PHOTORESPIRATION: explain why it happens and how plants avoid photorespiration spatially (C4 Plants) or temporally (CAM Plants)
Photorespiration is when Rubisco might pick up O2 instead of CO2 for the CC which causes to the loss of already fixed carbon, wasting energy and decreasing sugar synthesis, it occurs in hot, dry conditions where there is a lot of O2 and little CO2, which could be do the stomata being close din hot conditions to keep water in, allowing little O2 out
Rubisco has a high affinity for O2 when it is hot, but when it is mild or cool, it is easier for rubisco to bind to CO2
Most plants, C3 variety, have no special features to prevent this
C4 plants minimize photorespiration by using KRANZ ANATOMY separating the CO2 fixation site from the rest of the calvin cycle. In C4 plants, the light reaction occurs in mesophyll cells while the calvin cycle, instead of occurring in the stroma of those same cells, occurs in bundle-sheath cells (protecting cells around leaf veins). When the CO2 is received in the mesophyll, PEP Carboxylase (NOT RUBISCO) affixes it to oxaloacetate which is converted to malate then sent to the bundle sheath cell where it is broken down into CO2, which is used by rubisco to make sugars, but the remaining pyruvate is phosphorylated by ATP into PEP, which is affixed to CO2 by PEP carboxylase and is started all over. This ATP use can be a toll, but compared to reducing photorespiration, this is ok. This includes crabgrass, sugarcane, and corn, which are common in hot areas but not so much in cooler areas (SPATIAL SEPARATION)
CAM (crassulacean acid metabolism) pathways separate the light dependent and independent (CC) by time of day. At night, the CAM plants open stomata, bringing in CO2 which is fixed onto PEP by PEP carboxylase just like in C4, but then this oxaloacetate is converted into malate or another organic acid that is stored in the vacuoles until day. When day arrives, CAM plants keep stomata closed, but they photosynthesize by releasing these organic acids out of the vacuole which can be broken down to release CO2, which goes through CC. All of this is temporal spacing to maintain a high CO2 concentration around Rubisco. It also uses ATP like C4 plants. It is seen in plants like succulents or pineapples, or plants that are water efficient since they only open stomata on cooler, more humid nights compared to hot, dry days, which helps maintain water.