Carbon Dioxide
Carbon dioxide is a gas which is present in air but only in small amounts (about 0.04% of air is carbon dioxide). But carbon dioxide is continually added to air by respiration of all living organisms and by the burning of wood, coal, gas, oil and petrol used as fuel. There is no danger that carbon dioxide will run out in fact it is slowly increasing in the air because so much burning takes place. Carbon dioxide dissolves in water, plants living in water therefore also have a supply of carbon dioxide. Plants get their carbon dioxide usually from the air, through the stomata by diffusion. However water plants use the carbon dioxide which is dissolved in the water. Carbon dioxide is needed in photosynthesis to produce glucose which is what the plant is trying to make. This can be controlled by having a fixed amount of sodium hydrogen carbonate powder dissolved in the beaker of water – in this experiment I will use 0.6g of sodium hydrogen carbonate.
Light intensity
Light rays are a form of energy they are wave movements traveling at great speed. Those of a certain wavelength, which are seen by our eyes, are light rays. Plants have a green pigment called chlorophyll which can trap some of these light rays and use their energy to build up the simple sugar - glucose, from carbon dioxide and water. The light energy is trapped as chemical energy in the glucose molecule. Plants can use any source of light rays, but the source that does not run out is sunlight. Artificial light is used in glasshouses when extra light is needed. In water, only the top few meters get sufficient light for plants to use in photosynthesis. Plants that are able to detect a source of light grow towards it, for example, cress.
Temperature
Temperature affects the rate of photosynthesis because when cells are hotter they have more energy and therefore move around quicker. Enzymes change the starch into proteins and many other foods when needed and this is done much quicker when they are hotter.
What I am going to do
I plan to investigate how light intensity affects the rate of photosynthesis in Canadian pondweed. The equation shows that glucose and oxygen are made in the process of photosynthesis. Therefore the rate of photosynthesis can be measured by the amount of glucose or oxygen produced. Because oxygen is let out of the plant, this will be the easier of the two to measure. I will measure the amount of oxygen made by the number of bubbles given off from the plant. I will count the number of bubbles produced in 5 minutes light intensity is measured by the distance between the lamp and pondweed. This distance is then squared to give the light intensity.
How I will ensure a fair test
It is very difficult to keep the experiment free from any outside factors. Changes in temperature by as much as two or more degrees can affect the whole experiment e.g. draughts or another heat source. For example if the experiment is set up by a window and the sun comes out from behind a cloud then the plant receives extra heat and light. Also if the experiment is spread over two days, then the temperature is likely to be different. To compensate for this I will place a thermometer in the beaker and check that this does not rise above or fall below a one-degree boundary on either side of the room temperature at the beginning of the experiment. Also the experiment will be carried out away from the glare of all windows to ensure that no other changes will affect the results. Different pieces of pondweed have different masses and length. Each time I change the light intensity I will use the same piece of pondweed so the length and mass is always the same. If the pondweed was longer it would have more palisade cells and so more chloroplasts and if the mass was bigger then more starch would be used up.
My Prediction
On the left is my prediction of how the rate of photosynthesis will relate to the light intensity. They are directly proportional meaning that for example, if the light intensity increases the rate of photosynthesis will also increase. Also if one is doubled the other doubles as well. The reason for this relationship is that during photosynthesis light is a key element. If all other aspects of photosynthesis (carbon dioxide, water chlorophyll) are kept constant then light intensity is in control of the reaction. As the light intensity increases chlorophyll can trap enough light needed to form glucose quicker. Therefore the higher the light intensity the quicker that chlorophyll can trap sunlight so the rate of photosynthesis is faster also.
I predict that the closer the lamp is to the pondweed the faster photosynthesis will take place because light is needed for the reaction and there will be more input energy. When the lamp gets close to the pondweed the lamp will be giving all the light (energy) that the pondweed will need to photosynthesise at its optimum speed. It cannot photosynthesise any faster because there are only a certain number of chloroplasts containing a limiting amount of chlorophyll and this can only absorb a certain amount of light which is called the light saturation point. I think that the light saturation point will be at about 5cm judging by my preliminaries. The other limiting factors should be at their optimum or remain constant throughout the experiment and so should not affect the experiment. As a limiting factor is unlikely to occur I anticipate that there will be a positive correlation between the rate of photosynthesis and the light intensity. This means that as the light intensity doubles the rate of photosynthesis doubles also.
Apparatus
Method
- Get out the apparatus, which consist of a 250ml beaker, a test tube, a lamp, a thermometer, a stop clock, a funnel and a meter rule.
- Fill the beaker with 250ml of tap water, and check that its temperature is around 21ºc. Add 0.6grams of sodium hydrogen carbonate to the water and stir vigorously for 2 minutes to ensure that all of the powder has dissolved. This is important because otherwise the sodium hydrogen carbonate wouldn’t have a significant effect.
- Put the pondweed into the beaker and place a funnel over it. Make sure that the top of the funnel is under the water level in the beaker.
- Fill the test tube to the top and then invert it over the top of the funnel. Try to lose as little water as possible in the process.
- Start the stopwatch and turn on the lamp at the desired distance from the beaker.
- Start the stop clock and for the next 5 minutes count the bubbles of oxygen produced by the pondweed.
- Repeat this experiment with the lamp at different distances from the beaker: 10cm, 20cm, 30cm, 40cm and 50cm.
The reason why this experiment works is that during photosynthesis oxygen is given off. This works well as no extra oxygen or carbon dioxide interferes.
I will also use five lengths so a clear picture can be drawn as to the affect of the light intensity and I will repeat the whole experiment three times to make sure any anomalies are seen.
Observations
During the experiment I will observe bubbles of gas being given off by the pond weed and being collected at the top of the test tube. As the light intensity increases the number of gas bubbles given off per minute will increase. I will measure the number of gas bubbles given off per minute by use of tally marks. Every time a gas bubble is let off I will put a tally mark on a piece of paper. My original plan was to collect the bubbles in a measuring cylinder instead of a test tube and then find the volume of oxygen produced. However from completing my preliminary experiment I found out that not enough oxygen was produced to give an accurate reading of volume.
Range
In my preliminary work, I found that the best range for my experimental variable was to move the light source between 10cm and 50cm away from the pond weed because if I was to move the light source too close to the pond weed it would possibly increase the temperature therefore making it an unfair test as another factor is being altered. And if I was to move the light source to far away from the pond weed it would not affect the experiment. Therefore if I keep the light source within my distances I should get good results.
Number
To get more accurate results I will take five readings. This should be enough for me to plot a graph. I will also do the experiment three times at each distance so that I can get a reliable average.
Safety precautions
Precautions play an important part in every experiment.
- Bags and books must be kept away from the apparatus and the place where the practical is set up.
- The lamp must not be kept too close to the leaves for a long period of time. This is because chlorophyll is an enzyme and gets denatured by excessive heat.
- Bags and blazers must be kept under benches so that you are less likely to trip over things.
- Wear safety goggles and aprons so that the sodium hydrogencarbonate cannot get into your eyes as it would cause irritation.
- Never run in the laboratory because it could cause an accident.
- Tie long hair back so that it cannot get in the way and become dangerous.
- Wash hands after practical so that anything you have picked up from the pondweed is washed off.
- The lamp will become hot if it is left on for long periods of time so care must be taken at the end of the practical when it has to be moved.
Why I did a pilot
I did a pilot experiment so that I could make sure my method worked and also that I was getting sensible results. It gave me an idea of what to expect for my actual investigation. I also did a preliminary test to make sure I was comfortable with the experiment. My results, although they were not thorough, show a relationship, as I have already stated, forming. The rate of photosynthesis was going up at a fairly similar rate to the light intensity so I am confident that in the actual experiment the results will be closer to the graph above and show a directly proportional line on the graph.
The results for my preliminary experiment:
What I discovered from my preliminary measurements
In my preliminary experiment I learnt what to expect for the real experiment so this prepared me in many ways. From my preliminary experiments I discovered that I should not have the lamp any closer the 10cm because the light intensity is beyond the light saturation point so there was no change in the results and that the light will begin to heat the water bath making it inaccurate. I also found that there was not much point in having the lamp any further away than 50cm because very little photosynthesis occurred. My range of results will be from 10cm to 50cm. I found that the pondweed had to be put in upside-down as that is where the bubbles come out of. The pondweed shouldn’t be touching the side because otherwise the bubbles could get trapped. These all helped me design my final method, I decided to continue counting the number of bubbles produced instead of collecting the oxygen produced and giving my result as a volume because in my preliminary experiment I found that such a small volume was produced that it was hard to take an accurate reading that was meaningful.
Have I made enough measurements and repeated them?
In the time given I managed to take 3 readings for 5 different distances of the lamp. The results are shown in the table below. I have also worked out the average so that my graph is a representation of the whole experiment. I worked out the light intensity by using the formula:
1/d² for example: 1÷(0.1²)
I also worked out the rate of photosynthesis – I did this by using the formula 1/time
In this case it was the average number of bubbles ÷time taken (in seconds).
5 minutes = 300 seconds
For example:
108.6 ÷300 = rate of photosynthesis when the light intensity is 100
Accuracy
Obviously because I was counting bubbles the results are dependent on what I defined as being a bubble. This is it had to be big enough to be seen, therefore any tiny little streams of bubbles were not counted. This does not make my results unreliable because I tried to use the same criteria in each experiment.
Clearly and accurately recorded in a table
After collecting all my results I recorded them in a table so they were easier to understand, this also helped comparison. The table is shown below:
I used all the equipment as accurately as I could, making sure I did each experiment in the same conditions as the previous ones. This minimalised the risk of there being any major differences between experiments and so gave me much more accurate results. I kept to my original guidelines and I made sure all the other factors remained constant. Because of the small mass of sodium hydrogencarbonate used, I weighed it using a 2 decimal place balance. This made it possible for me to add exactly 0.6 grams to each experiment. I think that I chose an appropriate range of distances because if the lamp had been any further away it would not have affected my experiment and it is more likely that another lamp near by would have been interfering with my experiment. I think that the results I have collected are reliable because no other factors were introduced – this is why it was important that the lamp wasn’t too close to the beaker as it could introduce temperature because the lamp would be heating up the water enough to alter my results. I also think that they are reliable because when I repeated them I got the same kind of numbers.
Analysis
My graph clearly shows that as the distance of lamp from the pondweed increases, the number of bubbles decreases. In my second graph I can see that as the light intensity increases so does the rate of photosynthesis. However when the light intensity is at 30 it has a very gentle slope up to 100. This suggests that all available chloroplasts are fully occupied in light absorption so no matter how much the light intensity increases, no more light can be absorbed and used. This proves that my prediction is correct. It also provides sufficient evidence to back up my statement that that the closer a plant is to light the higher the light intensity and the higher the rate of photosynthesis
The graph comparing the distance of the lamp from the pondweed with the number of bubbles produced shows a steady inversely proportional line with the line of best fit lying close to all of the points. The graph definitely shows that there is a direct relationship between the distance of the lamp and the amount of oxygen produced. Therefore this shows that the further the light is from the pondweed the lower the number of bubbles produced in 5 minutes.
When the light is more intensive the chloroplast can absorb sufficient amount needed for photosynthesis to take place quicker. In the experiment all the factors of photosynthesis needed were present and the chloroplasts did indeed absorb the light faster which then sped up the whole reaction. The light is important as without it the chloroplasts could not absorb it and turn it into starch using carbon dioxide and water. As the distance increases the light intensity reduces so it will make the rate of photosynthesis decrease.
My results reflect my prediction; they show that the light intensity affects the rate of photosynthesis. However in my prediction I said that I thought as the light intensity doubled, so would the rate of photosynthesis, however this is not the case because I did not fully take into account the effect of the saturation point.
The procedure I used to complete the experiment worked well and no limiting factors came into contention. I found the procedure very suitable to the task in hand as it was quick and easy to set up so more results could be researched in the time available. There was only one problem with the procedure and that was the possibility of heat warming the beaker of water.
My expected graph is fairly similar to the outcome but it does not reflect the forty-five degree gradient causing a definite directly proportional graph.
My graphs show that, at first, the rate of photosynthesis increases rapidly and steadily, at light intensity 4 the bubble rate is 21.3 per 5 minutes but at light intensity 25 the bubble rate is 69.6 in 5 minutes. Increased light intensity produces a faster rate of photosynthesis because more light energy can be trapped in the chlorophyll which is needed for the reaction. However part of my prediction is wrong because when it reaches a light intensity of about 30 the rate of photosynthesis doesn’t increase as rapidly and the graph shows a very steady curve. By the time the light intensity reaches 100 the line on the graph is almost flat. This point is the light saturation point (where more light does not affect the rate of photosynthesis because all of the chlorophyll is being used to trap the sunlight.) In my prediction I said that I thought as the light intensity doubled, so would the rate of photosynthesis, however this is not the case.
From these results I can conclude that the greater the light intensity the faster plants can photosynthesize until a certain point when they photosynthesize the same amount even with more light. This is the light saturation point; the rate of photosynthesis can not improve with more light because it already has as much light as it needs.
The plants photosynthesize faster with more light because energy is needed for the reaction and light is the energy.
My results do agree with my predictions: I predicted that more light would speed up the rate of photosynthesis, I predicted that the rate of photosynthesis would not increase so rapidly when the lamp got closer (light saturation point). I did not predict correctly what distance the light saturation point will be, the light saturation point turned out to be closer than I had predicted.
My predicted graph and actual graph are quite similar in shape although in the actual graph the rate of photosynthesis goes up steeper than I had predicted to begin with and then much more steadily.
The procedure that I used was accurate enough for me to come up with good results and strong conclusions. I could identify trends and patterns in the graph that I had in my predicted graph. Because I can come to these conclusions the experiment was a success.
My evidence is sufficient enough to support my conclusion and it agrees with my theory so they must be quite accurate.
Errors, Limitations and Improvements
Like all experiments there is always room for improvement and this one is no exception. My main limitation was time. To improve the reliability of the results more tests could have been done but there was limited time so this became a problem.
To improve the reliability of the experiment I could do the experiment in a room by myself so my experiment doesn’t get affected by others. Also I could have tried to control the background light more efficiently by covering the whole experiment or by completing the experiment in a dark room.
Some further work I could carry out to provide additional and further evidence for the experiment would be to take more readings from the experiment and draw up a larger graph so it would be easier to make out the results and there would be more of them to make the experiment more accurate.
In my experiment there were a few things that may have made my results less accurate. I could have miscounted the number of bubbles or gone a few seconds longer than I was meant to because it is difficult to look at two things at once. The variables were controlled reasonably well, the amount of water and chloroplasts kept constant, the temperature was controlled quite well in a water bath, the amount of carbon dioxide was kept constant by using the same mass of sodium hydrogen carbonate in each experiment, the light intensity might have changed a bit because of the varying sunlight coming in the windows. The sunlight could have been controlled by closing the blinds on the windows. The temperature could have been controlled better by having a bigger water bath.
The method of counting bubbles was probably not very accurate because the bubbles can be of different sizes, this can be improved by leaving the plant to photosynthesize for longer and then the volume of oxygen given off can be measured.
Whilst carrying out my experiment I encountered no abnormal results, all complying with one another and with my prediction. However a problem that occurred with my experiment was that the temperature had been fluctuating whilst the lamp was at 10cm from the beaker. I think that if I were to carry out my experiment again I wouldn’t change the way I actually carried out the experiment because I found that the way I did it anyway was quite successful, instead I would just try to make my experiment even more accurate. I would do this by monitoring all the factors much more closely. I would use instruments/equipment that would allow me to make sure that every other factor did stay absolutely constant, such as a temperature sensor/probe to record even the slightest change in the water temperature. I would also try and find a way of preventing the heat from reaching the pondweed, for example a glass barrier/heatproof screen or a beaker of water which would only let light through.
Overall, I would state the experiment as a success since my predictions were supported by my results. This is important in reflecting success only if my prediction was sensible and logical. Just as important is where the experiment was not a success and why. This photosynthesis investigation was probably not performed as accurately as it could have been due to some controllable and uncontrollable conditions. Some mistakes can be corrected. A large factor in determining data accuracy is the amount of human error during experiments. The rate at which oxygen bubbles were being produced by my plant was so high that I found it difficult to count the amount of bubbles. I estimate a margin of error of at least 3 bubbles for each reading taken. To improve the accuracy of the results, the readings would have to be taken several more times. The entire experiment could have been performed again, and the new results could be combined if the same plant is used. But the photosynthetic rate of the same piece of pondweed would eventually decrease over time. Repetitions would, however, improve the overall reliability of the results. There are quite a few factors that could affect the results of my experiment. Some of these are variables that were mentioned earlier and could not be controlled, or they were variables that were not initially considered. While performing the experiment, some of the oxygen produced from photosynthesis may have dissolved into the water. Microorganisms living on the pondweed may have even used some oxygen. The amount of oxygen dissolved or used by microbes is probably insignificant to my results since the degree of accuracy at which I measured was not high enough. Some oxygen is also used during the respiration of the plant. But since only bubbles were counted, the volume of bubbles was not as important. But the volume of oxygen produced is important, since it was volume in terms of bubbles that were measured. As the rate of photosynthesis decreased due to a decrease in light intensity, the size of the bubbles produced also became smaller. This change in bubble size was not accounted for when the results were analyzed. For a more accurate analysis of the collected data, volume should have been measured instead of bubble quantity since the size of bubbles can vary. Using a measuring cylinder in place of the test tube so that the volume of each bubble could have been measured could have done this. During the high light intensities I had experienced counting difficulties of the bubbles being produced. There are also factors affecting accuracy at low light intensities. With low light intensity, the pondweed receives some light energy from background light such as sunlight seeping through curtains or the light from the lamp of another student's experiment. To eliminate almost all background light, the experiment must be performed in a completely dark room. Even then, some of the light from the lamp in my experiment would reflect of the beaker and reach the plant though this amount of light is probably insignificant in affecting the rate of photosynthesis. If during a repeated experiment, counting bubbles is still used, there is a smaller chance for human error when counting within a smaller time frame. If the measuring cylinder option was to be chosen, volume should be measured for a smaller time frame to reduce the overall time to complete the experiment. Also, during high rates of photosynthesis, it would still be difficult and impractical to measure the volume of oxygen produced for a long duration.
