The photosynthesis reaction begins with carbon dioxide taken from the air and water absorbed from the soil. Within the plant, special structures called chloroplasts contain sunlight-absorbing pigments, known as chlorophyll. The energy converted by these pigments from the sunlight powers the conversion of air and water to oxygen, which is released to the environment, and to a sugar called glucose, which is an excellent storage molecule for energy. Glucose can later be stored in the form of starch or transformed into ATP, which is used as the ‘fuel’ of all living things.
The chemical equation for photosynthesis is:
Carbon Dioxide + Water + Light Energy → Glucose and Oxygen
6CO2 + 12 H2O + light energy → C6H12O6 + 6O2 +6H2O
In the process of photosynthesis, several factors may affect its execution or results, including the temperature, light intensity and presence of CO2. Similarly, there are also several ways to determine the rate of photosynthesis, such as measuring the amount of oxygen produced, measuring the mass of the plant or testing to find an increase or decrease in pH. In this experiment, we are investigating the effects of light intensity on photosynthesis using pH as a form of measurement. As chlorophyll uses light energy to perform photosynthesis, it can only do it as fast as the light is arriving. Chlorophyll only absorbs the red and blue ends of the visible spectrum but not the green light in the middle, which is reflected back. If the light level is raised the rate of photosynthesis will increase steadily but only to a certain point. In water plants, photosynthesis can be measured using pH as carbon dioxide in water makes a slightly acidic solution - carbonated mineral water. After photosynthesis has occurred, a change will occur in the concentration of carbon dioxide and the rate of photosynthesis can be determined by recording the change in pH.
HYPOTHESIS:
If the photosynthesis has occurred, then the solution will be more basic (pH will increase).
In photosynthesis, carbon dioxide is exhausted and thus the combination of water and carbon dioxide (carbonic acid) will contain less carbon dioxide. The less amount of carbon dioxide in the solution, the less acidic it becomes and thus the solution will become more basic and the pH will increase.
If photosynthesis has not occurred, then the solution will be more acidic (pH will decrease).
If there is no photosynthesis, but there is a green plant present, the plant will assumedly undergo the process of respiration, that is, the reversal of photosynthesis, creating carbon dioxide and water. Therefore, with more carbon dioxide manufactured, the solution will become more acidic and the pH will decrease.
If there is no green plant present, then photosynthesis cannot occur.
Photosynthesis can only occur with the combination of a green plant, carbon dioxide and light energy.
VARIABLES:
Independent Variable: Exposure to light
Dependent Variable: Change in pH
Constant Variable: Amount of sodium bicarbonate, distilled water and plant/lack of plant
Uncontrollable Variable: Accuracy of pH meter
DATA COLLECTION:
TABLE 1 – pH of Solution Before and After Incubation in the Light or Dark
UNCERTAINTY: ± 0.05 pH units
CONCLUSION AND EVALUATION:
In this experiment, the effect of light intensity on photosynthesis in the plant Elodea was investigated using pH as a form of measurement. The results gained supported the hypothesis:
- In a darkened environment, the pH of the solution in which the plant was immersed was lower, and more acidic.
- In a lightened environment, the pH of the solution in which the plant was immersed was higher, and more basic.
- Photosynthesis did not occur without the presence of a green plant
However, there was a change recorded in the control, where there was no plant, therefore respiration or photosynthesis could not take place, however the solution became more acidic. This was probably due to contamination of the pH meter.
