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Biology 201 Chapter 8
- 1. Chapter 08 Lecture and AnimationOutline Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes and animations. To run the animations you must be in Slideshow View. Use the buttons on the animation to play, pause, and turn audio/text on or off. Please Note: Once you have used any of the animation functions (such as Play or Pause), you must first click on the slide’s background before you can advance to the next slide.
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- 3. 3 Photosynthesis Overview • Energyfor all life on Earth ultimately comes from photosynthesis 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2 • Oxygenic photosynthesis is carried out by – Cyanobacteria – 7 groups of algae – All land plants – chloroplasts
- 4. Chloroplast • Thylakoid membrane– internal membrane – Contains chlorophyll and other photosynthetic pigments – Pigments clustered into photosystems • Grana – stacks of flattened sacs of thylakoid membrane • Stroma lamella – connect grana • Stroma – semiliquid surrounding thylakoid membranes 4
- 5. 5 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Vascular bundle Stoma Cuticle Epidermis Mesophyll Chloroplast Inner membrane Outer membrane Cell wall 1.58 mm Vacuole Courtesy Dr. Kenneth Miller, Brown University
- 6. 6 Stages • Light-dependent reactions –Require light 1.Capture energy from sunlight 2.Make ATP and reduce NADP+ to NADPH • Carbon fixation reactions or light- independent reactions – Does not require light 3.Use ATP and NADPH to synthesize organic molecules from CO2
- 7. 7 O2 Stroma Photosystem Thylakoid NADP+ ADP + Pi CO2 Sunlight Photosystem Light-Dependent Reactions Calvin Cycle Organic molecules O2 ATPNADPH H2O Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 8. 8 Discovery of Photosynthesis •Jan Baptista van Helmont (1580–1644) – Demonstrated that the substance of the plant was not produced only from the soil • Joseph Priestly (1733–1804) – Living vegetation adds something to the air • Jan Ingenhousz (1730–1799) – Proposed plants carry out a process that uses sunlight to split carbon dioxide into carbon and oxygen (O2 gas)
- 9. • F.F. Blackman(1866– 1947) – Came to the startling conclusion that photosynthesis is in fact a multistage process, only one portion of which uses light directly – Light versus dark reactions – Enzymes involved 9 Maximum rate Temperature limited Excess CO2; 20ºC CO2 limited Light Intensity (foot-candles) 500 1000 1500 2000 2500 IncreasedRateofPhotosynthesis 0 Excess CO2; 35ºC Insufficient CO2 (0.01%); 20ºC Lightlim ited Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 10. 10 • C. B.van Niel (1897–1985) – Found purple sulfur bacteria do not release O2 but accumulate sulfur – Proposed general formula for photosynthesis • CO2 + 2 H2A + light energy → (CH2O) + H2O + 2 A – Later researchers found O2 produced comes from water • Robin Hill (1899–1991) – Demonstrated Niel was right that light energy could be harvested and used in a reduction reaction
- 11. 11 Pigments • Molecules thatabsorb light energy in the visible range • Light is a form of energy • Photon – particle of light – Acts as a discrete bundle of energy – Energy content of a photon is inversely proportional to the wavelength of the light • Photoelectric effect – removal of an electron from a molecule by light
- 12. 12 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 400 nm Visible light 430 nm 500 nm 560 nm 600 nm 650 nm 740 nm 1 nm0.001 nm 10 nm 1000 nm Increasing wavelength Increasing energy 0.01 cm 1 cm 1 m Radio wavesInfraredX-raysGamma rays 100 m UV light
- 13. 13 Absorption spectrum • Whena photon strikes a molecule, its energy is either – Lost as heat – Absorbed by the electrons of the molecule • Boosts electrons into higher energy level • Absorption spectrum – range and efficiency of photons molecule is capable of absorbing
- 14. 14 Wavelength (nm) 400 450500 550 600 650 700 Light Absorbtion low high carotenoids chlorophyll a chlorophyll b Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 15. • Organisms haveevolved a variety of different pigments • Only two general types are used in green plant photosynthesis – Chlorophylls – Carotenoids • In some organisms, other molecules also absorb light energy 15 Pigments in Photosynthesis
- 16. Chlorophylls • Chlorophyll a –Main pigment in plants and cyanobacteria – Only pigment that can act directly to convert light energy to chemical energy – Absorbs violet-blue and red light • Chlorophyll b – Accessory pigment or secondary pigment absorbing light wavelengths that chlorophyll a does not absorb 16
- 17. • Structure of chlorophyll •porphyrin ring – Complex ring structure with alternating double and single bonds – Magnesium ion at the center of the ring • Photons excite electrons in the ring • Electrons are shuttled away from the ring 17 H2C CH CH2CH3 H H H C O CH CCH3 CHCH3 CH2 CH2 CH2 CHCH3 CH2 CH2 CH2 CHCH3 CH3 O CO2CH3 O N N N N Mg H H Chlorophyll a: = CH3 Chlorophyll b: = CHO R R R H Porphyrin head H3C H3C CH3 CH2 CH2 CH2 CH2 CH2 CH2 Hydrocarbon tail Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 18. • Action spectrum –Relative effectiveness of different wavelengths of light in promoting photosynthesis – Corresponds to the absorption spectrum for chlorophylls 18 Light Absorbtion low high Oxygen-seeking bacteria Filament of green algae Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 19. • Carotenoids – Carbonrings linked to chains with alternating single and double bonds – Can absorb photons with a wide range of energies – Also scavenge free radicals – antioxidant • Protective role • Phycobiloproteins – Important in low-light ocean areas 19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Oak leaf in summer Oak leaf in autumn © Eric Soder/pixsource.com
- 20. 20 Photosystem Organization • Antennacomplex – Hundreds of accessory pigment molecules – Gather photons and feed the captured light energy to the reaction center • Reaction center – 1 or more chlorophyll a molecules – Passes excited electrons out of the photosystem
- 21. Antenna complex • Alsocalled light-harvesting complex • Captures photons from sunlight and channels them to the reaction center chlorophylls • In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins 21
- 22. 22 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. e– Photon Photosystem Thylakoid membrane Chlorophyll molecule Electron acceptor Reaction center chlorophyll Thylakoid membrane Electron donor e–
- 23. Reaction center • Transmembraneprotein–pigment complex • When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level • Light-energized electron can be transferred to the primary electron acceptor, reducing it • Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule 23
- 24. 24 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Light e– –+–+ Excited chlorophyll molecule Electron donor Electron acceptor Chlorophyll reduced Chlorophyll oxidized Donor oxidized Acceptor reduced e– e– e– e– e– e– e–
- 25. 25 Light-Dependent Reactions 1. Primaryphotoevent – Photon of light is captured by a pigment molecule 2. Charge separation – Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule 3. Electron transport – Electrons move through carriers to reduce NADP+ 4. Chemiosmosis – Produces ATP Captureoflightenergy
- 26. 26 • In sulfurbacteria, only one photosystem is used • Generates ATP via electron transport • Anoxygenic photosynthesis • Excited electron passed to electron transport chain • Generates a proton gradient for ATP synthesis Cyclic photophosphorylation
- 27. 27 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Energyofelectrons High Low e– Photon Photosystem Excited reaction center Electron acceptor Electron acceptor Reaction center (P870) b-c1 complex ATPe– e–
- 28. 28 Chloroplasts have twoconnected photosystems • Oxygenic photosynthesis • Photosystem I (P700) – Functions like sulfur bacteria • Photosystem II (P680) – Can generate an oxidation potential high enough to oxidize water • Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH
- 29. 29 • Photosystem Itransfers electrons ultimately to NADP+ , producing NADPH • Electrons lost from photosystem I are replaced by electrons from photosystem II • Photosystem II oxidizes water to replace the electrons transferred to photosystem I • 2 photosystems connected by cytochrome/ b6-f complex
- 30. 30 Noncyclic photophosphorylation • Plantsuse photosystems II and I in series to produce both ATP and NADPH • Path of electrons not a circle • Photosystems replenished with electrons obtained by splitting water • Z diagram
- 31. 31 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Energyofelectrons Photon Excited reaction center Excited reaction center Plastoquinone Plastocyanin Ferredoxin Photosystem II Photosystem I Photon b6-f complex 3. A pair of chlorophylls in the reaction center absorb two photons. This excites two electrons that are passed to NADP+ , reducing it to NADPH. Electron transport from photosystem II replaces these electrons. H2O H+ PC Fd 2H+ + 1 /2O2 NADP+ + H+ 2 2 2 2 2 1. A pair of chlorophylls in the reaction center absorb two photons of light. This excites two electrons that are transferred to plastoquinone (PQ). Loss of electrons from the reaction center produces an oxidation potential capable of oxidizing water. Reaction center Proton gradient formed for ATP synthesis Reaction center e– e– PQ e– NADP reductase NADPH e– 2. The electrons pass through the b6-f complex, which uses the energy released to pump protons across the thylakoid membrane. The proton gradient is used to produce ATP by chemiosmosis. e–
- 32. Two photosystems 32 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RateofPhotosynthesis low high Far-red light on Both lights onRed light onOff Off Time Off
- 33. 33 Photosystem II • Resemblesthe reaction center of purple bacteria • Core of 10 transmembrane protein subunits with electron transfer components and two P680 chlorophyll molecules • Reaction center differs from purple bacteria in that it also contains four manganese atoms – Essential for the oxidation of water • b6-f complex – Proton pump embedded in thylakoid membrane
- 34. 34 Photosystem I • Reactioncenter consists of a core transmembrane complex consisting of 12 to 14 protein subunits with two bound P700 chlorophyll molecules • Photosystem I accepts an electron from plastocyanin into the “hole” created by the exit of a light-energized electron • Passes electrons to NADP+ to form NADPH
- 35. 35 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Photosystem II Photosystem Ib6-f complex Stroma Plastoquinone Proton gradientPlastocyanin Ferredoxin H+ H+ H+ H+ NADPH ATP ADP + NADP+ NADPHNADP ATPADP + Pi Calvin Cycle Photon Photon H2O e– e– e– Fd PC PQ 1. Photosystem II absorbs photons, exciting electrons that are passed to plastoquinone (PQ). Electrons lost from photosystem II are replaced by the oxidation of water, producing O2 2. The b6-f complex receives electrons from PQ and passes them to plastocyanin (PC). This provides energy for the b6-f complex to pump protons into the thylakoid. 3. Photosystem I absorbs photons, exciting electrons that are passed through a carrier to reduce NADP+ to NADPH. These electrons are replaced by electron transport from photosystem II. 4. ATP synthase uses the proton gradient to synthesize ATP from ADP and Pi enzyme acts as a channel for protons to diffuse back into the stroma using this energy to drive the synthesis of ATP. NADP reductase ATP synthase 1 /2O2 2H+ Water-splitting enzyme Thylakoid space Antenna complexThylakoid membrane Light-Dependent Reactions H+ H+ e– 22 22 22 22
- 36. Chemiosmosis • Electrochemical gradientcan be used to synthesize ATP • Chloroplast has ATP synthase enzymes in the thylakoid membrane – Allows protons back into stroma • Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions 36
- 37. Production of additionalATP • Noncyclic photophosphorylation generates – NADPH – ATP • Building organic molecules takes more energy than that alone • Cyclic photophosphorylation used to produce additional ATP – Short-circuit photosystem I to make a larger proton gradient to make more ATP 37
- 38. 38 Carbon Fixation –Calvin Cycle • To build carbohydrates cells use • Energy – ATP from light-dependent reactions – Cyclic and noncyclic photophosphorylation – Drives endergonic reaction • Reduction potential – NADPH from photosystem I – Source of protons and energetic electrons
- 39. 39 Calvin cycle • Namedafter Melvin Calvin (1911–1997) • Also called C3 photosynthesis • Key step is attachment of CO2 to RuBP to form PGA • Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco
- 40. 40 3 phases 1. Carbonfixation – RuBP + CO2 → PGA 2. Reduction – PGA is reduced to G3P 3. Regeneration of RuBP – PGA is used to regenerate RuBP • 3 turns incorporate enough carbon to produce a new G3P • 6 turns incorporate enough carbon for 1 glucose
- 41. 41 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 Pi 12 NADP+ 12 12 ADP NADPHNADP+ ADP+ Pi Light-Dependent Reactions Calvin Cycle 6 molecules of 12 molecules of 12 molecules of 1,3-bisphosphoglycerate (3C) 12 molecules of Glyceraldehyde 3-phosphate (3C) (G3P) 10 molecules of Glyceraldehyde 3-phosphate (3C) (G3P) Stroma of chloroplast 6 molecules of Carbon dioxide (CO2) 12 ATP 6 ADP 6 ATP Rubisco Calvin Cycle Pi Ribulose 1,5-bisphosphate (5C) (RuBP) 3-phosphoglycerate (3C) (PGA) Glyceraldehyde 3-phosphate (3C) 2 molecules of Glucose and other sugars 12 NADPH ATP
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- 43. 43 Output of Calvincycle • Glucose is not a direct product of the Calvin cycle • G3P is a 3 carbon sugar – Used to form sucrose • Major transport sugar in plants • Disaccharide made of fructose and glucose – Used to make starch • Insoluble glucose polymer • Stored for later use
- 44. 44 Energy cycle • Photosynthesisuses the products of respiration as starting substrates • Respiration uses the products of photosynthesis as starting substrates • Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse • Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria
- 45. 45 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. O2 Heat ATP NADPH NADH ATP Sunlight Pyruvate CO2 Glucose ADP + Pi NAD+ NADP+ H2O Photo- system II Photo- system I Electron Transport System ADP + Pi ADP + Pi ATP ATP Calvin Cycle Krebs Cycle
- 46. 46 Photorespiration • Rubisco has2 enzymatic activities – Carboxylation • Addition of CO2 to RuBP • Favored under normal conditions – Photorespiration • Oxidation of RuBP by the addition of O2 • Favored when stoma are closed in hot conditions • Creates low-CO2 and high-O2 • CO2 and O2 compete for the active site on RuBP
- 47. 47 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Heat Stomata O2O2 CO2 CO2 Under hot, arid conditions, leaves lose water by evaporation through openings in the leaves called stomata. The stomata close to conserve water but as a result, O2 builds up inside the leaves, and CO2 cannot enter the leaves. Leaf epidermis H2OH2O
- 48. 48 Types of photosynthesis •C3 – Plants that fix carbon using only C3 photosynthesis (the Calvin cycle) • C4 and CAM – Add CO2 to PEP to form 4 carbon molecule – Use PEP carboxylase – Greater affinity for CO2, no oxidase activity – C4 – spatial solution – CAM – temporal solution
- 49. 49 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. CO2 RuBP 3PG (C3) a. C4 pathway Bundle-sheath cellMesophyll cell Stoma Vein G3P b. C4 pathway Stoma Vein Mesophyll cell G3P CO2 CO2 C4 Bundle- sheath cell Mesophyll cell Bundle- sheath cell Calvin Cycle Mesophyll cell Calvin Cycle a: © John Shaw/Photo Researchers, Inc. b: © Joseph Nettis/National Audubon Society Collection/Photo Researchers, Inc.
- 50. 50 C4 plants • Corn,sugarcane, sorghum, and a number of other grasses • Initially fix carbon using PEP carboxylase in mesophyll cells • Produces oxaloacetate, converted to malate, transported to bundle-sheath cells • Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2 • Carbon fixation then by rubisco and the Calvin cycle
- 51. 51 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Oxaloacetate Pyruvate Malate Glucose MalatePyruvate + Pi Mesophyll cell Phosphoenolpyruvate (PEP) Bundle-sheath cell Calvin Cycle AMP + PPi ATP CO2 CO2
- 52. • C4 pathway,although it overcomes the problems of photorespiration, does have a cost • To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone • C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone 52
- 53. 53 CAM plants • Manysucculent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups • Stomata open during the night and close during the day – Reverse of that in most plants • Fix CO2 using PEP carboxylase during the night and store in vacuole
- 54. • When stomataclosed during the day, organic acids are decarboxylated to yield high levels of CO2 • High levels of CO2 drive the Calvin cycle and minimize photorespiration 54
- 55. 55 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. night day CO2 CO2 C4 G3P Calvin Cycle (inset): © 2011 Jessica Solomatenko/Getty Images RF
- 56. Compare C4 andCAM • Both use both C3 and C4 pathways • C4 – two pathways occur in different cells • CAM – C4 pathway at night and the C3 pathway during the day 56