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Photosynthesis is the process by which photosynthetic organisms can convert carbon dioxide and water to sugar, oxygen, and water, using energy from light. The equation for photosynthesis is 2H2O + CO2 + light = carbohydrate (CH2O) + O2 + H2O (Investigation 5: Photosynthesis). Photosynthesis takes place in the chloroplasts of a photosynthetic organism, usually a plant. In the chloroplasts, disks called thylakoids hold a pigment called chlorophyll. Chlorophyll absorbs light (except green light, which it reflects) and the electromagnetic energy is transferred to chemical energy during the light reactions in the thylakoids. The chemical energy (in the form of ATP and NADPH) then moves to the Calvin Cycle in the stroma, where it is made into sugar using CO2.

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In this experiment, leaf chads  are placed in a CO2 containing solution (water and baking soda) and put under a lamp to induce photosynthesis. Before being placed in the solution, the leaf chads were vacuumed in order to replace the oxygen in them with water so that they would sink in the solution. The time it takes the chads to rise from the bottom of the beaker to the surface of the solution is measured. The rate of how many disks rise per minute is used as the rate of photosynthesis. The ET50 (amount of time it takes half of the chads to rise) will also be calculated. The first set of trials uses the CO2 solution with just baking soda (pH 6) and the second set uses a solution with pH 5 (addition of 25 mL vinegar- more acidic). This variation will reveal how pH effects the rate of photosynthesis of the leaf chads.

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If the rate of photosynthesis of leaf chads is measured using solutions of different pH, then the chads in lower/more acidic solutions will have a slower rates.

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The controlled variable of this experiment is the first set of trials with the pH 6 solution (baseline). The independent variable is the pH of the CO2 solution (decrease/more acidic). The dependent variable is the rate of photosynthesis and the ET50. The constants in the experiment are the size of the chads, the type of leaf, the intensity of the light, the amount of CO2 solution, and the temperature of the solution.

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According to the data, the rate of the baseline trials (pH of 6) were faster than the rate of the experimental trails (pH of 5). The average rate of photosynthesis for the baseline trials was 230% higher than the rate of the experimental trials. Comparing the average rate of the first trial, the baseline rate (1.32 d/min) was 175% higher than the experimental rate (.48 d/min). In the second trial, the baseline rate (4.02 d/min) was 458% higher than the experimental rate (.72 d/min). In the third trial, the average rate for the baseline trial was .6 d/min, and there was no third trial for the experimental trials. The ET50 is the time that it takes half of the chads to rise to the top. The fastest ET50 was trial two of the baseline experiment, with 1:35. The slowest was trial two of the experimental trials, with 6:40. The average ET50 for the baseline trials was 4:56 and for the experimental trials it was 6:37. The ET50 also reflects that the baseline trials were faster than the experimental trials.

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My hypothesis, that the rate of photosynthesis would decrease with a lower pH solution, was accepted by the data. The overall rate of the chads in the pH 6 solution (baseline) was 230% higher than the solution with pH 5 (experimental). The lower pH of 5 made the rate decrease because the proteins most likely denatured due to the solution becoming more acidic. The ET50 also reflects that a lower pH causes a lower rate. The average ET50 of the pH 6 solution was 4:56 for the solution with pH 5 it was 6:37, a difference of 1:41.

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The major error made in this experiment is not having a third trial for the experimental trials. Due to the error of not turning the lamp on, the second trial had to be redone, leaving no time to do a third one. This could be fixed by making sure all materials of your experiment are ready. Another error was not completely vacuuming the leaf chads. Although they appeared to sink in the syringe, a few of the chads floated around the bottom and would not sink completely. The fix for this error is just making sure all the chads are fully vacuumed.

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One way this experiment could be improved is to lower the pH of the solution used in the experimental trials even more, so that the change will be more drastic and visible in the data. Another improvement could be to do more trials than three because some trials go strangely fast or slow, so more trials would mean more accurate data.

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Investigation 5: Photosynthesis. Investigation 5: Photosynthesis, The College Board.

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The average rate of the baseline experiment was 1.98 d/min. The average rate for the experimental trials was .6 d/min. The average ET50 of the baseline was 4:56 and for the experimental it was 6:37.