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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 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 • 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 • 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 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 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 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 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
Examples • C3 • Basswood, beans, peas • C4 • Corn • CAM • Cactus
Resources • Adapted from Gary E. Kaiser • http://student.ccbcmd.edu/biotutorials