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Investigation on how light intensity affects the rate of Photosynthesis

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Introduction: Photosynthesis is the process by which green plants use the light energy to convert carbon dioxide and water into simple sugar called glucose, which provides a basic energy source for the plant. Photosynthesis occurs in leaves that contain specialized cell structures called chloroplasts. Chloroplasts contain molecules of a green pigment called chlorophyll, consisting largely of carbon and hydrogen. It gives green color and absorbs the necessary light for photosynthesis. Chloroplasts are found in palisade cells, they have a large surface area, which means there is more change of light hitting a chloroplast. Light is a very important factor when it comes to the process of photosynthesis, without light the reaction cannot be carried out and no glucose can be made. Light intensity is one of the factors that affect the rate of photosynthesis and that is what our experiment is based on:

Aim: In this experiment we shall investigate how the rate of photosynthesis (measured by the increased volume of oxygen) is affected by changes of light intensity in water plants.

Hypothesis: I predict that as the intensity of light increases, so will the rate of photosynthesis. Furthermore, I hypothesise that if the light intensity increases, the rate of photosynthesis will increase at a proportional rate.


Constant VariedMeasured

ShootLight intensityRate of photosynthesis

Number of lightsDistance between light and pondweedNumber of Bubbles

Water ——

Time between each result recording ——


– Beaker almost full of water

– Pondweed

– 60 watt Lamp

– Ruler

– Stopwatch

– Short-stemmed Funnel

– Test Tube

-paper clip

-pure CO2


1. First clip that end of the pondweed that was not cut.

2. Then place a short-stemmed funnel over some pondweed in a beaker of water. Add pure CO2 to the beaker of water.

3. Fill a test tube with water and place it upside-down over the funnel stem.

4. Place the 60watt lamp at these distances away from the beaker once you have started timing: 20cm; 15cm; 10cm; 5cm; 0cm (Decrease the distance every 3 minutes)

5. Oxygen bubbles should appear from the stems and collect in the test tube.

6. Count and record the number of bubbles, produced from the stem that as been cut, at the distances shown below in the table.


Distance between pondweed and light source (cm)Number of bubbles produced every three minsPercentage increase (%)

2014100 (from zero)






By analyzing my results I can say that my prediction is correct. The results indeed show that the amount of light intensity and oxygen go up in a directly proportional rate. I can tell this because as I increase the light intensity, the amount of bubbles increased also (oxygen). This is because photosynthesis is a reaction, which needs energy from light to work, so as the amount of energy available from light increased with the rise in light intensity, so did the amount of oxygen produced as a product of photosynthesis. Alhtough I am quite puzzled myself at why there was sometimes a decrease in percentage increase, even through extentive research I have been unable to find the answer. I am quite curious myself to know the reason.


Apart from testing the intensity of light affecting the rate of oxygen production during photosynthesis, different types of coloured lights may be used to test their effectiveness in bringing about photosynthesis. (Different coloured filters may be used in conjunction with normal ‘white’ light to generate coloured lights.)

We can start the experiment having the light source further away from the pondweed. This allows us to space out the distances between the light and the pondweed, we could the lamp at these distances every three mins away from the plant: 50cm; 40cm; 30cm; 20cm; 10cm; 0cm. The only reason why we should do this is to backup my hypothesis that “As the distance between the light source decreases, the light intensity increases and so does the rate of photosynthesis.”

We can test to see how the concentration of carbon dioxide affects the rate of photosynthesis. We can do the same for temperature.

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