I am going to make it a fair test by making sure the temperature is the same by using a heat filter to stop the water absorbing the heat. I will leave the thermometer in the beaker and check the temperature of the water is kept the same throughout the experiment (25°C). I will carry out the experiment in the dark so no other light will affect the temperature of the water. I will do this to ensure the temperature is kept the same for each of the distances of the lamp because increasing the temperature will also increase the number of oxygen bubbles being produced. For instance, increasing the temperature from 10ºC to 20ºC could double the rate of photosynthesis, as the plant's enzymes will be closer to their optimum working temperature. As the temperature is increased, molecules in the cells will be moving at a faster rate due to the kinetic theory. If the temperature is raised above a certain level, the rate of photosynthesis will drop as the plant's enzymes are denatured. They will therefore be more likely to join onto the enzymes and react.
The experiment will be carried out using the same equipment, which means that the same amount of water will be used. This is important because increasing the amount of water available to the plant will affect the rate of photosynthesis. If the plant does not have enough water, the plant's stomata will shut and the plant will be deprived of CO². This also means that the elodea is the same which is important as different plants will have different amounts of chlorophyll which affects the number of oxygen bubbles released by the plant. I will use a stop clock to measure the time because if I measure the amount of oxygen bubbles longer for one distance than another then this will obviously give me inaccurate results.
Safety Procedures:
The elodea should be cut with a blade, which is not too sharp.
The lamp should be kept within sufficient distance from the beaker as it contains
water.
The lamp bulb should not be touched with your hand, as it will be very hot.
The beaker and the test tube should be handled with care, as they are glassware.
Extra care should be taken with lamp wires as the experiment is taking place in the
dark.
I will take five readings for the experiment and I will count the bubbles for 20 seconds. The distances I am going to use are:
I will repeat the experiment once and take an average to make my results more accurate.
I will record my results in a table which resembles the following:
Prediction:
I predict that as the light intensity increases, so will the amount of oxygen bubbles being given off because more energy is given to the chloroplast so it can combine carbon dioxide and water to produce glucose and oxygen. The amount will only increase up to a point because it already has as much light as it can use. When it reaches this point the amount of oxygen bubbles will be the same and will not increase anymore unless the temperature or carbon dioxide increases which are now the limiting factors.
If the light intensity increases the number of bubbles will increase at a proportional rate until a certain level is reached, then the rate of photosynthesis will then level out. Eventually a level will be reached where increasing the light intensity will have no more effect on the rate of reaction, as there is some other limiting factor.
Light is needed for photosynthesis in plants. When chloroplasts in the leaf's cell are exposed to light they synthesise ATP from ADP. Oxygen is produced as a by-product of the photosynthesis reaction. Therefore increasing the concentration of light will increase the amount of ATP being synthesised from ADP and so more oxygen will be released as a by-product.
Obtaining Evidence
40 + 40 = 40
2
15 + 18 = 16.5
2
7 + 9 = 8
2
3 + 4 = 3.5
2
1 + 2 = 1.5
2
In this experiment I noticed that the oxygen bubbles were not all the same size. This makes my experiment less accurate as my results recorded the frequency of the emittance of oxygen bubbles rather than the volume of oxygen given off. Also more oxygen bubbles were collected on the second trial. I think this is because I didn’t leave the water to equilibrate for long enough so the plant didn’t absorb as much light.
Analysing Evidence and Drawing Conclusions
I have found out that the larger the amount of light given to the plant, the larger the number of oxygen bubbles given off. This means that as the light intensity increases so does the amount of oxygen given off.
I have plotted graphs showing the evidence I obtained. They are shown on the next page.
Graph Calculations:
Light intensity ∝ 1
d2
Light intensity ∝ 1 = 0.04
52
Because the number 0.04 is too small to plot on a graph, I will times the number by 1000.
Light intensity ∝ 1000= 40
52
Light intensity ∝ 1000 = 10
102
Light intensity ∝ 1000 = 4.4
152
From the graphs I noticed that more light intensity resulted in a greater amount of oxygen bubbles being collected. For example when the light intensity was 40 at 5cm the amount of oxygen bubbles collected was 40 whereas when the light intensity was 1.6 at 25cm there were only 1.5 oxygen bubbles given off. When I repeated the results a second time, they were fairly similar to the first trial, which proves my prediction was correct twice.
As predicted when the light intensity increases so does the rate of photosynthesis. I predicted that a level would be reached where increasing the light intensity would have no more effect on the rate of reaction as there would be some other limiting factor, which limits the rate of the reaction. The rate increases at a steady rate as the light intensity increases until near the end of each line where the rate of increase decreases. This is either because the photosynthesis reaction has reached its maximum rate of reaction or another factor is limiting the rate.
The fact that the curve levels off so quickly indicates that there is another limiting factor limiting the photosynthesis. It could be temperature. These tests are being carried out at room temperature so the temperature would have to be raised another 15ºC before the enzymes in the plant's cells were at their optimum working temperature. More tests could be done by using water that was at a higher temperature to see what effect this would have on the photosynthesis rate. It is however impossible to raise the plant's temperature without affect other factors. For instance the actual amount of oxygen released by the plant is slightly more than the readings would suggest as some of the oxygen would dissolve into the water. At a higher temperature less oxygen would be able to dissolve into the water so the readings for the photosynthesis rate could be artificially increased.
It is also possible that the photosynthetic reactions in the plant are occurring at their maximum possible rate and so can not be increased any more.
The light is probably not a limiting factor as all but one of the curves level off before the maximum light intensity of 400 is reached. The maximum light intensity that the plants can handle is therefore just below 400.
Water will not be a limiting factor as the plants are living in water. They therefore have no stomata and absorb all their CO2 by diffusion through the leaves.
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. My graphs showed that the relationship between the light intensity and the rate of photosynthesis was non-linear, as both graphs produced a best-fit curve. However, as I expected in my hypothesis, it does appear that for the very first part of the graph, the increase in rate is in fact proportional to the increase in light intensity (i.e. a straight line) From these results, I am able to say that an increase in light intensity does certainly increase the rate of photosynthesis. The gradual decrease in the rate of increase of the rate of photosynthesis (the shallowing of the curve) can be attributed to the other factors limiting the rate of photosynthesis. As light intensity increases, the photosynthetic rate is being limited by certain factors, such as carbon dioxide and temperature. These factors do not immediately limit the rate of photosynthesis, but rather gradually. As light intensity increases further, so the rate of photosynthesis is being limited by other factors more and more, until the rate of photosynthesis is constant, and so is almost certainly limited in full by another factor. Overall, both graphs and my results support my predictions fully. My idea that number of oxygen bubbles would increase with light intensity was comprehensively backed up by my results. This is because a higher light intensity involves a greater level of light energy, which can then be transferred to a special protein environment designed to convert the energy. The reason that light intensity does affect the rate of photosynthesis is because as light, and therefore energy, falls on the chloroplasts in a leaf, it is trapped by the chlorophyll, which then makes the energy available for chemical reactions in the plant. Thus, as the amount of sunlight, or in this case light from a bulb, falls on the plant, more energy is absorbed, so more energy is available for the chemical reactions, and so more photosynthesis takes place in a given time.
My results proved my prediction was correct because the graph showed that the number of oxygen bubbles were increasing steadily but then the amount of bubbles started to become similar and didn’t increase much. This happened because the plant was given all the light that it could use. It wouldn’t matter how much more light was given to the plant because the number of bubbles wouldn’t increase. This is because the temperature and the amount of carbon dioxide are now the limiting factors. The amount of oxygen bubbles didn’t completely stop increasing but if I had taken more readings it should have.
Conclusion:
I can conclude that the as the light intensity increases so will the amount of oxygen bubbles given off, but only up to a point because there is too little heat or carbon dioxide which are limiting the rate of photosynthesis to increase.
Evaluating Evidence
I think my method was fairly successful because there were no accidents, which showed that I took note of the safety precautions in my method. Also the results I obtained were what I expected. My results supported my prediction because my graph showed that the greater the light intensity, the greater the amount of oxygen bubbles given off of reaction up to a point.
I don’t think my results are good enough to draw a firm conclusion because this experiment was done at one time where the environment may have affected the results. Also I have only done one experiment using one method. I need to investigate other methods and try different experiments to draw a firm conclusion.
The accuracy of this experiment is limited by a number of factors.
- Some of the oxygen give off is used for respiration by the plant.
- Some of the oxygen dissolved into the water.
- Some was used by small invertebrates that may have been living within the pieces of elodea.
- The higher light intensities should be quite accurate but the smaller light intensities would be less accurate because the light spreads out. the elodea will also get background light from other experiments.
- Using the same piece of elodea for each experiment may have given inaccurate results as the elodea's photosynthesis rate decreased over time. By using a different piece of elodea for each experiment did create the problem of it being impossible for each piece to have the same surface area.
Although I feel that my experiment was sound overall, I thought there were many points at which the accuracy was not perfect. The fact that I was relying on all the bubbles being the same size, which they clearly weren’t led to inaccuracies. However many of the smaller inaccuracies also apply to my main experiment. Firstly, the distance between the light sources and elodea 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
Another error would have been due to background light in the vicinity. We tried to reduce this error by closing all blinds in the laboratory, but due to practical reasons, we could not all perform the experiment in a separate room, and we therefore experienced light pollution from other students experiments. This would have had a very marginal effect on my results as a whole, but to eliminate this problem completely, it would have been necessary to perform the experiment in a totally dark room. A further inaccuracy was in the heat generated by the lamp. As I have earlier described, temperature has a very noticeable effect on the
The last inaccuracy, though a small one, was in the time keeping. The main problem here was in when to begin the minute. If for one reading, the minute was started just after one bubble had been produced, and in another reading it was just before, this could have had a negative effect on the accuracy of my results. I therefore ensured that in each case I started the stopwatch just after a bubble had been produced, thus heightening the accuracy. Overall, I felt that due to the small volumes of oxygen involved, my experiment was not as accurate as it could have been, however I believe it was accurate enough to support and justify my hypotheses. Improvements could have been made as I have stated, mainly by simply increasing the time taken. However, due to practical time constraints in taking the readings for my investigation, and some consequential problems relating to time extension, I could not in fact make these adjustments. The other obvious way of increasing the reliability of my results would be to take many repeat readings and find an average. To extend my enquiries into the rate of photosynthesis, I could perhaps try to link in some of the other limiting factors to the same experiment, as well as investigating them in their own right. It could also be interesting to explore the effects of coloured lights on the rate of photosynthesis, which could lead to the question of whether or not other types of light, such as fluorescent lights or halogen lights, would have a different effect on the rate of photosynthesis
I think my results are fairly accurate because the results plotted on my graph show that they fit into a pattern. Also when I repeated the results a second time, they were fairly similar to the first trial, which proves my prediction was correct twice. If you repeated it more times you would get similar results, thus proving a successful method and reliable results. However, I realised that on my second trial, amount of bubbles was greater than the first trial so I think I should have let the water equilibrate for longer. I think my results are fairly accurate because most points fit on the line of best fit, and the other points are quite close to the line of best fit. The number of oxygen bubbles didn’t completely stop increasing but if I had taken more readings they would have.
To improve my experiment I could try other methods. For example, instead of counting the oxygen bubbles, I could measure the volume of oxygen collected. In this method the size of the bubbles would not affect the results so it would make the results more reliable.
To extend my investigation, I could investigate if different coloured lights affect the rate of photosynthesis. For this investigation I used white light, so as an extension I could split up this light into the colours of the spectrum by using a coloured filter and find out if one colour has a different effect to the rate of photosynthesis than another.
I know that plants reflect green light so I could also investigate if plants absorb one colour more than another does.