In this graph you can see that at a lower temperature the rate of photosynthesis is increasing with increasing light intensity and/or carbon dioxide availability. However the CPS/LSP is quickly reached. So the plant cannot photosynthesis at an accelerating rate anymore. But at the higher temperature you can see that the rate of photosynthesis increases further and it takes more time to reach the CSP/LSP. Meaning that the temperature also affects the rate of photosynthesis, as the temperature increases the rate of photosynthesis also increases.
So by looking at this information, I can see that if I only want to investigate the amount of light affecting the rate of photosynthesis in Elodea then I have to make sure that the amount of carbon dioxide and temperature is kept constant.
Prediction
I predict that as the light intensity is increased the rate of photosynthesis will also increase fairly constantly as long as it is the only factor affecting the rate of photosynthesis and all the other factors are kept constant. Until it gets to a stage where the rate of photosynthesis will stop increasing and be steady. This will mean that the amount of light intensity will have no further affect on the rate of photosynthesis. This will only be the case as long as light intensity is the only variable. If there are other variables acting on the rate of photosynthesis at the same time as the changing amount of light intensity this would affect the rate of photosynthesis meaning that the experiment would not be accurate or a fair test.
Preliminary Results
Looking at my preliminary results I can see that they do not fit my prediction. As the distance from the lamp gets closer the average number of bubbles should be rising fairly steadily. But as you can see in my preliminary results the trend is not followed. There are far too many anomalies making my experiment inaccurate. I think that one of the reasons the anomalies occurred was because of me only timing it for 1 minute at each distance, meaning that the Elodea did not have time to properly start producing oxygen at a steady rate. Another reason why my results did not follow the pattern could have been because of me only having a scale of measurement from 10cm to 30cm. Resulting in there being a low amount of bubbles in general. If I had moved the lamp even closer say… “5cm” then I could have maybe obtained more bubbles and so improved my results. Finally I think I should have let the Elodea start producing oxygen bubbles before I started measuring and timing. In other words I should of let the Elodea start to produce bubbles before I began my experiment because otherwise it was not a fair test owing to the fact that the first length (30cm) was being timed even before there had been any bubbles produced by the Elodea. So it did not take as much time for the other lengths to start producing bubbles as it did the first one. Not JUST because of lower light intensity but also the fact that the Elodea hadn’t started photosynthesising, resulting in the experiment being inaccurate and not a fair test as light intensity is supposed to be the only variable and in this case was not.
Learning from my past experiences I have decided to change the following things in order for me to obtain more accurate and fair results…
- I will time each length for 2 minutes using a stop watch for accuracy.
- I will use 6 lengths in total instead of 5.
- I will allow the Elodea to start producing oxygen as a result from photosynthesising due to light intensity (provided by my lamp) before I start my experiment.
Equipment List
- Large Beaker (Water Bath)
- Clamp Stand
- Boiling Tube
-
Na+ HCO3- solution
- Elodea- Canadian pond weed
- Electric lamp with 100W filament bulb
- Thermometer
- Stopwatch
Set up of Apparatus
Method
For this experiment I will use the “counting the bubbles” method. This involves first taking about a 5cm cutting of the Canadian pond weed Elodea and then placing it a boiling tube. The boiling tube is then clamped and placed in a large beaker of cold water. Now I place a thermometer in there just in case there is an unexpected rise in the temperature. So I will measure the temperature at the beginning and end of each length just to make sure that there has not been a significant rise in temperature, as that would mean that there were two variables affecting the rate of photosynthesis temperature and light intensity, meaning that my experiment would not be a fair test. Now all that is left to do is plug in the lamp fairly far from the Elodea, also make sure that when I am measuring the distance from the lamp to the Elodea then I measure from the bulb of the lamp to the plant. Not from the base of the lamp to the Elodea as that might not be accurate if the lamp is one of those that have an adjustable neck. I will use the same lamp to measure all of my lengths as the light intensity will always be the same at any given time in the experiment. The only thing that will change is the length from the lamp to the plant resulting in the light intensity acting on the plant to increase.
Before beginning measuring my first length I will make sure that the Elodea has started photosynthesising, I will do this by first adding the Na+ HCO3- solution so that the plant has a good supply of carbon dioxide meaning that it has everything it needs to photosynthesis now. I will wait until I can see some oxygen bubbles forming meaning that photosynthesis has started taking place. So now I measure my first distance 30cm and start my stop watch. My partner will count the amount of bubbles while I keep an eye on the time as the time limit is one minute. When the time has finished then I will switch the lamp of and record our results. Then I will clear out any visible bubbles from the top of the boiling tube so that I do not mistake them for my next experiment. I will repeat the whole process 2 more times so that I can work out an average result which will be a lot more accurate. Finally I will change my distance and repeat the whole thing again with all of my lengths.
The reason I choose this method was that it is a simple procedure which involves no complex apparatus. It is easy to carry out and very time efficient I also think that I can keep the other variables fairly constant so that light intensity is the only major factor and so come out with reliable and accurate results. So overall I choose this method because it was easy to set up and carry out. It is not expensive and uses replaceable equipment so no major loss can occur and is capable of obtaining accurate and reliable results if carried out carefully.
Safety Precautions and Fair Test
The safety precautions I will take are making sure that my lamp is kept well away from the water to prevent the risk of any electric shocks or fires. I will also make sure that my beaker is kept away from the edge of the table so that it does not fall over and hurt anybody.
To make my experiment a fair test I will make sure that the same lamp is used through out all the experiments this is to make sure that all the experiments are carried out with the same amount of light intensity (100w) at any given distance. I will also make sure that there is always the same person (me) stopping and starting the stopwatch because different people might stop or start the watch at different times. So if there is only one person starting and stopping then the results will all be obtained by that one person meaning that the experiment will be more accurate and reliable. Another thing that would be kept the same is the person counting the bubbles because different people have different eye sights and so someone might be able to see more bubbles then another, keeping the person the same will ensure that all the results are from the same source meaning that the results in the end will be more accurate and so more reliable.
The only variable that will be changed is Light intensity as I am trying to find the link between light intensity and photosynthesis. To make sure that light intensity is the only major variable I will place my boiling tube in a beaker of cold water (bath tub) so that the heat energy from the lamp is mostly ineffective due to the presence of the cold water acting as a heat shield. The amount of carbon dioxide needed should not be a problem as the sodium hydrogen carbonate should easily provide enough carbon dioxide needed for the duration of the experiment so it is not a problem. It would only be a problem if I was carrying out this experiment over a long period of time e.g. 24 hours.
Example of what my results table will look like…
My Results Table
My Graphs
Analysis
Looking at my graphs you can see that as the distance of the lamp from the Elodea increased so did amount of bubbles produced. Meaning that as the amount of bubbles increased the rate of photosynthesis also increased. The reason for this was because as the light falling on leafs of the plant increased, the chlorophyll in the chloroplast trapped more light energy. It was able to turn more light energy into chemical energy and therefore produce more oxygen and sugars. The bubbles that are produced are oxygen bubbles (The sugars are quickly turned in to starch) and so we can see that as the light intensity increases the amount of bubbles increase meaning that the rate of photosynthesis also increases. Similarly the reason for there being fewer bubbles as the lamp in further away from the Elodea is because of there being a decreased amount of light falling on the Elodea meaning that the chlorophyll trapped less light energy and so converted less light energy into chemical energy. In theory producing less oxygen bubbles meaning that the rate it is photosynthesising at is lower then that of the lamp being at a closer distance.
My graphs were both best fit curves; this is because I wanted to show a clear pattern of the points. E.g. you can clearly that in the “Average No of Bubbles against Rate” graph that as the number of bubbles increases this means that the rate of photosynthesis also increases. And that in the “Average No of Bubbles against Distance” you can clearly see that as the distance increases the no of Bubbles also increase meaning that more oxygen is being produced. Simply put, the Elodea is photosynthesising faster.
My prediction matched my results fairly accurately, as I predicted rate of photosynthesis increased as the distance and in result amount of light intensity increased. I also think that the other variables were kept fairly constant, as my results show a steady link between the light intensity and rate of photosynthesis. The temperature was kept constant by the water absorbing most of it, the level of carbon dioxide was kept constant as only a certain amount was added to the pond weed and the other sources of carbon dioxide e.g. me and my partner breathing out was too insignificant to have a major affect on the Elodea.
Overall I think that both my graphs and my results supported my predictions however there are some anomalies in my results, I think explaining them is pretty simple as Elodea is a living organism and so cannot be expected to follow a regimented pattern.
Evaluation
Even though I think that my experiment was successful due to the results I obtained I also think that there were couple of points where the experiment was not as perfect as it could have been. One of these points was that I think my experiment was not as accurate as it could have been due to a number of things. First of I think that the method I used “Counting the bubbles” method was far too prone to error and inaccuracy. As the size of the bubbles were not the same all the time, also counting the bubbles was hard as towards the end of the experiment there were a lot of bubbles being formed and it was hard to keep track of them all. Something that I could have done to fix the different “size of bubbles” problem is that I could have added a bit of detergent to the water as this helps for the bubbles to be a constant size. I could of also used the gas syringe method which measures the exact volume and so is more accurate compared to the “counting the bubble method”. Another inaccuracy was the presence of other light beside my lamp, e.g. the other student’s lamps however that was a very little amount of light but I think that I could have acquired even more accurate results by maybe performing the experiment in a totally dark room where the only light affecting the Elodea was from my lamp.
I did have some anomalies and I believe that they were because of those inaccuracies that I have just explained. Because of the fact that we might have missed some bubbles that were produced or the method of bubble counting not being an extremely accurate one. I could have maybe measured a lot more distances to make my experiment more accurate and so eliminate the anomalies, in other words I could have had a longer range which would of leaden me to have more results and my graphs to be more accurate. I could have also measured the time even more accurately, even though this is a small issue I think that measuring each length to exactly 2 minutes would have made my experiment more accurate and perhaps even produced results with no anomalies. However I do think that I was right to repeat the experiments 3 times each and so produce more accurate results as I had an average to work with, meaning that If I had different results when I repeated the experiments then I could find the average and so produce more accurate and reliable results.
Overall I think that my experiment was a success because of the results I obtained and them fully backing up my prediction. However I do think that I could of done more and found out at which point the light intensity does not have a affect on increasing the rate of photosynthesis, meaning that I could have found out at which point a limiting factor is formed. I could have also found out how different coloured lights affected the rate of photosynthesis and found out the most efficient one this could be benefit to plant growers and farmers.