conflicting requirements in plants water regulation in plants how do organisms ‘solve’ common...
TRANSCRIPT
• 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 CO2 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’ CO2
• 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 H2O) ‘excited’ by light energy
• 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)
• CO2 ‘fixed’ by rubisco to convert 5C RuBP into 3C PGA molecules (called C3 photosynthesis)
• 3 CO2 molecules needed to produce 1 G3P and restore 3 RuBP
• Photorespiration
Water regulation in plants
How do organisms ‘solve’ common problems?
– Rubisco also fixes O2 – Oxidation of RuBP causes additional reactions that release CO2
• But nearly 20% of CO2 originally fixed for Calvin Cycle is lost by photorespiration
– Under ‘normal’ conditions rubisco fixes CO2 at faster rate than O2
• 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
• Physiological solutions
Water regulation in plants
How do organisms ‘solve’ common problems?
– C4 photosynthesis • Uses new pathway (called C4 photosynthesis) to initially fix CO2
in mesophyll of cell– 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)
• Physiological solutions
Water regulation in plants
How do organisms ‘solve’ common problems?
– C4 photosynthesis
• Physiological solutions
Water regulation in plants
How do organisms ‘solve’ common problems?
– C4 photosynthesis • C4 pathway dumps
CO2 in bundle sheath cells where rubisco waiting to capture it for Calvin Cycle
• C4 photosynthesis separates CO2 fixation and Calvin Cycle in space
• Physiological solutions
Water regulation in plants
How do organisms ‘solve’ common problems?
– Advantages of C4 photosynthesis • C4 pathway dumps CO2 in bundle sheath cells
– 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
• 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 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
• Physiological solutions
Water regulation in plants
How do organisms ‘solve’ common problems?
– CAM photosynthesis • C4 pathway dumps
CO2 during day where rubisco waiting to capture it for Calvin Cycle
• CAM photosynthesis separates CO2 fixation and Calvin Cycle in time
• Physiological solutions
Water regulation in plants
How do organisms ‘solve’ common problems?
– Advantages of CAM photosynthesis • C4 pathway dumps CO2 during day when stomates closed
– 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
• Why don’t all plants use C4 or CAM photosynthesis?
Water regulation in plants
How do organisms ‘solve’ common problems?
– Change in biochemical pathway leading to C4 pathway did not arise in all families of plants
– Under ‘normal’ conditions, C3 plants have higher photosynthetic rate than C4 or CAM plants
– C4 pathway energetically more costly (requires more ATP) under ‘normal’ conditions