1. Lecture Outlines by Gregory Ahearn, University of North Florida Copyright © 2011 Pearson Education Inc. Chapter 7 Capturing Solar Energy: Photosynthesis

2. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Chapter 7 At a Glance  7.1 What Is Photosynthesis?  7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?  7.4 Why Do Some Plants Use Alternate Pathways for Carbon Fixation?

3. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.1 What Is Photosynthesis?  For most organisms, energy is derived from sunlight, either directly or indirectly  Those organisms that can directly trap sunlight do so by photosynthesis  Photosynthesis is the process by which solar energy is trapped and stored as chemical energy in the bonds of a sugar

4. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Photosynthesis

5. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.1 What Is Photosynthesis?  Leaves and chloroplasts are adaptations for photosynthesis –Photosynthesis in plants takes place in chlorophyll- containing organelles called chloroplasts, most of which are contained within leaf cells –Chloroplasts contain chemical reactions that are able to convert energy in sunlight into stored energy in sugars –Both the upper and lower surfaces of a leaf consist of a layer of transparent cells, the epidermis

6. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e An Overview of Photosynthetic Structures Fig. 7-1 cuticle (b) Internal leaf structure upper epidermis mesophyll cells lower epidermis chloroplasts stoma bundle sheath cells vascular bundle (vein) stoma outer membrane inner membrane thylakoid stroma channel interconnecting thylakoids (d) Chloroplast (a) Leaves (c) Mesophyll cell containing chloroplasts

7. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.1 What Is Photosynthesis?  Leaves and chloroplasts are adaptations for photosynthesis (continued) –The outer surface of both epidermal layers is covered by the cuticle, a transparent, waxy, waterproof covering that reduces the evaporation of water from the leaf –Leaves obtain CO 2 for photosynthesis from the air through pores in the epidermis called stomata (singular, stoma)

8. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Stomata Fig. 7-2

9. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.1 What Is Photosynthesis?  Leaves and chloroplasts are adaptations for photosynthesis (continued) –Inside the leaf are layers of cells called the mesophyll, where the chloroplasts are located and where photosynthesis occurs –Bundle sheath cells surround the vascular bundles, which form veins in the leaf and supply water and minerals to the mesophyll

10. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.1 What Is Photosynthesis?  Leaves and chloroplasts are adaptations for photosynthesis (continued) –Chloroplasts are organelles with a double membrane enclosing a fluid called the stroma –Embedded in the stroma are disk-shaped membranous sacs called thylakoids –The light-dependent reactions of photosynthesis occur in and adjacent to the membranes of the thylakoids –Reactions of the Calvin cycle that capture carbon dioxide and produce sugar occur in the stroma

11. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.1 What Is Photosynthesis?  Photosynthesis consists of the light reactions and the Calvin cycle –Starting with carbon dioxide (CO 2 ) and water (H 2 O), photosynthesis converts sunlight energy into chemical energy stored in bonds of glucose and releases oxygen (O 2 ) as a product by the following equation: 6 CO 2 +6 H 2 O+light energy  C 6 H 12 O 6 +6 O 2 carbonwatersunlightglucoseoxygen dioxide(sugar)

12. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e An Overview of the Relationship Between the Light Reactions and the Calvin Cycle Fig. 7-3

13. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.1 What Is Photosynthesis?  Photosynthesis consists of the light reactions and the Calvin cycle (continued) –During the light reactions, chlorophyll and other molecules embedded in the chloroplast thylakoid membranes capture sunlight energy and convert some of it into chemical energy stored in the energy-carrier molecules ATP and NADPH

14. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.1 What Is Photosynthesis?  Photosynthesis consists of the light reactions and the Calvin cycle (continued) –In the reactions of the Calvin cycle, enzymes in the stroma use CO 2 from the air and chemical energy from the energy-carrier molecules to synthesize a three-carbon sugar that will be used to make glucose

15. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.1 What Is Photosynthesis?  Photosynthesis consists of the light reactions and the Calvin cycle (continued) –The “photo” part of photosynthesis refers to the capture of sunlight in the thylakoids –The “synthesis” part of photosynthesis refers to the Calvin cycle, which synthesizes sugar from the energy captured in ATP and NADPH in the light reactions

16. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Reactions of Photosynthesis

17. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  Light is captured by pigments in chloroplasts –The sun emits energy within a broad spectrum of electromagnetic radiation –This electromagnetic spectrum ranges from short-wavelength gamma rays, through ultraviolet, visible, and infrared light, to long- wavelength radio waves

18. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Visible Light

19. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  Light is captured by pigments in chloroplasts (continued) –Light is composed of individual packets of energy called photons –Visible light has wavelengths with energies strong enough to alter biological pigment molecules such as chlorophyll a –Chlorophyll a is a key light-capturing pigment molecule in chloroplasts, absorbing violet, blue, and red light –Green light, however, is reflected, which is why leaves appear green

20. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  Light is captured by pigments in chloroplasts (continued) –Chloroplasts also contain accessory pigments, that absorb additional wavelengths of light energy and transfer them to chlorophyll a –Chlorophyll b absorbs blue and red-orange wavelengths of light missed by chlorophyll a –Carotenoids are accessory pigments that absorb blue and green light, and appear yellow or orange to our eyes because they reflect these colors –In autumn, more-abundant, green chlorophyll breaks down before the carotenoids do, revealing their yellow color, which in summer is masked

21. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Light and Chloroplast Pigments Fig. 7-4 gamma rays higher energylower energy X-raysUVinfrared micro waves radio waves wavelength (nanometers) chlorophyll b carotenoids chlorophyll a

22. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Loss of Chlorophyll Reveals Yellow Carotenoid Pigments Fig. 7-5

23. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  The light reactions occur in association with the thylakoid membranes –The thylakoid membranes contain many photosystems, each consisting of a cluster of chlorophyll and accessory pigment molecules surrounded by various proteins –Two photosystems, photosystem II (PS II) and photosystem I (PS I), work together during the light reactions

24. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  The light reactions occur in association with the thylakoid membranes (continued) –Each type of photosystem has a unique electron transport chain located adjacent to it –These electron transport chains (ETC) each consist of a series of electron carrier molecules embedded in the thylakoid membrane –Within the thylakoid membrane, the overall path of electrons is as follows: PS II ETC II PS I ETC I NADP +

25. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  Photosystem II uses light energy to create a hydrogen ion gradient and split water –The steps in the light reactions are as follows: 1.Photons of light are absorbed by pigment molecules clustered in photosystem II 2.The energy hops from one pigment molecule to the next until it is funneled into the reaction center 3a. The reaction center within each photosystem consists of a pair of specialized chlorophyll a molecules and a primary electron acceptor molecule embedded in a complex of proteins

26. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  The steps in the light reactions are as follows: (continued) 3b. When it reaches the reaction center, light energy boosts an electron from one of the reaction center chlorophylls to the primary electron acceptor, which captures the energized electron 3c. These lost electrons are constantly replaced by a PS II protein that splits water in a process that generates the electrons for PS II, the hydrogen ions for a hydrogen gradient used in ATP synthesis, and the oxygen that is released as a by-product

27. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  The steps in the light reactions are as follows: (continued) 4.The electron acceptor passes the electron on to the first molecule of ETC II, and it is then passed from one electron carrier molecule to the next, losing energy as it goes

28. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  The steps in the light reactions are as follows: (continued) 5a.Some of the energy is harnessed to pump hydrogen ions (H + ) across the thylakoid membrane and into the thylakoid space, where they will be used to generate ATP 5b.The energy-depleted electron leaves ETC II and enters the reaction center of photosystem I, where it replaces an electron ejected when light strikes photosystem I

29. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  The steps in the light reactions are as follows: (continued) 6.Light energy striking PS I is captured by its pigment molecules and funneled to a chlorophyll a molecule in the reaction center 7.This ejects an energized electron that is picked up by the primary electron acceptor of PS I

30. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  The steps in the light reactions are as follows: (continued) 8.From the primary electron acceptor of PS I, the energized electron is passed along ETC I until it reaches NADP + 9.The energy-carrier molecule NADPH is formed when each NADP + molecule picks up two energetic electrons, along with one hydrogen ion

31. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Light-Dependent Reactions

32. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Energy Transfer and the Light Reactions of Photosynthesis Fig. 7-6 NADP + ATP ADP NADPH Calvin cycle light reactions H2OH2O O2O2 CO 2 C 6 H 12 O 6 sugar ee primary electron acceptor pigment molecules light energy ee electron transport chain ATP ee ee Photosystem I NADP  NADPH ee electron transport chain ee H+H+ 2 1/21/2 ee H+H+ + high low reaction center H2OH2O O2O2 Photosystem II energy level of electrons 3 4 1 2 7 8 9 5 6

33. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  The hydrogen ion gradient generates ATP by chemiosmosis –The energy of electron movement through the thylakoid membrane creates an H + gradient that drives ATP synthesis in a process called chemiosmosis

34. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  The hydrogen ion gradient generates ATP by chemiosmosis (continued) –Chemiosmosis occurs in three steps: 1.As the energized electron travels along ETC II, some of the energy it liberates is used to pump hydrogen ions into the thylakoid space 2.This pumping creates a high concentration of H + inside the space relative to the surrounding stroma 3.H + flows down its concentration gradient through a thylakoid channel protein called ATP synthase, generating ATP from ADP

35. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Chemiosmosis

36. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Events of the Light Reactions Occur In and Near the Thylakoid Membrane H + are pumped into the thylakoid space ATP synthase photosystem I photosystem II thylakoid membrane light energy  + P ATP NADP + ADP NADPH Calvin cycle CO 2 C 6 H 12 O 6 sugar e–e– e–e– e–e– e–e– e–e– e–e– 1/21/2 2 H2OH2O chloroplast electron transport chain II electron transport chain I (stroma) (thylakoid space) thylakoid O2O2 A high H + concentration is created in the thylakoid space The flow of H + down their concentration gradient powers ATP synthesis H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ 1 2 3 Fig. 7-7

37. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.2 Light Reactions: How Is Light Energy Converted to Chemical Energy?  The hydrogen ion gradient generates ATP by chemiosmosis (continued) –The generation of ATP from ADP plus phosphate resembles the electrical energy obtained from water flowing downhill and driving an electrical turbine –ATP synthase captures the energy liberated by the flow of H + down its concentration gradient and uses it to drive ATP synthesis from ADP plus phosphate

38. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Energy is released as water flows downhill Energy is harnessed to rotate a turbine The energy of the rotating turbine is used to generate electricity 1 2 3 Energy Stored in a Water “Gradient” Can Be Used to Generate Electricity Fig. 7-8

39. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Oxygen Is a By-product of Photosynthesis Fig. 7-9

40. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?  The Calvin cycle captures carbon dioxide –ATP and NADPH synthesized from light reactions are used to power the synthesis of a simple sugar (gyceraldehyde-3-phosphate, or G3P) –This is accomplished through a series of reactions occurring in the stroma called the Calvin cycle

41. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?  The Calvin cycle captures carbon dioxide (continued) –The Calvin cycle occurs in three steps: 1.Carbon fixation 2.The synthesis of G3P 3.The regeneration of ribulose bisphosphate (RuBP)

42. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?  The Calvin cycle occurs in three steps: (continued) 1. Carbon fixation –During carbon fixation, carbon from CO 2 is incorporated, or “fixed,” into a larger organic molecule –The enzyme rubisco combines three CO 2 molecules with three RuBP molecules, forming three unstable six-carbon molecules that each quickly split in half –Six molecules of a three-carbon product, phosphoglyceric acid (PGA), result –The generation of this three-carbon PGA molecule is called the C 3 pathway

43. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?  The Calvin cycle occurs in three steps: (continued) 2.The synthesis of G3P –Energy donated by ATP and NADPH is used to convert six PGA molecules into six of the three-carbon sugar molecule G3P

44. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?  The Calvin cycle occurs in three steps: (continued) 3.The regeneration of RuBP –ATP from the light reactions is used with five of the six G3P molecules formed to regenerate the five-carbon RuBP necessary to repeat the cycle –The remaining G3P molecule, which is the end product of photosynthesis, exits the cycle

45. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?  Carbon fixed during the Calvin cycle is used to synthesize sugar –In reactions that occur outside the Calvin cycle, two G3P molecules can be combined to form one six-carbon glucose molecule –Glucose may then be converted to the disaccharide sucrose or linked to form starch (a storage molecule) or cellulose (a major component of plant cell walls)

46. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Light Independent Reactions

47. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e The Calvin Cycle Fixes Carbon from CO 2 and Produces G3P Fig. 7-10 C 3 CO 2 CCC 6 PGA NADP + ATP ADP NADPH 6 6 6 6 CCC 1 G3P CCC 5 ATP ADP 3 3 CCC 3 RuBP CCC 1 G3P + CCC 1 CCC 1 glucose CCC CCC 6 G3P Calvin cycle Energy from ATP and NADPH is used to convert the six molecules of PGA to six molecues of G3P Carbon fixation combines three CO 2 with three RuBP using the enzyme rubisco Using the energy from ATP, five of the six molecules of G3P are converted to three molecules of RuBP 4 One molecule of G3P leaves the cycle Two molecules of G3P combine to form glucose and other molecules CC NADP ATP ADP NADPH Calvin cycle light reactions H2OH2O O2O2 CO 2 C 6 H 12 O 6 sugar 5 3 4 2 1

48. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.4 Why Do Some Plants Use Alternate Pathways for Carbon Fixation?  When stomata are closed to conserve water, wasteful photorespiration occurs –When plant stomata are closed in hot environments to prevent water loss, oxygen builds up in the plant cells and RuBP combines with it, rather than CO 2, in a wasteful process called photorespiration –This process prevents the Calvin cycle from synthesizing sugar, and plants may die under these circumstances

49. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.4 Why Do Some Plants Use Alternate Pathways for Carbon Fixation?  When stomata are closed to conserve water, wasteful photorespiration occurs (continued) –Flowering plants have evolved two different mechanisms to circumvent wasteful photorespiration: –The C 4 pathway –Crassulacean acid metabolism (CAM)

50. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.4 Why Do Some Plants Use Alternate Pathways for Carbon Fixation?  C 4 plants capture CO 2 and synthesize sugar in different cells –In typical plants, known as C 3 plants, both carbon fixation and the Calvin cycle occur in mesophyll cells –Photorespiration in C 3 plants causes photosynthesis to slow dramatically –C 4 plants avoid photorespiration by using the C 4 pathway –The C 4 pathway performs CO 2 capture and the Calvin cycle in separate cell types

51. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.4 Why Do Some Plants Use Alternate Pathways for Carbon Fixation?  The C 4 pathway performs CO 2 capture and the Calvin cycle in separate cell types (continued) –CO 2 is captured in the mesophyll cells, as in the C 3 cycle, but with several differences –Instead of RuBP, a three-carbon intermediate, PEP, is used to capture CO 2, yielding the four-carbon oxaloacetate –Oxaloacetate undergoes rearrangement into malate –Instead of rubisco, a CO 2 -capturing enzyme, PEP carboxylase, is used that is not inhibited by the high oxygen levels generated when the stomata are closed –PEP is eventually regenerated after malate, serving as a shuttle, then passes a CO 2 molecule into a bundle sheath cell and is broken down into pyruvate, which is transported back to a mesophyll cell

52. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.4 Why Do Some Plants Use Alternate Pathways for Carbon Fixation?  The C 4 pathway performs CO 2 capture and the Calvin cycle in separate cell types (continued) –Unlike in C 3 plants, the bundle sheath cells of C 4 plants perform the Calvin cycle –Despite the low levels of CO 2 in the mesophyll cells, caused by the closed stomata, the C 4 cycle passes high levels of CO 2 into the bundle sheath cells –Rubisco is thus able to fix large amounts of CO 2 and create sugar despite the unfavorable conditions in the neighboring mesophyll cells

53. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e The C 4 Pathway and the CAM Pathway Fig. 7-11a CO 2 Calvin cycle mesophyll cell oxaloacetate (4C) malate (4C) PEP carboxylase PEP (3C) bundle sheath cell sugar malate (4C) pyruvate (3C) pyruvate (3C) CO 2 rubisco (a) C 4 plants

54. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.4 Why Do Some Plants Use Alternate Pathways for Carbon Fixation?  CAM plants capture carbon and synthesize sugar at different times –Although they use the C 4 cycle to generate CO 2, CAM plants do not use different cell types to capture carbon and to synthesize sugar –Instead, they perform both these activities in the same mesophyll cells, but at different times –Carbon fixation occurs at night, when the stomata are open and CO 2 is available –Sugar synthesis occurs during the day

55. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e CO 2 Mesophyll cell PEP carboxylase Calvin cycle sugar CO 2 rubisco central vacuole oxalo- acetate (4C) PEP (3C) pyruvate (3C) malic acid malate (4C) malate (4C) nightday (b) CAM plants The C 4 Pathway and the CAM Pathway Fig. 7-11b

56. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 7.4 Why Do Some Plants Use Alternate Pathways for Carbon Fixation?  Alternate pathways adapt plants to different environmental conditions –C 4 plants and CAM plants consume more energy than do C 3 plants; they have an advantage when light energy is abundant but water is not –In environments where water is abundant or light levels are low, plants using the C 3 carbon fixation pathway dominate