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Chapter 7.7. C4 and CAM Carbon-Fixing Pathways. AP Biology Fall 2010. Gas Entry. Stomata : small pores on underside of leaves Entry and exit of gasses CO2, gas required for Calvin cycle, is not very abundant in nature. Gas Entry.

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chapter 7 7

Chapter 7.7

C4 and CAM Carbon-Fixing Pathways

AP Biology

Fall 2010

gas entry
Gas Entry
  • Stomata: small pores on underside of leaves
    • Entry and exit of gasses
  • CO2, gas required for Calvin cycle, is not very abundant in nature
gas entry1
Gas Entry
  • Under hot and dry environmental conditions stomata close to reduce loss of water vapor, also results in diminished supply of CO2
  • Plants that normally live in these environments adapted different ways of initially fixing CO2 prior to it entering the Calvin cycle
  • Known as C4 and CAM pathways, takes place in cytoplasm of cell
c4 pathway
C4 Pathway
  • Designed to efficiently fix C2 at low concentrations
  • Plants that use this are known as C4 plants
  • First fix CO2 into a 4-C compound (C4) called oxalacetate
  • Occurs in cells called mesophyll cells
c4 pathway2
C4 Pathway
  • CO2 fixed to 3-C compound phosphoenolpyruvate to produce 4-C compound oxaloacetate
    • Enzyme catalyzing this reaction, PEP carboxylase, fixes CO2 very efficiently so C4 plants don’t need to have their stomata open as much
c4 pathway3
C4 Pathway
  • Oxaloacetate converted to 4-C compound malate
    • Requires reducing power of NADPH
    • Malate then exits mesophyll cells and enters chloroplasts of specialized cells called bundle sheath cells
    • Malate is decarboxylated to produce CO2, a 3-C pyruvate, and NADPH
c4 pathway4
C4 Pathway
  • Oxaloacetate converted to 4-C compound malate
    • CO2 combines with RuBP and goes through Calvin cycle
    • Pyruvate re-enters mesophyll cells, reacts with ATP, & converted back to phosphoenolpyruvate
      • Starting compound of C4 cycle
cam pathway
CAM Pathway
  • CAM plants live in very dry conditions
  • Open stomata to fix CO2 only at night
  • Use PEP carboxylase to fix CO2, forming oxaloacetate
  • Oxaloacetate is converted to malate which is stored in cell vacuoles
  • During the day when stomata are closed, CO2 is removed from the stored malate and enters Calvin cycle
  • C3
    • Basswood, beans, peas
  • C4
    • Corn
  • CAM
    • Cactus
  • Adapted from Gary E. Kaiser