An Investigation on the Effects of Light Intensity on Photosynthesis

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I plan to investigate how light intensity affects the rate of photosynthesis in pondweed. The rate of photosynthesis will be measured by the number of bubbles given off (considering that oxygen is produced as a waste gas during photosynthesis). The light intensity is measured by the distance between the lamp and pondweed.

Fair test:

Fix: the amount of pondweed, the amount of water and sodium hydrogen carbonate solution in the test tube, the time allowed for each experiment

Change: the distance from the lamp to the pondweed (light intensity)

Measure: the number of bubbles given off by the pondweed, the temperature of the water bath


The following are factors that affect the rate of photosynthesis:

Light – quality and quantity

Carbon dioxide




Light is needed to provide the energy for the reaction to take place because it is an endothermic reaction. This is the variable that I am going to change, the light intensity will be measured by the distance from the lamp to the pondweed. The quality of the light will be kept the same because the same lamp will be used.

Carbon dioxide is needed because the element are needed to produce glucose which is what the plant is trying to make. This can be controlled by having a fixed amount of sodium hydrogen carbonate solution in the test tube.

The temperature needs to be controlled at higher temperatures the rate of reaction speeds up. It will be controlled by putting the plant in a water bath to maintain the same temperature

Water is needed for photosynthesis because the elements are needed to make glucose. This can be controlled by having a fixed amount of water in the test tube for each experiment.

If there is more chlorophyll then more light can be trapped which will speed up the process. This needs to be controlled by using the same piece of pondweed for the experiments.

The limiting factor is the that slows the reaction down so the factor that there is least of will be the one that changes the rate of the reaction. All the factors (other than the light intensity) must be giving plenty otherwise they will change the rate o the reaction.

The equation for photosynthesis is:

Carbon dioxide + water glucose + oxygen

6CO2 + 6HO2 C6H12O6 + 6O2

the equation shows that glucose and oxygen are made in the process of photosynthesis. Therefore the rate of photosynthesis can be found by the amount of glucose or oxygen produced. Because oxygen is let out of the plant, this will be the easiest of the two to measure. I will measure the amount of oxygen made by the number of bubbles given off from the plant.

Preliminary experiments:

In my preliminary experiment I learnt what to expect for the real experiment so this prepared me in many ways. From my preliminary experiments I discovered that I should not have the lamp any closer the 5cm because the light intensity is beyond the light saturation point so there was no change in the results and that the light will begin to heat the water bath making it inaccurate. I also found that there was not much point in having the lamp any further away than 70cm because very little photosynthesis occurred. My range of results will be from 5cm to 70cm. I found that the pondweed had to be put in upside-down as that is where the bubbles come out of. The pondweed shouldn't be touching the side because otherwise the bubbles sometimes gets trapped. The pondweed should also be placed a fair distance below the water so that when there is a bubble it can be seen clearly. These all helped me design my final method.

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