Find out how light affects the rate of photosynthesis.
Photosynthesis
Introduction
Photosynthesis is the process plants use to produce their own food, turning Carbon Dioxide and water into glucose and oxygen. This process can be explained using the formula:
Sunlight
6CO2+6H2O C6 H12 O6 +6O2
In this piece of coursework I am going to find out how light affects the rate of photosynthesis.
The energy from sunlight is absorbed into the plant by a green chemical called chlorophyll, which is found in chloroplasts in the plant's cells. Plants need a healthy supply of carbon dioxide and water to make photosynthesis possible, the water is drawn in by the roots, and the carbon dioxide is taken in by the leaf. The chlorophyll is able to split water in to hydrogen and oxygen, the 'light' reaction. The oxygen leaves the leaf and the hydrogen and carbon dioxide combine to make glucose, the 'dark' reaction.
The leaf is specially adapted for photosynthesis as shown in the cross section of the leaf (taken from www.bbc.co.uk/schools/gcsebitesize). One adaptation of the leaf is that the chloroplasts, which contain chlorophyll, are found in the palisade layer, close to the surface of the leaf, so they can absorb sunlight easily. Another adaptation of the leaf is that the cuticle and the epidermis are transparent to allow light to pass through to the palisade cells.
Preliminary work
We have completed some preliminary work to identify what is needed to ensure a fair test and get accurate results.
Firstly we did a trial experiment to help us decide what distances to use in the real experiment. We found that it was impossible to get the lamp any closer than 5 cm and the elodea stopped producing oxygen bubbles after 30 cm. We decided to use these distances - 6,9,12,15,18,21 and 24 cms; we chose those as they are within the 5- 30 cm boundaries and we can easily find a pattern. Now we know the distances we are going to use we can work out the light intensity for each distance, by using the formula- Light intensity =1/d2
6cm - Light intensity = 1/36 = 0.027 Lux
9cm - Light Intensity = 1/81 = 0.012 Lux
2cm - Light Intensity= 1/144 =0.0069 Lux
5cm - Light Intensity = 1/225 = 0.0041 Lux
8cm - Light Intensity = 1/324 = 0.0031 Lux
21cm - Light Intensity = 1/441 = 0.0023 Lux
24cm - Light Intensity = 1/576 = 0.0017 Lux
To make these numbers easier to graph we can divide by 1000 instead of 1 this changes the results to:
6cm - Light intensity = 1000/36 = 27 Lux
9cm - Light Intensity = 1000/81 = 12 Lux
2cm - Light Intensity= 1000/144 =6.9 Lux
5cm - Light Intensity = 1000/225 = 4.1 Lux
8cm - Light Intensity = 1000/324 = 3.1 Lux
21cm - Light Intensity = 1000/441 = 2.3 Lux
24cm - Light Intensity = 1000/576 = 1.7 Lux
As part of this experiment we need to look at limiting factors and how to control them. There are a number of different limiting factors - temperature, light, Carbon Dioxide and water. After a lot of research we found that ...
This is a preview of the whole essay
9cm - Light Intensity = 1000/81 = 12 Lux
2cm - Light Intensity= 1000/144 =6.9 Lux
5cm - Light Intensity = 1000/225 = 4.1 Lux
8cm - Light Intensity = 1000/324 = 3.1 Lux
21cm - Light Intensity = 1000/441 = 2.3 Lux
24cm - Light Intensity = 1000/576 = 1.7 Lux
As part of this experiment we need to look at limiting factors and how to control them. There are a number of different limiting factors - temperature, light, Carbon Dioxide and water. After a lot of research we found that the optimum temperature for Elodea in 20oC as this is the temperature at which they flourish in the wild. To keep the Carbon Dioxide levels constant we shall use a chemical called Sodium Hydrogen Carbonate, although we must be careful when adding it a too much can poison the plant. Another element that can affect photosynthesis is the stomata in the leaf, that, when closed prevent water entering the leaf and thus stopping photosynthesis. The opening and closing of the stomata can be controlled by using light, the stomata only open when light shines on the plant.
One of the control elements of the experiment is the length of time the light shines on the plant at each of the distances. We decided to allow 3 minutes of light at each distance as we chose to split the one-hour lesson into 3 sections, 15 minutes to prepare the experiment, half an hour to carry out the experiment and 15 minutes to analyse the results.
Prediction
I predict that my results will show that the greater the light intensity, the faster the rate of photosynthesis. We can work out light intensity using the formula:
Light intensity=1/d2
Although, this formula can be changed to 100/d2 or possibly even 1000 /d2 to make it easier to graph. From my preliminary work I know that when the distance from the light source to the elodea is halved the light intensity is quadrupled. I predict the end result will look like this:
I believe my graph will look like this because, as I have previously explained the greater the light intensity the greater the rate of photosynthesis. This statement explains the initial rise, until a point where I believe limiting factors may affect the results. There are four factors that limit the rate of photosynthesis, these are: Water, carbon dioxide, light and temperature. In this experiment we are going to change one factor and try to keep the others as constant as possible.
Method
I am going to carry out an experiment to see how light affects the rate of photosynthesis. I shall use Canadian Pond Weed (Elodea) and change the light intensity and measure the rate of photosynthesis. To begin the experiment I will need:
* Measuring beaker
* Stop watch
* Measuring cylinder
* Elodea
* Thermometer
* Ruler
* Lamp
* Filter Funnel
* Sodium Hydrogen Carbonate
* Spatula
* Plasticine
Firstly I will need to set up the apparatus, I will fill the beaker with water and place the Elodea in the water. I will put half a spatula of Sodium hydrogen Carbonate in the water and place the funnel on top of the Elodea, securing with plasticine, then finally fill the measuring cylinder with water and place the measuring cylinder on the end of the funnel. To complete the set-up I shall put the ruler beside the beaker, place a thermometer in the beaker and put the lamp 6cm away from the plant. Before setting this up I shall ensure that the Elodea has been in light for at least 24 hours so the stomata will be open.
To carry out this experiment I shall turn the lamp on and begin timing, once the light has been shining for 3 minutes I will mote the amount of oxygen given off. To ensure I measure the rate of photosynthesis accurately I will measure it two ways, I will count the number of bubbles given off and use the measurement on the side of the measuring cylinder to gauge the amount of oxygen released. After recording the results, I shall remove the measuring cylinder and move the lamp to the next distance, which in this case is 9cm. I will leave the Elodea for one minute to adjust to the new light intensity. After the minute is over I shall fill the measuring cylinder with water and once again I will place it on the end of the funnel. I will then begin timing for three minutes and note the rate of photosynthesis. I shall carry out these steps twice for each distance and find the average. I have decided to use the distances 6,9,12,15,18,21,and 24 as they are within the 5-30 cm boundaries, I found that it is impossible to get the lamp closer than 5 cm and beyond 30cm the Elodea stopped producing oxygen. Once I have found the averages for my experiment I shall put them into a table of results and compare them to another groups results.
To ensure reliable results I will need to keep this a fair test. I will need to keep certain factors constant, such as temperature, Carbon Dioxide levels and P.H. levels. I will need to use the same plant as different plants my carry out photosynthesis at different rates. To maintain the temperature I will add cold water if the temperature greatly exceeds 20oC or add warm water if the temperature significantly drops below 20oC. To keep the CO2 levels constant we shall add Sodium Hydrogen Carbonate as this produces Carbon dioxide. I will also need to prevent other light affecting my experiment so I will have to ensure the room is completely dark, except from the lamp I am using. Finally, I will need to maintain P.H. levels; I predict it will be slightly acidic to begin with due to the Carbon Dioxide levels present. This theory can be checked using litmus paper.
Below is a diagram of my experiment:
My results
This is the table of results for my experiment followed by a graph:
Number of bubbles
Distances (cm)
Light intensity (lux)
st
2nd
3rd
Average
Diameter of bubble
Initial temp
Final temp
P.H.
6
27
390
402
412
401.3
0.5
22
24
Neutral
9
2
372
362
378
370.6
0.7
25
25
Neutral
2
6.9
366
354
351
357
0.8
25
26
Neutral
5
4
312
300
304
305.3
0.8
26
26
Neutral
8
3.1
280
285
255
273.3
0.8
26
25
Neutral
21
2.3
20
90
78
62.6
0.8
25
24
Neutral
24
.7
45
45
49
46.3
0.7
24
25
Neutral
Other group's results
These are the other group's result table and graph:
Number of bubbles
Distances (cm)
Light intensity (lux)
st
2nd
3rd
Average
Diameter of bubble
Initial temp
Final temp
P.H.
3
00
7
28
34
26
0.4
30
27
Neutral
6
27
90
96
98
95
0.4
30
27
Neutral
9
2
61
73
59
64
0.3
30
28
Neutral
2
6.9
43
41
39
41
0.2
30
27
Neutral
5
4
34
28
27
30
0.1
30
27
Neutral
Analysis
My graph shows that the rate of photosynthesis rises quite steadily until a point where limiting factors force it to level off. I decided to get another group's results and drew a graph for those results. On the comparison graph the leveling off is not as apparent as in mine, conducting the experiment with higher light intensities and establishing if it leveled off could remedy this.
I looked at the light intensity and average number of bubbles released to try and find a pattern. I found that:
Lux difference
Percentage difference in bubbles
5
7.7
5.1
4.7
2.9
4.5
0.9
0.5
0.8
40.61
0.6
71.47
This indicated that there was no apparent trend apart from the percentage difference in bubbles increases as the difference in lux decreases.
The only other pattern that was obvious throughout the experiment, is when the distance is halved the light intensity is quadrupled although I had already proved this in my prediction.
The oxygen that was given off in the experiment is produced as a waste product of photolysis. Photolysis, also known as 'the Light Stage', is when water is split into hydrogen and oxygen by light. The oxygen is then given off and the hydrogen is used for making glucose for the plant, this 'light stage can be described using this diagram:
Evaluation
My experiment was quite successful as our results were reasonably accurate. I believe they were accurate as I did the experiment three times at each distance and then found the average. I also feel my experiment was a success due to the extensive planning and preliminary work carried out before the experiment could take place. For example I researched the optimum temperature to make the experiment more productive.
On my graph I had one anomalous result at 1.7 lux, this could be due to human errors or could be due to limiting factors stetting in.
To improve his procedure and make it more reliable there are several changes we could make. Such as putting a shield around the plant to block out any excess light or conducting the experiment in a water bath to maintain a constant temperature. Another change that could be made is, changing the lamp for a light box and focusing one beam of light onto the plant to prevent losing any light energy into the surroundings.
I believe my results were quite reliable and I think they are strong enough to make a firm conclusion. I have concluded that as the light intensity increases so does the rate of photosynthesis. To cancel out any anomalous results I could repeat the experiment for just those intensities and recording my results.
There are many other ways of conducting this experiment, such as collecting the oxygen in a syringe. But if I were asked to repeat this experiment I would keep the light the same distance away from the Elodea and change the light intensity by changing the amount of current flowing through the lamp. I believe this would make the light intensities more accurate. I could also try the experiment at a different temperature that was easier to maintain.
Laura Gales
Photosynthesis Investigation
___________________________________________________________________________________
______________________________________________________