IV) Light Dependent rxn a) harvests energy for next step - splits water releasing electrons, O2 and H+ b) Light dep rxn is what releases O2 to atmosphere i) resp for our atmosphere being hospitable to aerobic life c) Happens in thylakoid membrane compartments (grana) d) Can be non-cyclic or cyclic i) Non-cyclic electron flow (1) Involves two types of photosynthesis ( I and II) starts with what happens in II first (2) Sun light (photon of light energy) hits a special chlorophyll molecule = P680 (photosynthesis II) (3) causes P680 to give up an 2e- elevated to excited state (4) photolysis (water is split) H2O + sunlight —> 2 H+ + 1/2O2 + 2 electrons (these replace those lost by P680) (5) excited electrons move down the electron transport (cytochrome) system in thyl. Membrane (a) (sim. To respiration) (6) energy ADP → ATP ( used in the next step) (a) called photosynthesis (7) receptor is P700- it is also receiving photons that excite electrons (8) electrons from the transport system replaces the excited one (9) excited electron passes down another electron transport system (10) NADP+ (nicotinamide adenine dinucleotide phosphate) → NADPH (reduced) - carries e-s to next step ii) Cyclic electron flow (1) Involves photosynthesis I only (2) Produces ATP but no NADPH (3) Light independent rxn (next step) requires more ATP than NADPH (4) Cyclic provides extra ATP iii) ATP formation in both is by chemiosmosis (similar to mitochondria during respiration) (1) H+s from water build up in compartment (2) also as steps of e- transport system receive e-s - H+s are drawn across membrane from stroma (3) creates electrical and chemical gradient of H+ (conc higher inside membrane (different from resp)) (4) flow down gradient through ATP Synthase (proteins in membrane) (a) H+s pass out into stroma (5) synthases catalyze ADP à ATP (a) ATP is released in stroma (6) ATP and NADPH provide energy for next step V) Light independent rxn (Calvin cycle) a) Happens in stroma b) CO2 diffuses from air into stroma. C from CO2 is fixed Product is a 3 carbon sugar G3P c) made into sugars and other organic compounds later i) starches etc. d) Steps i) to make this 3CO2 molecules enter and react with 3RuBP (5carbon sugar) – total C=18 (1) rxn catalyzed by rubisco ii) 6ATP provide energy and 6NADPH provide electrons to make 6G3P (18Cs) iii) ________________________________________________________ iv) ____________________________________________________ (15Cs) to start cycle again (1) __________________________________________________ e) Overview of Calvin Cycle i) Reactants: (1) _________________ from atmosphere (2) _________________ regenerated from cycle (3) __________________ from light rxn (4) __________________ from light rxn ii) products (1) ________________ (3C sugar that can be made into glucose and complex carbohydrates – food for nearly all organisms on earth) (2) __________________ to be used in the cycle again (3) ____________________ – regenerated by light rxn later (4) ____________________ - regenerated by light rxn later VI) Some plants in arid climates have alternate methods for fixing C c) Leaves of plants have stomata i) Opening in leaves to allow gas exchange ii) Close when hot and dry to prevent water loss iii) Also reduces CO2 (1) Rubisco does non fix C well at low conc. Because it also reacts with O2 but does not produce anything useful iv) C4 and CAM plants convert CO2 to an organic acid before the Calvin cycle (1) Catalyzed by enzymes that work well at low CO2 conc. v) Organic acid releases CO2 to Calvin cycle d) C4 plants i) Grasses, sugar cane, corn ii) CO2 fixed into 4C organic acid in special cells near leaf surface (1) Mesophyll cells iii) Acid is transported to Bundle sheath-cells where Calvin cycle happens e) CAM plants i) Succulents (cacti, pineapples etc.) ii) Open stomata at night and close during the day (opposite of most plants) iii) Convert CO2 to organic acids iv) Acids release CO2 to Calvin cycle during the day