Conflicting requirements in plants

1
Water regulation in plants

• Conflicting requirements in plants

• Water lost by transpiration through stomata

• 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

• Overview of Photosynthesis

• Light-dependent reactions

• Electrons (from H2O) excited by light energy

• Energy of excited electrons used to
  chemiosmotically produce ATP and form NADPH

How do organisms solve common problems?
3
Water regulation in plants

• Overview of Photosynthesis

• Light-independent reaction (Calvin Cycle)

• CO2 fixed by rubisco to convert 5C RuBP into 3C
  PGA molecules (called C3 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

• Photorespiration

• Rubisco also fixes O2

• Oxidation of RuBP causes additional reactions
  that release CO2

• 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

• Physiological solutions

• 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)

How do organisms solve common problems?
6
Water regulation in plants

• Physiological solutions

• C4 photosynthesis

How do organisms solve common problems?
7
Water regulation in plants

• Physiological solutions

• 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

How do organisms solve common problems?
8
Water regulation in plants

• Physiological solutions

• 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

How do organisms solve common problems?
9
Water regulation in plants

• Physiological solutions

• 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

How do organisms solve common problems?
10
Water regulation in plants

• Physiological solutions

• 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

How do organisms solve common problems?
11
Water regulation in plants

• Physiological solutions

• 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

How do organisms solve common problems?
12
Water regulation in plants

• Why dont 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?