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  • Level: GCSE
  • Subject: Science
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Rate of photosynthesis experiment

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Stuart Green 11AK Rate of photosynthesis experiment Aim: To investigate whether wavelength affects the rate of photosynthesis. 6CO2 + 6H2O light energy & chlorophyll C6H12O6 + 6O2 Variables: Dependant Variable- Wavelength of light is to be the variable explored in this investigation. Changing the wavelength of the light source to the plant should vary the rate of photosynthesis. Light Wavelength (colour)- Light energy is absorbed by pigments in the leaf such as chlorophyll. Chlorophyll easily absorbs blue light, in the 400-450 nm range, and also easily absorbs red light in the 650-700 nm range. Chlorophyll does not absorb green light or yellow light effectively but tends to reflect them, decreasing the amount of light absorbed and decreasing the rate of photosynthesis. Why the rate of photosynthesis increases or decreased from the amount of light energy absorbed is what is being investigated in this experiment. The distance of the lamp, from the plant will be kept the same throughout the experiment. Dependant Variables: Carbon dioxide Concentration- CO2 concentration can affect the rate of photosynthesis since the more CO2 in the air, the more CO2 that can diffuse into the leaf. This variable can be fixed by adding a fixed amount of sodium hydrogen carbonate to the beaker and plant. The experiment should also be completed in one session so the plant doesn't absorb a significant percentage of the CO2. ...read more.


Our results were not exactly the same as the prediction graph, but several different things could cause this. Human error may have caused the results to be inaccurate. Some white light could have been used by the pondweed, causing the rate of photosynthesis to go up slightly. The heat being produced from the light may have also affected the rate of photosynthesis. Since temperature affects the movement of molecules and the reactions worked by enzymes, the heated molecules and enzymes may be the cause of the increasing rate of photosynthesis, because photosynthesis works using enzymes. This may account for why the number of bubbles being made was higher on the 3rd repeat, compared to the first repeat The photosynthetic rate increases when you are testing certain wavelengths; this means that plants can absorb certain wavelengths better than others. Our results show that orange light was the wavelength (630-700 nm) that had the highest rate of photosynthesis. We also found out that green light (500-600 nm) is the wavelength that has the lowest rate of photosynthesis. This is correct, according to our prediction graph. The only anomalies in our results are for the red wavelength, these are as low as the green results, which is not right. If you compare the results graph to the prediction graph, you can see that the red wavelength should be much higher. ...read more.


If during a repeated experiment, counting bubbles is still used, there is a smaller chance for human error when counting within a smaller time frame. If the capillary tube option was to be chosen, volume should be measured for a smaller time frame to reduce the overall time to complete the experiment. Also, during high rates of photosynthesis, it would still be difficult and impractical to measure the volume of oxygen produced for a long duration. Due to the nature and convenience of the experiment, it could be easily modified to investigate another variable of photosynthesis. Since sodium hydrogen carbonate (NaHCO3) is used to provide the pondweed with carbon dioxide. Performing the experiment with different volumes of NaHCO3 could vary the amount of CO2. The plant would be kept at a constant distance from the lamp and a constant volume of water would be added to the sodium hydrogen carbonate. Another experiment using almost identical apparatus would be to vary the intensity of the light the plant absorbs. Using a LVPSU (Low Voltage Power Supply Unit) you could vary the voltage going to the lamp, allowing you change the light intensity, like a dimmer switch. Since light intensity has already been identified as a variable of photosynthesis, it would be interesting to actually test it. The only way the measure the amount of light being produced by the lamp would be to use a Lux meter. Stuart Green 11AK Page 1 Of 3 ...read more.

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