C4 and CAM Carbon-Fixing Pathways

AP BiologyFall 2010

Gas EntryStomata: small pores on underside of leaves

Entry and exit of gassesCO2, gas required for Calvin cycle, is not

very abundant in nature

Gas EntryUnder 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 PathwayDesigned to efficiently fix C2 at low

concentrationsPlants that use this are known as C4 plants First fix CO2 into a 4-C compound (C4) called

oxalacetateOccurs in cells called mesophyll cells

C4 Pathway

C4 Pathway1. 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 Pathway2. 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 Pathway2. 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

CAM Pathway

ExamplesC3

Basswood, beans, peasC4

Corn CAM

Cactus

ResourcesAdapted from Gary E. Kaiserhttp://student.ccbcmd.edu/biotutorials