Inefficiency of RubisCORibulose-1,5-bisphosphate carboxylaseoxygenase • RubisCO has 2 catalytic properties • In C3 plants rubisCO catalyzes carbon fixation: CO2+ RuBP 2 3-PGA (2- 3Ccompounds) • O2competeswith CO2 for the active site on rubisCO and leads to: O2+RuBP 2-phosphoglycolate + 3-PGA (a 2C compound and only 1- 3C compound)
Photorespiration • The reaction of oxygen with RuBP is called photorespiration and it reduces the efficiency of the Calvin cycle and the regeneration of RuBP.
The effect of the environment • In hot dry conditions C3 plants close their stomata to prevent water loss.
The effect of the environment • This traps the oxygen inside the leaf and blocksCO2 from entering. • Photorespiration increases and photosynthesisdecreases.
The effect of the environment • Plants native to regions with typically hot and dry climates (over 28°C) have developed adaptations to avoid photorespiration.
1) C4Plants • Ex. Foodcropslike corn and millet, grasses like crabgrass • Uptake of CO2 is separatedfrom the Calvin Cycle by cell types (and therefore also by location of the enzymes)
C3 Plant Leaf Structure • The mesophyll cellsare arranged in 2 layers (palisade and spongy), with vascular bundles in the middle, wrapped in a layer of cells called the bundle sheath cells. • The bundle sheath cells do NOT have chloroplasts.
C4 Plant Leaf Structure • Have one layer of mesophyll cells that surround the bundle sheath cells and the vascular bundle. • The bundle sheath cells have chloroplasts.
C3 plant leaf C4 plant leaf Histology
Carbon Fixation in C4Plants • CO2 diffuses into mesophyll cells and is accepted by PEP producing a 4 carbon compound, oxaloacetate (OAA) – therefore C4 plants • (vs. plants that use RuBP and make PGA (3C’s) -- C3 plants)
Carbon Fixation in C4 Plants • Oxaloacetate is converted (reduced by NADPH) to malate (4C) • Malate is activelytransportedinto bundle sheath cells
Carbon Fixation in C4 Plants • In the bundle sheath cells, malate is decarboxylated to produce CO2 and pyruvate. • The bundle sheath cells are impermeable to CO2 so it can’t exit. • CO2 is used in the Calvin Cycle.
Carbon Fixation in C4 Plants • Pyruvate is transported backinto the mesophyll cells and phosphorylated to regenerate PEP.
2) CAM Plants • Succulent plants (water-storing) like pineapple and cacti; live in hot, arid desert • Stomata are closedall day and only openat night (opposite to most plants) • Same Carbon fixation pathway as C4 plants butall done in the same cell – separated by time of day
Carbon Fixation in CAM Plants • CO2 fixation to malate (C4 compound) done at nightwhile stomata are open • Malate is stored in a vacuole at night
Carbon Fixation in CAM Plants • C4is converted to CO2 for Calvin Cycleduring the day while stomata areclosed and CO2cannot escape • Malate exits vacuole and is converted into pyruvate and CO2
Carbon Fixation in CAM Plants • Storing malate takes energy therefore these plants would be very inefficient in a cool climate.
Homework Questions: • Define photorespiration and explain how it affects the Calvin cycle. • What happens in C3 plants when temperature rises above 28°C? • How does a C4 plant reduce the amount of photorespiration that occurs in the leaves even when the air is hot and dry? • In the leaves of CAM plants, carbon dioxide fixation and the Calvin cycle occur inside the same cell. How then, do CAM plants minimize the photorespiration? • Would CAM plants have an advantage over native species in a Boreal forest? Explain why or why not.