BIOL 1411 1st Edition Lecture 14 Outline of Last Lecture I Photosynthesis a Light Reactions b Light dependent Reactions Outline of Current Lecture I Photosynthesis a Calvin Cycle b C4 photosynthesis c CAM Lecture Use of light Reaction Products to synthesize carbohydrates Light independent reaction o CO2 fixation and Calvin cycle o CO2 is reduced to carbohydrates o Enzymes in the stroma use the energy in ATP and NADPH to reduce CO2 Calvin and Benson used the 14C radioisotope to determine the sequence of reactions in CO2 fixation o They exposed Chlorella to 14CO2 then extracted the organic compounds and separated them by paper chromatography The Calvin Cycle o CO2 is first added to an acceptor molecule 5 C RuBP the 6 C compound immediately breaks down into two molecules of 3PG o The enzyme catalyzing the intermediate formation is rubisco ribulose biophosphate carboxylase oxygenase o Takes place in the stroma not thylakoid lumen How is chemical energy used to synthesize carbohydrates o G3P Glyceraldehyde 3 phosphate is the product of the Calvin cycle o Most is recycled into RuBP the rest is used to make sugars or stored starch Sugar synthesis o CO2 enters leaf through stomata o Intercellular air spaces o CO2 mesophyll cells chloroplasts These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o Water via veins Photosynthetic carbon reduction o Carboxylation carbon fixation o Phosphorylation uses ATP o Reduction uses NADPH o Regeneration uses ATP o For each CO2 3 ATP 2 NADPH used Keeping track o 1x through cycle 1 CO2 incorporated 2 NADPH used 3 ATP used o 3x 3 CO2 incorporated 6 NADPH used 9 ATP used 6 G3P generated 3 carbon molecules 5 G3P 5 x 3C used to regenerate 3 ribulose bisphosphate 3 x 5C 1 G3P is precursor to sugars o For 6C sugars like glucose 6CO2 are incorporated photochemical reaction stimulate the calvin cycle o the calvin cycle is not light dependent but is stimulated by light protons pumped form stroma into thylakoids increase the pH which favors the activation of Ribisco elctron flow from photosystem I reduces disulfide bonds to activate Calvin Cylce enzymes Photosynthesis concepts o Light energy is used to move elctrons o ATP o NADPH o E path o Cyclic electron flow provides extra ATP s Photorespiration o Ribulose bisphosphate CO2 3PG First step in Calvin cycle o Catalyzed by enzyme Most abundant enzyme on earth Bibulose bisphosphate carboxylase rubisco Binds ribuose bisphosphate and CO2 Active site also binds O2 Wasteful o Rubisco bind O2 catalyzes an oxygenation reaction Sugar is burned O2 added But no ATP is generatued wasteful photo occurs in light o o o o o o respiration uses O2 gives off CO2 Conditions that favor photorespiration Sunny dry windy Plant closes stomata to conserve water Photosynthesis continues CO2 is depleted O2 builds up Up to 50 loss of sugars Why does this wasteful process occur Evolutionary remnant Primitive Earth no O2 in atmosphere No selection A locked in defect Adaptations to avoid PR Protect rubisco from low CO2 high O2 Calvin cycle is not affected Take extra steps use extra energy Initial carboxylation Spatial separation C4 Photosynthesis Temporal separation CAM C4 CAM geographically restricted Use extra energy Found in sunny locales C4 photosynthesis Initial product 4 carbon molecule 2 photosynthetic cell type Bundle sheath cells Mesophyll cells Both have chloroplasts different enzymes in 2 cell types CORN CAM Crassulacean acid metabolism Crassulacea family of deser dwelling secculents Open stomata at night rather than day Reuces transpiration Gas exchange at night CO2 in O2 out Initial carboxylation yields organic acid How does photosynthesis interact with other pathways Photosynthesis and respiration are closely linked through Calvin cycle G3P is important Some takes part in glycolysis and cellular respiration for energy or can make other compounds Some is involved in gluconeogenesis the reverse of glycolysis supplying nonphotosynthestic parts o Energy Loss in Photosynthesis Wavelengths of light not part of absorbed due to plant structure Inefficiency of light reactions converting light to chemical energy Inefficiency of CO2 fixation pathways