| Explore | We got our idea for our lab from our two previous labs we did in class: peroxidase and floating leaf disk. From the peroxidase lab we used the idea of using different pHs. In that lab we recorded the color change from an indicator after we combined the different pHs with a .2 bicarbonate solution but in our experiment we are just using the different pH too see how it affects the rate of photosynthesis. The floating leaf disk lab provided us with the leaf disks to experiment on and the method in order to vacuum the oxygen out of the leaf disks in order to form our experiment. From the leaf disk lab we experimented with and without CO2 and found that in order for the leaf disks to rise to the surface we must use CO2. So in our experiment we will use the different pHs to judge the rate of photosynthesis from the leaf disks under the lamp. |
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| Research Question | Question: How does pH affect the rate of photosynthesis? From previous experiments, we observed how differing pHs affect the rate of oxygen released from the decomposition of H2O2. We also knew that oxygen is released during the process of photosynthesis and wondered if pH would also have an effect on the rate of photosynthesis. We observed how CO2 affects the rate of photosynthesis in our floating leaf disk lab. From this lab, we formed the backbone of our experiment through the use of measuring the rate of photosynthesis by placing hole punched leaf disks under a light source and placing them in differing solutions. We adapted these solutions to fit our experiment by also including pHs. |
| Predictions | We predict that of the tested pHs, pH 7 will be the most effective in catalyzing the rate of photosynthesis due to its neutrality; it is pure water without any contaminants. We hypothesize that having a pH closer to the highly acidic or highly basic ends of the pH scale could denature the enzymes in the plant. |
| Experimental Design | Materials: Spinach leaves, Hole puncher,, Sodium Bicarbonate 1.5%, 8 500 mL flasks, 8 500 mL clear plastic cups, pH buffers 8-10, Scale, 400 mL of DI water, Graduated cylinder, stir rods, weighboats, parafilm, 2 desk lamps (60 Watts), Syringes, Timer, Masking tape. Variables: Our independent variable will be the pH buffer used and our dependent will be the rate of photosynthesis that occurs. Variables kept in constant will be the amount of solution and pH buffer in each cup, the wattage and type of desk lamp used, the amount of spinach disks used, the temperature and distance from the light source to the cup, the type of cup. Procedure: 1) Measure 100mL of DI H2O into a clear, plastic cup using a graduated cylinder. Twist open pH 10 buffer and pour contents into the DI H2O. Stir with stirring rod until dissolved. 2) Pour half of the DI solution (50 mL) into a separate plastic cup. 3) Using a weigh boat and scale, measure 0.75 g of Sodium Bicarbonate. Pour into one 50- mL portion and thoroughly dissolve. 4) Using masking tape, label the cup containing the Sodium Bicarbonate "with CO2." Label the cup without sodium bicarbonate "without CO2." Be sure to include pH 10 on both labels. 5) Using a hole puncher, punch out 20 spinach leaves; 10 per cup. Take plunger out of empty syringes and insert leaf disks. Reinsert plunger. 6) push plunger in until little air (<10%) remains in the barrel. Insert tip of syringe into "without CO2" cup and draw some of the solution into the barrel, about half way. 7) Invert syringe so tip faces the ceiling. Push plunger in to rid excess air. Place thumb over tip and draw back plunger, shaking the syringe. Hold for ten seconds. Release plunger and invert syringe so no leaf disks are stuck to sides. Repeat this process until all leaf disks have sunk to the bottom of the solution. 8) Repeat this process with the "With CO2" solution. Try to act quickly so the disks do not begin to photosynthesize. 9) Empty disks into respective solutions. Place cups under desk lamp so the bulb is 1 inch away from the top of the cup. Make sure no disks are stuck to sides and are all sunk on the bottom. Turn on light and start timer. 10) Record how many leaf disks have floated to the surface after every minute. Swirl solution to dislodge leaf disks every minute. Continue until all 10 disks are floating from the "with CO2" solution. 11) Repeat this process for each pH. You may want to store solutions in 500 mL flasks and cover with Parafilm while not in use to prevent evaporation. Note: you do not need a pH 7 buffer; DI water is sufficient. Note: you may also want to test other pHs to broaden your result pool. For our experiment, we also tested pH 2 and 12 to gain a comparison on highly acidic and highly basic pHs on the rate of photosynthesis. |
| Conclusion | From our experiment, we concluded that a slightly basic solution is the best for photosynthesis based on our data on pH’s 7-10 . The reaction occurred the fastest at pH 9 and the slowest at ph 7. The fastest rates were around pH 8 and 9 but slower at pH 7 and 10. PHs 8 and 9 have the optimal pH for photosynthesis to take place the fastest it can and catalyze the rate of the reaction. This is because rubisco, an important enzyme involved in the Calvin Cycle, functions at its optimum rate around pH 8 and 9. Increasing or decreasing the pH causes the reaction to occur more slowly. The data we collected is present in our 2 pictures we have, a data table and a graph of time vs number of floating leaf disks in CO2. As you can see in the multi colored graph, pH 9 made more leaf disks float in the shortest amount of time. In order to expand results on this experiment we could test different pHs but with the same experiment. The range of PHs was not very wide, so in order to see how photosynthesis is affected by more acidic PHs we would have to run the experiment again. Also, with a bit of extra time, we tried to expand our knowledge on the experiment and tested pH 2 and pH 12. pH 12 supported our claim and it took longer time for all of the leaf disks to float because of the higher pH. On the other hand, pH 2 was very acidic and the leaf disks floated almost immediately, which did not support our claim. We think that with the pH 2 the solution was extremely carbonated so the leaves photosynthesized quickly in the presense of so much added CO2. To be able to apply these results to the rest of our experiment, in the future we would test a wider range of pH levels and conduct more research on the effects of acid solutions on plants. Within our experiment, we had some sources of error. The spinach that we used may have started to go bad and wilt towards the end of our experiment, which could have altered our results and may have affected the time taken to photosynthesize. Also, we had to restart the experiment due to contamination of the solutions that did not contain CO2. We witnessed the leaf disks within the solutions photosynthesizing though there was not meant to be any carbon dioxide present. To sum it up, our core data supported our conclusion that spinach photosynthesizes ideally around pH 8-9, and that the rate of this reaction slows as the pH becomes further from this ideal pH. In the future, we would like to expand our results, especially to include more focus on acidic solutions, and compare them to our results from this experiment. |
| Investigation Theme | POS |
| Grade Level | High School Students (Grades 9,10,11,12) |
| School Name | Mother of Mercy High School |
| Session | Fall 2017 |