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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 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