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Photosynthesis: Life from Light and Air

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  1. Photosynthesis:LifefromLight andAir

  2. Energy Free energy NECESSARY for life Used to grow, maintain organization, reproduce Offsets increased entropy, maintains order Energy input must exceed free energy lost to entropy If more free energy is expended than acquired -> loss of mass -> eventual death of organism

  3. Energy All organisms capture and store (in organic molecules) free energy for use in biological processes Autotrophs (Auto = self) Produce organic energy storage molecules themselves Photoautotrophs (Photosynthesis) capture energy from sunlight Chemoautotrophs (Chemosynthesis) capture energy from inorganic molecules (hydrogen sulfide, ammonia) Heterotrophs (Hetero = different) Acquire organic energy storage molecules from other organisms

  4. Common Chemical “Themes” • Coupled reactions • An energetically unfavorable reaction (endergonic) can be driven (“powered”) by coupling it mechanistically with an energetically favorable one (exergonic) • The energy released by the exergonic rxn is used to drive the endergonic rxn, rather than being lost as heat • Analogy: instead of burning a can of fuel, couple it with a car engine to drive a car!

  5. Common Chemical “Themes” • Redox reactions (oxidation-reduction reactions) • Oxidation = loss of electrons. Reduction = gain in electrons. (Think charge. If you gain an electron, your charge is “reduced,” becomes more negative.)

  6. Common Chemical “Themes” • We can see coupling within redox reactions. • Ex: cellular respiration. Glucose is oxidized, electrons are transferred out as it’s converted to CO2. Oxygen is reduced, is gains those electrons and is converted to water. oxidation C6H12O6 + 6O2 -> 6CO2 + 6H2O reduction

  7. Common Chemical “Themes” • How is energy “moved” and “released” in photosynthesis and respiration? • Electron donors and receptors • Electron donor in a reaction = gives away electrons. • Electron receptor = acquires electrons. • Discussion: is an electron donor oxidized or reduced in the course of a reaction?

  8. Common Chemical “Themes” • Electron donors and acceptors • Electrons (sometimes via an atom/ion – phosphate acceptors are also electron acceptors!) can be thought of as our principal energy “units” • If you’ve accepted electrons, you now have options in forming or breaking new bonds • And remember: energy is the ability to do work. Chemical energy is the ability to change bond configuration! • Electron transfer = energy transfer

  9. DON’T FORGET THIS! • In both photosynthesis and respiration: • Electron transfer = Energy transfer! • Electron carriers = energy carriers! Electrons = Energy

  10. Discussion • So what does oxidation/reduction have to do with energy? How could a cell use a redox reaction to manage energy?

  11. Common Chemical “Themes” • This electron transfer = energy transfer also connects back to… • Making bonds requires free energy • Bonds store free energy • Breaking bonds releases free energy

  12. ATP Adenosine triphosphate is a basic energy transfer molecule. ATP = electron donor ADP = electron acceptor The phosphate groups are highly charged and electron-rich (i.e. energy rich), and when broken off by hydrolysis release large amounts of energy. (ΔG = -7.3 kcal/mole)

  13. O– O– O– O– O– O– O– O– P P P P P P P P –O –O –O O– O– O– –O –O –O O– O– O– –O –O O– O– O O O O O O O O How does ATP store energy? • Each negative Phosphate more difficult to add • a lot of stored energy in each bond • most energy stored in 3rd Pi • 3rd Pi is hardest group to keep bonded to molecule • Bonding of negative Pi groups is unstable • spring-loaded • Pi groups “pop” off easily & release energy AMP ADP ATP Instability of its P bonds makes ATP an excellent energy donor

  14. O– O– O– O– P P P P –O O– –O O– –O –O O– O– O O O O How does ATP transfer energy? • ATP  ADP • releases energy • This is ATP hydrolysis (being hydrolysis, it does consume a molecule of water) 7.3kcal/ mole + ADP ATP

  15. How ATP does work With the help of enzymes, the cell can couple the energy release of ATP hydrolysis with an endergonic process by transferring the 3rd phosphate group from ATP to another molecule. The recipient of the phosphate is phosphorylated and becomes a more reactive intermediate ATP is like an electron donor, but with phosphate

  16. + Pi ATP / ADP cycle ATP • Can’t store ATP • good energy donor, not good energy storage • too reactive • transfers Pi too easily • only short term energy storage • carbohydrates & fats are long term energy storage cellularrespiration 7.3 kcal/mole ADP A working muscle recycles over 10 million ATPs per second

  17. Regeneration of ATP An organism at work (alive) uses ATP continuously, but ATP is a renewable resource and can be regenerated from ADP by adding a phosphate (Pi) The energy required to phosphorylate ADP comes from the break down (catabolism) of molecules in the cell Very quick processA working muscle cell regenerates ALL its ATP in under 1 minute (10 million molecules/second)

  18. Discussion • Suppose I use ATP hydrolysis to power the synthesis of molecule B from two molecule As. Is this a coupled reaction? How? • Redox? Where’s the oxidation vs. the reduction? ATP + 2MolA -> ADP + MolB

  19. Photosynthesis • OVERALL GOAL: synthesize an energy-rich monosaccharide to serve as an energy-storage molecule • INTERMEDIATES • Light reactions: capture energy from sunlight, temporarily store in ATP and NADPH • Calvin cycle: use the energy from ATP and NADPH to power simple sugar anabolism • Takes place in chloroplasts

  20. Overview • We will do this in two parts. • 1) Make ATP • ADP and Pi are always floating around, but the enzyme that makes ATP needs protons and electrons in order for it to work • 2) Use energy from ATP to make glucose • Build glucose from CO2, protons, and electrons

  21. H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ outer membrane inner membrane stroma thylakoid granum chloroplast Plant structure ATP thylakoid • Chloroplasts • double membrane • stroma • fluid-filled interior • thylakoid sacs • stacks of grana • Thylakoid membrane contains • chlorophyll molecules • electron transport chain • ATP synthase • H+ gradient built up within thylakoid sac

  22. Step 1: capture light energy • Chlorophylls & other pigments • embedded in thylakoid membrane • arranged in a “photosystem” • collection of molecules

  23. A Look at Light • The spectrum of color V I B G Y O R

  24. Light: absorption spectra • Photosynthesis gets energy by absorbing wavelengths of light • chlorophyll a • absorbs best in red & blue wavelengths & least in green • accessory pigments with different structures absorb light of different wavelengths • chlorophyll b, carotenoids, xanthophylls Why areplants green?

  25. Photosystems of photosynthesis • 2 photosystems in thylakoid membrane • collections of chlorophyll molecules • act as light-gathering molecules • Photosystem II • chlorophyll a • P680 = absorbs 680nm wavelength red light • Photosystem I • chlorophyll b • P700 = absorbs 700nm wavelength red light reactioncenter antennapigments “a” - “earlier letter” absorbs the “earlier frequency” and comes first yet is labeled “second!” Argh!

  26. Photosystem II Photosystem I ETC of Photosynthesis chlorophyll a chlorophyll b

  27. e e e e H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ ETC of Photosynthesis sun sun O to Calvin Cycle split H2O ATP

  28. http://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynthesis.swfhttp://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynthesis.swf http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter39/photosynthetic_electron_transport_and_atp_synthesis.html ETC of Photosynthesis • Electron transport chain (ETC) uses light energy to produce • ATP & NADPH -> go to Calvin cycle • PSII absorbs light photon, which excites an electron in that molecule • excited electron passes from chlorophyll to “primary electron acceptor,” which passes it down ETC, providing energy for moving H+ into thylakoid interior for ATP synthesis • PSI absorbs light photon, which excites an electron in that molecule • Electron passes further down ETC, used with a proton to synthesize NADPH • NADP+ + H+ + e- -> NADPH • Now need to replace electrons in chlorophyll, so… • enzyme extracts electrons from H2O & supplies them to chlorophyll • This splits H2O • O combines with another O to form O2 • O2 released to atmosphere • H+ used to power ATP synthase proton pump • Enzyme which synthesizes ATP

  29. H+ H+ H+ H+ H+ H+ H+ H+ ADP + Pi H+ http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter39/proton_pump.html The ATP Synthase Enzyme Complex - Chemiosmosis photosynthesis respiration sunlight breakdown of C6H12O6 • moves the electrons • runs the pump • pumps the protons • builds the gradient • drives the flow of protons through ATP synthase • bonds Pi to ADP • generates the ATP ATP

  30. Experiment 1 Experiment 2 light energy light energy light energy    6CO2 6CO2 6CO2 + + + 6H2O 6H2O 6H2O + + + + + + 6O2 6O2 6O2 C6H12O6 C6H12O6 C6H12O6 Experimental evidence • Where did the O2 come from? • radioactive tracer = O18 Showed O2 came from H2O not CO2 = plants split H2O!

  31. Discussion Where did the energy come from? Where does it go? Trace its pathway. Where did the electrons come from? Where did the H2O come from? Where did the O2 come from? Where did the O2 go? Where did the H+ come from? Where did the ATP come from? Where did the NADPH come from?

  32. Photosynthesis: The Calvin Cycle LifefromAir

  33. + water + energy  glucose + oxygen carbon dioxide light energy  6CO2 + 6H2O + + 6O2 C6H12O6 Remember what it means to be a plant… • Need to produce all organic molecules necessary for growth • carbohydrates, lipids, proteins, nucleic acids • Need to store chemical energy (ATP) produced from light reactions • in a more stable form that can be moved around plant • saved for a rainy day

  34. Light reactions • Convert solar energy to chemical energy • ATP • NADPH • What can we do now? ATP  energy  reducing power/H   build stuff !! photosynthesis

  35. C6H12O6 NADP How is that helpful? • Want to make C6H12O6 • synthesis • How? From what? What raw materials are available? CO2 carbon fixation = taking inorganic carbon and “fixing” it in a biomolecule NADPH reduces CO2 NADP

  36. From CO2 C6H12O6 • CO2 has very little chemical energy • fully oxidized • C6H12O6contains a lot of chemical energy • highly reduced • Synthesis = endergonic process • put in a lot of energy • Reduction of CO2C6H12O6proceeds in many small uphill steps • each catalyzed by a specific enzyme • using energy stored in ATP & NADPH

  37. stroma thylakoid From Light reactions to Calvin cycle • Calvin cycle • chloroplast stroma • Need products of light reactions to drive synthesis reactions • ATP • NADPH ATP

  38. 5C 1C 3C 3C CO2 C 5C C 3 ATP C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C 3 ADP 3C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C 6C = = C C C 6 ATP 6 NADPH H H H H H H | | | | | | – – C C C 6 NADP 6 ADP C C Calvin cycle C 1. Carbon fixation 3. Regenerationof RuBP RuBP RuBisCo ribulose bisphosphate starch,sucrose,cellulose& more ribulose bisphosphate carboxylase used to makeglucose glyceraldehyde-3-P PGA G3P phosphoglycerate 2. Reduction

  39. To G3P and Beyond! • Glyceraldehyde-3-P • end product of Calvin cycle • energy rich 3 carbon sugar • G3Pis an important intermediate • G3P  glucose   carbohydrates   lipids  phospholipids,fats, waxes   amino acids  proteins   nucleic acids  DNA, RNA

  40. RuBisCo • Enzyme which fixes carbon from air • ribulose bisphosphate carboxylase • Very important enzyme to all life! • it makes life out of air! • definitely the most abundant enzyme It’s not easy being green!

  41. Accounting • The accounting is complicated • 3 turns of Calvin cycle = 1G3P • 3 CO2 1G3P (3C) • 6 turnsof Calvin cycle = 1C6H12O6(6C) • 6 CO2 1C6H12O6(6C) • 18 ATP+ 12 NADPH 1C6H12O6 • anyATPleft over from light reactions will be used elsewhere by the cell

  42. NADP ADP Photosynthesis summary • Light reactions • produced ATP • produced NADPH • consumed H2O • produced O2as byproduct • Calvin cycle • consumed CO2 • produced G3P (sugar) • regenerated ADP • regenerated NADP

  43. light energy  H2O + + + O2 ATP NADPH sunlight Light Reactions H2O • produces ATP • produces NADPH • releases O2 as a waste product Energy Building Reactions NADPH ATP O2

  44. CO2 + + + + ATP NADPH C6H12O6 ADP NADP Calvin Cycle • builds sugars • uses ATP & NADPH • recycles ADP & NADP • back to make more ATP & NADPH CO2 ADP NADP SugarBuilding Reactions NADPH ATP sugars

  45. light energy  CO2 + H2O + C6H12O6 + O2 sunlight Putting it all together H2O CO2 ADP NADP SugarBuilding Reactions Energy Building Reactions NADPH ATP sugars O2

  46. sun light energy  CO2 + H2O + + O2 C6H12O6 glucose H2O ATP energy  + O2 + CO2 + H2O C6H12O6 Energy cycle Photosynthesis plants CO2 O2 animals, plants Cellular Respiration ATP

  47. light energy  6CO2 + 6H2O + C6H12O6 + 6O2 Discussion • Where did the CO2 come from? • Where did the CO2 go? • Where did the H2O come from? • Where did the H2O go? • Where did the energy come from? • What’s the energy used for? • What will the C6H12O6be used for? • Where did the O2 come from? • Where will the O2 go? • What else is involved…not listed in this equation?