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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

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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 pathway1

C4 Pathway

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

Cam pathway1

CAM Pathway



  • C3

    • Basswood, beans, peas

  • C4

    • Corn

  • CAM

    • Cactus



  • Adapted from Gary E. Kaiser


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