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UA BIOC 460 - Photosynthesis

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PowerPoint PresentationSlide 2Slide 3Net reaction of photosynthesis and carbon fixationThe Biosphere Experiment, then and now!1. What do the photosynthetic electron transport system and Calvin cycle accomplish for the cell?2. What are the overall net reactions of photosynthetic electron transport system and the Calvin cycle?3. What are the key enzymes in the photosynthetic electron transport system and the Calvin cycle?4. What are examples of the photosynthetic electron transport system and Calvin Cycle in real life?Overview of PhotosynthesisPay attention to the compartmentalization inside and outside of the chloroplast. The product of the carbon fixation is glyceraldehyde-3P (GAP) which is converted to hexose sugars for use as chemical energy at night.Peter Mitchell’s chemiosmotic theory was actually first proven using photosynthetic systems in which an artificial proton gradient was established using buffered solutions at different pH values.Slide 13Slide 14Slide 15Slide 16Slide 17Organization of Photosynthetic pigmentsLight harvesting pigment molecules transfer energy to nearby molecules which eventually results in photooxidationThe Z Scheme of Photosynthetic Electron TransportSlide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Functional organization of the cytochrome b6f complexThe light-driven Q cycle is responsible for translocation of 8 H+ from the stroma to the lumenSlide 31Functional organization of the PSI complexFerrodoxin NADP+ reductase plays a crucial role in converting redox energy into a useable form for the Calvin Cycle by generating NADPH. Since e- arrive in PSI one at a time, the FAD coenzyme must store on e- in a semiquinone chemical structure.Slide 34Photosynthesis:Overview of photosynthesis, conversion of light energy to redox energy, and the Z schemeBioc 460 Spring 2008 - Lecture 31 (Miesfeld)Solar power can provide a sustainable energy source for some areas in the worldParaquat is an inhibitor of PSI that was used to kill marijuana plantsAgricultural crops are the primary means for converting solar energy into chemical energy for life on earthParaquat•The energy from sunlight is used to initiate photooxidation reactions in light-absorbing pigments that convert light energy into chemical energy. •The light reactions of photosynthesis are similar to the electron transport system in that they require a proton impermeable membrane and a series of linked redox reactions to generate proton motive force used for ATP synthesis.•Photooxidation uses the oxidation of H2O to produce O2 in a process that provides electrons for photophosphorylation and the reduction of NADP+ to produce NADPH. •Chemical energy in the form of ATP and NADPH is used to convert CO2 to glyceraldehyde-3-P using enzymes in the Calvin Cycle pathway. Plants use sunlight during the day and aerobic respiration at night.Key Concepts in PhotosynthesisThe photosynthetic electron transport system is often referred to as the light reactions of photosynthesis, whereas, the Calvin cycle has been called the dark reactions.However, the term "dark reactions" can be misleading because the Calvin cycle is most active in the light when ATP and NADPH levels are high.Net reaction of photosynthesis and carbon fixation The O2 generation is the result of H2O oxidation, whereas, the CO2 is used to synthesize carbohydrate (CH2O) It takes 2 H2O to make an O2 and six CO2 molecules are required for the synthesis of each molecule of glucose, therefore:ΔGº' for this reaction is +2868 kJ/mol!This is overcome by the energy potential stored in the products of photosynthetic electron transport, namely, ATP and NADPH. H2O + CO2 ─(light energy)→ (CH2O) + O26 H2O + 6 CO2 ─(light energy)→ C6H12O6 + 6 O2The Biosphere Experiment, then and now!Biosphere 2 did not actually work very well because CO2 levels built up inside the sealed environment and periodic CO2 removal was required. The first “Biospherian” came out after ~1 year.Joseph Priestly - 1772Biosphere 2 - 19911. What do the photosynthetic electron transport system and Calvin cycle accomplish for the cell?•The photosynthetic electron transport system converts light energy into redox energy which is used to generate ATP by chemiosmosis and reduce NADP+ to form NADPH. •Calvin cycle enzymes use energy available from ATP and NADPH to reduce CO2 to form glyceraldehyde-3-P, a three carbon carbohydrate used to synthesize glucose. •Photosynthetic cells use the carbohydrate produced by the Calvin cycle as a chemical energy source for mitochondrial respiration in the dark. Photosynthetic organisms are autotrophs because they derive energy from light rather than from organic materials (as food).2. What are the overall net reactions of photosynthetic electron transport system and the Calvin cycle?Photosynthetic electron transport system (production of ATP and O2):2 H2O + 8 photons + 2 NADP+ + ~3 ADP + ~3 Pi → O2 + 2 NADPH + ~3 ATPCalvin cycle (six turns of cycle yields glucose):6 CO2 + 12 NADPH + 18 ATP + 12 H2O → Glucose + 12 NADP+ + 18 ADP + 18 Pi3. What are the key enzymes in the photosynthetic electron transport system and the Calvin cycle?Protein components of the photosynthetic electron transport system – three protein complexes are required for the oxidation of H2O and reduction of NADP+; photosystem II (P680 reaction center), cytochrome b6f (proton pump) and photosystem I (P700 reaction center).Chloroplast ATP synthase – enzyme responsible for the process of photophosphorylation which converts proton-motive force into net ATP synthesis; this enzyme is very similar to mitochondrial ATP synthase.Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) - is responsible for CO2 fixation in the first step of the Calvin cycle. Rubisco activity is maximal in the light when stromal pH is ~8 and Mg2+ levels are elevated due to proton pumping.4. What are examples of the photosynthetic electron transport system and Calvin Cycle in real life?DCMU (dichlorophenyl dimethylurea) is a broad spectrum herbicide that functions by blocking electron flow through photosystem II and is used to reduce weeds in non-crop areas. Another herbicide, paraquat, prevents reduction of NADP+ by accepting electrons from intermediate reductants in photosystem I. PSIIPSIOverview of Photosynthesis Light energy is used to oxidize 2 H2O which releases 4e- and 4H+The photosynthetic electron transport system and photophosphorylation work togethe to generate ATP and


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UA BIOC 460 - Photosynthesis

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