1. Variations in Photosynthesis Lecture 9 Fall 2008

2. Variation in Photosynthesis Three types of photosynthesis C3 C4 CAM Why do plants need three different types of photosynthesis? 1

3. Water Balance in Plants Transpiration –Loss of water vapor from the leaf through diffusion and evaporation Plants need to balance loss of water to atmosphere with uptake of water from soil 2

4. Water Loss from Leaves Occurs through stomata (Path) 1.Water vapor in leaf spaces diffuses out of stomata Leaf air spaces are moist, air is dryer Driving force: Concentration gradient of water vapor inside vs. outside 2. Liquid water in leaf evaporates to replace water vapor that diffused out Blue dots = water vapor 3

5. The role of stomata Gas Exchange CO 2 enters, O 2 leaves via open stomata Water Balance Majority of water loss through stomata Stomata typically open during the day and closed at night Plant can control whether stomata are open or closed –Can close stomata during the day to prevent water loss 4

6. Balance between photosynthesis and water loss Stomata need to be open for gas exchange –No incoming CO 2, then low input for Calvin cycle Stomata need to be closed to prevent excessive water loss –If water in the soil is limited, and –The day is hot or dry 5

7. Balance between photosynthesis and water loss C3 Uses a 3-carbon sugar as substrate in Calvin cycle (3-phosphoglycerate) Stomata open during day, closed at night Not typically found in dry (arid) environments In hot temperatures, must close stomata –Photosynthesis reduced, so productivity reduced 6

8. Photorespiration A metabolic pathway that consumes O 2 and ATP, releases CO 2 and reduces photosynthetic output –~50% in some crop plants –Produces no ATP –Produces no sugar –Releases CO 2 rather than fixing it 7

9. Photorespiration Rubisco (RuBP carboxylase) adds CO 2 to ribulose bisphosphate (RuBP) Rubisco can also bind O 2 O 2 and CO 2 compete for binding at active site As concentrations of CO 2 decline, O 2 gets added to Calvin cycle instead Fig. 10.18 8

10. Photorespiration Results of ribulose bisphosphate & O2 –3-phosphoglycerate (3 carbon) –Phosphoglycolate (2 carbon) Phosphoglycolate not used in Calvin cycle Carbon in phosphoglycolate must be salvaged –Needed for regeneration of RuBP Fig. 10.18 9

11. Photorespiration Salvage pathway involves chloroplasts, mitochondria and peroxisomes 10

12. Why Photorespiration? Rubisco evolved in bacteria before O2 in atmosphere –Enzyme had affinity for both Modification of active site to reduce access to oxygen may also reduce the access to CO2 Plants compensate by increasing the concentration of rubisco. –Half of the protein in the chloroplast is rubisco PR may be protective in terms of preventing damage from excessive light 11

13. Alternative Pathways: C 4 C4 First carbon fixation produces a 4-carbon sugar (oxaloacetate) Can do photosynthesis with less CO 2 coming into leaf Can close stomata during day, without losing much productivity Plants found in hot/dry environments –E.g., many grasses, sugar cane, corn 12

14. Alternative Pathways: C 4 Two types of cells –Mesophyll cells Carbon fixation (C 4 pathway) –Bundle-sheath cells Calvin cycle in chloroplasts Spatial separation Keeps high CO 2 levels in bundle-sheath cells cycle See Fig. 10.19 13

15. Alternative Pathways: C 4 In mesophyll cells C4 Pathway CO 2 fixed to PEP (phosphoenolpyruvate) –Requires PEP carboxylase –Forms oxaloacetate (4 carbon) PEP carboxylase –No affinity for O 2 –Higher affinity for CO 2 than rubisco has –Able to add CO 2 to PEP even in low CO 2 concentrations Fig. 10.19 14

16. Alternative Pathways: C 4 Oxaloacetate converted to malate or aspartate Exported from mesophyll cell to bundle sheath cell –Plasmodesmata In bundle-sheath cells Malate broken down into pyruvate and CO 2 CO 2 to Calvin cycle Pyruvate to mesophyll cells –Regenerates to PEP –Requires ATP Fig. 10.19 15

17. Cyclic Electron Flow Uses cyclic electron flow to generate extra ATP Occurs in bundle-sheath cells –Thylakoids only have PS1 (and cytochrome complex) Fig. 10.15 17

18. Alternative Pathways: C 4 PS2 primarily in well developed granum PS1 and ATP synthase primarily in unstacked areas of membrane Cytochrome complexes distributed evenly Fig. 10.17 18

19. Alternative Pathways: C 4 19

20. Cyclic Electron Flow Linear electron flow produces 6 ATP & 6 NADPH –Calvin cycle requires 9 ATP Cyclic electron flow produces extra ATP –No water split –No O2 produced –No NADPH Fig. 10.15 20

21. Alternative Pathways: C 4 Besides the 4-carbon product, what else must travel between the mesophyll cells and the bundle sheath cells? 16

22. Alternative Pathways: CAM CAM (Crassulacean acid metabolism) Stomata open during night CO 2 stored as 4-carbon compound to be used the next day Stomata can be closed during the day Plants found in hot/dry environments –Cactus, pineapple, many orchids –(some are members of plant family Crassulaceae) 21

23. Alternative Pathways: CAM Temporal separation of carbon fixation and Calvin cycle Takes in CO2 at night Uses C4 pathway to fix CO2 into organic acids –malate Stored in vacuole –Malic acid –Active transport 22

24. Alternative Pathways: CAM Malic acid leaves vacuole Broken down to form pyruvate and CO2 CO2 to Calvin cycle Pyruvate converted to starch and stored 23

25. Alternative Pathways: CAM Fig. 10.20 24

26. Trade Offs Why aren’t all plants C4 or CAM? Type of CO 2 fixation Energy to fix one CO 2 Water Transpired per CO 2 fixed C33 ATP400 – 500 g C45 ATP250-300 g CAM6.5 ATP50-100 g 24

27. Importance of Photosynthesis Provides the carbon compounds for most organisms on the planet Changed Earth’s atmosphere to the current one Important part of the CO 2 cycle – Moderates temperature on the planet 26