Transcript CO 2
Photorespiration: * Rubisco catalyze its oxygenation ability omnipresent, even in anaerobic, autotrophic bacteria when exposed to oxygen * Loss of CO2 from cells * Competition: decrease the efficiency of photosynthesis * Interconnection: determined by the kinetic properties of rubisco, the concentration of substrates, and temperature * C2 oxidative photosynthetic carbon cycle: act as a scavenger operation to recover fixed carbon lost during photorespiration Three organelles Carbon flow 22C13C+CO2 75% Nitrogen flow no changed Oxygen flow 3 O2/2 RuBP Malate-OAA shuttle supply NADH Web Topics 8.6 [gas] µM = Pgas 106/ V0 In vitro vs. In vivo Pgas: partial pressure; : absorption coefficient Solubility of CO2 and O2 as a function of temperature T tilt toward the C2 oxidative photosynthetic cycle ROS Photorespiration depends on the photosynthetic electron transport system The biological function of photorespiration is under investigation ＊ a protective, to dissipate excess ATP and reducing power, especially under high light intensity and low [CO2]inter (e.g., water stress) ＊ mutants lack glycerate kinase, not viable in normal air ＊ linked photorespiration to nitrate assimilation a full understanding is still not at hand CO2-concentrating mechanisms: 0.03% CO2 / 21% O2 A. C4 photosynthetic carbon fixation (C4), in hot environment; B. Crassulacean acid metabolism (CAM), in desert environment; C. CO2 pumps at the plasma membrane. In aquatic plants, such as unicellular cyanobacteria and algae. In aquatic environment, [CO2] low rubisco specificity activity low CO2-HCO3- pumps at the plasma membrane are induced, to accumulate inorganic carbon light energy provide ATP to uptake CO2 and HCO3carbonic anhydrase: HCO3- + H+ → H2O + CO2 → Calvin cycle [CO2] ↑ suppress photorespiration Cyanobacterial CO2 concentrating mechanism — high homologous to the Rheus, a protein in erythrocytes The C4 carbon cycle spatial Kranz (wreath) cells: present two distinct chloroplast-containing cells, mesophyll and bundle sheath cells OAA Calvin cycle sugarcane Flaveria australasica Poa sp The C4 photosynthetic pathway: Hatch and Slack Gramineae (corn, millet, sorghum, sugarcane); Chenopodiaceae (Atriplex); Cyperaceae (sedges). external plasmodesmata specific translocators vascular NADP-ME: in chloroplast NAD-ME: in mitochondria PEP carboxykinase: in cytosol Web Topic 8.7 (?) Three variations of C4 metabolism The form of transportation The manner of decarboxylation (1) maize, crab grass, sugarcane, sorghum; Aspartate aminotransferase (2) pigweed, millet; (3) guinea grass. PEP carboxykinase Alanine aminotransferase Kranz anatomy: mesophyll and bundle-sheath cells carbon concentrating mechanism / suppressed photorespiration Photosynthetic Carbon assimilation reduction plasmodesmata Borszczowia aralocaspica Bienertia cycloptera Chloroplasts containing rubisco are near mitochondria with NAD-ME