Water regulation in plants

1. Water regulation in plants • Water lost by transpiration through stomata • Conflicting requirements in plants • But water needed for metabolic activities and to maintain water balance in cells • If plants prevent water loss by closing guard cells then no CO2 can enter for photosynthesis • How do plants (in arid habitats) solve the conflict? • Surprise, surprise … different plants have different solutions • Morphological adaptations: sunken stomata, extensive roots • Physiological adaptations: alternative ways to ‘fix’ CO2 How do organisms ‘solve’ common problems?

2. Water regulation in plants • Light-dependent reactions: • Electrons (from H2O) ‘excited’ by light energy • Overview of Photosynthesis • Energy of excited electrons used to chemiosmotically produce ATP and form NADPH How do organisms ‘solve’ common problems?

3. Water regulation in plants • Light-independent reaction (Calvin Cycle): • CO2 ‘fixed’ by rubisco to convert 5C RuBP into 3C PGA molecules (called C3 photosynthesis) • Overview of Photosynthesis • ATP and NADPH used to convert PGA into G3P molecules (later converted to glucose) • 3 CO2 molecules needed to produce 1 G3P and restore 3 RuBP How do organisms ‘solve’ common problems?

4. Water regulation in plants • Rubisco also fixes O2 • Oxidation of RuBP causes additional reactions that release CO2 • Photorespiration • Under ‘normal’ conditions rubisco fixes CO2 at faster rate than O2 • But nearly 20% of CO2 originally fixed for Calvin Cycle is lost by photorespiration • Photorespiration increases substantially at high temperature and at low CO2 concentrations • Photorespiration decreases efficiency of C3 photosynthesis; stomates must remain open (risk water loss) to get enough CO2 How do organisms ‘solve’ common problems?

5. Water regulation in plants • C4 photosynthesis • Uses new pathway (called C4 photosynthesis) to initially fix CO2 in mesophyll of cell • Physiological solutions • uses enzyme PEP carboxylase to fix CO2 • CO2 combines with phosphoenolpyruvate (PEP) • PEP converted to a 4C oxaloacetate (OAA) • PEP carboxylase has no affinity for O2, so no photorespiration in C4 pathway • PEP carboxylase has greater affinity for CO2 than rubisco, so more effective at capturing CO2 from environment • During C4 pathway, OAA modified so that CO2 released in bundle sheath cells (deeper in leaf tissue) How do organisms ‘solve’ common problems?

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

7. Water regulation in plants • C4 photosynthesis • C4 pathway dumps CO2 in bundle sheath cells where rubisco waiting to capture it for Calvin Cycle • Physiological solutions • C4 photosynthesis separates CO2 fixation and Calvin Cycle in space How do organisms ‘solve’ common problems?

8. Water regulation in plants • Advantages of C4 photosynthesis • C4 pathway dumps CO2 in bundle sheath cells • Physiological solutions • Builds up concentration of CO2, making rubisco more efficient • PEP carboxylase has greater affinity for CO2 than rubisco, so stomates can be closed more than in C3 photosynthesis • Disadvantage of C4 photosynthesis • 12 additional ATP required to produce 1 glucose molecule • Additional ATP needed in C4 pathway to regenerate PEP • Cost of producing glucose by C4 photosynthesis nearly twice that of C3 photosynthesis How do organisms ‘solve’ common problems?

9. Water regulation in plants • CAM photosynthesis (Crassulacean Acid Metabolism) • CAM pathway adopted by many succulent plants such as cacti and stonecrops • Physiological solutions • CAM separates CO2 fixation and Calvin Cycle in time • Stomates open only at night • CO2 fixed at night by compounds like those in C4 pathway • CO2 released during day in mesophyll cells where Calvin Cycle proceeds as usual How do organisms ‘solve’ common problems?

10. Water regulation in plants • CAM photosynthesis • C4 pathway dumps CO2 during day where rubisco waiting to capture it for Calvin Cycle • Physiological solutions • CAM photosynthesis separates CO2 fixation and Calvin Cycle in time How do organisms ‘solve’ common problems?

11. Water regulation in plants • Advantages of CAM photosynthesis • C4 pathway dumps CO2 during day when stomates closed • Physiological solutions • Builds up concentration of CO2, 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 C4 pathway How do organisms ‘solve’ common problems?

12. Water regulation in plants • Why don’t all plants use C4 or CAM photosynthesis? • C4 pathway energetically more costly (requires more ATP) under ‘normal’ conditions • Under ‘normal’ conditions, C3 plants have higher photosynthetic rate than C4 or CAM plants • Change in biochemical pathway leading to C4 pathway did not arise in all families of plants How do organisms ‘solve’ common problems?