1. Section 1 The Light Reactions Chapter 6 Objectives Explain why almost all organisms depend on photosynthesis. Describe the role of chlorophylls and other pigments in photosynthesis. Summarize the main events of the light reactions. Explain how ATP is made during the light reactions.

2. Section 1 The Light Reactions Chapter 6 Obtaining Energy All organisms use energy to carry out functions of life. This energy comes directly or indirectly from the sun. –Energy from the sun enters living systems when plants, algae, some unicellular protists, and some prokaryotes absorb sunlight and use it to make organic compounds –In essence, all energy on our planet comes from the SUN –Energy is required for a variety of life processes including growth and development, movement, and transport of certain materials across cell membranes Photosynthesis converts light energy from the sun into chemical energy in the form of organic compounds through a series of reactions known as biochemical pathways. The biochemical pathway is a series of reactions that could be compared to an assembly line that has workers completing tasks. Without this “assembly line” photosynthesis would not happen as smoothly

3. Section 1 The Light Reactions Chapter 6 Obtaining Energy, continued Autotrophs are organisms that use energy from sunlight or from chemical bonds in inorganic substances to make organic compounds. Most plants would be considered autotrophs Animals and other organisms that must get energy from food instead of directly from sunlight or inorganic substances are called heterotrophs. Humans are heterotrophs –Page 113, Figure 6-1

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

5. Section 1 The Light Reactions Chapter 6 Overview of Photosynthesis The major atmospheric byproduct of photosyntheis is oxygen The oxygen (O 2 ) and some of the organic compounds produced by photosynthesis are used by cells in a process called cellular respiration. During cellular respiration, CO 2 and water are produced So the reactants of cellular respiration are products of photosynthesis, and vice versa

6. Section 1 The Light Reactions Chapter 6 Overview of Photosynthesis This diagram is very similar to Figure 6-2, pg 114. Many autotrophs produce organic compounds and oxygen through photosynthesis. Both autotrophs and heterotrophs produce carbon dioxide through cellular respiration

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

8. Section 1 The Light Reactions Chapter 6 Overview of Photosynthesis, continued Photosynthesis can be divided into two stages: Light Reactions and Calvin Cycle –In the light 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 CO 2 and the chemical energy stored in ATP and NADPH.

9. Section 1 The Light Reactions Chapter 6 Overview of Photosynthesis, continued Equation of Photosynthesis: 6CO 2 + 6H 2 O light energy C 6 H 12 O 6 + 6O 2

10. Section 1 The Light Reactions Chapter 6 Capturing Light Energy 1 st stage of photosynthesis includes light reactions, which are triggered by the absorption of light and need light to occur The light reactions begin with the absorption of light in chloroplasts organelles found in the cells of plants, some bacteria, and algae.

11. Section 1 The Light Reactions Chapter 6 Capturing Light Energy Most chloroplasts are similar in structure Each chloroplast is surrounded by a pair of membranes –Inside the inner membrane is another system of membranes called thylakoids that are arranged as flattened sacs –The thylakoids are connected and layered to form stacks called grana –Surrounding the grana is a solution called the stroma

12. Section 1 The Light Reactions Chapter 6 Capturing Light Energy Photosynthesis in eukaryotes occurs inside the chloroplast. The light reactions of photosynthesis take place in the thylakoids, which are stacked to form grana. Pg. 114 Fig. 6-3

13. Chapter 6 Parts of a Chloroplast Section 1 The Light Reactions Visual Concept Click below to watch the 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. –When light strikes an object, the light may be reflected, absorbed, and/or transmitted. –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 the thylakoids of chloroplasts are several pigments, including chlorophylls (such as chlorophyll a and chlorophyll b, role is to absorb light) and carotenoids (yellow, orange, brown pigments) –In the leaves of a plant, the chlorophylls are generally much more abundant and therefore mask the colors of the other pigments. During the fall, many plants lose their chlorophylls, and their leaves take on the rich hues of the carotenoids

16. Section 1 The Light Reactions Chapter 6 Capturing Light Energy, continued Chlorophyll a absorbs less blue light but more red light than chlorophyll b absorbs. Neither chlorophyll a nor b absorbs much green light. Instead, they allow green light to be reflected or transmitted. For this reason, leaves and other plant structures contain large amounts of chlorophyll look green. Figure 6-5, pg.115 The three curves on this graph show how three pigments involved in photosynthesis differ in the colors of light they absorb. Where a curve has a peak, much of the light at that wavelength is absorbed. Where a curve has a trough, much of the light at that wavelength is reflected or tranmitted.

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

18. 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. Follow along in your book, pgs 116-118

19. 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.

20. 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 (in chloroplasts, an acceptor of electrons lost from chlorophyll a; found in thylakoid membrane), which donates the electrons to the electron transport chain (series of molecules, found in the inner membranes of mitochondria and chloroplasts, through which electrons pass in a process that causes protons to build up on one side of the membrane) –When electrons of a chlorophyll molecule are raised to a higher energy level, they enter an 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.

21. Section 1 The Light Reactions Chapter 6 Light Reactions in Photosynthesis Pg 116, Fig 6-6 The light reactions, which take place in the thylakoid membrane, use energy from sunlight to produce ATP, NADPH, and oxygen In step 1, light 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.” In step 2, these excited electrons have enough energy to leave the chlorophyll a molecules. Some substance must accept the electrons that the chlorophyll a molecules have lost. The acceptor of the lost molecules in the thylakoid membrane is called the primary electron acceptor. In step 3, the primary electron acceptor donates the electrons to the first of a series of molecules located in the thylakoid membrane. These molecules are called an electron transport chain because they transfer electrons from one molecule to the next. In step 4, light is absorbed by photosystem I. This happens at the same time that light is absorbed by photosynthesis II. Finally in step 5, the primary electron acceptor of photosystem I donates electrons to a different electron transport chain and finally NADP + can be reduced to NADPH.

22. Section 1 The Light Reactions Chapter 6 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. Converting Light Energy To Chemical Energy, continued

23. Section 1 The Light Reactions Chapter 6 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. –NADP + is important in photosynthesis because it provides protons and electrons for some reactions Converting Light Energy To Chemical Energy, continued

24. 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. –At the thylakoid membrane, electrons return to their original energy levels

25. 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.

26. Section 1 The Light Reactions Chapter 6 Summary of Processes in Light Reactions

27. 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 C 3, C 4, 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.

28. Chapter 6 Carbon Fixation The ATP and NADPH produced in the light reactions drive the second stage of photosynthesis, the Calvin cycle. In the Calvin cycle, CO 2 is incorporated into organic compounds, a process called carbon fixation. –All organic molecules contain carbon atoms that ultimately can be traced back in the food chain to carbon dioxide from the atmosphere »Page 120-121 discusses the starting steps of Calvin cycle Section 2 The Calvin Cycle

29. 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 This 3-carbon sugar is a complex carbohydrate that is needed for energy and growth of plant 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. Section 2 The Calvin Cycle

30. Chapter 6 Misc. Calvin Cycle The Calvin cycle of photosynthesis requires ATP and NADPH, can occur in both light and dark conditions, and generates glucose The energy used in the Calvin cycle for the production of carbohydrate molecules comes from ATP made in the light reactions of photosynthesis During the Calvin cycle, carbon-containing molecules are produced from carbon atoms from carbon dioxide in the air and hydrogen atoms from NADPH Section 2 The Calvin Cycle

31. Chapter 6 Section 2 The Calvin Cycle The Calvin Cycle Figure 6-9 Page 120

32. Chapter 6 Alternative Pathways The C 4 Pathway –Some plants that evolved in hot, dry climates fix carbon through the C 4 pathway. These plants have their stomata partially closed during the hottest part of the day. C 3 and C 4 have their stomata open during the day –Certain cells in these plants have an enzyme that can fix CO 2 into four-carbon compounds even when the CO 2 level is low and the O 2 level is high. These compounds are then transported to other cells, where the Calvin cycle ensues. Section 2 The Calvin Cycle

33. Chapter 6 Alternative Pathways, continued The CAM Pathway –Some other plants that evolved in hot, dry climates fix carbon through the CAM pathway. These plants carry out carbon fixation at night and the Calvin cycle during the day to minimize water loss. Their stomata is open during the night Section 2 The Calvin Cycle

34. Chapter 6 Section 2 The Calvin Cycle Ongoing Cycle of Photosynthesis

35. Chapter 6 A Summary of Photosynthesis Photosynthesis happens in two stages, both of which occur inside the chloroplasts of plant cells and algae –The light reactions – Energy is absorbed from sunlight and converted into chemical energy, which is temporarily stored in ATP and NADPH –Calvin cycle – Carbon dioxide and the chemical energy stored in ATP and NADPH are used to form organic compounds Products of light reactions are used in Calvin cycle and some of the products of Calvin cycle are reused in light reactions Other products of Calvin cycle create organic compounds like carbohydrates, lipids, and proteins Section 2 The Calvin Cycle

36. 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. Section 2 The Calvin Cycle

37. 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. Section 2 The Calvin Cycle

38. 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. Section 2 The Calvin Cycle

39. Factors That Affect Photosynthesis, cont. Page 124, Figure 6-12 Environmental factors affect the rate of photosynthesis in plants. (1) As light intensity increases, the rate of photosynthesis increases and then levels off at a maximum. (2) As temperature increases, the rate of photosynthesis increases to a maximum and then decreases with further rises in temperature

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