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Photorespiration. Packet #34 Chapter #10. Introduction. In the 1960’s, it was discovered that illuminated plants consume and use O 2 and produce CO 2 .

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Packet #34

Chapter #10

  • In the 1960’s, it was discovered that illuminated plants consume and use O2 and produce CO2.
  • With low CO2 levels and high O2 levels, this photorespiration overwhelms photosynthetic CO2 fixation (light reactions + Calvin cycle).
introduction ii
Introduction II
  • In a very lengthy and costly process, O2 is converted into CO2 and 3-phosphoglyceraldehyde.
  • Photorespiration involves the use of three organelles
    • Chloroplast
    • Peroxisome
    • Mitochondria
  • Photorespiration also requires the use of ATP and NADPH.
    • Reducing the number of those molecules readily available for the Calvin cycle (photosynthesis).
introduction iii
Introduction III
  • With higher levels of O2, O2 competes with CO2 as a substrate for the enzyme rubisco (RuBPcarboxylase/oxygenase).
    • This is the foundation of photorespiration.
    • O2 binds to rubisco and starts the series of reactions that eventually lead to the production of CO2 and 3-phosphoglyceraldehyde.
      • A wasteful process that undoes the some of the work of photosynthesis.
introduction iv
Introduction IV
  • Plants can be divided into three categories based on how they deal with photorespiration.
    • C3 plants
      • No mechanism developed to decrease photorespiration.
    • C4 plants
    • CAM plants
      • Both C4 and CAM plants have mechanisms in place to decrease photorespiration.
photorespiration in c 3 plants
Photorespiration in C3 Plants
  • On dry, hot days in the presence of light C3 plants close their stomata.
  • This causes the plant to use O2 retain as much H2O.
    • O2 binds to rubisco and starts the series of reactions.
    • H2O is retained for use in the light reactions to fill the ATP and NADPH used as a result of photorespiration.
c4 cam plants introduction
C4 & CAM Plants Introduction
  • C4 and CAM plants have devised mechanisms that prevent/reduce the impact of photorespiration.
introduction i
Introduction I
  • C4 plants occur largely in tropical regions because they grow faster under hot and sunny conditions.
    • C3 plants live in cooler climates where photorespiration is less of a burden and less ATP is required to fix carbon.
introduction ii1
Introduction II
  • On a hot bright day, when photosynthesis has depleted the level of CO2 at the chloroplast and raised that of O2, the rate of photorespiration reaches the rate of photosynthesis.
  • However, C4 plants have leaves that are different anatomically to that of C3 plants and have devised a mechanism to reduce the impact of photorespiration.
introduction iii1
Introduction III
  • Photorespiration is negligible in C4 plants because the concentration of carbon dioxide is always high in the bundle sheath cells.
    • C4 plants “concentrate” CO2.
c4 plants process i
C4 Plants Process I
  • Phosphoenol-pyruvate (PEP)  Oxaloacetate
    • PEP carboxylase adds CO2 to PEP to produce Oxaloacetate
    • Occurs in the mesophyll cell.
c4 plants process ii
C4 Plants Process II
  • OxaloacetateMalate
    • Malate dehydrogenase reduces oxaloacetate into malate.
      • NADPH is used during this step.
c4 plants process iii
C4 Plants Process III
  • Malate is transported from mesophyll cell into the bundle sheath cell.
c4 plants process iv
C4 Plants Process IV
  • Malate  Pyruvate
    • Malic enzyme converts malate into pyruvate.
      • Two byproducts are made.
        • CO2
          • The CO2 is now considered to be concentrated.
        • NADPH
          • Both are used in the Calvin Cycle that occurs within the bundle sheath cell.
c4 plants process v
C4 Plants Process V
  • Pyruvate leaves the bundle sheath cell and enters the mesophyll cell.
c4 plants process vi
C4 Plants Process VI
  • PyruvatePEP
    • Pyruvate-phosphate dikinase converts pyruvate into PEP.
c4 plants process vii
C4 Plants Process VII
  • Process is repeated to concentrate more CO2.
additional information on c4 plants
Additional Information on C4 Plants.
  • At lower light levels and temperature, C4 plants will utilize the traditional C3 pathway.
  • Examples of C4 plants
    • Sugarcane
    • Corn
    • Crab grass.
introduction i1
Introduction I
  • CAM is an acronym for crassulacean acid metabolism.
  • Examples
    • Succulent plants {family Crassulaceae}
    • Family Cactaceae
    • Family Lilaceae
    • Family Orchidaceae
    • Many others in 25 families.
introduction ii2
Introduction II
  • CAM plants exhibit a pathway similar to C4 plants and allow them to live in highly xeric conditions.
    • CAM plants store CO2 using a variation of the technique used by C4 plants.
cam plants night
CAM Plants—Night
  • CAM plants open their stomata at night.
    • If these plants, living in the xeric conditions opened their stomata during the daytime, would lose large amounts of H2O through osmosis and then evaporation.
  • PEP carboxylase fixes carbon at night in the mesophyll cells
    • Stomata are open at night
      • Minimizes water loss and allows the entry of CO2
  • Calvin Cycle occurs during the daytime
cam plants night ii
CAM Plants—Night II
  • PEPoxaloacetatemalate
    • PEP carboxylase and malate dehydrogenase fixes CO2 at night in the mesophyll cells.
    • Malate is stored in vacuoles.
cam plants daytime i
CAM Plants—Daytime I
  • During the daytime, while the stomata is closed, malate is converted into pyruvate in the bundle sheath cell.
    • This allows the production of CO2.
    • CO2 is used to drive the Calvin cycle in the bundle sheath cell.
  • Types of plants
    • C3
    • C4
      • Concentrate CO2 in the bundle sheath cells.
    • CAM
      • Store CO2 in the form of malate by having the stomata only open at night.
homework assignment
Homework Assignment
  • What are some of the similarities, and differences, between C4 and CAM plants?