1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 10 Photosynthesis

2. Overview: The Process That Feeds the Biosphere Photosynthesis converts solar energy to chemical energy Autotrophs- producers for the biosphere; make organic molecules from CO 2 & other inorganic molecules. They don’t eat anything of other organisms Most plants are photoautotrophs- use solar energy to make organic molecules from H 2 O and CO 2 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

3. Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes These organisms feed not only themselves but also most of the living world BioFlix: Photosynthesis BioFlix: Photosynthesis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. Fig. 10-2 (a) Plants (c) Unicellular protist 10 µm 1.5 µm 40 µm (d) Cyanobacteria (e) Purple sulfur bacteria (b) Multicellular alga

5. Heterotrophs get organic material from other organisms Heterotrophs- consumers in the biosphere Almost all heterotrophs, including humans, depend on photoautotrophs for food and O 2 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

6. Concept 10.1: Photosynthesis converts light energy to the chemical energy of food Leaves- major location for photosynthesis Chlorophyll- the green pigment in chloroplasts – Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast Stomata- microscopic pores through which CO 2 enters & O 2 exits leaf Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

7. Chloroplasts- structurally similar to(evolved from?) photosynthetic bacteria Chloroplasts are found mainly in the mesophyll – Mesophyll-interior tissue of leaf – typical mesophyll cell has 30–40 chloroplasts Thylakoids- contain chlorophyll – connected sacs in the chloroplast – may be stacked in columns called grana Chloroplasts also contain stroma, a dense fluid Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Chloroplasts: The Sites of Photosynthesis in Plants

8. Fig. 10-3 Leaf cross section Vein Mesophyll Stomata CO 2 O2O2 Chloroplast Mesophyll cell Outer membrane Intermembrane space 5 µm Inner membrane Thylakoid space Thylakoid Granum Stroma 1 µm

9. Tracking Atoms Through Photosynthesis: Scientific Inquiry Photosynthesis can be summarized in the equation: Photosynthesis is a redox process in which H 2 O is oxidized and C O 2 is reduced 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

10. The Splitting of Water Chloroplasts split H 2 O into hydrogen and oxygen, incorporating electrons from hydrogen into sugar molecules Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Reactants: 6 CO 2 Products: 12 H 2 O 6 O 2 6 H 2 O C 6 H 12 O 6 Fig. 10-4

11. The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The light reactions (in the thylakoids): – Split H 2 O – Release O 2 – Reduce NADP + to NADPH – Generate ATP from ADP & P i by photophosphorylation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

12. The Calvin cycle (in the stroma) forms sugar from CO 2, using ATP and NADPH – begins with carbon fixation – carbon fixation- incorporating CO 2 into organic molecules Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

13. Light Fig. 10-5-4 H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2 [CH 2 O] (sugar)

14. Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH Chloroplasts are solar-powered chemical factories Their thylakoids transform light energy into the chemical energy of ATP and NADPH Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

15. The Nature of Sunlight Light - electromagnetic energy, also called electromagnetic radiation – light travels in rhythmic waves Wavelength- the distance between crests of waves Wavelength determines the type of electromagnetic energy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

16. Electromagnetic spectrum- the entire range of electromagnetic energy, or radiation Visible light- wavelengths (including those that drive photosynthesis) that produce colors we see Light also behaves as if it is made of discrete particles, called photons Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

17. UV Fig. 10-6 Visible light Infrared Micro- waves Radio waves X-rays Gamma rays 10 3 m 1 m (10 9 nm) 10 6 nm 10 3 nm 1 nm 10 –3 nm 10 –5 nm 380 450 500 550 600 650 700 750 nm Longer wavelength Lower energyHigher energy Shorter wavelength

18. Photosynthetic Pigments: The Light Receptors Pigments -substances that absorb visible light – Different pigments absorb different wavelengths Wavelengths not absorbed are reflected or transmitted Leaves appear green because chlorophyll reflects and transmits green light Animation: Light and Pigments Animation: Light and Pigments Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

19. Fig. 10-7 Reflected light Absorbed light Light Chloroplast Transmitted light Granum

20. Spectrophotometer- measures a pigment’s ability to absorb various wavelengths – Sends light through pigments and measures the fraction of light transmitted per wavelength Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

21. Fig. 10-8 Galvanometer Slit moves to pass light of selected wavelength White light Green light Blue light The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. Refracting prism Photoelectric tube Chlorophyll solution TECHNIQUE 1 23 4

22. Absorption spectrum -graph plotting a pigment’s light absorption versus wavelength Chlorophyll a spectrum indicates violet-blue and red light work best for photosynthesis An action spectrum shows the effectiveness of different wavelengths in driving a process Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

23. Fig. 10-9 Wavelength of light (nm) (b) Action spectrum (a) Absorption spectra (c) Engelmann’s experiment Aerobic bacteria RESULTS Rate of photosynthesis (measured by O 2 release) Absorption of light by chloroplast pigments Filament of alga Chloro- phyll a Chlorophyll b Carotenoids 500 400 600700 600 500 400

24. Theodor W. Engelmann first demonstrated action spectrum of photosynthesis in 1883 He exposed different segments of filamentous alga to different wavelengths – Areas receiving wavelengths favorable to photosynthesis produced excess O 2 – He used the growth of aerobic bacteria clustered along the alga as a measure of O 2 production Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

25. Chlorophyll a is the main photosynthetic pigment Accessory pigments, like chlorophyll b, broaden light spectrum used for photosynthesis Accessory pigments called carotenoids absorb excessive light to prevent damage to chlorophyll Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

26. Fig. 10-10 Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center in chlorophyll a CH 3 Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown CHO in chlorophyll b

27. Excitation of Chlorophyll by Light A pigment absorbs light & goes from stable ground state to excited, unstable state When excited electrons fall back to ground state, photons are given off causing fluorescence If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

28. Fig. 10-11 (a) Excitation of isolated chlorophyll molecule Heat Excited state (b) Fluorescence Photon Ground state Photon (fluorescence) Energy of electron e–e– Chlorophyll molecule

29. A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes A photosystem is a reaction-center complex (a type of protein complex) surrounded by light- harvesting complexes Light-harvesting complexes (pigment molecules bound to proteins) funnel photon energy to reaction center Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

30. Primary electron acceptor in the reaction center accepts excited electron from chlorophyll a Solar-powered transfer of an electron from a chlorophyll a molecule to the primary acceptor is first step of the light reactions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

31. Fig. 10-12 THYLAKOID SPACE (INTERIOR OF THYLAKOID) STROMA e–e– Pigment molecules Photon Transfer of energy Special pair of chlorophyll a molecules Thylakoid membrane Photosystem Primary electron acceptor Reaction-center complex Light-harvesting complexes

32. Two different photosystems are in the thylakoid membrane Photosystem II (PS II) functions first (numbers reflect order of discovery – best at absorbing a wavelength of 680 nm – The reaction-center chlorophyll a of PS II is called P680 Photosystem I (PS I) – best at absorbing a wavelength of 700 nm – The reaction-center chlorophyll a of PS I is called P700 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

33. Linear Electron Flow During light reactions, there are two possible routes for electron flow: cyclic and linear Linear electron flow – primary pathway – involves both photosystems – produces ATP and NADPH using light energy Photon hits a pigment & its energy is passed by pigment molecules until it excites P680 P680 ‘s excited electron is transferred to a primary electron acceptor Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

34. Pigment molecules Light P680 e–e– 2 1 Fig. 10-13-1 Photosystem II (PS II) Primary acceptor

35. Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 Fig. 10-13-2 Photosystem II (PS II) P680 + (P680 missing an electron)- very strong oxidizing agent H 2 O is split by enzymes -Electrons from H 2 O are transferred from hydrogen atoms to P680 + reducing it to P680 -O 2 is released as a by- product

36. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane Diffusion of H + (protons) across the membrane drives ATP synthesis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

37. Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Fig. 10-13-3 Photosystem II (PS II)

38. In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor P700 + (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

39. Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Photosystem I (PS I) Light Primary acceptor e–e– P700 6 Fig. 10-13-4 Photosystem II (PS II)

40. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd) The electrons then transfer to NADP + and reduce it to NADPH The electrons of NADPH are available for the reactions of the Calvin cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

41. Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Photosystem I (PS I) Light Primary acceptor e–e– P700 6 Fd Electron transport chain NADP + reductase NADP + + H + NADPH 8 7 e–e– e–e– 6 Fig. 10-13-5 Photosystem II (PS II)

42. Fig. 10-14 Mill makes ATP e–e– NADPH Photon e–e– e–e– e–e– e–e– e–e– ATP Photosystem IIPhotosystem I e–e–

43. Cyclic Electron Flow Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH – It generates surplus ATP, for needs of the Calvin cycle – It may protect cells from light-induced damage – It is thought to have evolved before linear electron flow – Some organisms (ex. purple sulfur bacteria) have PS I but not PS II Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

44. Fig. 10-15 ATP Photosystem II Photosystem I Primary acceptor Pq Cytochrome complex Fd Pc Primary acceptor Fd NADP + reductase NADPH NADP + + H +

45. A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy – Mitochondria transfer chemical energy from food to ATP – Chloroplasts transform light energy into chemical energy (ATP) Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but shows similarities Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

46. In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

47. Fig. 10-16 Key Mitochondrion Chloroplast CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE Intermembrane space Inner membrane Electron transport chain H+H+ Diffusion Matrix Higher [H + ] Lower [H + ] Stroma ATP synthase ADP + P i H+H+ ATP Thylakoid space Thylakoid membrane

48. ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H 2 O to NADPH Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

49. Fig. 10-17 Light Fd Cytochrome complex ADP + i H+H+ ATP P synthase To Calvin Cycle STROMA (low H + concentration) Thylakoid membrane THYLAKOID SPACE (high H + concentration) STROMA (low H + concentration) Photosystem II Photosystem I 4 H + Pq Pc Light NADP + reductase NADP + + H + NADPH +2 H + H2OH2O O2O2 e–e– e–e– 1/21/2 1 2 3

50. Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO 2 to sugar Calvin cycle regenerates its starting material after molecules enter and leave the cycle The cycle builds sugar from smaller molecules using ATP and the reducing power of electrons from NADPH Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

51. Carbon enters the cycle as CO 2 and leaves as a sugar- glyceraldehyde-3-phospate (G3P) To synthesize 1 G3P, the cycle must occur three times, fixing 3 molecules of CO 2 Three phases of Calvin cycle: – Carbon fixation (catalyzed by rubisco) – Reduction – Regeneration of the CO 2 acceptor (RuBP) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

52. Fig. 10-18-3 Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Short-lived intermediate Phase 1: Carbon fixation (Entering one at a time) Rubisco Input CO 2 P 3 6 3 3 P P P P ATP 6 6 ADP P P 6 1,3-Bisphosphoglycerate 6 P P 6 6 6 NADP + NADPH i Phase 2: Reduction Glyceraldehyde-3-phosphate (G3P) 1 P Output G3P (a sugar) Glucose and other organic compounds Calvin Cycle 3 3 ADP ATP 5 P Phase 3: Regeneration of the CO 2 acceptor (RuBP) G3P

53. Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates Dehydration- a problem for plants, may require trade-offs with other metabolic processes, especially photosynthesis On hot, dry days, plants close stomata; conserves H 2 O but limits photosynthesis Closing stomata reduces CO 2 access and causes O 2 build up These conditions favor a seemingly wasteful process called photorespiration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

54. Photorespiration: An Evolutionary Relic? In most plants (C 3 plants), initial fixation of CO 2, via rubisco, forms a three-carbon compound In photorespiration, rubisco adds O 2 instead of CO 2 in the Calvin cycle Photorespiration – consumes O 2 & organic fuel – releases CO 2 without producing ATP or sugar Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

55. Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O 2 and more CO 2 Photorespiration limits harmful light reaction products (they build up in the absence of the Calvin cycle) Photorespiration may be a problem because it can drain as much as 50% of the carbon fixed by the Calvin cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

56. C 4 Plants C 4 plants minimize the cost of photorespiration by incorporating CO 2 into four-carbon compounds in mesophyll cells This step requires the enzyme PEP carboxylase PEP carboxylase has a higher affinity for CO 2 than rubisco does; it can fix CO 2 even when CO 2 concentrations are low These four-carbon compounds are exported to bundle-sheath cells, where they release CO 2 that is then used in the Calvin cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

57. Fig. 10-19 C 4 leaf anatomy Mesophyll cell Photosynthetic cells of C 4 plant leaf Bundle- sheath cell Vein (vascular tissue) Stoma The C 4 pathway Mesophyll cell CO 2 PEP carboxylase Oxaloacetate (4C) Malate (4C) PEP (3C) ADP ATP Pyruvate (3C) CO 2 Bundle- sheath cell Calvin Cycle Sugar Vascular tissue

58. CAM Plants Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, incorporating CO 2 into organic acids Stomata close during the day, and CO 2 is released from organic acids and used in the Calvin cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

59. Fig. 10-20 CO 2 Sugarcane Mesophyll cell CO 2 C4C4 Bundle- sheath cell Organic acids release CO 2 to Calvin cycle CO 2 incorporated into four-carbon organic acids (carbon fixation) Pineapple Night Day CAM Sugar Calvin Cycle Calvin Cycle Organic acid (a) Spatial separation of steps (b) Temporal separation of steps CO 2 1 2

60. The Importance of Photosynthesis: A Review The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits In addition to food production, photosynthesis produces the O 2 in our atmosphere Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings