pH Effects on Photosynthesis Noah and Max

Research Question How does an acidic or basic pH affect the rate of photosynthesis in spinach? This is spinach

Hypothesis If leaf disks are placed in three cups, with pH's of 4, 7, and 10, respectively, the leaf disks will float more quickly in the neutral pH than in the acidic and basic environments because pure water is neutral and plants have evolved over time to photosynthesize as effectively as possible using neutral water. Thus, the rate of photosynthesis will be slower in the basic and acidic pH’s, so the leaf disks will float most quickly in the neutral cup.

Materials • Spinach leaves • Baking soda (sodium bicarbonate) • Liquid dish or hand soap • Water (room temperature) • pH probe • Single-hole paper punch • 20 mL disposable syringe, without a needle • 3 clear, wide-mouth plastic cups • Sharpie marker • Light source • Ruler • Stopwatch • Pipette • Bottle of 0.1 M HCl • Bottle of 0.1 M NaOH pH probe

Procedure 15. Hold the vacuum for about 10 seconds. Gently release. 1. Add 1/10 gram of baking soda to one plastic cup. 2. Add water 6 cm deep. 3. Stir to dissolve thoroughly. 4. Add a drop of liquid soap to the solution and stir thoroughly. 5. Divide the solution into the 3 cups, and use a marker to label them as acidic, neutral, and basic. 6. Add 0.1 M HCl and 0.1 M NaOH to the acidic and basic cups until the pH levels are 4 and 10, respectively. 7. Use the hole-puncher to punch 30 disks out of the spinach leaves. 8. Remove the plunger from the syringe. 9. Pour 10 leaf disks into the barrel of the syringe. 10. Tap the barrel on the table or in your hand, so that the leaf disks move toward the narrow end where a needle would normally go. 11. Carefully push the plunger back into the syringe, about 1/10th of the way from the tip. 12. Place the syringe tip into the neutral cup and extract the bicarbonate + soap solution until the syringe is 1/3 full. 13. Tap the syringe to drop the leaf disks down into the liquid in the syringe. 14. Holding the syringe with the tip upwards, cover the open tip with your finger. Pull back on the plunger a bit with your finger over the open tip. A vacuum will be created. 15. Hold the vacuum for about 10 seconds. Gently release. 16. Pull a vacuum 3 more times, or until all leaf disks sink in the solution. 17. Pour the infiltrated leaf disks into the neutral cup. 18. Infiltrate the other 2 sets of leaf disks as described above, then pour one set into the acidic cup and the other set into the basic cup. 19. Place the 3 cups under the light source. 20. Prepare a data table to record the number of floating leaf disks. 21. Turn the light source on and begin the stopwatch. 22. After each minute, count the number of leaf disks that are floating in each cup and record this amount in the data table. 23. After 30 minutes, stop recording and end the experiment. Procedure

Control The neutral cup was the control group in this experiment. No treatment was given to the neutral cup, whereas the acidic and basic cups were given treatment to modify their pH’s. By ensuring that all other variables were constant, the experiment displayed results that occurred solely based on the pH of the water. These results could be compared to the control group to analyze their effects.

Results Day 1 Day 2 pH Leaf Disk Floatation Data

Results (Continued) Day 1 Day 2 This graph shows the number of floating leaf disks in all 3 cups over the 30 minutes on day 1. At the end of the experiment, the neutral cup had all 10 disks floating, the acidic cup had 3 floating, and the basic cup had none floating. This graph shows the number of floating leaf disks in all 3 cups over the 30 minutes on day 2. At the end of the experiment, the neutral cup had all 10 disks floating, the basic cup had 2 floating, and the acidic cup had none floating.

Analysis According to the data collected in both experiments, the leaf disks floated the most quickly in the neutral cup. On day 1, all 10 of the leaf disks in the neutral cup were floating by 15 minutes. However, by the end of the experiment, only 3 disks were floating in the acidic cup and none were floating in the basic cup. On day 2, all 10 of the leaf disks in the neutral cup were floating by 10 minutes. However, by the end of the experiment, only 2 disks were floating in the basic cup and none were floating in the acidic cup. Both of these results confirm that the leaves floated the most quickly in the neutral cup. There were some unusual data points in the tables and graphs. The two experiments produced different results in terms of the acidic and basic cups. On day 1, 3 disks were floating in the acidic cup and none were floating in the basic cup at the end of the experiment, but on day 2, 2 disks were floating in the basic cup and none were floating in the acidic cup at the end of the experiment. These differences are irregular and are probably the result of an experimental error. On the whole, however, both graphs had the same trend. In the neutral cup, all the leaf disks floated quickly, and in the acidic and basic cups, only a few disks were floating after the 30 minutes. This common trend can be explained scientifically. The key carbon-fixing enzyme in the process of photosynthesis, RuBisCO, has an optimal pH of around 8. When the pH is raised or lowered, the rate of photosynthesis is slower because the RuBisCO works slower. Furthermore, many of the other enzymes and proteins that aid photosynthesis have optimal pH’s of around 7 or 8. Thus, these enzymes denature under pH’s that are either very acidic or basic, which is why the leaf disks floated the most quickly in the neutral environment.

Possible Error The pH of the water in the cups may have been inaccurate on day 2. The distilled water was measured at around an 8.6 pH on the pH probe, so it is suspected that the pH probe was not taking accurate measurements. This could have caused the leaf disks to rise at a different rate because the pH of the water could have been closer or farther from the optimal pH of the enzymes that help photosynthesis. This could explain the discrepancy between the two experiments concerning the acidic and basic cups.

Conclusion The hypothesis claimed that the rate of photosynthesis would be slower in acidic and basic environments, so the leaf disks would float most quickly in the neutral cup. By examining the data, the hypothesis appears to be correct. In both experiments, all of the leaves floating after the 30 minutes. On the other hand, in the acidic cup, there were 3 disks floating by the end of the experiment in day 1 and none in day 2, and in the basic cup, there were no disks floating by the end of the experiment in day 1 and 2 in day 2. Thus, the collected data supports the hypothesis in claiming that the rate of photosynthesis in spinach is slower under a basic or acidic pH.