Photosynthesis Light Dependent Reactions

PHOTOSYNTHESIS Stages of Photosynthesis Photosynthesis is a two-step process: • The light dependent reactions occur in the thylakoids and convert light energy into chemical energy (ATP + NADPH) • The light independent reactions occur in the stroma and use this chemical energy to make organic compounds Light Dependent Reactions Non-Cyclic Photophosphorylation • Chlorophyll in Photosystems I and II absorb light, which triggers the release of energised electrons (photoactivation) • The electrons from PS I reduce NADP (forms NADPH) • The electrons from PS II move through an electron transport chain before replacing the electrons from PS I • The transport chain produces ATP (photophosphorylation) • Electrons lost from PS II are replaced by water (photolysis) STROMA Cyclic Photophosphorylation • Only chlorophyll in Photosystem I are activated by light • The electrons move through an electron transport chain before returning to their original location (i.e. cyclic) • The transport chain produces ATP (photophosphorylation) • Cyclic photophosphorylation does not produce NADPH • Hence, while cyclic photophosphorylation can produce usable energy (ATP), it cannot produce organic molecules STROMA NADPH Light Light NADP Light ATP ATP e- e- e- e- e- e- H2O O2 H+ Photosystem II (P680) H+ THYLAKOID THYLAKOID Photosystem I (P700) Photosystem I (P700) Light Independent Reactions Step 1: Carbon Fixation • Rubisco catalyses the carboxylation of RuBP (requires CO2) • This forms two 3C compounds called GP Step 2: Reduction of GP • GP is phosphorylated by ATP and reduced by NADPH • This converts each GP molecule into a TP molecule Step 3: Regeneration of RuBP • One molecule of TP is used to form half a sugar (two complete cycles are needed to form a glucose molecule) • The remaining TP molecules are used to reform RuBP 3 × CO2 3 × RuBP ATP ADP + Pi 1 × TP ½ × sugar Step 1: Carbon Fixation Step 3: Regeneration of RUBP GP × 6 Step 2: Reduction of GP ATP ADP + Pi NADPH TP ×6 NADP+ + H+ Lollipop Experiment The light independent reactions were elucidated by Melvin Calvin (i.e. Calvin cycle) using a ‘lollipop’-shaped apparatus • Radioactive CO2 was incorporated to identify the different carbon compounds (involves chromatography and autoradiography)