Investigation into the effect light intensity has on the process of photosynthesis
Biology AT1
Investigation into the effect light intensity has on the process of photosynthesis
SECTION 1: PLAN
Introduction
I plan to investigate the effect that light intensity has upon photosynthesis.
In order to properly investigate this, I will collect some background information.
According to Microsoft's Encarta 98, this is what photosynthesis is:
Photosynthesis, the process by which chlorophyll-containing organisms-green plants, algae, and some bacteria-capture energy in the form of light and convert it to chemical energy. Virtually all the energy available for life in the earth's biosphere-the zone in which life can exist-is made available through photosynthesis.
Also according to Microsoft's Encarta 98, here is some information on how photosynthesis takes place:
Photosynthesis consists of two stages: a series of light-dependent reactions that are temperature independent and a series of temperature-dependent reactions that are light independent. The rate of the first series, called the light reaction, can be increased by increasing light intensity (within certain limits) but not by increasing temperature. In the second series, called the dark reaction, the rate can be increased by increasing temperature (within certain limits) but not by increasing light intensity.
Light Reaction
The first step in photosynthesis is the absorption of light by pigments. Chlorophyll is the most important of these because it is essential for the process. It captures light energy in the violet and red portions of the spectrum and transforms it into chemical energy through a series of reactions. Different forms of chlorophyll and other pigments known as carotenoids and phycobilins absorb slightly different wavelengths of light and pass the energy to chlorophyll a for the completion of the transformation process. These accessory pigments thus broaden the spectrum of light energy that can be fixed through photosynthesis.
Photosynthesis takes place within cells in organelles called that contain the chlorophylls and other chemicals, especially enzymes, necessary for the various reactions. The chemicals involved are organized into units of the chloroplasts called thylakoids, and the pigments are embedded in the thylakoids in subunits called photosystems. Light is absorbed by the pigments, raising their electrons to higher energy levels. The energy is then transferred to a special form of chlorophyll a called a reaction center.
Here is the equation for Photosynthesis.
Sunlight
Carbon dioxide + Water -------------|> glucose + oxygen
Chlorophyll
OR
6CO2 + 6H20 --> C6H12O6 + 6O2
Leaf surface area: The higher the surface area, the more surface there is to exchange gases or absorb sunlight. The higher the surface area, the larger the Gas Exchange System, or GES.
Temperature: The higher the temperature, the more kinetic energy is present, and so molecules move faster. This increases the rate of reaction, as there are more molecular collisions per second, and those that collide, collide with a greater force. The enzymes that are used during photosynthesis function at their optimum level, around 35-40 degrees Celsius. Above 40 degrees Celsius, they start to be denaturised, meaning the rate of photosynthesis goes down.
Light: This is the factor I wish to investigate, and more specifically I wish to investigate into light intensity, rather than colour of light etc. The reason why this affects the rate of photosynthesis is that the chlorophyll trap sunlight, and this light energy is used to convert simple chemicals (Carbon dioxide and Water) into energy rich compounds (glucose) which the plant uses as food. Due to class work, I know that light Rate of Photosynthesis\Light intensity ? 1/d. As long as this formula is correct, I would expect the rate of photosynthesis to quadruple if I were to double the distance. However, all of this is dependant on the limiting factor, as the graph on the next page shows. As you can see, if all other factors are limitless (e.g. C02 ) ,or at their optimum (e.g. temperature), the rate of photosynthesis is reliant on the limiting factor.
Carbon Dioxide levels: Carbon Dioxide is one of the substances taken in during photosynthesis. The process turns it into Glucose, using energy from light. The more Carbon Dioxide there is available, the longer the plant will be able to photosynthesise for.
In order for me to be able to measure the rate of photosynthesis, I need a way of being able to see the reaction. As one of the by-products of photosynthesis is Oxygen, I can take advantage of this. By using pondweed, I can see the oxygen bubbles raise from the plant as they are released through the stomata. I did a quick experiment to see if this would work.
Revised plan (basic): I will need to keep the pondweed underwater throughout this experiment; therefore weighing it down with something would be a good idea. The pondweed ...
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In order for me to be able to measure the rate of photosynthesis, I need a way of being able to see the reaction. As one of the by-products of photosynthesis is Oxygen, I can take advantage of this. By using pondweed, I can see the oxygen bubbles raise from the plant as they are released through the stomata. I did a quick experiment to see if this would work.
Revised plan (basic): I will need to keep the pondweed underwater throughout this experiment; therefore weighing it down with something would be a good idea. The pondweed will, just as before, need to have a clean cut that is not obstructed. Also, as it needs to photosynthesise, I cannot use just ordinary tap water, as there will not be enough carbon dioxide in there for photosynthesis to be carried out effectively. So, I will need to supply some CO2. I could do this by adding some ordinary fizzy table water. And by doing it this way, I don't have the advantage of establishing an easy average.
Plan
I plan to do the following in order to produce a good set of results: (see next heading for potential problems + measurements)
) Set up the equipment as follows.
2) Make sure the liquid in the test tube is the correct concentration of Carbon Dioxide.
3) Make a small incision in the stem of the pondweed, to insure that gases can escape.
4) Place the lamp the required distance away for this result.
5) Start the stop clock and record the number of bubbles that are photosynthesised from the weed.
6) After the set time, stop recording.
7) Start from Stage 1 for the next result.
8) Calculations: Using the formula Light intensity= 1/d, I can calculate the light intensity, and then plot this against the average number of bubbles a minute.
Problems
) Volume of Water Against Concentration of Fizzy Water: The more CO2 that is present in the water, the higher the rate of photosynthesis (up to a point, depending on the limiting factor). This must be kept constant throughout the experiment otherwise the results are useless. If I use too much carbonated water, it will become impossible to tell which is a transpired bubble, and which is a bubble from the carbonated water. If I use too little, the rate of photosynthesis will be too small, and thus makes the experiment very inaccurate (see point 5). I will try 20% carbonated water in my preliminary experiment. A good way to make sure it is 20% through all the experiments I do, would be to mix up a big batch, before I begin my experiment. Filling a 100cm3 measuring cylinder with 20cm3 of carbonated water and 80cm3 of normal tap water would be sensible. This would be a good way of making sure the experiment is a fair test.
2) Distance of Lamp From Weed: My first reading should be with the lamp adjacent to the weed, but from then on it depends on what intervals I think I should do. The experiment must be carried out in lesson time, so I can't move the lamp one centimetre along at a time. However, moving the lamp 30cm a time would be equally stupid, because that would be widely inaccurate and could miss out all the possible results. A distance of 10cm between each reading would be a good distance to try out in my preliminary experiment. I will also need to make sure that my lamp is pointing directly at my weed, and not pointing directly above or below it (this may be more difficult as the distances increase)
3) Time: In order for the experiment to be carried out in class effectively, I need to time the weed long enough to get a large number of bubbles (thus making it more accurate, see point 5), but not too long making repeat experiments impossible.
4) Environmental factors: The temperature\wind speed etc of the room I carry the experiment in WILL affect the results but so minutely that I don't believe it will carry much significance at this level of accuracy. However, in order to make the variation of light intensity from my lamp give any satisfactory change, I will need to make sure that I carry out the experiment in a darkened room.
5) Overall Accuracy: There is a limit to the accuracy that can be achieved using measuring cylinders, stop clocks and the human eye. However, I think that the methods I am using are satisfactory considering the nature of the experiment, and the usage of the less accurate equipments. i.e. the measuring cylinder has only got to be accurate to 0.5 cm3, and the human reaction speed when it comes to stop counting is by far fast enough not to cause any problems. However, when it comes to size of results (e.g. if the rate of photosynthesis is too slow, or I don't leave it running long enough), this relatively small margin of error will be percentagely much, much greater.
Factors:
These factors must be kept constant:
) The piece of pondweed.
2) The lamp I use.
3) The concentration of the water\carbonated water.
4) The brand of carbonated water I use (different brands may have different concentrations).
5) The way that the pondweed is cut.
6) Amount of water I use.
7) The temperature that the water is (in this case room temperature)
8) Time
The dependant variable is the rate of photosynthesis..
The independent variable is the light intensity.
I decided to carry out a preliminary experiment in order for me to help decide on what values to set these factors at.
Preliminary Experiment:
I carried out the experiment exactly as I would the real thing, using the values I have suggested in "problems" section.
Prelim results:
#
Distance (cm)
Number of Bubbles
Time (min)
% of Carbonated Water
0
0
20
2
0
8
20
3
20
7
20
4
30
5
20
5
40
3
20
6
50
0
20
Using these results, I have decided to stick to my original plan.
After this preliminary experiment, I conclude that these values I have chosen are the optimum for a good experiment. They do not take too long to carry out, but yet they also retain accuracy. I have decided not to change my plan, but stick to this one. To further increase the accuracy, I will repeat the experiment 3 times, and therefore I can analyse the results effectively.
Safety Issues:
Care must be taken so I don't burn myself on the lamps, as they will get hot. Also, if any breakages occur, I must be very careful of broken glass. I'm also using an electrical lamp near water, which can be very dangerous. I must also be very certain to follow all the normal laboratory rules when carrying out an experiment: Not to run, not to leave an experiment unattended, to keep my tie tucked into my blazer egc.
Hypothesis:
Using all this information, I think that the higher the light intensity, the higher the rate of photosynthesis, up to a point. At this point, limiting factors will began to affect the result. In this experiment, light intensity is unlikely to be the limiting factor, but could be it nonetheless. As the light intensity increase, chlorophyll molecules become proportionally more excited. This is because more energy is received to proportionally to the finite number of chlorophyll molecules. Therefore, the light intensity increases. However, a some point the chlorophyll molecules become saturated with energy, and cannot process any more. At this point, the rate of photosynthesis has reached its maximum. In fact, if you were to continue increasing the light intensity into infinity, you would damage the chlorophyll. However, other limiting factors may play a role in this experiment, such as CO2 level, temperature etc. I also think that the number of bubbles will be proportional to the light intensity, because as light intensity increases, so does the amount of energy available for photosynthesis. By the same token, the lower the light intensity, the lower the amount of energy available for photosynthesis.
Over leaf is a graph of predicted results.
SECTION 2: OBTAINING EVIDENCE
To carry out my experiment, I will be using certain pieces of equipment. These are as follows, including a short justification as to why I chose these and not others:
Lamp: I am using this to provide the light for the weed. I think this is a good piece of equipment to use for this purpose because it is easily directed, and thus very mobile. Due to its conical shape, it distributes light over an increasingly wide area, the further away it gets from its target. This is ideal for my experiment since I am testing light intensity.
Stop Clock: I am using this to time how long I count the transpired bubbles. It is as accurate as my reflexes are, and from my preliminary experiment I have decided that this is by far accurate enough.
Test Tube, Test Tube Rack: This holds the pondweed, and is just big enough. This is good because it gives me a small area to work on. It could be a lot bigger, and it would not affect the experiment's results, (it is just the pond weed that transpires, a greater volume of water would not affect the rate of photosynthesis). However, having a very large container would a) be more cumbersome
b) take longer to fill up (as I have stated in the Plan section, time is vital)
Plastocene, string: These are good way to weigh the weed done, because they are quick and easy to set up, and don't release any chemicals (or not in great enough quantities in the short term) to affect the results.
Ruler: Again, this is a quick and efficient way of measuring distance, and is accurate enough for this experiment.
I then carried out my experiment, using all the values I decided upon in my plan. I repeated it three times for accuracy.
As I have already stated, in order to find out the light intensity from these values, I must use the following calculation: 1/d2.
I got these results:
Experiment #1
#
Distance (cm)
Light intensity
Bubble Rate per minute
0.00
-
20.00
2
0.00
0.01
5.00
3
20.00
0.00
5.00
4
30.00
0.00
.00
5
40.00
0.00
0.00
6
50.00
0.00
0.00
Experiment #2
#
Distance (cm)
Light intensity
Bubble Rate per minute
0.00
-
9.00
2
0.00
0.01
8.00
3
20.00
0.00
4.00
4
30.00
0.00
2.00
5
40.00
0.00
.00
6
50.00
0.00
0.00
Experiment #3
#
Distance (cm)
Light intensity
Bubble Rate per minute
0.00
-
7.00
2
0.00
0.01
5.00
3
20.00
0.00
0.00
4
30.00
0.00
7.00
5
40.00
0.00
3.00
6
50.00
0.00
0.00
After looking at these results, I decided that 3 repeats was sufficient to give me a good spread of results, that I can analyse. There were no results that were drastically anomalous, and so I did not need to carry out any more readings.
SECTION 3: ANALYSIS
In order to properly analyse my results, I am going to plot a graph. First, I will draw up a table of the average results.
Experiment (average)
#
Exp. 1
Exp. 2
Exp.3
Distance (cm) | LI
Average:
20
9
7
0 | 0.04
9
2
5
8
5
0 | 0.01
6
3
5
4
0
20 |0.0025
6
4
2
7
30 |0.001111
3
5
0
3
40 |0.000625
6
0
0
0
50 |0.0004
0
I will now produce a series of graphs to show this information in a graphical representation, to make it easier to analyse. However, there is a problem when taking the distance from the pondweed to the lamp as "zero" (i.e. adjacent). Because you cannot divide any number by zero, the calculation does not work. In order to get around this problem, I am taking the light intensity to be 4, as each light intensity is a quarter of the previous light intensity (as I predicted). Also, as all the results are so small, I have multiplied them by a constant of 100.
Analysis of Graphs:
To analyse these graphs, I will mainly use the single lined, average graph.
The shape of this graph is almost that of a reciprocal graph. As the light intensity gets smaller and smaller, so does the bubble rate per minute. However, as the light intensity increases, even if it is a very small increase, the Bubble rate per minute increases hugely. This is because we are dealing with 1 divided by a square number. When the light intensity is 0.1, the bubble rate per minute is only about 0.5. However, when the light intensity is 0.5, the bubble rate per minute is about 11. When it is 1 the bubble rate per minute is 16. From then on, the graph levels off as dramatically as it rose. This is probably due to the presence of a limiting factor. In this case, it could be CO2 level, the surface area of the leaf, the temperature, number of chlorophyll, or even the colour of the light.
But what is clear, that up to a point, if you increase the light intensity, the rate of photosynthesis increase. This is what I predicted, more or less. It also complies with all the theory work I have done. However, the graph of bubble rate against distance, is different to the theory graph. The practical graph firstly fits properly on the scale. Secondly, the curve is a lot less like a reciprocal than on the theory. The Bubbles per minute doesn't stretch off into infinity in either direction. The area which best shows the theoretical curve, is the area between 5-15cm.
In real life, a plant cannot absorb an infinite amount of energy, and so the Bubble rate per minute will get anywhere near the theoretical infinity. Also in real life, there is a limiting factor. My prediction was that the higher the light intensity, the higher the rate of photosynthesis.
Here is the equation for Photosynthesis.
Sunlight
Carbon dioxide + Water -------------|> glucose + oxygen
Chlorophyll
As this shows, light energy is a vital part of the equation. Without it, there would be no reaction. My results fit into this very well. There is some difference in my predicted graph and actual graph, but this can be explained.
On either extreme side of the practical graph, the decline (or incline) in Bubble rate slows down. This is, as I have said, due to other factors. There comes a point that no matter how much energy you supply the plant with, something else holds it back. In any plant, there is a finite number of chlorophyll. A finite number of chlorophyll cannot process an infinite amount of energy and resources. If you were to divide this finite number by the infinite amount of energy (to find out the ratio of energy and chlorophyll) the number would be so close to zero on one side, and so huge on the other that it makes no odds.
Also, the theoretical graph does not make any assumptions when it comes to environmental factors. There were other experiments going on around me, and as such there have been an adverse effect on the accuracy of these results.
SECTION 4: EVALUATION
I think that my experiment was, on the whole a success.
Firstly, I was able to make accurate results. The pondweed did not photosynthesise so much that there were too many bubbles to count. Since there is either a bubble there or not (unlike if I was measuring temperature, where between 1oC and 2oC, there is an infinite range of inaccuracy that depends on my eyesight), it is very easy to count all of the bubbles that were produced. After that, I had to measure time. This was done using a stop clock. Depending on my reaction speed, I may have been up to 1 second out on either side. However, as I have already said, the plant was not photosynthesising so fast, that that one second made me miss out on more than one bubble. At the very most, I may have counted an extra bubble for one reading, and missed it out on the next, but even that is unlikely.
After that, the only other thing I had to measure was the distance that the lamp was from the pondweed. This was done using a ruler, and so was accurate to about half a centimetre. If I was half a centimetre out at each reading, that equates to 2.5 centimetres overall, and that in turn equates to a light intensity margin of error, of around .1 per result (when multiplied by the constant of 100). While this is relatively large, it is not large enough to produce results that are wildly off target.
Having said that, there are ways to make the experiment more accurate. Rather than using a lamp that has a large cone of light, something that is much more direct (similar to a Maglite Torch), that could have its power reduced increased accordingly. This way, not only is the light more direct and thus more accurate in that way, but you don't have the inaccuracy of having to measure the distance all the time, meaning that the relatively small margin of error of 0.1, wouldn't even apply here. Using a computer with data logging equipment could also produce very accurate results.
I have found no anomalous results, or at least none that are too anomalous. There WERE variations in the results, but that is what can be expected in an environment that is not totally controlled, like a classroom. However, I think I have acceptable variation, especially considering that my experiments had to be carried out on different days, and as such with different pond weed and lamp.
After looking back on this experiment, I realise that I did indeed carry out enough results to draw an accurate conclusions, and those results I carried out were accurate enough to be relevant. So, I conclude that the higher the light intensity, the higher the rate of photosynthesis up to a point, determined by either the limiting factor, or extreme temperature\overload of light energy (of the chlorophyll) which causes denaturation of the enzymes.
In order to further investigate the effect light intensity, I could investigate whether high light intensity do in fact damage chlorophyll, by overloading the system. I would need a very powerful lamp to do this, and as such, I would need to pay special attention to its heat emissions, as if chlorophyll does suffer damage, it could be from the heat not the light.
