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Photosynthesis Investigation

Our aim in this investigation is to test the prediction that the closer the lamp is to a piece of pondweed in a tube of water the less amount of oxygen will be produced.

Scientific Knowledge:

The equation for photosynthesis:

Photosynthesis is a chemical reaction occurring in the leaves of green plants. Using the energy from sunlight, it changes carbon dioxide and water into glucose and oxygen.

Oxygen is a ‘waste’ product of photosynthesis.

Glucose can be converted to sucrose and carried to other parts of the plant in phloem vessels.

Glucose can also be converted into starch and stored (the starch can later be turned back into glucose and used in respiration).

A cross section of a leaf

Photosynthesis happens in cells in the middle of the leaf. These cells contain tiny bodies called chloroplasts; these contain a

green chemical called chlorophyll.

Chlorophyll is used to convert light energy into chemical energy.

The intensity of light falling on the plant is inversely proportional to the distance, which is called the inverse square law.

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Doubling the distance between the lamp and the plant reduces the light intensity received by the plant by one quarter.


By trebling the distance between the lamp and the plant reduces the light intensity received by the plant by one ninth.


The inverse square law can be presented as:

L is the light intensity where as d is the distance between the lamp and the plant. Graphs can be plotted of the amount of gas produced against .

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This law assumes that the plant is not affected by any light other than that whose distance is being altered.

Fair test:

I will make it a fair test by using the approximately the same size of the leaf, trying to cut out extra light for example the windows and experimenting the project at room temperature.


Input variables – light intensity is to be varied by increasing and decreasing the distance from the light source to the plant

Output variables – volume of oxygen produced (rate of photosynthesis) is to be measured by finding the volume of oxygen produced in a minute, and thus finding the rate of photosynthesis

Control variables -Light wavelength (colour) – light energy is absorbed by the pigment, chlorophyll, in the leaf. Chlorophyll easily absorbs blue light, in the 400-450 nm range, and also easily absorbs red light, in the 650-700 nm range. However it does not easily absorb green or yellow light, rather it reflects them, decreasing the amount of light absorbed, and therefore the rate of photosynthesis. This can easily be controlled, simply by using the same lamp throughout the experiment.

Rate Increasing CO2

Increasing Light

Carbon dioxide concentration – This can affect the rate of photosynthesis, since if there is too little CO2, it can become the limiting factor, thus impeding the viability of the experiment. In this case, as long as the experiment is done over a short period of time, the amount of carbon dioxide used up by the plant will not be sufficient enough to cause the carbon dioxide concentration to become the limiting factor. If my experiment were to be performed over a longer period of time, for instance 24 hours, I would add a fixed amount of Sodium hydrogen carbonate to the water, thus ensuring a large enough supply of carbon dioxide.

Water availability – water is also required in the photosynthesis reaction, and when it is lacking, the plants’ stomata close to prevent further water loss. This closing of the stomata cells also leads to little carbon dioxide being able to diffuse through. Clearly, in a water plant, like the pondweed, as long as the plant is fully submerged in water at all times, this will not be a problem.

Temperature – Enzymes are used in the photosynthesis reactions of a plant. Therefore, temperature will increase the rate of photosynthesis, until a point at which the enzymes denature. Although performing the experiment at a temperature slightly higher than room temperature, perhaps 25�C, would have a positive effect on the accuracy of the readings I took, as it would reduce the percentage error, by increasing the volumes, I decided that the inaccuracy of maintaining a constant temperature would outweigh any advantages. I am therefore going to perform the experiment at room temperature, checking the temperature frequently, in case the heat given off from the light should slightly raise the temperature, in which case I shall simply refill the beaker with more water after each experiment.

Equipment & Method-


“Pond” Water

250ml Beaker

Stop Watch


Sodium hydrogen carbonate


Boiling tube


200w lamp


Card to use as a light shield for unwanted light

Lead strip

1)Add, to 200cm3 of “pond” water in a beaker, one spatula full of sodium carbonate.

2)Fill a boiling tube with this “pond” water.

1) Cut a sprig of Elodea crispa, 5 cm long.

2) Wrap the strip of lead round the top of the sprig.

3) Place the sprig, cut end uppermost, into the boiling tube full of “pond” water

4) Add a few drops of detergent to the contents of the boiling tube.

5) Place the boiling tube in a beaker of water (this will act as a heat shield)

6) Place the thermometer in the water in the beaker. (Record the temperature)

7) Allow the plant to produce a steady stream of bubbles.

8) Shield the set up from unwanted light.

9) Set up the 200w lamp at the furthest distance investigated. (Measure the distance)

10) Allow the plant time to adjust to the new conditions.

11) Count the number of bubbles produced in a known period e.g. 5-10 mins.

12) Measure and record the temperature of the water in the beaker.

13) Repeat these measurements.

14) Repeat steps 11-15 for a range of closer distances.


I predict from our aim that the closer the lamp from the pondweed the More Bubble or oxygen will be produced. I predict this reason because from my scientific knowledge because light is a limiting factor in slowing down photosynthesis, because a plant may have lots of water and carbon dioxide, but it will not photosynthesis very fast if there is not enough light; increasing the light intensity will make the process faster. I think that if the light intensity increases, the rate of photosynthesis will increase at a proportional rate until a certain level is reached, and the rate of increase will then go down. Eventually, a level will be reached where an increase in light intensity will have no further effect on the rate of photosynthesis, as there will be another limiting factor, in this case probably temperature.

Distance between lamp and plant


Number of bubbles produced in . 5 minutes
















Distance between lamp and plant


Number of bubbles produced in 5 minutes (2nd Attempt)















Distance between lamp and plant


Average number of bubbles produced in 5 minutes















Graph showing the results:

Evaluating Results-

The relationship I can see from the three tables are that as the distance between the lamp and the plant are increasing the number of bubbles being produced is decreasing. The same thing happened again as the experiment was repeated.

The scientific knowledge showed that it was right when saying that light is a limiting factor in slowing down photosynthesis, because a plant may have lots of water and carbon dioxide, but it will not photosynthesis very fast if there is not enough light; increasing the light intensity will make the process faster.

I was right when I said that our aim (to find out the further away the light is from the pondweed the more bubble produced) will come out negative. This shows that the more closer or the more light is focused on the plant the rate of photosynthesis will become faster.

My graph was in the form of a best-fit curve. I drew it as a curve rather than a straight line because of the clear pattern of the points. This meant that the rate of photosynthesis increased as the light intensity increased. This was 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.


To conclude this experiment or investigation I can say that I did not meet my objective as described in the introduction.

Because we only tried only one variable our experiment wasn’t accurate and a fair one because there were other variables. We can say that the room we were doing the experiment was too hot, resulting the enzymes working faster or we can also say that because we’ve got big windows in our class that the plant was getting light from the sun.

The results I got was accurate and reliable because we did the experiment twice and the pattern was pretty much the same: As the light was getting further away the less amount of bubbles were being produced.

If I was to do the experiment again I would block light coming from the sun and try out the experiment 3 times (excluding other attempts)

1. ………. At a cold room.

2. ………. At room temperature without the heating on

3. And at a fairly hot room.

so we can see that if the results are same as now when trying it other ways/ using different variables.

The most important problem in this experiment was the distance between the light sources and the Pondweed were not measured to a very high degree of accuracy, especially when you note the fact that the distance should have been measured exactly from the filament of the light bulb to the centre of the plant

A further inaccuracy was in the heat generated by the lamp. As I have earlier described, temperature has a very noticeable effect on the rate of photosynthesis, and so any increase in the temperature of the pond water would have had serious effects on the accuracy of my results. To ensure that this does not happen again I would monitor the temperature of the water before and after every reading, to check that the temperature rises or not

Overall I feel my experiment was not as accurate as it could have been, however I believe it was accurate enough to support and justify my aim in this investigation.

Cite this page

Photosynthesis Investigation. (2020, Jun 02). Retrieved from

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