The Effect of pH on Photosynthesis

By: Matthew Holmes, Rohit Kusupudi, Dylan Joyce

How does a change in pH affect the overall rate of photosynthesis in Spinach leaves?

Prediction:Like in enzymes, pH will also affect the rate of photosynthesis. It is predicted that the optimal pH will be seven, where the leaves photosynthesis the best. A higher or lower pH will make the photosynthesis rate lower and not as efficient. Also, if the pH is really high or really low, the photosynthesis rate might stops completely.

The independent variable in this experiment was the pH of the liquid. The dependent variable was the rate of photosynthesis. The control was the pH of 7.

Hypothesis:The pH of the liquid affected the rate of photosynthesis significantly. The rate (average leaf disks rising per minute) was the highest at a pH of 7 at 1.875 disks/minute. The rate was 0.56 disks/min at a pH of 10. It was 0.44 disks/minute at a pH of 4. The rate at a pH of 1 was the lowest as no leaf disks rose at all during the 25 minute duration.

Photosynthesis worked best at the pH of 7 because that is the optimal pH for spinach plants. Acidity level affects the dispersal of important nutrients. An imbalance can block a plant's ability to absorb them. Since all other factors such as light were plentiful. The change in pH inhibited the photosynthesis rate. This did not happen at the optimal pH of 7.

Procedure:

First, using a hole puncher, punch about 15 holes out of a spinach leaf to create the needed amount of leaf disks for the experiment. Once the leaf disks are ready make a dish soap and baking soda solution (1 drop of soap, and 1 pinch of baking soda in 50 mL of water). Then, obtain a syringe and put the 15 leaf disks inside. Push the syringe down as are as possible without touching the leaves. Then use infiltration to release all of the CO2 from the disks. In order to do this put the syringe with the disks in the solution and pull back. Once the solution is in the syringe with the disks with NO air space, infiltration can begin. Cover the tip of the syringe with a finger and pull back. This will create a suction in the syringe. After 15 seconds release the finger and the leaf disks should fall from the top of the syringe. If all of the leafs did not fall repeat the process until they do. Once the leaves are infiltrated, the experiment is ready to start. Get a 100 ml beaker and fill it up with 50 mL of tap water. Using HCl, NaOH, and a pH scale of some sort. Change the pH of the water to 7 and then place the water under a light. Then quickly, pour the filtered leaves into the 7 pH water and start recording. For 25 minutes record the total number of leaves at the top. Once the 25 minutes have passed pour out the 7 pH solution and repeat the process again multiple times. However these times change the pH to 4, and then 10. Once completed repeat the process 2 more times with pure HCl, and pure NaOH.

Minute Total Leaf Disks for pH 1 Total Leaf Disks for pH 4 Total Leaf Disks for pH 7 Total Leaf Disks for pH 10

1 0 0 0 0

2 0 0 0 0

3 0 0 0 0

4 0 0 0 3

5 0 0 2 5

6 0 0 6 5

7 0 1 10 6

8 0 2 15 8

9 0 3 15 9

10 0 3 15 10

11 0 3 15 10

12 0 5 15 10

13 0 9 15 10

14 0 9 15 10

15 0 10 15 10

16 0 10 15 10

17 0 10 15 10

18 0 10 15 10

19 0 10 15 10

20 0 10 15 10

21 0 10 15 11

22 0 11 15 12

23 0 11 15 14

24 0 11 15 14

25 0 11 15 14

The graph shows the total leaf disks that have risen overall in the total amount of time at each minute. It shows that a more basic pH results in a larger effect on photosynthesis. The reason that the pH of 7 is the best is because it it’s the optimal pH for the spinach plant to photosynthesize.

pH Tested Rate of Photosynthesis (disks/min)1 04 0.67 1.875

10 0.56

The graph explains the rate of photosynthesis by finding the number of disks per minute. Like enzymes plants need an optimal pH to create the best result. The pH of 7 works the best at 1.875 d/m while the more extreme pH result in a very low rate. The rate with a pH of 1 was 0.

Discussion/Analysis

The data collected showed a clear trend that was expected. The rate (average leaf disks rising per minute) was the highest at a pH of 7 at 1.875 disks/minute. The rate was 0.56 disks/min at a pH of 10. It was 0.44 disks/minute at a pH of 4. The rate at a pH of 1 was the lowest as no leaf disks rose at all during the 25 minute duration. When plotted on a graph, the rate of photosynthesis’ coincide with the points on a pH graph. The reason for this is that the enzymes that are responsible for photosynthesis work best at a certain pH. Since spinach grows in mild climates, the optimal pH for this plant was a pH of seven like most plants. Changes in pH cause amino acids' atoms and molecules to ionize. This can change the enzyme’s active site. Enzymes' active sites are responsible for their function, so changing the active site makes the enzyme work less efficiently or make it stop working entirely. So when the pH changed, one enzyme that was affected was rubisco. Rubisco is very important since it is responsible for the fixation of carbon dioxide in the Calvin cycle which makes G3P. Since carbon dioxide was not being fixated, no gases(or not as much gas) was being released causing not as many leaf disk to float. The data collected was mostly correct except the data collected when the pH of 10 was tested. The photosynthesis rate was slightly higher than it should have been at 0.56 disks/min. It is still in the correct range, so the graph coincide with the standard pH graph, but it should have been lower. This could have been due to a fluctuation in the baking soda used. Overall, the data was correct. The rate was highest at a pH of seven since it was the optimal pH. The rate was lowest(rate of zero)at a pH of one since most of the enzymes completely denatured. The rates were low at a pH of 4 and 10, since the enzyme were still functional, but not working at their best.

In the experiment, when infiltrating the spinach leaves a sodium bicarbonate solution was used. However, exact measurements were not used to make the solution and because of this there had been too much baking soda that was added. This caused leaf disks to rise regardless of photosynthesis giving invalid data. Fortunately, this error was made aware of and was fixed.

Along with that, in some of the tests the leaves were moving up and down from floating to being in the middle of the solution. This caused the data to be incorrect because the total of the floating leaves could possibly go down from one minute to another.

Another error was that in the experiment HCl and NaOH were added in different amounts to change the pH of tap water. This lead to each solution having a different amount of liquid due to the drops added. The difference in amount of liquid could have changed the rate of photosynthesis for the leaves and been another factor other than pH.

Lastley, while doing the experiment for a pH of 14 all of the spinach leaves went to the top and floated immediately. On Top of this, other groups with the same experiment got the same results. This means there was a reaction not related to photosynthesis which was affecting the leaves rising and this could have occurred in some of the other pH’s.

Errors:

Conclusion

The hypothesis was that pH affects the rate of photosynthesis greatly. It was understood towards the conclusion of the experiment that plants (spinach in this case) need an optimal pH to make glucose most efficiently. The rate of photosynthesis graph explained this. A pH of 7 works the best because it is generally what most plants photosynthesize with. This means that each pH further away from 7 will not work as well. This pH is best for the plant as was about 1 disk/min average better than the other pH tested. The hypothesis is correct since the acidity level significantly affects the way plants photosynthesize. The pH of 10 had a rate of 0.56 d/m as the pH of 1 had a rate of 0. It is seen that a more basic pH works slightly better, but a pH of 7 allows the plants to photosynthesise with the most efficiency.

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