PHOTOSYNTHESIS Student Packet SUMMARY LIVING SYSTEMS REQUIRE ENERGY AND MATTER TO MAINTAIN ORDER, GROW AND REPRODUCE Energy input must exceed energy lost and used by an organism. Organisms use various energy-related strategies to survive. Energy deficiencies may be detrimental to individual organisms, populations and ecosystems. Autotrophic cells capture free energy through photosynthesis and chemosynthesis. (In contrast, heterotrophs harvest free energy from carbon molecules produced by other organisms). While chemosynthesis captures energy present in inorganic chemicals, photosynthesis traps free energy present in sunlight and uses it to produce carbohydrates: CO 2 + H 2 O + light energy C 6 H 12 O 6 + O 2. Glucose can be converted to other organic molecules and provide the energy and building materials for ecosystems. Photosynthesis takes place in chloroplasts and consists of two main parts: light-dependent reactions and lightindependent reactions (a.k.a. Calvin or Calvin-Benson cycle). Light Light reactions thylakoids ATP NADPH Calvin cycle stroma Organic molecules (carbohydrates) The light reactions in the thylakoid membrane split water, releasing O 2 and converting light energy into chemical energy in the form of ATP and NADPH. NADPH is an electron acceptor/coenzyme. NADP + + H + + 2e - NADPH The Calvin cycle in the stroma forms carbohydrates from CO 2, using ATP for energy and NADPH for reducing power. Photosynthesis is a redox reaction: H 2 O is oxidized as it loses electrons during the light reactions and CO 2 is reduced as it accepts electrons during the Calvin cycle. LIGHT REACTIONS: Light is a form of electromagnetic radiation. Molecules that absorb light in the visible spectrum are called pigments. The most important photosynthetic pigment is chlorophyll. The absorption of a photon puts a chlorophyll pigment into an excited state that has more energy. This energy can be transferred through other chlorophylls to one in the reaction center of a photosystem. An excited chlorophyll pigment transfers electrons to electron carriers embedded in the electron transport chain of the thylakoid membrane. In photosystem II, the oxidized chlorophyll regains electrons by splitting H 2 O. The flow of electrons in the electron tranport chain is used to generate an H + gradient across the thylakoid membrane. ATP synthase uses this proton-motive force to synthesize ATP. Excited electrons from photosystem I also pass through several membrane-bound carriers and end up reducing NADP + to NADPH. The electrons lost from photosystem I are then replaced by the electrons that were excited and subsequently lost from photosystem II. 1
CALVIN CYCLE: The cycle consists of carbon fixation, reduction and regeneration of RuBP. RuBP is the initial CO 2 acceptor and the enzyme rubisco catalyzes this reaction. ATP and NADPH from light reactions are used to form G3P (a.k.a. PGAL). One molecule of G3P exists the cycle per three CO 2 molecules fixed and is then converted to glucose and other organic molecules. LINK TO EVOLUTION: Photosynthesis first evolved in prokaryotic organisms; scientific evidence supports that bacterial photosynthesis was responsible for the production of an oxygenated atmosphere and prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis. 2
MULTIPLE CHOICE QUESTIONS 1. Photosynthesis has two separate processes that work together to convert solar energy into chemical energy: light-dependent reactions (light reaction) and light-independent reactions (Calvin Cycle). Which of the following statements is incorrect about these two processes? a. During light reactions, light energy is converted into chemical energy in the form of ATP and NADPH. b. During light reactions, an enzyme catalyzes splitting of a water molecule. Chlorophyll molecules absorb the electrons, H + ions are released into the thylakoid space and oxygen gas is formed. c. During the Calvin cycle, energy of ATP and NADPH is used to form carbohydrates. d. During the Calvin cycle, water is split, releasing oxygen gas. The hydrogen atoms are used in carbohydrate synthesis. Questions 2-3 The pigment molecules in photosynthetic organisms absorb specific wavelengths of light. In 1883, Thomas Engelmann devised an experiment to learn which wavelengths of light were the most effective in carrying out photosynthesis in the filamentous alga Spirogyra. Engelmann illuminated the algae with light that passed through a prism, thus exposing different segments of algae to different wavelengths of light. He added aerobic bacteria and then noted in which areas the bacteria congregated. violet blue green yellow red 2. What did Engelmann conclude about the congregation of bacteria in the areas of red and blue light? a. Bacteria congregated in these areas due to an increase in the temperature of the red and blue light. b. Bacteria congregated in these areas due to an increase in the temperature caused by an increase in photosynthesis. c. Bacteria congregated in these areas because these areas had the most oxygen. d. Bacteria are attracted to red and blue light and thus these wavelengths are more reactive than other wavelengths. 3
3. Consider the two different types of algae shown below. Each type has the same number of chloroplasts but their chloroplast distribution in the cell varies. Which of the following statements correctly describes which cell(s) would be best suited for Engelmann s experiment? Assume that about three algal cells would cover the width of the visible spectrum on a microscope slide. a. Either of the two cells could be used because both are capable of absorbing light and producing oxygen in photosynthesis. b. Either of the two cells could be used because they both contain the same number of chloroplasts. c. The top cell should be used because the arrangement of chloroplasts allows for the entire length of the cell to respond to light. d. The bottom cell should be used because the high concentration of chloroplasts in one area allows for all wavelengths of light to be absorbed. 4. Cyanobacteria, also known as blue-green algae, are responsible for the initial oxygenation of the Earth's atmosphere through photosynthesis. Cyanobacteria are also thought to be the precursors of the chloroplasts that are found in true algae and plants. The photosynthetic pigments in cyanobactera include phycocyanin, carotenoids and chlorophyll. In contrast, plant cells contain carotenoids and chlorophylls but not phycocaynin. The absorption spectra of these pigments are shown in the graph below. violet blue green yellow orange red Which of the following is the least likely explanation for the lack of phycocyanin in plants? a. Genetic drift eliminated pigments that reflected blue wavelengths of light. b. Natural selection favored organisms with pigments that reflect blue wavelengths of light. c. Mutations altered the pigments used by the organisms. d. Chloroplasts evolved from cyanobacteria with pigments that absorbed mostly blue and red wavelengths of light. 4
5. Chemiosmosis drives the synthesis of ATP in photosynthesis and cellular respiration. Which statement is incorrect about chemiosmosis in photosynthesis? a. The movement of electrons through the electron transport chain is used to energize the proton pumps and generate a proton concentration gradient. The proton gradient is necessary for the proton-motive force to synthesize ATP. b. Movement of hydrogen protons down their electrochemical gradient is used to synthesize ATP. c. The active transport of protons into the thylakoid, followed by the passive transport of protons out of the thylakoid is an example of energy coupling. d. ATP synthesis is driven by diffusion of electrons through the ATP synthase. 6. The following diagram represents an experiment with isolated thylakoids. The thylakoids were first made acidic by soaking in a solution at ph 4. After the thylakoid space reached ph 4, the thylakoids were transferred to a basic solution at ph 8. The thylakoids then made ATP in the dark. How were the thylakoids able to make ATP in the dark? a. Soaking thylakoids in the ph 4 solution created high proton concentration inside the thylakoid. Since the thylakoids were placed in the ph 8 solution with lower proton concentration, protons were able to diffuse out of the thylakoids through ATP synthase and provide energy for ATP synthesis. b. Soaking thylakoids in the ph 4 solution created low proton concentration inside the thylakoid. Since the thylakoids were placed in the ph 8 solution with higher proton concentration, protons were able to diffuse into the thylakoids through ATP synthase and provide energy for ATP synthesis. c. Soaking thylakoids in the two ph solutions excited electrons which then combined with ADP and Pi to form ATP. d. Light is not necessary for the production of ATP in the thylakoids; the thylakoids were able to produce ATP by Calvin cycle. 7. What are the final electron acceptors of the electron transport chain in photosynthesis and in cellular respiration? a. H 2 O in photosynthesis and O 2 in cellular respiration b. NADP + in photosynthesis and NAD + /FAD in cellular respiration c. CO 2 in photosynthesis and O 2 in cellular respiration d. NADP + in photosynthesis and O 2 in cellular respiration 5
MATH GRID IN 1. An experiment involving a species of aquatic plant Alternanthera philoxeroides (alligator weed) was conducted to measure primary productivity. The plant was placed in a mason jar and then covered with the same Everglades swamp water in which the plant was growing. The initial amount of dissolved oxygen in the water was determined to be 5.64 mg O 2 /L. The mason jar was then placed under a light source for 24 hours. The final reading for dissolved oxygen was 11.83 mg O 2 /L. Calculate how much carbon was fixed in mg/l over the 24-hour period. Record your answer to the nearest hundredth. 2. In an experiment measuring the rate of photosynthesis, trapped air was removed from the spaces in the photosynthetic tissue of leaf disks and the spaces were infiltrated with a sodium bicarbonate solution. The leaf disks were then exposed to light and observations were made as the cells underwent photosynthesis. # of Floating Disks Over Time Time (min) 1 2 3 4 5 6 7 8 9 10 Number of 0 0 1 3 5 7 9 10 10 10 floating disks Determine the mode, median and range for the data set. Give your answers to the nearest whole number. Mode Median Range 6
SHORT FREE RESPONSE QUESTIONS 1. The total amount of energy that producers capture and convert to chemical energy by photosynthesis is called gross primary productivity. Not all of the captured energy is available to primary consumers because primary producers use some of it to fuel cellular respiration and other catabolic processes. The amount of energy stored in primary producers tissues per unit of time is called net primary productivity. The graph below depicts the net primary productivity in a freshwater pond. Explain the data presented by the graph, including a description of the relative rates of metabolic processes occurring at different depths of the pond. 7
2. Chloroplasts are specialized organelles found in algae and higher plants that capture the energy in sunlight and convert it to chemical energy in carbohydrates. Discuss THREE structural features of chloroplasts that allow photoautotrophs to capture free energy and convert it to chemical energy. LONG FREE RESPONSE QUESTION A controlled experiment was conducted to analyze the effects of darkness and boiling on the photosynthetic rate of incubated chloroplast suspensions. The dye reduction technique was used. Each chloroplast suspension was mixed with DPIP, an electron acceptor that changes from blue to clear when it is reduced. Each sample was placed individually into a spectrophotometer and the percent transmittance was recorded. The three samples used were prepared as follows: Sample 1 Chloroplasts + DPIP and placed in front of a light source Sample 2 Chloroplasts + DPIP and wrapped in foil and set in front of a light source Sample 3 Boiled chloroplasts + DPIP and placed in front of a light source Time (minutes) Sample 1 transmittance Sample 2 transmittance Sample 3 transmittance 0 28.8 29.2 28.8 5 48.7 30.1 29.2 10 57.8 31.2 29.4 15 62.5 32.4 28.7 20 66.7 31.8 28.5 a. On the axes provided, construct and label a graph showing the results for the three samples. b. Identify and explain the control or controls for this experiment. c. The differences in the curves of the graphed data indicate that there were differences in the number of electrons produced in the three samples during the experiment. Discuss how electrons are generated in photosynthesis and why the three samples gave different transmittance results. 8
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