Preliminary work
Initially, to ascertain a suitable range of distances at which to record results for my experiment, I did a preliminary investigation in which I recorded the number of bubbles of oxygen given off in a given time at various light intensities. To alter the light intensity, I placed a lamp at various distances from the plant. I got the following results:
Results of preliminary experiment
10 16
Although this is a very quick, simple and efficient way of obtaining an idea of the trends for the graph, and the boundaries for the measurements, this experiment was not in itself in my opinion accurate enough to be the basis of my main experiment. This lack of accuracy was mainly due to the fact that by simply counting the bubbles, I was relying on each bubble being exactly the same size, which they clearly were not. The preliminary experiment will, however, help me to determine if factors such as temperature or carbon dioxide concentration have become a limiting factor. It also shows that while my outer limits are justified, it would be better to take more readings between the distances of 10 and 20 centimetres, as the distance between the points is large at this point, and so I have decided to take readings at intervals of 5 from 0-100.
Method
Below is an apparatus list constructed of every piece of equipment used during the experiment:
Funnel
Tube
- Plasticine
- Pondweed
- Beaker
- Water
- Sodium bicarbonate
- Stopwatch
- 1 metre ruler in cm
- Lamp
The above apparatus was used in a unique way in order to complete the experiment successfully. Below is a diagram assembling all the equipment in their correct places:
Step 1 Firstly we have to set the apparatus according to the above diagram. Than we have to darken the room (no excess light energy reaching the plant) by putting blinds down and turning all light off.
Step 2 Turn lamp on, 100cm away from the pondweed.
Step 3 Leave for two minutes.
Step 4 Then count the amount of oxygen bubbles in the tube, and record the results in a table.
Step 5 Then move the lamp 5cm closer and leave for another two minutes.
Step 6 Count oxygen bubbles in tube and record results in a table.
Step 7 Keep moving the lamp 5cm closer and leaving for two minutes and counting the bubbles until the lamp is 5cm away from the pondweed.
Fairness
Fairness in my investigation is a major part of the experiment. Without fairness in my experiment the results are valid for correct answers. Below are the precautions taken in order to be safe about my experiment:
- I made sure that I had the same pondweed throughout the whole experiment and that it was always at room temperature.
- I reassured that the time spent counting the bubbles was precisely the same throughout the whole experiment. Also the distance the plant was being moved was at an accurate and equal interval.
- I made sure that the power of the light was always the same. I also made sure I only changed one thing, which was the light intensity towards the plant.
- I made sure that the amount of water surrounding the plant was kept equal throughout the entire investigation.
Above I have shown the precautions taken in order to assure that my investigation was as fair as possible. I will now look into further detail about each specific Variable:
Independent Variable:
light intensity is to be varied by increasing and decreasing the distance from the light source to the plant
Dependent Variable:
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 Variable:
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.
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.
Safety
Safety in an experiment is always a key aspect. The experiment I undertook was not that of a dangerous calibre, but safety precautions were still needed to be taken:
Chairs put to one side, away from the experiment table/area.
- Care to be taken whilst handling the light bulb, as it may get hot.
Results
= Anomalous result
I did not include the anomalous results when I was working out the averages.
Analysis
The results and graph show that the closer the lamp is to the plant, the higher the rate of photosynthesis. The graph is showing that more bubbles are being released when the lamp is closer to the plant and are decreasing the further away it gets. Thus showing that the stronger the light intensity the higher the rate of photosynthesis.
The further the distance of the lamp the less bubbles being released. This is due to there being less energy to breakdown bonds and also to make new bonds. Therefore when the lamp is closer to the plant and there is more energy the more bonds of CO2 and H20 will be broken and more bonds will be made.
Comparing my results to my prediction, I can see that my prediction was correct by stating that the more light intensity i.e. more light energy, will increase the rate of photosynthesis. This because more bonds will be broken down in water and carbon dioxide and also because more bonds will be made between glucose and oxygen atoms. Thus, more food being produced.
Evaluation
There were two anomalous results, which I circled in my results table; limiting factors could have caused them.
My method was fair because I only changed one factor, which was light intensity, I kept the temperature room temperature, but any change of temperature would probably be due to the light. To overcome this I would use a square glass heat sink, which absorbs heat and allows light to pass through. It would be square to stop refraction of light.
This experiment not entirely fair because we could not see all the bubbles produced due to some of them being too small and some too quick, also some may have stuck underneath the plant. To trounce this I would use a gas syringe as this avoids human errors such as blinking and also allows for more accurate measurements, we would also leave the plant for longer this will allow for a fairer result.
Looking at the three sets of results I would say they are fairly reliable but they is still little doubt that they’re similar but could be more analogous. To surmount this I would repeat the experiment more times to have more comprisable results.
Extension
To extend this experiment I would use different coloured lights to see if it affected the rate of photosynthesis.
I think due to the fact that chlorophyll is green it when green light is shone on a plant very little photosynthesis if any will take place, also I think that white light will give the best results.
To do this experiment I will repeat as I did in the last but instead of changing the distance I will use light filters to change the colour of the light