AP Curriculum Alignment

Transcription

1 Chapter 7: Photosynthesis AP Curriculum Alignment Chapter 7 begins with an introduction that connects to Big Idea 1 because there are photosynthetic organisms in all of the three domains. A photosynthetic bacterium is thought to have been the precursor to the modern chloroplast. Big Idea 2 focuses on the utilization of free energy and the curriculum framework gets very detailed as to the types of information that students are required to know. Chapter 7 covers how organisms obtain energy, either as autotrophs or heterotrophs. The steps involved in photosynthesis to convert solar energy into chemical bonds are covered in detail. The light reaction and the Calvin cycle are the two parts to photosynthesis but students are required to know more detail about the light reaction. Knowledge of photosystems I and II, the electron carriers, and the role of membranes in the electron transport chain are crucial to the connection of ATP production through the enzyme ATP synthase. It is not necessary for student to memorize the intermittent stages and enzymes involved in the Calvin cycle. Big Idea 4 contains detailed information about the structure of the chloroplast as it relates to photosynthesis. The role of the membranes within the chloroplast is to allow a buildup of hydrogen ions. This concentration gradient is then used as potential energy that is harvested by the enzyme ATP synthase in the production of ATP that is then used to power the Calvin cycle. Sections of the framework that are applicable to the concepts found in Chapter 7 are shown in the table below. ALIGNMENT OF CONTENT TO THE CURRICULUM FRAMEWORK Big Idea 1: The process of evolution drives the diversity and unity of life. Enduring Understanding (EU); 1.B: Organisms are linked by lines of descent from common ancestry. Essential knowledge (EK)1.B.1: Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. a. Structural and functional evidence supports the relatedness of all domains. 3. Metabolic pathways are connected across all currently recognized domains. Big Idea 2: Biological systems utilize free energy and molecular building blocks to grow, to reproduce and to maintain dynamic homeostasis. Enduring understanding (EU) 2.A: Growth, reproduction and maintenance of the organization of living systems require free energy and matter. Essential knowledge (EK) 2.A.1: All living systems require constant input of free energy. c. Energy-related pathways in biological systems are sequential and may be entered at multiple points in the pathway. To foster student understanding of this concept, instructors can choose an illustrative example such as: 106 Mader, Biology, 12 th Edition, Chapter 7

2 Krebs cycle Glycolysis Calvin cycle Fermentation Essential knowledge (EK) 2.A.2: Organisms capture and store free energy for use in biological processes. a. Autotrophs capture free energy from physical sources in the environment. Evidence of student learning is a demonstrated understanding of each of the following: 1. Photosynthetic organisms capture free energy present in sunlight. 2. Chemosynthetic organisms capture free energy from small inorganic molecules present in their environment, and this process can occur in the absence of oxygen. b. Heterotrophs capture free energy present in carbon compounds produced by other organisms. c. Different energy-capturing processes use different types of electron acceptors. To foster student understanding of this concept, instructors can choose an illustrative example such as: NADP+ in photosynthesis Oxygen in cellular respiration d. The light-dependent reactions of photosynthesis in eukaryotes involve a series of coordinated reaction pathways that capture free energy present in light to yield ATP and NADPH, which power the production of organic molecules. Evidence of student learning is a demonstrated understanding of each of the following: 1. During photosynthesis, chlorophylls absorb free energy from light, boosting electrons to a higher energy level in Photosystems I and II. 2. Photosystems I and II are embedded in the internal membranes of chloroplasts (thylakoids) and are connected by the transfer of higher free energy electrons through an electron transport chain (ETC). 3. When electrons are transferred between molecules in a sequence of reactions as they pass through the ETC, an electrochemical gradient of hydrogen ions (protons) across the thykaloid membrane is established. 4. The formation of the proton gradient is a separate process, but it is linked to the synthesis of ATP from ADP and inorganic phosphate via ATP synthase. 5. The energy captured in the light reactions as ATP and NADPH powers the production of carbohydrates from carbon dioxide in the Calvin cycle, which occurs in the stroma of the chloroplast. Memorization of the steps in the Calvin cycle, the structure of the molecules and the names of enzymes (with the exception of ATP synthase) are beyond the scope of the course and the AP Exam. e. Photosynthesis first evolved in prokaryotic organisms; scientific evidence supports that prokaryotic (bacterial) photosynthesis was responsible for the production of an oxygenated atmosphere; prokaryotic photosynthetic pathways Mader, Biology, 12 th Edition Chapter 7 107

3 were the foundation of eukaryotic photosynthesis. g. The electron transport chain captures free energy from electrons in a series of coupled reactions that establish an electrochemical gradient across membranes. Evidence of student learning is a demonstrated understanding of each of the following: 1. Electron transport chain reactions occur in chloroplasts (photosynthesis), mitochondria (cellular respiration) and prokaryotic plasma membranes. 3. The passage of electrons is accompanied by the formation of a proton gradient across the inner mitochondrial membrane or the thylakoid membrane of chloroplasts, with the membrane(s) separating a region of high proton concentration from a region of low proton concentration. In prokaryotes, the passage of electrons is accompanied by the outward movement of protons across the plasma membrane. 4. The flow of protons back through membrane-bound ATP synthase by chemiosmosis generates ATP from ADP and inorganic phosphate. Enduring understanding (EU) 2.B: Growth, reproduction and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments. Essential knowledge (EK) 2.B.1: Cell membranes are selectively permeable due to their structure. b. Selective permeability is a direct consequence of membrane structure, as described by the fluid mosaic model. Evidence of student learning is a demonstrated understanding of each of the following: 4. Small, uncharged polar molecules and small nonpolar molecules, such as N 2, freely pass across the membrane. Hydrophilic substances such as large polar molecules and ions move across the membrane through embedded channel and transport proteins. Big Idea 4: Biological systems interact, and these systems and their interactions possess complex properties. Enduring understanding (EU) 4.A: Interactions within biological systems lead to complex properties. Essential knowledge 4.A.2: The structure and function of subcellular components, and their interactions, provide essential cellular processes. g. Chloroplasts are specialized organelles found in algae and higher plants that capture energy through photosynthesis. Evidence of student learning is a demonstrated understanding of each of the following: 1. The structure and function relationship in the chloroplast allows cells to capture the energy available in sunlight and convert it to chemical bond energy via photosynthesis. 2. Chloroplasts contain chlorophylls, which are responsible for the green color of a plant and are the key light-trapping molecules in photosynthesis. There are several types of chlorophyll, but the predominant form in plants is chlorophyll a. 108 Mader, Biology, 12 th Edition, Chapter 7

4 The molecular structure of chlorophyll a is beyond the scope of the course and the AP Exam. 3. Chloroplasts have a double outer membrane that creates a compartmentalized structure, which supports its function. Within the chloroplasts are membranebound structures called thylakoids. Energy-capturing reactions housed in the thylakoids are organized in stacks, called grana, to produce ATP and NADPH 2, which fuel carbon-fixing reactions in the Calvin-Benson cycle. Carbon fixation occurs in the stroma, where molecules of CO 2 are converted to carbohydrates. Enduring understanding (EU) 4.C: Naturally occurring diversity among and between components within biological systems affects interactions with the environment. Chlorophylls are an illustrative example. Concepts covered in Chapter 7 also align to the learning objectives that provide a foundation for the course, an inquiry-based laboratory experience, class activities, and AP exam questions. Each learning objective (LO) merges required content with one or more of the seven science practices (SP), and one activity or lab can encompass several learning objectives. The learning objectives and science practices from the Curriculum Framework that pertain to photosynthesis are shown in the table below. Note that other learning objectives may apply as well. LO 2.4 The student is able to use representations to pose scientific questions about what mechanisms and structural features allow organisms to capture, store and use free energy. LO 2.5 The student is able to construct explanations of the mechanisms and structural features of cells that allow organisms to capture, store or use free energy. LO 4.4 The student is able to make a prediction about the interactions of subcellular organelles. LO 4.5 The student is able to construct explanations based on scientific evidence as to how interactions of subcellular structures provide essential functions. LO 4.6 The student is able to use representations and models to analyze situations qualitatively to describe how interactions of subcellular structures, which possess specialized functions, provide essential functions. Mader, Biology, 12 th Edition Chapter 7 109

5 Key Concepts Summary Evolution of autotrophs Plants, algae, and cyanobacteria are photosynthetic. The earliest autotrophs were chemosynthetic bacteria that used H 2 S as an initial source of energy to produce carbohydrates. Autotrophs and heterotrophs are tied together by energy requirements for life. Photosynthesis The overall equation for photosynthesis shows that it is a redox reaction in which carbon dioxide is reduced, and water is oxidized: Energy + 6 CO H2O + C 6 H 12 O O 2 Photosynthesis consists of two stages: the light-dependent or light-capturing reactions (nicknamed the light reactions) and the Calvin cycle. The light reactions take place in the thylakoid membranes of the chloroplasts o Light-capturing pigments, chlorophyll a, chlorophyll b, and the carotenoids, and photosystems I and II, are embedded in the thylakoid membranes o These pigment complexes that capture solar energy and excite electrons to higher energy levels. o The oxidation (splitting) of water replaces these electrons in the reactioncenter chlorophyll a molecules o Oxygen is released to the atmosphere, and hydrogen ions (H+) remain in the thylakoid space. Chemiosmosis occurs which moves the hydrogen ions through ATP synthase to produce ATP for the dark reaction (Calvin cycle) o Free energy from the light reactions stored in ATP and NADPH are used to power the reactions of the Calvin cycle for carbon fixation o Carbon fixation is the conversion of atmospheric oxygen to a carbohydrate C 4 and CAM plants have alternative photosynthetic pathways that provide evolutionary advances to withstand harsh environmental conditions. Key Terms absorption spectrum ATP synthase autotrophs C4 Pathway Calvin cycle CAM photosynthesis carbon dioxide fixation carotenoids chemiosmosis chlorophyll a chlorophyll b cyanobacteria electron transport chain (ETC) grana heterotrophs light reactions mesophyll NADP + ozone shield photorespiration photosystem spectrophotometer stomata stroma thylakoids 110 Mader, Biology, 12 th Edition, Chapter 7

6 Teaching Strategies It is easiest to start with something that the students are familiar with, such as heterotrophs and autotrophs, including a discussion about chemosynthesizers. You will want to explain both the light and dark reactions, emphasizing the importance of the light reaction as the energy source for the dark reaction. The structure of the chloroplast needs to be fully appreciated because of the role of membranes in the function of the light reaction. Lab Investigation 5 focuses on photosynthesis. In addition to learning about the connection between light and photosynthesis, this lab provides the opportunity to work on the following quantitative skills: calculating rate, preparing solutions, preparing serial dilutions, measuring light intensity, developing and applying indices to represent the relationship between two quantitative values, using reciprocals to modify graphical representations, utilizing medians, graphing. Class time: Six 45-minute class periods Day 1: lecture on energy use and the light and dark reaction of photosynthesis 25 minutes. Activity 1: Web-based photosynthesis review 20 min. Day 2: Activity 2 10 minutes for set up, 30 minutes to let it run, 10 minutes to observe/discuss results. While Activity 2 runs: Lecture on light and photosynthesis, the cross-section of a leaf, and the structure of chlorophyll 10 minutes Class discussion on factors that can affect photosynthesis 5 minutes Activity 3: students diagram the electron transport chain 10 minutes Class discussion on factors that can affect photosynthesis 5 minutes Day 3: lecture on evolutionary responses to environmental conditions in photosynthesis and the wavelengths of light used in photosynthesis 15 minutes Class discussion to review of tissues in at leaf cross section 5 minutes Activity 4: stomata number and structure 25 minutes Day 4: lecture on data analysis and ET 50 ; inverse relationships vs. direct relationships 25 minutes Mader, Biology, 12 th Edition Chapter 7 111

7 Investigation 5 of the AP Biology Laboratory Manual 20 minutes to divide students into lab groups of three or four. Have students choose their IVs and design their own experiment, with teacher approval. Day 5: Complete Investigation 5, Part 2 45 minutes Day 6: Lab group presentation and analysis of Investigation 5 45 minutes Suggested Approaches One of the hardest concepts for students to grasp is that the function of chlorophyll is to donate electrons after solar energy has been absorbed. These initial electrons start the whole process of photosynthesis and without the initial electrons, there would not be carbon fixation. A simple demonstration can focus student awareness on these electrons. The teacher can make a solution from ethanol (250 ml) and spinach leaves (50 g) that will provide the evidence of excited electrons to students. Put spinach in a mortar and add alcohol, and mash the spinach until you form a green solution, then strain it through cheesecloth. Avoid a blender because of the possibility of sparks close to the alcohol. Decant the solution into a jar or test tube that can be tightly sealed to prevent vaporization of the alcohol while students observe the florescence. Turn off the lights in your room and shine a UV light onto the solution and it will glow red. This is impressive and students interest is captured. You can be as dramatic as you want with this and may even want to stage a crime scene and let the students use pocket UV light to find clues. For more information on data analysis techniques before assigning students Investigation 5, see the teacher information in Investigation 5 and the AP Quantitative Skills Guide that can be downloaded from the College Board website. Student Misconceptions and Pitfalls When talking about the evolution of C 4 and CAM plants, students will want to view these adaptations as something that plants changed in order to survive in a dry, hot climate. It is hard for students to remember that plants with a possible mutation that would give them a different structure actually were the plants that survived in the dry, hot climates. Several changes that were probably caused by mutations led to the different enzymes and structures that constitute today s C 4 and CAM plants. Plants did not choose to change but they changed due to mutations that gave them an advantage in their environment and enabled them to live and pass on their genes. 112 Mader, Biology, 12 th Edition, Chapter 7

8 Suggested Activities Activity One: Molecular Workbench review of photosynthesis: This website is a great location to review the process of photosynthesis and the interaction of light. It will require downloading their free, open-source software. Activity 2: The importance of membrane systems in photosynthesis Students will observe the color change of an electron acceptor (which takes the place of NADP) in the presence of a chloroplast solution (with membranes) and a chlorophyll solution (without membranes). This is a teacher-lead demonstration. Full instructions below. Demonstration: The importance of membrane systems in photosynthesis This activity will drive home the importance of the chloroplast membrane to the process of photosynthesis. It can be done as a demonstration in front of the class. The chemicals listed below (phosphate buffer, DPIP) can be ordered from a chemical supply company. The chemical DPIP can be used to take the place of NADP in photosynthesis. DPIP is an electron acceptor and receives electrons from chlorophyll and hydrogen ions to become DPIPH. It will change from blue to clear as it accepts electrons. The membranes in the chloroplast are required for the light reaction to occur. If we only chlorophyll, DPIP will not change to clear. Without the membranes there's no hydrogen ion gradient and the reaction will not occur. Supplies: Spinach leaves (100 g) Ethanol (250 ml) 0.5 M cold sucrose (250 ml) 0.1 M phosphate buffer (at least 2 ml) DPIP (at least 2 ml) Distilled water (at least 6 ml) 1. Make a solution of just chlorophyll by mashing spinach leaves (50 g) and ethanol (250mL), and then straining the solution through cheesecloth. Make a second solution of chloroplast using 50 g spinach and 250 ml of cold sucrose (0.5 M). Do not strain the second solution! 2. Set two test tubes up in a rack where students can see them. To each test tube, add 1 ml phosphate buffer, 1 ml DPIP, and 3 ml distilled water. 3. Add seven drops of chlorophyll solution (strained) to one test tube, and seven drops of chloroplast solution to the other. Mader, Biology, 12 th Edition Chapter 7 113

9 4. Place both tubes in front of a light source and check for color change after 30 minutes. Note to Teachers: The DPIP will not change color (be reduced) in the presence of only chlorophyll. This is because there is no membrane system to build up the H + concentration. The DPIP will change color in the presence of the chloroplast because there is still an intact membrane system that allows for the movement of the H + ions. Activity 3: Membrane Structure Illustration Students will illustrate the process of photosynthesis. Drawings should include a chloroplast with its membrane structure, correct placement of the electron transport chain, and ATP synthase. The drawings should include photosystem I and photosystem II, water, the production of ATP, and the production NADPH. Activity 4: Stomata Number and Structure This activity can be conducted as an inquiry based activity or as a simple search for the stomata structure. Due to the rigors of the AP curriculum, I suggest the simple activity describe below for viewing guard cells. The inch plant, Tradescantia zebrina, which is easily purchased from a local plant store, contains purple leaves with green guard cells embedded in the epidermal tissue on the lower side of each leaf. This makes viewing of the stomata very easy. Using this leaf for viewing a guard cell allows you to simply view the leaf on an slide and place it under a microscope. Required materials: 1plant (inch plant, Tradescantia zebrina) for each lab group 2 microscopes for each lab group Microscope Slides To add inquiry, you may suggest that students place the plant in various conditions (dark, colored light, bright white light) for 24 hours prior to viewing the stomata and guard cells. Place a small portion of the leaf on a slide with the lower side of the leaf facing upwards. Using their cell phone cameras, students should document the appearance of the stomata. Pictures should be identified with the conditions in which the plant existed for 24 hours prior to viewing. These pictures should be shared with the other lab groups. Students should come up with an explanation for why the guard cells are different in each condition. 114 Mader, Biology, 12 th Edition, Chapter 7

10 Student Edition Chapter Review Answers Answers to Assess Questions 1. d; 2. a; 3. c; 4. d; 5. d; 6. a; 7. d; 8. T; 9. F; 10. F; 11. d; 12. c; 13. d Answers to Applying the Big Ideas Questions 1. Scientists claim that organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Defend this claim using TWO pieces of evidence from metabolic pathways. Essential Knowledge Science Practice Learning Objective 1.B.1: Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. 6.1: The student can justify claims with evidence. 1.16: The student is able to justify the scientific claim that organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. 2 points maximum. Description of the mechanisms or structural features and their explanation may include (1 point each): Photoautotrophs capture free energy from sunlight. Plants have evolved the ability to capture solar energy and store it in carbon-based organic nutrients. With few exceptions, it is possible to trace any food chain back to plants and algae. Autotrophs that captured free energy from the environment evolved early in Earth s history, likely using reducing agents such as hydrogen sulfide as a source of electrons. The ability to use water as a source of electrons, followed by the release of oxygen into the atmosphere, likely evolved in a common ancestor of cyanobacteria, plants, algae and certain other unicellular eukaryotes. Metabolic pathways are conserved across all currently recognized domains. Mader, Biology, 12 th Edition Chapter 7 115

11 2. Construct an explanation of the mechanism and structural features of CELLS that allow organisms to capture, store or use free energy. a) Describe TWO mechanisms or structural features of cells employed for use in photosynthesis. b) Explain how the two features you described in part (a) function to allow organisms to capture, store or use free energy. Essential Knowledge Science Practice Learning Objective 2.A.2: Organisms capture and store free energy for use in biological processes. 6.2: The student can construct explanations of phenomena based on evidence produced through scientific practices. 2.5: The student is able to construct explanations of the mechanisms and structural features of cells that allow organisms to captures, store or use free energy. 4 points maximum. Description of the mechanisms or structural features and their explanation may include: Descriptions of mechanisms or Explanations (1 point each) feature (1 point each) Conversion of ATP to ADP by the freeing of a phosphate (mechanism) Free energy becomes available for metabolism by this conversion, which is coupled to many steps in metabolic pathways. Photosynthesis (mechanism) uses chlorophyll (feature) to capture energy using NADP + as its electron acceptor. The Calvin cycle (mechanism) occurs in the stroma of the chloroplast (feature). The electron transport chain (mechanism) occurs in chloroplasts (features) and prokaryotic plasma membranes. In photosynthesis, chlorophylls absorb free energy from light, boosting electrons to a higher energy level in Photosystems I and II. This metabolic pathway uses the energy captured during the light reactions of photosynthesis to produce carbohydrates (sugar) from carbon dioxide. The ETC captures free energy from electrons in a series of coupled reactions that establish an electrochemical gradient across membranes. 116 Mader, Biology, 12 th Edition, Chapter 7

12 ATP synthase is an enzyme bound to the membrane. The flow of protons back through ATP synthase by chemiosmosis generates ATP from ADP and inorganic phosphate. 3. Based on your understanding of pigment molecules and the absorption of light by chloroplasts, construct an explanation for how variation in molecular units (chlorophylls) provides cells with a wider range of functions. Feel free to refer to evidence produced through scientific practices. Essential Knowledge Science Practice Learning Objective 4.C.1: Variation in molecular units provides cells with a wider range of functions. 6.2: The student can construct explanations of phenomena based on evidence produced through scientific practices. 4.22: The student is able to construct explanations based on evidence of how variation in molecular units provides cells with a wider range of functions. 3 points maximum. Explanation may include the following (1 point each): Evidence referred to by student might include classroom investigations where chlorophyll is removed from plant cells and observations under white light and black light are compared, or where pigments from a green plant are separated using paper chromatography, or a graph might be drawn to illustrate the absorption spectrum of photosynthetic pigments where light ranges from 390 to 750 nm wavelengths and the relative absorption of chlorophyll a, chlorophyll b and carotenoids are compared. Photosynthetic organisms differ in the types of chlorophyll they contain. In plants, chlorophyll a and chlorophyll b play prominent roles in photosynthesis. Carotenoids play an accessory role. Pigment molecules absorb wavelengths of light. Most pigments absorb only some wavelengths; they reflect or transmit the other wavelengths. The pigments in chloroplasts are capable of absorbing various portions of visible light. Both chlorophylls a and b absorb violet, blue and red light better than the light of other colors. The carotenoids are able to absorb light in the violetblue-green range. Carotenoid pigments (yellow and orange) become noticeable in the fall when chlorophyll breaks down. If only chlorophyll pigments were used by a plant, solar energy would not be captured by plants in fall and winter. Mader, Biology, 12 th Edition Chapter 7 117

13 Answers to Applying the Science Practices Questions Think Critically 1. Increasing concentrations of cadmium decreased leaf size, chlorophyll content, and the rate of photosynthesis. 2. The highest concentration had the greatest effect on all three variables tested. 3. Since the light-dependent reaction and the electron transport chain are very similar, cellular respiration may decrease. 118 Mader, Biology, 12 th Edition, Chapter 7

14 Additional Questions for AP Practice 1. Describe the characteristics of the thylakoid that enable chemiosmosis to occur. 2. A genetic mutation has caused the signaling mechanism of the stoma is in a leaf to no longer function. Select the effect on photosynthesis. A) Water and oxygen and will be prevented from entering the leaf. Photosynthesis will stop. B) The leaf will not lose as much water so photosynthesis will speed up. C) The CO 2 already in the leaves will be used for photosynthesis. Photosynthesis will not be affected. D) Carbon dioxide will not be able to enter the leaf. Photosynthesis will stop. 3. Explain why the light reaction is necessary in order for the dark reaction to occur. 4. Explain how photosynthesis is a redox reaction. 5. Justify that C 4 and CAM plants have an evolutionary advantage. Mader, Biology, 12 th Edition Chapter 7 119

15 Grid-In Questions 1. Both the level of light and the biochemical pathway of photosynthesis can have an effect on the rate of energy production by plants. How many times greater is the rate of photosynthesis in C 4 sugarcane plants than C 3 maple trees at a light intensity of 0.5 cal/cm 2 /min? Express your answer to the nearest whole number. 120 Mader, Biology, 12 th Edition, Chapter 7

16 Answers to Additional Questions for AP Practice 1. The membranes are made of phospholipids. The partial charge of the phospholipid prevents hydrogen ions from moving across the membrane and a concentration gradient of hydrogen ions can be established. The movement of hydrogen ions through ATP synthase provides the energy to produce ATP. 2. D is the correct answer. 3. There are many enzymes working in the dark reaction but energy is still required in order for carbon dioxide to become fixed into a carbohydrate. The light reaction provides this energy. 4. Carbon dioxide is reduced to a carbohydrate and water is oxidized into oxygen. 5. Both of these plants are more successful in hot arid conditions than are C 3 plants. In C 3 plants, the molecule RuBP combines with CO 2 to produce 3PG which is them reduced to G3P. The molecule G3P is then converted to the other molecules that a plant needs such as glucose. However, when it is too hot outside, C 3 plants close their stoma and oxygen builds up in the airspace in the leaves. RuBP uses the oxygen and releases CO 2 which causes a decrease in photosynthesis. C 4 plants use a different molecule, PEP, to fix CO 2 and avoid photorespiration. CO 2 fixation occurs in the mesophyll cells even when the stoma are closed. The CO 2 delivered to the Calvin cycle in the bundle sheath cells in the form of malate. This method of photosynthesis produces more sugar when the conditions are hot and dry than does the C 3 pathway. CAM plants fix CO 2 at night when their stomas are open and store the C 4 molecules that are produced in vacuoles. During the day when the stoma are closed, the Calvin cycle takes place. By opening their stoma only at night, CAM plants conserve water and are able to live in stressful conditions. Answers to Grid-In Questions 1. Chapter: 7 Photosynthesis Answer: 4 =~3300 mg CO 2 /cm 2 /hr/~900 mg CO 2 /cm 2 /hr = ~3.6 Mader, Biology, 12 th Edition Chapter 7 121