Also affecting the rate of photosynthesis is the amount of water and nutrients available to the plant. For example, if the plant does not have enough water, the plants stomata will shut and the plant will be deprived of carbon dioxide, therefore preventing the photosynthesis process.
Another factor which must be considered is the piece of pondweed. The larger the pondweed the larger the surface area for photosynthesis to occur in. In addition, we must use the same time scale/span in which we count the number of bubbles of oxygen produced in.
The final factor is light. This being the factor I intend to investigate. Light is an essential requirement for photosynthesis. In our experiment, the power of the light bulb must be equal throughout because the stronger the light bulb the more energy emitted by the bulb and the more light energy possibly absorbed by the pondweed. Therefore the pondweed would produce more food through photosynthesis.
Therefore to maintain a fair experiment all of these factors must remain fixed. The temperature will be constant at room temperature. The amount of water in the test tube will remain the same. The same piece of pondweed will be used throughout so that I don’t use different sizes. The bubbles will always be counted for one minute, and also the same desk lamp will be used throughout so I don’t use light bulbs of different powers. Therefore the only changing variable will be the distance of the desk lamp from the pondweed.
Prediction
In order to photosynthesise, plants need light as it provides the energy for the process to happen. I predict that as the light source (desk lamp) is moved closer to the pondweed, the rate that oxygen is produced will increase. Therefore more oxygen will be produced and also more glucose. I believe this will happen because when the light source is nearer to the pondweed, more of the pondweeds surface area is coming into contact with the light from the lamp, and more photosynthesis will take place meaning more oxygen and glucose will be produced.
For that reason, when the light is further away from the pondweed, we will not see as many bubbles of oxygen being produced. This is because the pondweed will not have so much energy (from the light) to activate photosynthesis.
I also believe that as we move the lamp further away from the pondweed, therefore reducing the light intensity, the number of bubbles produced will gradually decrease. For example, if at 20 cm, 80 bubbles are counted, then it is likely that at 40 cm, 40 bubbles are counted, as the desk lamp is twice the distance away meaning the rate of photosynthesis is halved.
Method
In order to carry out this experiment, I will have to ensure that everything is done safely and fairly. To guarantee it is a fair test I will have to use the same pondweed every time, even when we change the light distance. I must also carry out the experiment at least two times for each distance to get an accurate average. I must make sure that there is only one changing variable, which will be the distance of the desk lamp from the pondweed. In addition, the experiment must be at a constant room temperature because otherwise at hotter temperatures more oxygen will be produced.
Furthermore, I must make certain that I am working safely. Lab safety rules (eg no running etc). Also, the desk lamp must be kept out of contact with water to ensure safety.
It is also necessary that I use the same equipment for the experiment throughout. (This equipment is shown in the diagram attached).
To begin the experiment I need to collect the apparatus and set it up as shown in the diagram attached. The equipment needed to carry out the experiment is as follows: Stopwatch, light source (desk lamp), pondweed and paper clip to hold it down, boiling tube, test tube rack, water, and also a meter ruler.
If the pondweed is already set up in the test tube of water, held down by a paper clip as mine will be I will then put it in the test tube rack, prior to setting up the desk lamp. It must be set down firmly in place with a paper clip, upside down in the test tube, as leaves have a greater number of stomata on the underside of their leaves, so gas will be released more readily. I will set up the desk lamp 15 cm away from the pondweed and every time I will move it 5cm further away measuring the distance with a meter ruler. Then I will turn on the desk lamp and start the stop watch, and immediately begin counting the bubbles for 1 minute. After recording my results, showing distance in cm and number of bubbles I will repeat the experiment for distance 25 cm, and then do the same with distances of 20, 25, 30, 35, 40, 45, 50 cm.
Carrying Out
I had no problems during my experiment. To ensure that it was a fair test I was carrying out I did the experiment all together, instead of carrying it out over for example 2 days. This ensured that the conditions were relatively the same throughout. I also used the same piece of pondweed throughout. This was important because the size and surface area of the leaves on the pondweed could have changed on a different piece of pondweed and therefore would have altered the result. To overcome this problem I could have weighed the different pieces of pondweed and made sure they were the same weight, but this wouldn’t have been totally accurate. Therefore using the same piece throughout was the best decision. I always counted the bubbles given off from the pondweed for the same amount of time (1 minute), and also every time I did a repeat I turned off the lamp so that the pondweed had stopped photosynthesising before starting the repeat. Therefore I made no alterations to my plan and followed the safety rules I previously named in my method.
Results
Refer to Graph 1, and Table 1.
Conclusion
From observing my results I can see a significant increase in the rate of photosynthesis as the distance decreases and all of my results show this pattern.
For photosynthesis to occur, plants require light and chlorophyll. They are not constituents of glucose but are still vital. So, in the experiment when the distance was at the largest we tested (50 cm), there was not much photosynthesis taking place, as only about 43 bubbles of oxygen were produced. However when the desk lamp was at a small distance of 15 cm away from the pondweed, a greater amount of photosynthesis was occurring.
On average, 93.5 bubbles in 1 minute is quite a substantial increase from 50 cm. Therefore the range of number of bubbles of oxygen was, on average, 50.5. This is because when the desk lamp is close to the pondweed, more of the pondweeds surface area has light shining on it, which means more photosynthesis will occur in a shorter time.
This becomes obvious when looking at my results. Because it is evident that the number of bubbles produced is much greater when the distance of the pondweed from the desk lamp is less, my prediction was therefore correct. This is evidently true as my prediction was that “as the light source (desk lamp) is moved closer to the pondweed, the rate that oxygen is produced will increase. Therefore more oxygen will be produced and also more glucose. I believe this will happen because when the light source is nearer to the pondweed, more of the pondweeds surface area is coming into contact with the light from the lamp, and more photosynthesis will take place meaning more oxygen and glucose will be produced”.
My results also prove that my further prediction that “as we move the lamp further away from the pondweed, therefore reducing the light intensity, the number of bubbles produced will gradually decrease” was also correct. However I then continued in my prediction by saying “for example, if at 20 cm, 80 bubbles are counted, then it is likely that at 40 cm, 40 bubbles are counted, as the desk lamp is twice the distance away meaning the rate of photosynthesis is halved”. But my results weren’t that accurate and it is clear from my table that this didn’t quite happen, although it was a fair further prediction and I had reason to believe it.
If you refer to Graph 1, you will see that there is a downward trend, therefore proving that as the distance increases the rate of photosynthesis decreases. Also, if you refer to the scientific information throughout the start of my investigation you will see that the closer the light is to the pondweed the larger the surface area gaining energy to photosynthesise. Furthermore as the energy was absorbed by the chlorophyll in the pondweed, the energy combined with the water molecules and carbon dioxide, and as a result the oxygen was produced (as bubbles) along with the other product of photosynthesis, glucose.
Evaluation
From my results I have proven that the larger the distance the light travels the slower the rate of photosynthesis. Once again, this is due to the fact that when the light is close to the pondweed, more of the pondweeds surface area has light energy shining on it which means more photosynthesis taking place in a shorter time. Therefore I have succeeded in what I planned to do, which was to determine how light affects the amount of oxygen produced during photosynthesis, therefore investigating the rate of photosynthesis.
I think that on a whole my experiment was quite successful. This is because the results that I produced were what I had expected and they also proved my prediction correct.
Furthermore I did not get any anomalous results, as the results I produced were what I had anticipated. Also with the two sets I produced I was able to calculate accurate averages which followed my chief prediction. I also believe my results are reliable due to the fact that I conducted the same experiment twice producing two sets of results shown in Table 1. This then enabled me to produce a graph by7 calculating the average number of bubbles of oxygen. To do this I used the following equation:
Average number of bubbles = 1st result + 2nd result
2
I think my procedure used was quite suitable, as I managed to achieve fairly reliable results. However when dealing with living things the results can never be that accurate, because living things do not work at a constant rate. So I can’t know if the pondweed was working at a low or high rate or somewhere in between when I did the experiment. There’s no way I could solve this problem, because living things are so unreliable. However we could take results over a period of several days or weeks for more accuracy.
Another point is that the way I chose to measure the rate of reaction was not suitable. This is because counting bubbles got very tiresome and I could have miscounted. A better way would be to collect the oxygen using a capillary tube. I would have to half fill two test tubes with water, one also containing the pondweed and connect the two test tubes with a capillary tube. I would then measure the amount of water in the first test tube prior to leaving it for an amount of time before measuring the difference. This would certainly be more reliable.
Had no anomalous results, but if there were there are several reasons why it could be. Firstly, I could have miscounted the bubbles. Also the pondweed might not have been producing oxygen as properly due to an airlock I did not notice. Furthermore, lots of people were crowded in the same room carrying out the experiment. Now I can’t be certain that the position of my equipment remained the same as it is possible that part of the apparatus could have been moved. To overcome this problem I could have used chalk to mark the position of everything. In addition, with the curtains open, extra light from the windows or other experiments could have easily affected the rate of photosynthesis. Also, as I did each repeat, the pondweed most likely gained heat energy affecting the rate of photosynthesis.
To develop and extend my experiment there are several different tests for photosynthesis I could carry out. The first test I could do is to put a piece of foil on a destarched leaf and leave the plant in the sun for three days. Then test the leaf for starch using iodine. If it turns blue-black there is starch present. The second is to test for chlorophyll. I would take a variegated leaf (one with white and green leaves) and test it for starch, remembering that only green parts of plants contain chlorophyll. The last is to leave a destarched leaf in a plastic bag that contains soda, lime, and crystals for three days. This would absorb carbon dioxide and so there should be no carbon dioxide for the plant to use. Also next time I would do more repeats to make my results more accurate.
