The purpose of this experiment is to observe the factors affecting the Photosynthetic rate of leaves, which is measured in two ways. Firstly changing the light intensity, this will determine the rate of increase or decrease in photosynthesis. Secondly changing the availability of nutrients (Concentration of CO2) to the plants, this will directly affect the photosynthetic rate. To test the light intensity, an elodea submerged in a beaker was placed at different measurement away from the plant, to see if oxygen (bubbles) is produced. To test the availability of nutrients, different molarities of Sodium Bicarbonate was diluted in 500 ml of water with Elodea, to see if rate of photosynthesis increased or decreased.
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The result shows, as light intensity increases, the rate of reaction will increase at a proportional rate until a certain level is reached. At a light intensity of 400 the average increase in rate of reaction was 746v. At 4 the average increase in rate of reaction was 8676v, a difference of 7930v, which shows the rate of reaction is greatly influenced by light intensity.
As the molarity of Sodium Bicarbonate increases, the rate of reaction will also increase at a proportional rate with respect to light intensity. At 0.05M the average increase in rate of reaction was 0.80r. At 0.1M the average increase in rate of reaction was 1.90r, a difference of 1.1r, which shows the rate of reaction is also significantly influenced by the availability of nutrients.
Both light intensity and availability of nutrients are important factors that affect the rate of photosynthesis.
To investigate how different factors affect the rate of photosynthesis. The variables that will be changed are different intensity of light and different molarities of Sodium bicarbonate and then measuring the rate of reaction (photosynthetic rate).
Throughout this experiment the light intensity and different molarities of Sodium Bicarbonate will be varied. The variable that is measured will be time, for rate of reaction.
As light intensity increases the rate of reaction will increase at a proportional rate. As the concentration of NaHCO3 increases the rate of the reaction will also increase at a proportional rate.
Every species on earth needs some kind of energy source in order to survive. In animal cells, the mitochondria produce ATP from cellular respiration. However, the plant cells have a different type of center that produces energy-chloroplasts. Plants go through the process of photosynthesis. The main process of photosynthesis is the absorption of light by chlorophyll, found in leaves and the immersion of carbon dioxide from the environment, and together they produce oxygen and sugar (energy). The equation below represents the photosynthesis reaction:
The purpose of this experiment is to test whether factors such as light intensity and level of Carbon dioxide, will affect the rate of photosynthesis, which are the two most important variables in the photosynthesis process. This was demonstrated by Robert Hill in 1938, known as “The Hill Reaction”. Robert Hill and his associates at the University of Illinois found that the photosynthetic rate varies with light intensity, and as the light intensity increases, the reaction rate also increases up to a certain point.
Apparatus needed for the Experiment
Figure A) Potometer
The apparatus is set (see Fig. A) with the syringe full of the 0.01M solution of NaHCO3 solution. Two marks 10cm apart are made on the capillary tubing.
The syringe is placed 0.05m away from the lamp.
Using the syringe plunger the meniscus of the NaHCO3 is set so that it is level with the first mark.
A stopwatch is then started. The meniscus should gradually move down the capillary tube as the elodea produces oxygen as a by-product of photosynthesis. As the oxygen is produced it increases the pressure in the syringe and so the meniscus is pushed down the tube.
Light Intensity = 1 / Distance² (m)
When the meniscus reaches the level of the bottom mark the stopwatch should be stopped.
Light intensities have been worked out using the following equation:
6. Using the same piece of elodea and the same distance between the lamp and the syringe the experiment (steps 1 to 5) should be repeated for the other concentration of NaHCO3.
7. The experiment (steps 1 to 6) should then be repeated at each different distance between the syringe and the light for all the NaHCO3 concentrations. The remaining distances are 0.05m, 0.06m, 0.07m, 0.08m, 0.1m, 0.2m, 0.3m, and 0.5m.
8. The entire experiment should then be repeated three times in order to obtain more accurate data and to get rid of any anomalies that may occur in a single experiment.
In order to make this experiment as accurate as possible a number of steps must be taken.
The same piece of elodea should be used each time in order to make sure that each experiment is being carried out with the same leaf surface area.
The amount of NaHCO3 solution should be the same for each experiment. 20mm² should be used each time.
The distance should be measured from the front of the lamp to the syringe. Although taking these steps will make the experiment more accurate, its accuracy is still limited by several factors.
From these recorded times I will work out the rate of the reaction using the following equation.
Rate of the Reaction = 1 / Time (s)
Table1. (Average of the 4 trails of Molarity against Light intensity):
Molarity of NaHCO3
Light Intensity 1/d² (m)
Using these results I worked out the rate
Rate Of the Reaction = 1 / Time(s) x 1000
The rate was multiplied by 1000 to make the numbers easier to handle.
Table2.Average of the 4 trails in rate of reaction:
Molarity of NaHCO3
Light Intensity 1/d² (m)
Light intensity against NaHCO3
Analysis & Discussion of Results
Distilled water: With the distilled water the rate of reaction went up from 0.1 to 0.4 when the light intensity was increased from 100 to 400. This is a 4 times rise which is quite large. The curve on the graph does however level out quite soon showing that the rate is being limited by the lack of NaHCO3 in the water.
0.01M NaHCO3: At a light intensity of 4 the rate is 0.06 but this rises to 0.6 when the light intensity is brought up to 400. The curve is very shallow and levels off towards a light intensity of 350 – 400.
0.02M NaHCO3: The amount of NaHCO3 is double that of the 0.01M NaHCO3 experiment. The rate also finishes off twice that of the 0.01M experiment. This would suggest that there was a directly proportional relationship between the amount of NaHCO3 and the rate of reaction.
0.05M NaHCO3: The curve for the 0.05M NaHCO3 is steeper than the previous curves. The rate rises to 1.9 at a light intensity of 400.
0.07M NaHCO3: The 0.07M NaHCO3 test produces a line which is steeper than all the previous curves. The plant is using the extra CO2 to photosynthesize more. As the plant has more CO2 the limiting factor caused by the lack of CO2 is reduced. This test did produce a big anomaly. The rate for a light intensity of 400 is 5. By following the line of best fit I can see that this result should be more like 3.5. The elodea for this test was very close to the light source. It is possible that it had been left here for a while which caused the lamp to heat the elodea up. This would have increased the rate of reaction of the plant’s enzymes which would have increased the photosynthesis rate.
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0.1M NaHCO3: The 0.1M NaHCO3 produced the steepest line. Near the end of the line it looks as if the rate of reaction is hit by another limiting factor. The line goes up steadily but then between a light intensity of 300 and 400 levels off very quickly. This would suggest that at a 0.1M NaHCO3 is sufficient for the plant to photosynthesize at its maximum rate with its current environmental conditions. Increasing the NaHCO3 concentration after this level would therefore have no effect unless the next limiting factor was removed.
The hypothesis was that the rate of photosynthesis would increase if the light intensity and NaHCO3 levels were increased (please refer to Graph1). As the elodea absorbed the light and CO2 it produced oxygen gas which increased the pressure in the syringe. This pushed the air bubble in the capillary tube down. The chloroplasts produce ATP and reduce NADP to NADPH2 when exposed to light. It is at this stage of the reaction that oxygen is produced as a waste product, furthermore, the data collected was supported by the results obtained by Robert Hill and his associates at the University of Illinois, where they predicted, as the light intensity and NaHCO3 levels increased, the rate of photosynthesis will also increase up to a certain level (please refer to Graph2)
As predicted when the light intensity increases so does the rate of photosynthesis. It was 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 decreases. This is either because the photosynthesis reaction has reached it’s maximum rate of reaction or another factor is limiting the rate. As 6 different CO2 concentrations were used I can see that the first five reactions are not occurring at their maximum rate as there is the 0.1M NaHCO3 rest which is occurring at a faster rate then the other 5. The photosynthesis reactions of the other five tests must therefore be limited by the concentration of CO2 to the plant.
As predicted when the NaHCO3 concentration is increased the plant in able to get more CO2 which causes the rate of reaction to go up. It was predicted that once the NaHCO3 had been raised above a certain level increasing the rate further would have no effect as there would be other limiting factors limiting the rate of the reaction. As the NaHCO3 concentration the water was increased the rate of photosynthesis also increased. The plant therefore made more oxygen as a waste product. At a NaHCO3 concentration of 0.1M once the light intensity gets above 300 the rate of reaction decreases significantly. This could be because photosynthesis is occurring at it’s maximum possible rate or because another limiting factor is restraining the rate of reaction.
The fact that the curve levels off so quickly indicates that there is another limiting factor restraining photosynthesis. It could be temperature. These tests are being carried out at room temperature so the temperature would have to be raised another 15°C 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 cannot 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.
Graph1. Light intensity against NaHCO3 – MY RESULTS
Graph2. Light intensity against NaHCO3 – SOURCE
Limitations and Improvement
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 were found 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.
The lights are also a source of heat which will affect the experiments with only a small distance between the light and the syringe. This heating could affect the results.
Using the same piece of elodea for each experiment was impractical 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.
This experiment could be improved in a number of ways.
It could be repeated more times to help get rid of any anomalies. A better overall result would be obtained by repeating the experiment more times because any errors in one experiment should be compensated for by the other experiments.
Each person should have done their experiments in a different room to cut out all background light.
All the experiments should be done sequentially.
A perspex screen could have been placed between the light and the syringe to reduce any heating effect that the light may have.
The experiment could have been carried out with higher NaHCO3 to see if increasing the concentration would increase the rate of photosynthesis, or if a concentration of 0.1M NaHCO3 produces the maximum rate of photosynthetic reaction.
The intention of this experiment was to investigate different factors that affect the rate of photosynthesis. The hypothesis was, as light intensity increases the rate of reaction will increase at a proportional rate. As the concentration of NaHCO3 increases the rate of the reaction will also increase at a proportional rate. This was correct, supported by the data collected which shows at a light intensity of 400 the average increase in rate of reaction was 746v. At 4 the average increase in rate of reaction was 8676v, a difference of 7930v, which shows the rate of reaction is greatly influenced by light intensity. This was demonstrated by Robert Hill and his associates, with similar results to this experiment, which they found that the photosynthetic rate varies with light intensity, and as the light intensity increases, the reaction rate also increases up to a certain point.
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