Carbohydrate Biosynthesis in Plants Photosynthesis Photosynthesis • The light-independent reactions make sugars – take place in stroma – needs carbon dioxide from atmosphere – use energy to build a sugar in a cycle of chemical reactions Photosynthesis • The equation for the overall process is: 6CO2 + 6H2O  C6H12O6 + 6O2 granum (stack of thylakoids) 1s un li chloroplast gh t 6H2O thylakoid 6CO2 6O2 energy stroma (fluid outside the thylakoids) six-carbon sugar C6H12O6 Photosynthesis Left side: Light Dependent Rxs Right side: Light Independent Rxs Photosynthesis  Photosynthesis produces sugars that store chemical energy  Photosynthesis captures energy from sunlight to make sugars  Chlorophyll is a molecule that absorbs light energy  In plants, chlorophyll found in chloroplasts chloroplast leaf cell leaf Photosynthesis  Photosynthesis in plants occurs in chloroplasts • Photosynthesis takes place in two parts of chloroplasts – grana (thylakoids) grana (thylakoids) – stroma chloroplast stroma Photosynthesis • The light-dependent reactions capture energy from sunlight – – – – – take place in thylakoids water and sunlight are needed chlorophyll absorbs energy energy is transferred along thylakoid membrane for reactions oxygen is released Photosynthesis vs Cell Respiration Photosynthesis: From Light and CO2 to Glyceraldehyde 3-phosphate • The photosynthesis of one molecule of glyceraldehyde 3phosphate requires the capture of roughly 24 photons Synthesis of Glyceraldehyde-3 Phosphate (First Stage) • Three rounds of the Calvin cycle fixes three CO2 molecules and produces one molecule of 3phosphoglycerate Stoichiometry of Carbon Assimilation Overall Process: CO2 + ATP + NADPH → glyceraldehyde 3-phosphate + ADP + NADP+ + Pi • • Assimilation of carbons and phosphorous per cycle Inorganic phosphate must be replaced for sustained ATP synthesis in chloroplast Stoichiometry and Energetics • Fixation of three CO2 molecules yields one glyceraldehyde 3phosphate • Nine ATP molecules and six NADPH molecules are consumed Rubisco (ribulose 1,5-bisphosphate carboxylase) • Rubisco is a large Mg++-containing enzyme that makes a new carbon-carbon bond using CO2 as a substrate • Inefficient (kcat~ 3s-1) • Ru1,5P2 + CO2 –> 3-phosphoglycerate (3PG) Glyceraldehyde 3phosphate • Converted to starch in the chloroplast • Converted to sucrose for export • Recycled to ribulose 1,5bisphosphate Starch and Sucrose Biosynthesis • Excessive amounts of triose and monosaccharide phosphates are converted to alternative forms in the light • Liberates phosphate for ATP synthesis Starch Biosynthesis • Carbohydrate storage • Occurs in plastids • ADP-glucose substrate • Adds to reducing end (unlike glycogen synthesis) • α(1→4) glucose (amylose) with α(1→6) branches (amylopectin) Sucrose Biosynthesis • Carbohydrate transport • Occurs in cytoplasm • Fructose 6-phosphate & UDP-glucose • Joins reducing (anomeric) hydroxyls • Glucose(α1↔β2)Fructose Cellulose Biosynthesis • Cell wall structure • Occurs in cytoplasm and at plasma membrane • Lipid-linked carrier and membrane protein complex • UDP-glucose is generated from sucrose and UDP by sucrose synthase • UDP-glucose is substrate for cellulose synthase; adds glucose monomers to nonreducing end • Cellulose is β(1→4) linked glucose Glycolate Pathway • Salvage of 2-phosphoglycolate • Involves metabolite transport and enzymes in chloroplast, peroxisome and mitochondrion • Glycine decarboxylase is key enzyme • Process consumes O2 and evolves CO2 “Photorespiration” • Wastes energy and fixed carbon and nitrogen Glyceraldehyde 3phosphate • Converted to starch in the chloroplast • Converted to sucrose for export • Recycled to ribulose 1,5bisphosphate