Photorespiration

1. Photorespiration Packet #34 Chapter #10

2. Introduction • In the 1960’s, it was discovered that illuminated plants consume and use O2 and produce CO2. • With low CO2 levels and high O2 levels, this photorespiration overwhelms photosynthetic CO2 fixation (light reactions + Calvin cycle).

3. Introduction II • In a very lengthy and costly process, O2 is converted into CO2 and 3-phosphoglyceraldehyde. • Photorespiration involves the use of three organelles • Chloroplast • Peroxisome • Mitochondria • Photorespiration also requires the use of ATP and NADPH. • Reducing the number of those molecules readily available for the Calvin cycle (photosynthesis).

4. Introduction III • With higher levels of O2, O2 competes with CO2 as a substrate for the enzyme rubisco (RuBPcarboxylase/oxygenase). • This is the foundation of photorespiration. • O2 binds to rubisco and starts the series of reactions that eventually lead to the production of CO2 and 3-phosphoglyceraldehyde. • A wasteful process that undoes the some of the work of photosynthesis.

5. Introduction IV • Plants can be divided into three categories based on how they deal with photorespiration. • C3 plants • No mechanism developed to decrease photorespiration. • C4 plants • CAM plants • Both C4 and CAM plants have mechanisms in place to decrease photorespiration.

6. Photorespiration & C3 Plants

7. Photorespiration in C3 Plants • On dry, hot days in the presence of light C3 plants close their stomata. • This causes the plant to use O2 retain as much H2O. • O2 binds to rubisco and starts the series of reactions. • H2O is retained for use in the light reactions to fill the ATP and NADPH used as a result of photorespiration.

8. C4 & CAM Plants Introduction • C4 and CAM plants have devised mechanisms that prevent/reduce the impact of photorespiration.

9. C4 Plants & Photorespiration Concentrating CO2

10. Introduction I • C4 plants occur largely in tropical regions because they grow faster under hot and sunny conditions. • C3 plants live in cooler climates where photorespiration is less of a burden and less ATP is required to fix carbon.

11. Introduction II • On a hot bright day, when photosynthesis has depleted the level of CO2 at the chloroplast and raised that of O2, the rate of photorespiration reaches the rate of photosynthesis. • However, C4 plants have leaves that are different anatomically to that of C3 plants and have devised a mechanism to reduce the impact of photorespiration.

12. Introduction III • Photorespiration is negligible in C4 plants because the concentration of carbon dioxide is always high in the bundle sheath cells. • C4 plants “concentrate” CO2.

13. C4 Plants Process I • Phosphoenol-pyruvate (PEP)  Oxaloacetate • PEP carboxylase adds CO2 to PEP to produce Oxaloacetate • Occurs in the mesophyll cell.

14. C4 Plants Process II • OxaloacetateMalate • Malate dehydrogenase reduces oxaloacetate into malate. • NADPH is used during this step.

15. C4 Plants Process III • Malate is transported from mesophyll cell into the bundle sheath cell.

16. C4 Plants Process IV • Malate  Pyruvate • Malic enzyme converts malate into pyruvate. • Two byproducts are made. • CO2 • The CO2 is now considered to be concentrated. • NADPH • Both are used in the Calvin Cycle that occurs within the bundle sheath cell.

17. C4 Plants Process V • Pyruvate leaves the bundle sheath cell and enters the mesophyll cell.

18. C4 Plants Process VI • PyruvatePEP • Pyruvate-phosphate dikinase converts pyruvate into PEP.

19. C4 Plants Process VII • Process is repeated to concentrate more CO2.

20. Additional Information on C4 Plants. • At lower light levels and temperature, C4 plants will utilize the traditional C3 pathway. • Examples of C4 plants • Sugarcane • Corn • Crab grass.

21. CAM Plants & Photorespiration Storing CO2

22. Introduction I • CAM is an acronym for crassulacean acid metabolism. • Examples • Succulent plants {family Crassulaceae} • Family Cactaceae • Family Lilaceae • Family Orchidaceae • Many others in 25 families.

23. Introduction II • CAM plants exhibit a pathway similar to C4 plants and allow them to live in highly xeric conditions. • CAM plants store CO2 using a variation of the technique used by C4 plants.

24. CAM Plants—Night • CAM plants open their stomata at night. • If these plants, living in the xeric conditions opened their stomata during the daytime, would lose large amounts of H2O through osmosis and then evaporation. • PEP carboxylase fixes carbon at night in the mesophyll cells • Stomata are open at night • Minimizes water loss and allows the entry of CO2 • Calvin Cycle occurs during the daytime

25. CAM Plants—Night II • PEPoxaloacetatemalate • PEP carboxylase and malate dehydrogenase fixes CO2 at night in the mesophyll cells. • Malate is stored in vacuoles.

26. CAM Plants—Daytime I • During the daytime, while the stomata is closed, malate is converted into pyruvate in the bundle sheath cell. • This allows the production of CO2. • CO2 is used to drive the Calvin cycle in the bundle sheath cell.

27. Review

28. Review • Types of plants • C3 • C4 • Concentrate CO2 in the bundle sheath cells. • CAM • Store CO2 in the form of malate by having the stomata only open at night.

29. Homework Assignment • What are some of the similarities, and differences, between C4 and CAM plants?