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The Effect of pH on Photosynthetic Rate

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Isabel D'Alessandro

on 31 December 2013

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Transcript of The Effect of pH on Photosynthetic Rate

The Effect of pH on Photosynthetic Rate in Spinach Leaves
Question: What is the effect of pH on photosynthetic rate as measured through the rising of leaf disks?
Dependent Variable: Time Necessary for Leaf Disks to Float (T50) (seconds)
-indicative of photosynthetic rate
Independent Variable: pH
-temperature of water 22°C
-light source/ distance from light
-sodium bicarbonate concentration
-number of leaf disks and method of preparation
-time of exposure to light
-type and size of leaf disks
cup with neutral pH(water and sodium bicarbonate only) with pH of 8
cups of each pH covered with tin foil
demonstrates that rising of leaf disks is a result of interaction with light, not pH only

Experimental Design
optimal pH
photosynthesis requires the actions of enzymes which have an optimal pH at which they work
most plants typically photosynthesize in the environment with water at a pH between 5.5 and 7.0
If the pH of the solution is changed(raised above or below the baseline of 8), the rate of photosynthetic rate will decrease and the T50 for the leaf disks will be lower. If a neutral solution is used, then the rate of photosynthesis will be maximized.
1. 40 spinach leaf disks were cut out using a hole punch and kept moist on water-soaked paper towels

2. A 0.5% solution of sodium bicarbonate(NaHCO3) was prepared by adding 3.0g of sodium bicarbonate to 600mL of H2O
This step fills the air spaces in the spongy mesophyll of the leaf with water rich in carbon dioxide to facilitate photosynthesis
When oxygen is produced through photosynthesis, oxygen gas will fill the air space and the leaf disks will float again
3. 1 drop of dish detergent was added to the solution to decrease the hydrophobic bonds in the leaf and allow water to penetrate the leaf cells
5. 100mL of the sodium bicarbonate solution were poured into each of 4 plastic cups
First Cup: marked Light Control [nothing was added]
Second Cup: marked Dark Control [nothing was added]
Third Cup: marked Acid; 92 drops of HCl were added to t he solution until the pH strips showed a pH of 11
Fourth cup: marked Base; 84 drops of NaOH were added to the solution until the pH strips showed a pH of 5
4. The pH of this stock solution was measured using pH indicator strips to be 8
6. 10 leaf disks were placed in each of 4 plastic syringes

7. 15mL of the remaining 0.5% sodium bicarbonate stock solution was drawn up in each of the syringes

8. Air was expelled from the syringes by pushing the plunger towards the tip

9. A vacuum was created in each of the syringes by covering the tip of the syringe, pulling the plunger back, and releasing it, until all of the leaf disks in all of the syringes had sunk to the bottom

Intention: to suck the existing oxygen out from inside the leaf disks and replace it with water and sink, in order that their rate of floating could be measured
Data Tables
Time v. Number of Leaf Disks Floating
*Time necessary for 50% (5) of the leaf disks to float to the surface of the cup
*Time necessary for 100% (10) of the leaf disks to float to the surface of the cup
Rate of Photosynthesis(Rate of O2 production)
Acid (pH 5)
Rate of Photosynthesis(Rate of O2 production)
Dark Control(with foil)
Rate of Photosynthesis(Rate of O2 production)
Base(pH 11)
Rate of Photosynthesis (Rate of O2 production)
Combined Results
Summary/ Analysis
The results of this experiment, which demonstrated that the rate of photosynthesis and O2 production was highest in an acidic environment, followed by a basic environment, and lowest in a nearly neutral environment(pH 8), rejects our original hypothesis, which stated that photosynthetic rate would be highest at a neutral pH.

Experimental Conclusions
Scientific Theory
Explanation of Experimental Results
Other experiments have shown trends similar to this
Future Study
The experiment could be repeated using very low and very high pHs to see if in fact,enzymes were denatured, and what the effect would be on photosynthetic rate
An experiment could be performed in which sodium bicarbonate was not used(and thus would not combine with the HCl and NaOH to form carbon dioxide)
An experiment could be performed in which the pH of the soil or the water used to moisten the plant were used, and the rate of growth could be assessed
The experiment could be repeated using leaves from other types of plants to determine if the effect of pH varies based on the plant species
Experimental Design Changes/ Critiques
When preparing the syringe chambers, all of the chambers should have been kept in an equally dark environment, to prevent the leaves from undergoing photosynthesis before the timed experiment starts

A greater number of leaf disks should have been placed in each cup to eliminate the variability of individual leaf disks that could have been damaged or otherwise abnormal

A more quantitative method of measuring pH should have been used instead of the indicator strips, to eliminate the error of human judgment

More caution should have been taken to eliminate all of the soap bubbles in the cups, which could have interfered with the experiment
Future Tests and Experiments
10. Most of the water was expelled from each of the syringes. The plungers were then removed to empty 10 disks(1 syringe) into each of the 4 plastic cups

11. One of the cups( marked Dark Control) was covered with tin foil

12. All of the cups were placed 6 inches below a table light

13. A timer was used to record the time necessary for half of the leaf disks in each of the cups to rise to the surface(T50) and the time necessary for all of the leaves in each cup to rise to the surface
The results of this experiment showed that the fastest rate of photosynthesis( and O2 production) was brought about in the acidic environment(pH 5).
-The overall rate of photosynthesis as reflected by the rising leaf disks was 2.64 leaf disks per minute.
-The acidic environment also had the fastest T50(180 seconds).
The second fastest rate of photosynthesis was brought about in the basic environment(pH 11)
Rate of Photosynthesis[ slope]=0.84 leaf disks per minute
second fastest T50(577 seconds)
The slowest rate of photosynthesis was brought about in the neutral/slightly basic environment(pH 8) obtained by adding only sodium bicarbonate to water
Rate of photosynthesis[slope]=0.70 leaf disks per minute
slowest T50(715 seconds)
Dark Control:
The results of the dark control(covered in tin foil) demonstrated that a change in pH(the dark control had a pH of 8) was not sufficient to cause the leaf disks to rise. Rather, photosynthesis only occured when the plants were exposed to the light.
Katie Foster, Nicole Devaney, Jojo
Gleason-Vergados, Bridget Kelly, Isabel D'Alessandro
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