Photosynthesis: The Light Reactions

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2. Resources Chapter Presentation Visual Concepts Standardized Test Prep Transparencies

3. Chapter 6 Photosynthesis Table of Contents Section 1 The Light Reactions Section 2 The Calvin Cycle

4. Section 1 The Light Reactions Chapter 6 Objectives • Explainwhy almost all organisms depend on photosynthesis. • Describethe role of chlorophylls and other pigments in photosynthesis. • Summarizethe main events of the light reactions. • Explainhow ATP is made during the light reactions.

5. Section 1 The Light Reactions Chapter 6 Obtaining Energy • Photosynthesisconverts light energy from the sun into chemical energy in the form of organic compounds through a series of reactions known as biochemical pathways.

6. Section 1 The Light Reactions Chapter 6 Obtaining Energy,continued • Autotrophsuse energy from sunlight or from chemical bonds in inorganic substances to make organic compounds. • Animals and other organisms that must get energy from food instead of directly from sunlight or inorganic substances are calledheterotrophs.

7. Section 1 The Light Reactions Chapter 6 Comparing Autotrophs and Heterotrophs Click below to watch the Visual Concept. Visual Concept

8. Section 1 The Light Reactions Chapter 6 Overview of Photosynthesis • The oxygen (O2) and some of the organic compounds produced by photosynthesis are used by cells in a process calledcellular respiration.

9. Section 1 The Light Reactions Chapter 6 Linking Photosynthesis and Respiration Click below to watch the Visual Concept. Visual Concept

10. Section 1 The Light Reactions Chapter 6 Overview of Photosynthesis, continued • Photosynthesis can be divided into two stages: Light Reactions and Calvin Cycle • In thelight reactions,light energy is converted to chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH. • In the Calvin Cycle, organic compounds are formed using CO2 and the chemical energy stored in ATP and NADPH.

11. Section 1 The Light Reactions Chapter 6 Overview of Photosynthesis, continued • Equation of Photosynthesis: 6CO2 + 6H2O light energy C6H12O6 + 6O2

12. Section 1 The Light Reactions Chapter 6 Capturing Light Energy • The light reactions begin with the absorption of light in chloroplasts, organelles found in the cells of plants, some bacteria, and algae.

13. Section 1 The Light Reactions Chapter 6 Parts of a Chloroplast Click below to watch the Visual Concept. Visual Concept

14. Section 1 The Light Reactions Chapter 6 Capturing Light Energy, continued • Light and Pigments • White light from the sun is composed of an array of colors called the visible spectrum. • Pigments absorb certain colors of light and reflect or transmit the other colors.

15. Section 1 The Light Reactions Chapter 6 Capturing Light Energy, continued • Chloroplast Pigments • Located in the membrane of thethylakoids of chloroplastsare several pigments, including chlorophylls (such as chlorophyll a and chlorophyll b) and carotenoids.

16. Section 1 The Light Reactions Chapter 6 Spectrum of Light and Plant Pigments Click below to watch the Visual Concept. Visual Concept

17. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy • The pigments are grouped in clusters of a few hundred molecules in the thylakoid membrane. Each cluster and the proteins that the pigment molecules are embedded in are referred to collectively as a photosystem. • By absorbing light, pigment molecules in photosystem I and photosystem II acquire some of the energy carried by the light.

18. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued • In each photosystem, the acquired energy is passed quickly to other pigment molecules until it reaches a specific pair of chlorophyll a molecules. • The acquired energy forces electrons to enter a higher energy level in the two chlorophyll a molecules of photosystem II. These energized electrons are said to be “excited.” The excited electrons have enough energy to leave the chlorophyll a molecules.

19. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued • The acceptor of these electrons from photosystem II is a molecule called the primary electron acceptor, which donates the electrons to the electron transport chain. • As the electrons move from molecule to molecule in this chain, they lose most of the acquired energy. The energy they lose is used to move protons into the thylakoid.

20. Section 1 The Light Reactions Chapter 6

21. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued • Light is absorbed by photosystem I at the same time it is absorbed by photosystem II. Electrons move from chlorophyll a molecules to another primary electron acceptor. • The electrons lost from photosystem I are replaced by electrons that have passed through the electron transport chain from photosystem II.

22. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued • These electrons are then donated to another electron transport chain, which brings the electrons to the side of the thylakoid membrane that faces the stroma. • In the stroma, the electrons combine with a proton and NADP+. This causes NADP+ to be reduced to NADPH.

23. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued • Replacing Electrons in Light Reactions • Electrons from photosystem II replace electrons that leave photosystem I. Replacement electrons for photosystem II are provided by the splitting of water molecules. • Oxygen produced when water molecules are split diffuses out of the chloroplast and then leaves the plant.

24. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy, continued • Making ATP in Light Reactions • An important part of the light reactions is the synthesis of ATP. During chemiosmosis, the movement of protons through ATP synthase into the stroma releases energy, which is used to produce ATP.

25. Section 1 The Light Reactions Chapter 6

26. Section 2 The Calvin Cycle Chapter 6 Objectives • Summarize the main events of the Calvin cycle. • Describe what happens to the compounds that are made in the Calvin cycle. • Distinguish between C3, C4, and CAM plants. • Summarize how the light reactions and the Calvin cycle work together to create the continuous cycle of photosynthesis. • Explain how environmental factors influence photosynthesis.

27. Section 2 The Calvin Cycle Chapter 6 Carbon Fixation • The ATP and NADPH produced in the light reactions drive the second stage of photosynthesis, theCalvin cycle. • In the Calvin cycle, CO2 is incorporated into organic compounds, a process calledcarbon fixation.

28. Section 2 The Calvin Cycle Chapter 6 Carbon Fixation, continued • The Calvin cycle, which occurs in the stroma of the chloroplast, is a series of enzyme-assisted chemical reactions that produces a three-carbon sugar. • Most of the three-carbon sugars (G3P) generated in the Calvin cycle are converted to a five-carbon sugar (RuBP) to keep the Calvin cycle operating. But some of the three-carbon sugars leave the Calvin cycle and are used to make organic compounds, in which energy is stored for later use.

29. Section 2 The Calvin Cycle Chapter 6

30. Section 2 The Calvin Cycle Chapter 6 The Calvin Cycle

31. Section 2 The Calvin Cycle Chapter 6

32. Section 2 The Calvin Cycle Chapter 6 Factors That Affect Photosynthesis • Light Intensity • The rate of photosynthesis increases as light intensity increases, because more electrons are excited in both photosystems. • However, at some point all of the available electrons are excited, and the maximum rate of photosynthesis is reached. The rate then stays level regardless of further increases in light intensity.

33. Section 2 The Calvin Cycle Chapter 6 Factors That Affect Photosynthesis, continued • Carbon Dioxide Levels • As with increasing light intensity, increasing levels of carbon dioxide also stimulate photosynthesis until the rate levels off.

34. Section 2 The Calvin Cycle Chapter 6 Factors That Affect Photosynthesis, continued • Temperature • As temperature increases, the rate of photosynthesis increases to a maximum and then decreases with further rises in temperature. • The rate peaks at a certain temperature, at which many of the enzymes that catalyze the reactions become ineffective. Also, the stomata begin to close, limiting water loss and entry of carbon dioxide.

35. Section 2 The Calvin Cycle Chapter 6 Environmental Influences on Photosynthesis Click below to watch the Visual Concept. Visual Concept

36. Standardized Test Prep Chapter 6 Multiple Choice 1. Which of the following is a reactant in the Calvin cycle? A. O2 B. CO2 C. H2O D. C6H12O6

37. Standardized Test Prep Chapter 6 Multiple Choice, continued 1. Which of the following is a reactant in the Calvin cycle? A. O2 B. CO2 C. H2O D. C6H12O6

38. Standardized Test Prep Chapter 6 Multiple Choice, continued 2. Which of the following statements is correct? F. Accessory pigments are not involved in photosynthesis. G. Accessory pigments add color to plants but do not absorb light energy. H. Accessory pigments absorb colors of light that chlorophyll a cannot absorb. J. Accessory pigments receive electrons from the electron transport chain of photosystem I.

39. Standardized Test Prep Chapter 6 Multiple Choice, continued 2. Which of the following statements is correct? F. Accessory pigments are not involved in photosynthesis. G. Accessory pigments add color to plants but do not absorb light energy. H. Accessory pigments absorb colors of light that chlorophyll a cannot absorb. J. Accessory pigments receive electrons from the electron transport chain of photosystem I.

40. Standardized Test Prep Chapter 6 Multiple Choice, continued 3. Oxygen is produced at what point during photosynthesis? A. when CO2 is fixed B. when water is split C. when ATP is converted into ADP D. when 3-PGA is converted into G3P

41. Standardized Test Prep Chapter 6 Multiple Choice, continued 3. Oxygen is produced at what point during photosynthesis? A. when CO2 is fixed B. when water is split C. when ATP is converted into ADP D. when 3-PGA is converted into G3P

42. Standardized Test Prep Chapter 6 Multiple Choice, continued 4. Which of the following correctly identifies the structure marked X and the activities that take place there? F. stroma—Calvin cycle G. stroma—light reactions H. thylakoid—Calvin cycle J. thylakoid—light reactions The diagram below shows a portion of a chloroplast. Use the diagram to answer the question that follows.

43. Standardized Test Prep Chapter 6 Multiple Choice, continued 4. Which of the following correctly identifies the structure marked X and the activities that take place there? F. stroma—Calvin cycle G. stroma—light reactions H. thylakoid—Calvin cycle J. thylakoid—light reactions The diagram below shows a portion of a chloroplast. Use the diagram to answer the question that follows.

44. Standardized Test Prep Chapter 6 Multiple Choice, continued 5. light reactions : ATP :: Calvin cycle : A. H+ B. O2 C. G3P D. H2O

45. Standardized Test Prep Chapter 6 Multiple Choice, continued 5. light reactions : ATP :: Calvin cycle : A. H+ B. O2 C. G3P D. H2O

46. Standardized Test Prep Chapter 6 Multiple Choice, continued The diagram below shows a step in the process of chemiosmosis. Use the diagram to answer the question that follows. 6. What is the substance identified as Y in the image? F. H+ G. NAD+ H. NADPH J. ADP synthase

47. Standardized Test Prep Chapter 6 Multiple Choice, continued The diagram below shows a step in the process of chemiosmosis. Use the diagram to answer the question that follows. 6. What is the substance identified as Y in the image? F. H+ G. NAD+ H. NADPH J. ADP synthase

48. Standardized Test Prep Chapter 6 Short Response Chloroplasts are organelles with areas that conduct different specialized activities. Where in the chloroplast do the light reactions and the Calvin cycle occur?

49. Standardized Test Prep Chapter 6 Short Response, continued Chloroplasts are organelles with areas that conduct different specialized activities. Where in the chloroplast do the light reactions and the Calvin cycle occur? Answer: The light reactions of photosynthesis occur along the thylakoid membrane. The Calvin cycle occurs in the stroma, surrounding the thylakoids.

50. Standardized Test Prep Chapter 6 Extended Response The reactions of photosynthesis make up a biochemical pathway. Part A What are the reactants and products for both the light reactions and the Calvin cycle? Part B Explain how the biochemical pathway of photosynthesis recycles many of its own reactants, and identify the recycled reactants.