2. Conflicting requirements in plants Water regulation in plants How do organisms ‘solve’ common problems? –Water lost by transpiration through stomata –If plants prevent water loss by closing guard cells then no CO 2 can enter for photosynthesis How do plants (in arid habitats) solve the conflict? –But water needed for metabolic activities and to maintain water balance in cells Surprise, surprise … different plants have different solutions –Morphological adaptations: sunken stomata, extensive roots –Physiological adaptations: alternative ways to ‘fix’ CO 2

3. Overview of Photosynthesis Water regulation in plants How do organisms ‘solve’ common problems? –Light-dependent reactions: Energy of excited electrons used to chemiosmotically produce ATP and form NADPH Electrons (from H 2 O) ‘excited’ by light energy

4. Overview of Photosynthesis Water regulation in plants How do organisms ‘solve’ common problems? –Light-independent reaction (Calvin Cycle): ATP and NADPH used to convert PGA into G3P molecules (later converted to glucose) CO 2 ‘fixed’ by rubisco to convert 5C RuBP into 3C PGA molecules (called C 3 photosynthesis) 3 CO 2 molecules needed to produce 1 G3P and restore 3 RuBP

5. Photorespiration Water regulation in plants How do organisms ‘solve’ common problems? –Rubisco also fixes O 2 –Oxidation of RuBP causes additional reactions that release CO 2 But nearly 20% of CO 2 originally fixed for Calvin Cycle is lost by photorespiration –Under ‘normal’ conditions rubisco fixes CO 2 at faster rate than O 2 Photorespiration increases substantially at high temperature and at low CO 2 concentrations Photorespiration decreases efficiency of C 3 photosynthesis; stomates must remain open (risk water loss) to get enough CO 2

6. Physiological solutions Water regulation in plants How do organisms ‘solve’ common problems? –C 4 photosynthesis Uses new pathway (called C 4 photosynthesis) to initially fix CO 2 in mesophyll of cell –uses enzyme PEP carboxylase to fix CO 2 –CO 2 combines with phosphoenolpyruvate (PEP) –PEP converted to a 4C oxaloacetate (OAA) –PEP carboxylase has no affinity for O 2, so no photorespiration in C 4 pathway –PEP carboxylase has greater affinity for CO 2 than rubisco, so more effective at capturing CO 2 from environment –During C4 pathway, OAA modified so that CO 2 released in bundle sheath cells (deeper in leaf tissue)

7. Physiological solutions Water regulation in plants How do organisms ‘solve’ common problems? –C4 photosynthesis

8. Physiological solutions Water regulation in plants How do organisms ‘solve’ common problems? –C4 photosynthesis C 4 pathway dumps CO 2 in bundle sheath cells where rubisco waiting to capture it for Calvin Cycle C 4 photosynthesis separates CO 2 fixation and Calvin Cycle in space

9. Physiological solutions Water regulation in plants How do organisms ‘solve’ common problems? –Advantages of C 4 photosynthesis C 4 pathway dumps CO 2 in bundle sheath cells –Builds up concentration of CO 2, making rubisco more efficient –PEP carboxylase has greater affinity for CO 2 than rubisco, so stomates can be closed more than in C 3 photosynthesis –Disadvantage of C 4 photosynthesis 12 additional ATP required to produce 1 glucose molecule –Additional ATP needed in C 4 pathway to regenerate PEP –Cost of producing glucose by C 4 photosynthesis nearly twice that of C 3 photosynthesis

10. Physiological solutions Water regulation in plants How do organisms ‘solve’ common problems? –CAM photosynthesis (Crassulacean Acid Metabolism) CAM pathway adopted by many succulent plants such as cacti and stonecrops CAM separates CO 2 fixation and Calvin Cycle in time –Stomates open only at night –CO 2 fixed at night by compounds like those in C4 pathway –CO 2 released during day in mesophyll cells where Calvin Cycle proceeds as usual

11. Physiological solutions Water regulation in plants How do organisms ‘solve’ common problems? –CAM photosynthesis C 4 pathway dumps CO 2 during day where rubisco waiting to capture it for Calvin Cycle CAM photosynthesis separates CO 2 fixation and Calvin Cycle in time

12. Physiological solutions Water regulation in plants How do organisms ‘solve’ common problems? –Advantages of CAM photosynthesis C 4 pathway dumps CO 2 during day when stomates closed –Builds up concentration of CO 2, making rubisco more efficient –Open stomates during night when more humidity and less water loss by transpiration –Disadvantage of CAM photosynthesis 12 additional ATP required to produce 1 glucose molecule because of C 4 pathway

13. Why don’t all plants use C 4 or CAM photosynthesis? Water regulation in plants How do organisms ‘solve’ common problems? –Change in biochemical pathway leading to C 4 pathway did not arise in all families of plants –Under ‘normal’ conditions, C 3 plants have higher photosynthetic rate than C 4 or CAM plants –C 4 pathway energetically more costly (requires more ATP) under ‘normal’ conditions