Water regulation in plants
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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 CO 2 can enter for photosynthesis.

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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?


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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?


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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?


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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?


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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?


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Water regulation in plants

  • C4 photosynthesis

  • Physiological solutions

How do organisms ‘solve’ common problems?


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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?


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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?


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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?


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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?


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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?


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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?


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