Review Calvin Cycle begins with adding a CO2 to RuBP This reaction is catalyzed by the an enzyme. Can you name it?

Rubisco Rubisco stands for: Ribulose bisphosphate carboxylase oxygenase. RuBP either gets a carbon dioxide added or an oxygen. This means that Rubisco isn't choosy. It will bind to either a carbon dioxide or an oxygen molecule, whichever it interacts with first.

Photorespiration When Rubisco binds to an oxygen instead of a carbon dioxide, the newly formed molecule (still 5C like RuBP) splits. A 2C compound leaves the chloroplast and is rearranged and split by peroxisomes and mitochondria to form CO2, which is released. This process occurs in the light (photo) and involves using O2 and creating CO2 (respiration), but does NOT generate any ATP.

Consequences What is the problem with Rubisco binding to oxygen instead of carbon dioxide?

Recall Gas Exchange in the Leaf When stomata are open plants take in CO2 for the Calvin Cycle and release the O2 produced during the light reactions. What else happens while the stomata are open?

Balancing Act – CO2 in vs. H2O out When it is hot and dry plants must close their stomata to prevent dehydration. Without input of CO2, eventually O2 begins to build up in the leaf and binds with Rubisco more often, resulting in photorespiration. Can you see a problem with this?

Alternatives Because of this problem, plants have evolved alternative methods of carbon fixation (taking inorganic carbon and getting organic carbon). When the first compound made in carbon fixation has 3 carbons (like we saw yesterday), we call the plant a C3 plant. There are two main types of adaptations to hot and dry conditions: C4 plants and CAM plants.

C3 vs. C4 Leaf Structure Key difference = Bundle Sheath cells. C4 plants have much bigger and tightly packed bundle sheath cells In C4 plants the Calvin Cycle only occurs in bundle sheath cells, NOT mesophyll

C4 Plants First compound in carbon fixation has 4C In mesophyll cells CO2 is bound to PEP by PEP carboxylase. PEP carboxylase has much higher affinity for CO2 than Rubisco and NO affinity for O2.

C4 plants Crabgrass, corn and sugarcane are all C4 plants.

CAM Plants CAM = Crassulacean Acid Metabolism Crassulaceae are a family of succulent plants. To minimize water loss they close stomata during the day, open them at night (opposite). At night they take in CO2 and store it as organic acids in mesophyll cells. During the day CO2 is released from these acids and the Calvin Cycle takes place to create sugar.

C4 CAM CO2 first incorparated into organic acids Separated from Calvin cycle until ready for use: Spatially Bundle sheath cells CO2 first incorparated into organic acids Separated from Calvin cycle until ready for use: Temporally Stomata open at night and CO2 stored until day.