enzyme; and, in Panicum maximum, it is PEP carboxykinase.

20. As determined by 14CO2-fixation experiments, the main initial product is malate in C4 species of the NADP-ME type, whereas aspartate is the major product in the NAD-ME and PEP-CK types.

22. C4 plants have a competitive advantage over plants possessing the more common C3 carbon fixation pathway under conditions of drought, high temperatures, and nitrogen or CO2 limitation. When grown in the same environment, at 30 °C, C3 grasses lose approximately 833 molecules of water per CO2 molecule that is fixed, whereas C4 grasses lose only 277. This increased water use efficiency of C4 grasses means that soil moisture is conserved, allowing them to grow for longer in arid environments.

23. CAM Pathway • Fix CO2 at night and store as a 4 carbon molecule • Keep stomates closed during day to prevent water loss • Same general process as C4 Pathway • Has the same leaf anatomy as C3 plants

24. How does the CAM Pathway limit photorespiration? • Collects CO2 at night so that it can be more concentrated during the day • Plant can still do the calvin cycle during the day without losing water

25. Summary of C4 Photosynthesis • C4 Pathway • Separates by space (different locations) • CAM Pathway • Separates reactions by time (night versus day)

26. Comparison • Light intensity is directly related to temperature • C4 Plants (CAM and C4 Pathway) are able to do more photosynthesis at high temperatures