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Chapter 08 Lecture and Animation Outline Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. See separate PowerPoint.

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Presentation on theme: "Chapter 08 Lecture and Animation Outline Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. See separate PowerPoint."— Presentation transcript:

1 Chapter 08 Lecture and Animation Outline 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.

2 Chapter 8 Photosynthesis 2

3 3 Photosynthesis Overview Energy for all life on Earth ultimately comes from photosynthesis 6CO 2 + 12H 2 O C 6 H 12 O 6 + 6H 2 O + 6O 2 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 © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vascular bundleStoma 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 CO 2

7 7 O2O2 Stroma Photosystem Thylakoid NADP + ADP + P i CO 2 Sunlight Photosystem Light-Dependent Reactions Calvin Cycle Organic molecules O2O2 ATP NADPH H2OH2O 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 (O 2 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 CO 2 ; 20ºC CO 2 limited Light Intensity (foot-candles) 5001000 150020002500 Increased Rate of Photosynthesis 0 Excess CO 2 ; 35ºC Insufficient CO 2 (0.01%); 20ºC Light limited 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 O 2 but accumulate sulfur –Proposed general formula for photosynthesis CO 2 + 2 H 2 A + light energy → (CH 2 O) + H 2 O + 2 A –Later researchers found O 2 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 that absorb 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 © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 400 nm Visible light 430 nm500 nm560 nm600 nm650 nm740 nm 1 nm0.001 nm10 nm1000 nm Increasing wavelength Increasing energy 0.01 cm1 cm1 m Radio wavesInfraredX-raysGamma rays 100 m UV light

13 13 Absorption spectrum When a 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) 400450500550600650700 Light Absorbtion low high carotenoids chlorophyll a chlorophyll b Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

15 Organisms have evolved 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 H2CH2C CH CH 2 CH 3 H H H C O CH CCH 3 CHCH 3 CH 2 CHCH 3 CH 2 CHCH 3 CH 3 O CO 2 CH 3 O NN NN Mg H H Chlorophyll a: = CH 3 Chlorophyll b: = CHO R R R H Porphyrin head H3CH3C H3CH3C CH 3 CH 2 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 –Carbon rings 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 Antenna complex –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 Also called 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 © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. e–e– Photon Photosystem Thylakoid membrane Chlorophyll molecule Electron acceptor Reaction center chlorophyll Thylakoid membrane Electron donor e–e–

23 Reaction center Transmembrane protein–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 © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Light e–e– – + – + Excited chlorophyll molecule Electron donor Electron acceptor Chlorophyll reduced Chlorophyll oxidized Donor oxidized Acceptor reduced e–e– e–e– e–e– e–e– e–e– e–e– e–e–

25 25 Light-Dependent Reactions 1.Primary photoevent –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 Capture of light energy

26 26 In sulfur bacteria, 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 © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Energy of electrons High Low e–e– Photon Photosystem Excited reaction center Electron acceptor Electron acceptor Reaction center (P 870 ) b-c 1 complex ATP e–e– e–e–

28 28 Chloroplasts have two connected photosystems Oxygenic photosynthesis Photosystem I (P 700 ) –Functions like sulfur bacteria Photosystem II (P 680 ) –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 I transfers 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/ b 6 -f complex

30 30 Noncyclic photophosphorylation Plants use 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 © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Energy of electrons Photon Excited reaction center Plastoquinone Plastocyanin Ferredoxin Photosystem II Photosystem I Photon b 6 -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. H2OH2O H+H+ PC Fd 2H + + 1 / 2 O 2 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– e–e– PQ e–e– NADP reductase NADPH e–e– 2. The electrons pass through the b 6 -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–e–

32 Two photosystems 32 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Rate of Photosynthesis low high Far-red light onBoth lights onRed light onOff Time Off

33 33 Photosystem II Resembles the reaction center of purple bacteria Core of 10 transmembrane protein subunits with electron transfer components and two P 680 chlorophyll molecules Reaction center differs from purple bacteria in that it also contains four manganese atoms –Essential for the oxidation of water b 6 -f complex –Proton pump embedded in thylakoid membrane

34 34 Photosystem I Reaction center consists of a core transmembrane complex consisting of 12 to 14 protein subunits with two bound P 700 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 © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Photosystem IIPhotosystem Ib 6 -f complex Stroma Plastoquinone Proton gradient PlastocyaninFerredoxin H+H+ H+H+ H+H+ H+H+ NADPH ATP ADP +NADP + NADPH NADP ATP ADP + P i Calvin Cycle Photon H2OH2O e–e– e–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 O 2 2. The b 6 -f complex receives electrons from PQ and passes them to plastocyanin (PC). This provides energy for the b 6 -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 P i 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 1/2O21/2O2 2H + Water-splitting enzyme Thylakoid space Antenna complex Thylakoid membrane Light-Dependent Reactions H+H+ H+H+ e–e– 22 22 2 2 2 2

36 Chemiosmosis Electrochemical gradient can 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 additional ATP 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 Named after Melvin Calvin (1911–1997) Also called C 3 photosynthesis Key step is attachment of CO 2 to RuBP to form PGA Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco

40 40 3 phases 1.Carbon fixation –RuBP + CO 2 → 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 © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 PiPi 12 NADP + 12 12 ADP NADPH NADP + ADP+PiPi Light-Dependent Reactions Calvin Cycle 6 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 (CO 2 ) 12 ATP 6 ADP 6 ATP Rubisco Calvin Cycle PiPi Ribulose 1,5-bisphosphate (5C) (RuBP) 3-phosphoglycerate (3C) (PGA) Glyceraldehyde 3-phosphate (3C) 2 molecules of Glucose and other sugars 12 NADPH ATP

42 42 Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.

43 43 Output of Calvin cycle 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 Photosynthesis uses 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 © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. O2O2 Heat ATP NADPH NADH ATP Sunlight Pyruvate CO 2 Glucose ADP + P i NAD + NADP + H2OH2O Photo- system II Photo- system I Electron Transport System ADP + P i ATP Calvin Cycle Krebs Cycle

46 46 Photorespiration Rubisco has 2 enzymatic activities –Carboxylation Addition of CO 2 to RuBP Favored under normal conditions –Photorespiration Oxidation of RuBP by the addition of O 2 Favored when stoma are closed in hot conditions Creates low-CO 2 and high-O 2 CO 2 and O 2 compete for the active site on RuBP

47 47 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Heat Stomata O2O2 O2O2 CO 2 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, O 2 builds up inside the leaves, and CO 2 cannot enter the leaves. Leaf epidermis H2OH2OH2OH2O

48 48 Types of photosynthesis C 3 –Plants that fix carbon using only C 3 photosynthesis (the Calvin cycle) C 4 and CAM –Add CO 2 to PEP to form 4 carbon molecule –Use PEP carboxylase –Greater affinity for CO 2, no oxidase activity –C 4 – spatial solution –CAM – temporal solution

49 49 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CO 2 RuBP 3PG (C 3 ) a. C 4 pathway Bundle-sheath cellMesophyll cell Stoma Vein G3P b. C 4 pathway Stoma Vein Mesophyll cell G3P CO 2 C4C4 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 C 4 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 CO 2 Carbon fixation then by rubisco and the Calvin cycle

51 51 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Oxaloacetate PyruvateMalate Glucose Malate Pyruvate + P i Mesophyll cell Phosphoenolpyruvate (PEP) Bundle-sheath cell Calvin Cycle AMP + PP i ATP CO 2

52 C 4 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 C 4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C 3 pathway alone 52

53 53 CAM plants Many succulent (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 CO 2 using PEP carboxylase during the night and store in vacuole

54 When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO 2 High levels of CO 2 drive the Calvin cycle and minimize photorespiration 54

55 55 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. night day CO 2 C4C4 G3P Calvin Cycle (inset): © 2011 Jessica Solomatenko/Getty Images RF

56 Compare C 4 and CAM Both use both C 3 and C 4 pathways C 4 – two pathways occur in different cells CAM – C 4 pathway at night and the C 3 pathway during the day 56


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