Section 1 The Light Reactions. Section 2 The Calvin Cycle. Resources

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1 How to Use This Presentation To View the presentation as a slideshow with effects select View on the menu bar and click on Slide Show. To advance through the presentation, click the right-arrow key or the space bar. From the resources slide, click on any resource to see a presentation for that resource. From the screen click on any lesson to go directly to that lesson s presentation. You may exit the slide show at any time by pressing the Esc key. Chapter Presentation Transparencies Visual Concepts Photosynthesis Table of Contents 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 Obtaining Energy 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. Obtaining Energy, continued Autotrophs use 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 called heterotrophs. Comparing Autotrophs and Heterotrophs Overview of Photosynthesis The oxygen (O 2 ) and some of the organic compounds produced by photosynthesis are used by cells in a process called cellular respiration.

3 Linking Photosynthesis and Respiration 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 CO 2 and the chemical energy stored in ATP and NADPH. Overview of Photosynthesis, continued Equation of Photosynthesis: 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. 6CO 2 + 6H 2 O light energy C 6 H 12 O 6 + 6O 2

4 Parts of a Chloroplast 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. Capturing Light Energy, continued Spectrum of Light and Plant Pigments Chloroplast Pigments Located in the membrane of the thylakoids of chloroplasts are several pigments, including chlorophylls (such as chlorophyll a and chlorophyll b) and carotenoids.

5 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. 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. 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.

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. 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. 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. 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.

7 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. 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. 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.

8 The Calvin Cycle 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. 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. 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.

9 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. 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. 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.

10 Environmental Influences on Photosynthesis Multiple Choice 1. Which of the following is a reactant in the Calvin cycle? A. O 2 B. CO 2 C. H 2 O D. C 6 H 12 O 6 1. Which of the following is a reactant in the Calvin cycle? A. O 2 B. CO 2 C. H 2 O D. C 6 H 12 O 6 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.

11 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. 3. Oxygen is produced at what point during photosynthesis? A. when CO 2 is fixed B. when water is split C. when ATP is converted into ADP D. when 3-PGA is converted into G3P 3. Oxygen is produced at what point during photosynthesis? A. when CO 2 is fixed B. when water is split C. when ATP is converted into ADP D. when 3-PGA is converted into G3P The diagram below shows a portion of a chloroplast. Use the diagram to answer the question that follows. 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

12 The diagram below shows a portion of a chloroplast. Use the diagram to answer the question that follows. 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 5. light reactions : ATP :: Calvin cycle : A. H + B. O 2 C. G3P D. H 2 O 5. light reactions : ATP :: Calvin cycle : A. H + B. O 2 C. G3P D. H 2 O 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

13 Short Response 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 Chloroplasts are organelles with areas that conduct different specialized activities. Where in the chloroplast do the light reactions and the Calvin cycle occur? 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. 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.

14 Extended Response, continued Answer: Part A The reactants for the light reactions of photosynthesis are sunlight, water, NADP +, and ADP. The products are oxygen, ATP, and NADPH. The reactants for the Calvin cycle are ATP, NADPH, CO 2, and RuBP. The products are NADP +, ADP, and organic compounds. Part B ADP/ATP, NADP + /NADPH, and electrons are recycled during photosynthesis. RuBP, which reacts with CO 2 in the Calvin cycle, is regenerated at each turn of the cycle.