Photosynthesis Lecture Outline Overview Chloroplasts Pigment molecules The light reactions Calvin cycle and carbon fixation C4 and CAM metabolism The overall chemical reaction of photosynthesis is CO2 2 H2O light energy CH2O n H2O 2S Photosynthesis is made by two linked set of reactions Light dependent reactions It produces oxygen ATP and NADPH Light independent reactions It produces sugar from carbon dioxide The two set of reactions are linked by electrons Two linked processes of photosynthesis During the Calvin Cycle the electrons from NADPH and the potential energy of ATP is used to reduce CO2 to carbohydrate The resulting sugar is used in cellular respiration Plants oxidize sugars in their mitochondria The Structure of the Chloroplast Thylakoids are vesicle like structures Often the thylakoid are interconnected in stacks called grana singular granum Lumen is the internal space in the thylakoid Stroma is the fluid filled space between the thylakoids and the inner membrane Pigments are molecules capable of absorbing certain wavelengths of light Photosynthesis is divided Light dependent reactions Thylakoid Light independent reactions Stroma How Does Chlorophyll Capture Light Energy Light is a type of electromagnetic radiation energy Photosynthesis converts this energy into the C C and C H bonds of sugar Absorption Spectrum and Pigments Shorter wavelengths of electromagnetic radiation have more energy The structure of Chlorophyll Both chlorophyll a b have the same structures A long tail of isoprene subunits A head which is a large structure with magnesium atom in is center The tail keeps the molecule embedded in the thylakoid membrane The head is where the light is absorbed Photosystems In the thylakoid membrane between 200300 chlorophyll molecules and accessory pigments are grouped together in an array of proteins This complex is known as a photosystem Photosystems have two major elements An antenna complex The reaction center When light is absorbed electrons are excited The Three Fates of Excited Electrons in Photosynthetic Pigments What happens in the reaction center When the energy of the reaction reaches the reaction center of a photosystem excited electrons are transferred to a molecule that acts as an electron acceptor When this molecule becomes reduced absorption of light becomes permanent At this point we can say that electromagnetic energy has become chemical energy Pheophytin is similar to chlorophyll molecules but they lack the central magnesium atom This molecule acts as an electron acceptor When the electrons are excited in the reaction center pheophytin gets reduced and the reaction center is oxidized Electron transport chain Once pheophytin is reduced the electron is transferred to an electron transfer chain This ETC is similar in structure and function to the ECT studied in cellular respiration Photosystems in the thylakoid membrane Stroma Photosystem II Electron Transport Chain Reaction center H ee Antenna complex Photosystem I ATP Synthase H H H H Electron Carrier Molecule Plastocyanin Lumen Sun Light Photosystem II ADP Pi e e H ADP Pi ADP Pi Stroma ATP ATP H H ee H H e ee eH2O H2O ATP H H H H H H O O H H H H H H H H H H H H H Lumen Sun Light Photosystem I Stroma 0 0 NADPH H ee H e e H H H e e e e NADP Lumen Sun Light Photosystem I Photosystem II 0 0 ATP ATP ATP ATP H Stroma ATP ATP ATP NADPH H e e H ee H O O H e e e e NADP Lumen The ETC in PS II Pump Protons Photophosphorylation Production of ATP molecules using the energy released as light excited electrons flow through an electron transport chain during photosynthesis Involves the generation of a proton motive force during electron transport and is used to drive ATP synthesis Photosystem I Produces NADPH The Z Scheme Model Links Photosystem I and II Plastoquinine Plastocyanin Connects both systems Cyclic phosphorylation Instead of Photosystem I transfers electrons back to the electron transport chain in photosynthesis II This will increase ATP generation Cyclic phosphorylation coexists with the Z scheme and produces additional ATP Light Independent Reactions the Calvin Cycle Calvin Cycle Fixes Carbon RuBP ribulose bisphosphate RuBP Glyceraldehyde 3 phosphate G3P The Three Phases of the Calvin Cycle Fixation CO2 reacts with RuBP This fixes carbon into a more complex molecule Carbon fixation is the addition of a carbon dioxide to an organic compound Reduction 3 phosphoglycerate is phosphorylated by ATP and then reduced by an electron from NADP producing gluceraldehyde 3phosphate G3P Photorespiration Rubisco is a carboxylase This means that it adds CO2 to RuBP However It can also be an oxygenase adding O2 to RuBP These two reactions compete with each other When RuBP reacts with O2 it cannot react with CO2 which reduces the rate of CO2 fixation Photorespiration and Its Consequences Rubisco acts as an oxygenase if the CO2 levels are very low and the O2 levels are very high O2 levels become very high when stomata are closed to prevent water loss when the weather is hot and dry In some plants CO2 can be fixed by rubisco or by pep carboxylate They are located in different cell subtypes