Photosynthesis 6CO2 12H2O Light C6H12O6 6O2 6 H2O 12 water molecules consumed 6 generated Simplifies to CO2 H2O CH2O O2 but Photosynthesis CO2 2H2O CH2O O2 H2O CO2 2H2O CH2O O2 H2O Photosynthesis Photosynthesis Photosynthesis Light reactions Use photons to create energy carrier molecules ATP Adenosine tri phosphate NADPH Nicotinamide adenine dinucleotide phosphate Dark reactions Actually occur in the light Burn ATP and NADPH to reduce CO2 to carbohydrates Photosynthesis Spatial separation of light and dark reactions Photosynthesis Photosynthesis Photosynthesis ADP P ATP Phosphodiester bonds are high energy Photosynthesis Photosynthesis ATP synthesis in the light reactions is essentially identical to ATP synthesis during respiration Photosynthesis NADP 2e H NADPH Energy stored as reducing power Photosynthesis Cyclic electron flow Photosynthesis Cyclic electron flow Photosynthesis The Dark reactions AKA the Calvin Cycle Use energy from ATP reducing power of NADPH Incorporates CO2 into carbohydrate molecules Photosynthesis Carbon fixation 1C 5C acceptor 6C species is transient Photosynthesis Doing the math Per carbon fixed 3 ATP 2 NADPH Per each glucose 6C 18 ATP 12 NADPH Photosynthesis Photorespiration Rubisco combines O2 with 5C acceptor A mistake the enzyme is sloppy Result 3C sugar OK normal 2C molecule not useful Photosynthesis Photorespiration The 2C compound needs extensive salvage metabolism COST lose one CO2 burns ATP consumes O2 Photosynthesis Photorespiration Rubisco is about 100 times as likely to select CO2 than O2 BUT O2 is at much higher atmospheric concentrations Ratio of photorespiration carbon fixation is 3 2 Photosynthesis Photorespiration In hot sunny areas Drought stress causes stomatal closure during the day Light reactions continue Gas exchange limited Photosynthesis C4 plants First product incorporating newly fixed carbon has 4 Carbons An adaptation to concentrate CO2 and decrease photorespiration Photosynthesis C4 plants The Bundle Sheath Photosynthesis C4 plants PEP carboxylase combines 3C acceptor with CO2 no O2 issues This reaction is specific to mesophyll cells Photosynthesis C4 plants 4C compound is shuttled to bundle sheath cells Disassembled to release CO2 and a 3C compound Photosynthesis C4 plants Calvin cycle restricted to bundle sheath cells Photosynthesis C4 plants ATP consumed to regenerate the 3C acceptor Bundle sheath cells lack PSII no O2 production by light reactions Photosynthesis Photosynthesis C4 plants are more successful at lower latitudes Photosynthesis Crassulacean acid metabolism CAM Named for plant family Crassulaceae Jade Plant Crassula ovata Photosynthesis Most CAM plants grow in the desert Stomata are closed in the day open at night Photosynthesis Most CAM plants grow in the desert Night Open stomata allow gas exchange Store CO2 in a storage molecule Photosynthesis Most CAM plants grow in the desert Day Close stomata run light and dark reactions Get CO2 from reserved captured in daytime Photosynthesis CAM plants grow very slow limited CO2 uptake Only competitive in environments which are too extreme for C3 or C4 plants in general Photosynthesis Both C4 and CAM optimize photosynthesis in hot dry climates
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