Photosynthesis ada

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University of Maryland Global Campus (UMGC) *

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1407

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Biology

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Apr 3, 2024

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Photosynthesis Photosynthesis is the process in which plants, algae, and some microorganisms capture light energy and convert it to chemical energy. The chemical reaction for this process is: 6CO 2 + 6H 2 O + light energy  C 6 H 12 O 6 + 6O 2 Carbon dioxide + water + light energy  glucose + oxygen In plants, photosynthesis takes place in organelles called chloroplasts . The structure of the chloroplast includes two membranes, a fluid called stroma , and flattened membranous sacs named thylakoids which are stacked in structures called grana . Chloroplasts are mainly found in the leaves of plants, but can also be present in other green parts of a plant. Figure 1: Chloroplast Structure Photosynthesis is divided into two major reactions: the light-dependent reactions that occur in the thylakoid membrane of the chloroplast and the light-independent reactions , or Calvin Cycle , that occur in the stroma of the chloroplast. Observe Figure 1 and the chloroplast model in the lab room to identify the location of these reactions within this organelle. During the light-dependent reactions of photosynthesis, light energy from the sun hits the plant leaf and is absorbed by the pigment chlorophyll . Chlorophyll transfers the light energy through a series of steps to the energy carriers, ATP and NADPH. In the second set of reactions, known as the light-independent reactions or the Calvin Cycle, CO 2 from the atmosphere enters the plant leaf and uses the energy from the light-dependent reactions, ATP and NADPH, to make a glucose molecule. Figure 2: Overview of Photosynthesis 1

LEARNING OUTCOMES Students will determine the function of CO 2 in the process of photosynthesis, by measuring its consumption by a plant exposed to light. Plot spectrophotometer data of chlorophyll a and chlorophyll b to determine the absorption spectrum for these pigments. MATERIALS NEEDED FROM YOUR ESCIENCE LAB KIT Bromothymol blue 5 test tubes and rack 5 mL bromothymol blue large plastic beaker 10 mL graduated cylinder 100 mL graduated cylinder PPE kit (lab coat, lab gloves, and lab goggles) MATERIALS NEEDED FROM THE GROCERY STORE (OR FROM HOME ) Elodea ( Anacharis ) – from pet supply store Another plant type – leaf from tree/ bush/ flower around house; spinach leaves paper towels PICTURES NEEDED FOR LAB REPORT photograph of you holding your TWO DIFFERENT types of plants: plant leaves and aquatic plant (Elodea/ Anacharis ) with your face clearly visible photograph of ALL of the test tubes for the experiment showing the initial color of the bromothymol blue at the BEGINNING of the experiment. photograph of ALL of the test tubes for the experiment showing the final color of the bromothymol blue in all test tubes at the END of the experiment. Lift plant leaves so color of bromothymol blue is visible. Investigation 1: Observing Photosynthesis through CO2 Consumption Photosynthesis uses carbon dioxide and water to produce glucose and oxygen. In this investigation, we will use a living plant, such as the water plant, Elodea , to observe the process of photosynthesis. The pH indicator bromothymol blue will estimate the amount of CO 2 present in the test tubes. When CO 2 is added to a solution in the test tube, it causes a decrease in pH because the CO 2 reacts with the H 2 O to produce carbonic acid. CO 2 + H 2 O  H 2 CO 3 (carbonic acid) This reaction is reversible, so when CO 2 is removed from the solution (such as when it is used during photosynthesis), the acid is removed and the pH increases, or becomes basic. Bromothymol blue is an acid base indicator. It changes colors based on the solutions pH. ‐ 2

Table: Blue in a base (pH > 7) Green at neutral pH (pH 7) Yellow in an acid (pH < 7) Figure 1: Bromothymol blue indicator When you blow exhaled air through a straw into a solution containing bromothymol blue, you are adding CO 2 which reacts with the water to create carbonic acid, thus lowering the pH and turning the solution green first, then yellow if you continue to add CO 2 . CO 2 + Bromothymol blue+ H 2 O  H 2 CO 3 (carbonic acid) Blue Bromothymol blue  Yellow Bromothymol blue Base  Acid Procedure: 1. Obtain two different plants. You will need two leaves (if medium/large) or sprigs (short stem segment with several small leaves) from each plant. For best results, it is preferable if your first plant is an aquatic/aquarium plant (row 1 below). Table 1: Live Plant Options NAME PICTURE WHERE CAN I GET IT? Anacharis Also called Elodea (or any variety of live aquatic/aquarium plant) PetSmart PetCo Any pet store with a fish section Fresh spinach leaves Any grocery store Sold as a bunch or in bags Leaves from household plants or outside plants N/A 2. Using a graduated cylinder, measure out 5 mL of bromothymol blue into a large beaker. Add 45 mL of water to the beaker and swirl to mix. The solution will be light blue (basic pH). 3

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3. Insert one end of a drinking straw into the beaker so the bottom end sits just above the surface of the solution. Do not immerse the straw into the bromothymol blue. Making sure your safety glasses are in place, gently blow through the straw onto the surface of the bromothymol blue. When you need more air, make sure to remove your mouth from the straw before breathing in. 4. Keep blowing until the solution in the beaker changes to a green color (indicating the pH has decreased from basic to neutral). DO NOT keep blowing after it turns green or you will add too much CO 2 and your experiment will take too long to finish. 5. Swirl the solution to mix and ensure it is uniformly green. 6. Measure out 10 mL of the green bromothymol blue into each of your 5 test tubes. Number the tubes 1-5. 7. If you were able to obtain an aquatic plant such as Elodea , place a sprig of it into Tubes 1 and 2 (if you don’t have an aquatic plant, place either a sprig or two good-sized leaves of your first plant into Tubes 1 and 2). Place your second plant leaf into Tubes 3 and 4. Tube 5 will not have any plant leaves added – it will be a negative control with solution only. 8. Take Tubes 1 and 3 and cover the two test tubes with Aluminum foil so no sunlight can reach the solution. Place them back on the test tube rack. If you do not have aluminum foil, you can place Tubes 1 and 3 into a dark cabinet for the duration of the experiment instead. 9. Keep Tubes 2, 4, and 5 (two with plant leaves and one with only green bromothymol blue) uncovered and on the test tube rack. 10. Leave the test tube rack in full view of bright sunlight for at least two hours. If sunlight is not available, you may use a bright desk lamp ONLY if you can safely bend the lamp so the light shines directly towards the test tubes. 11. After 2 hours, examine all 5 test tubes. Note any changes in the color or appearance of the bromothymol blue solutions in each tube. Record your results in Table 1. Table 2: Photosynthesis and Bromothymol Blue Test tube Initial Bromothymol blue Color Final Bromothymol blue Color Conclusions: What happened? Why? Plant Type #1: Light Green Dark Blue/ Green Due to the light the plant had given off chorophyll into the water. Plant Type #2: Light Green Dark Blue/Green Due to the light the plant had given off chorophyll into the water Plant Type #1: Dark Green Slightly Green Due to the light the plant had given off chorophyll into the water Plant Type #2: Dark Green Light Blue/Green The plant absorbs the bromothymol blue 4

Test tube Initial Bromothymol blue Color Final Bromothymol blue Color Conclusions: What happened? Why? Bromothymol blue only Green Light Blue The water made some of the bromothymol evaporate Conclusion Questions: 1. Why did the bromothymol blue change colors when we blew through the straw into the Erlenmeyer flask?  When I had blew into the bromothymol with a starw, that gave the color change due to the carbon dioxide and its natural pH 2. What is bromothymol blue an indicator for?  It indicates pH 3. What does the color change in bromothymol blue in the white light indicate in terms of the Elodea?  Decrease of pH 4. Did you see a change in the color of the bromothymol blue test tube wrapped in Aluminum foil? Why or why not?  Yes, during the two hour I did see a change in color while being covered in foil. Because it is the Calvin cycle and the light independent reaction of photosynthesis 5. Identify and explain the control in this experiment. Why do we have a control?  6. Carbon dioxide is a greenhouse gas, in part responsible for the warming of the planet. Why do some people believe that planting trees can help in this matter? 7. How does a plant use carbon? What is meant by the term carbon fixation? 5

8. Hypothesize what would happen to the bromothymol blue, if you had placed your test tube in front of a green light? Would you see a change in the color of the bromothymol blue? Why or why not? Investigation 2: The Absorption of Light by Chlorophyll A pigment is a substance that absorbs light of particular wavelengths. For example, the green color of leaves is due to a pigment called chlorophyll. When white light hits the chlorophyll pigment molecule, the chlorophyll absorbs most of the red, orange, blue and violet wavelengths and it reflects most of the green and yellow wavelengths. That is why the leaf appears green. Pigments absorb to some degree all of the colors of the white length spectrum accept the one you see. A spectrophotometer is an instrument that is used to measure the amount of light absorbed by a pigment. It works by dispersing visible light into its component colors or wavelengths. The disappearance from the spectrum of various colors, or wavelengths, as a result of passing light through a pigment solution, means those wavelengths were absorbed by the pigment. Taking the ability of a chlorophyll pigment solution to absorb light and plotting it against the wavelengths of light shows the absorption spectrum for chlorophyll. Procedure: 1. Plot the data in Table 2 on the provide graph. Answer the conclusion questions once completed. Table 2: Chlorophyll a and b absorption Wavelength (nm) Chlorophyll A % Absorption Chlorophyll B % Absorption 400 nm 30 0 425 nm 60 30 450 nm 10 70 500 nm 5 0 550 nm 5 5 600 nm 10 10 625 nm 10 30 650 nm 45 10 700 nm 10 10 Graph 1: Absorption Spectrum of Chlorophyll 6

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400 450 500 550 600 650 700 0 20 40 60 80 100 Absorption Spectum of Chlorophyll a and b Wavelength (nm) % Absorption Violet Indigo Blue Green Yellow Orange Red Conclusion Questions: 1. What wavelengths of light does chlorophyll a absorb best? What color are these? 2. What wavelengths of light does chlorophyll b absorb best? What colors are these? 3. What do you think happens to the light that is not absorbed? 4. Using the information from the graph above, explain why plants are green. 7

5. The plant pigment xanthophyll absorbs light in the range of 400 – 550 nm and reflects the other wavelengths. What color light does xanthophyll absorb? What color do you think xanthophyll is? 6. Explain the benefit of having different kinds of pigment in the chloroplast, instead of only having chlorophyll a. 8

Graph 2: Rate of Photosynthesis 400 450 500 550 600 650 700 0 20 40 60 80 100 Rate of Photosynthesis in visible light spectrum Wavelength (nm) % Photosynthesis occuring Violet Indigo Blue Green Yellow Orange Red 7. In Graph 2: Rate of Photosynthesis, why does photosynthesis occur at a 30% rate in the green wavelengths of light and at a 90-100% rate in the blue and red wavelengths of light? Use your knowledge of plant pigments to explain. 8. What would be the best color of light for photosynthesis? Explain why. Created by Pebble Barbero 9

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