Biology Lab Report- Photosynthesis
Palak Jolly, IB-1C i) Research Question:
How do different light intensities affect the rate of photosynthesis in plants?
ii) Aim:
To investigate the effect of light intensity on the number of bubbles of oxygen produced by an aquatic pondweed in a period of time of 15 seconds, by changing the brightness level of a lamp between 0, 10, 20, 30, 40 and 50. The number of bubbles counted acts as an indicator of the rate of photosynthesis of the plant.
iii) Introduction:
Photosynthesis is the process that produces carbon compounds in plants by utilising light energy present in sunlight along with the inorganic compounds of carbon dioxide and water. The energy conversion is from light energy to chemical energy, and the carbon compounds produced as a result of this process are carbohydrates, lipids and proteins.1 The simplest equation that represents photosynthesis is as follows:
6CO2 + 6H2O à C6H12O6 + 6O2 . The light energy used in this reaction is captured by photosynthetic pigments, prominently chlorophyll. Photolysis is the process by which electrons needed to convert carbon dioxide into
carbohydrates are produced, through the splitting of water molecules. Oxygen is a waste product of photolysis2.
Photosynthesis is affected by several factors, however at a given time only the factor that is farthest away from the optimum point actually limits photosynthesis. Therefore, that factor is known as the limiting factor. In this experiment, we investigate the effect of just one of the limiting factors- light intensity2. There are two more limiting factors, the concentration of carbon dioxide dissolved in the water and temperature, both of which will be controlled at an optimum point so as to not affect experimental results. At low light intensities, the rate of light absorption is lower. This results in a lower rate of photolysis and consecutively a lower rate of photosynthesis. Increasing light intensity should therefore result in an increasing rate of photosynthesis until some other factor limits it, and only extremely high light intensities that do not occur in nature, if synthetically provided, can actually inhibit the rate of photosynthesis3.
Research conducted in the past using light intensity units of meter candles and several trials under various different carbon dioxide concentrations seems to exhibit a similar, consistent trend of increasing rate of photosynthesis with increasing light intensity4. The results obtained in these conducted researches will be discussed in comparison to the results obtained through this experiment in the conclusion.
1 Allot, Andrew and Mindorff, David. Biology: Course Companion. Oxford University Press, 2014 Edition.
2 Allot, Andrew. Biology for the IB Diploma: Study Guide. Oxford University Press, 2014 Edition.
3 “What happens during photosynthesis? - OCR 21C.” BBC Bitesize, BBC, www.bbc.co.uk/bitesize/guides/z9pjrwx/revision/5, Last accessed 3 November 2020.
4 Smith, Emil L. “Limiting Factors In Photosynthesis: Light And Carbon Dioxide.” Journal of General Physiology, vol. 22, no. 1, 1938, pp. 21–35., doi:10.1085/jgp.22.1.21.
iv) Hypothesis:
a) Null hypothesis: Changing the light intensity has no effect on the number of oxygen bubbles released by the pondweed.
b) Alternate hypothesis: As light intensity increases, the number of bubbles released increases until a certain point at which the plant reaches its maximum absorption capacity, beyond which the number of bubbles released remains the same. Light intensities at this point and beyond will be the optimum range for the pondweed.
v) TABLE 1 - Independent and Dependent Variables:
Variable Units Method
Independent Light Intensity None provided Varying the brightness level of the lamp at 10, 20, 30, 40 and 50 on the scale provided in the simulation. One
trial is conducted at 0 as a control.
Dependent Number of Bubbles Released
- Counting the bubbles released and observed visually in a set period of time
of 15 seconds and recording them in a data table.
vi) TABLE 2 – Controlled Variables:
Variable Units Reason Method to Control
Temperature of water
°C At low temperature below 5°C, the enzymes catalysing conversion of CO2 are very slow,
almost inactive and at high temperatures above 30°C, the enzyme rubisco that fixates CO2
denatures5. Therefore, an optimum point for the enzymes
needs to be maintained.
Using a thermostatically controlled hot plate with the thermostat kept throughout at
25°C5, which is room temperature and therefore an
optimum point for photosynthesis enzymes.
Carbon dioxide concentration
% Low CO2 concentrations, such as 10 mmol dm-3, often do not allow for sufficient collisions between reactants and therefore
a very low reaction rate, acting as a limiting factor. Therefore, CO2 concentration needs to be maintained at a high enough level for it not to be a limiting
factor5.
Starting with a large volume of boiled, cooled water that
has no carbon dioxide dissolved and adding NaHCO3 (the source of
carbon dioxide for the pondweed) until it forms a high concentrated solution, so
that the carbon dioxide concentration is not a limiting
factor.5 Using the large volume in parts, by replacing
the fluid after every 3 trials.
5 Allot, Andrew. Biology for the IB Diploma: Study Guide. Oxford University Press, 2014 Edition.
Time period when counting bubbles
s Given more time, the pondweed will photosynthesise more.
Therefore, there needs to be a standard time frame in which the
bubbles are counted.
Using a digital stopwatch, starting counting when the stopwatch is started and finishing counting when the stopwatch reaches 15 seconds.
Distance of lamp from pondweed
beaker
cm With a higher distance between the lamp (light source) and the
beaker, the light intensity lessens. Since this experiment is
varying light intensity using a simulation scale, the distance has
to remain constant so as to not distort the results.
Maintaining the lamp at a distance of 5 cm from the beaker, measured with a ruler.
Amount of pondweed used
- More pondweed present in the beaker will photosynthesise more, therefore the same amount
of pondweed has to be used in each of the trials.
Using the same plant for every trial.
vii) TABLE 3 – Materials Required:
Materials Magnitude Quantity Least Count
1. Glassware
Beaker 200 mL 1 5 mL
Graduated measuring cylinder
1000 mL 1 10 mL
Thermometer Range: -10°C to 110°C
1 1°C
2. Chemicals Sodium bicarbonate (NaHCO3)
- 20 g -
Water - 700 mL -
3. Miscellaneous Thermostatically controlled hot plate
- 1 -
Lamp with variable brightness
- 1 -
Pondweed - - -
Stopwatch - 1 1 s
viii) Procedure:
1. Prepare a solution of sodium bicarbonate in water by taking around 700 mL of water in a graduated measuring cylinder, and adding around 20 g of sodium bicarbonate to form a solution with a sufficiently high concentration of around 3%.
2. Add 100 mL of this solution to a beaker and place it on a thermostatically controlled hot plate, with the thermostat set to 25°C. Measure the temperature of the water till it settles to 25°C.
3. Set the light intensity scale to 0%, and add the pondweed to the beaker. Start the stopwatch.
4. Wait 15 seconds and count the number of bubbles released during this time period.
Record the count in the raw data table.
5. Repeat steps 3-4 twice at the same intensity level.
6. Replace the solution in the beaker with another 100 mL of the sodium bicarbonate solution to replenish the CO2 concentration that might have been reduced by the pondweed photosynthesising.
7. Repeat the procedure step 3 onwards, setting light intensities to 10%, 20%, 30%, 40%
and 50% respectively for every repeat.
ix) Precautions & Risk Assessment:
For safety precautions, protective eyewear, a lab coat and latex gloves were worn throughout the duration of the experiment. The light source was touched only over the insulated regions. None of the materials used in the experiment are health hazards, and there are no guidelines attached to the disposal of sodium bicarbonate in the location of the experiment, India; the solution can be stored or disposed in a drain after use. There are no ethical issues associated with this experiment.
Experimental precautions include ensuring the thermometer is functional by first testing it with melting ice and ensuring that the reading is not too far from 0°C. The pondweed should be green and kept in an aquatic medium at all times, it should not be aged but rather a fresh plant to provide accurate results. Additionally, the stopwatch timing and counting should be as precise as possible, if required aid can be taken from another person to start and stop the stopwatch. As the experiment does not deal with precise measurements, a graduated measuring cylinder is sufficient for measuring the solution volumes.
x) FIGURE 1 – Picture of experimental setup:
xi) TABLE 4 – Raw Data:
Trial Number
Number of Bubbles Released
0% 10% 20% 30% 40% 50%
1 0 8 13 14 15 17
2 0 7 12 15 16 17
3 0 7 13 14 16 17
xii) TABLE 5 – Processed Data:
Light Intensity/% Mean average number of bubbles released (correct to 2 sf)
Standard Deviation (for the 3 trials)
0 0.0 0
10 7.3 0.58
20 13 0.58
30 14 0.58
40 16 0.58
50 17 0
xiii) FIGURE 2 – Graph of Light intensity vs. Mean of number of bubbles released:
xiv) Statistical Testing:
A T-test was performed using the data at 2 different data point pairs to confirm the hypotheses.
Sample calculations:
For light intensity 10% and 20%:
X1 = 13, X2 = 7.3 S1 = 0.58, S2 = 0.58 n1, n2 = 3
tcal = ("!#"")
%(($!)"&! &($")"
&"
tcal = ('(#).()
%(('.)*)"
+ &('.)*)"
+
= 5.7 / √(0.6728/3)
= 12.04
Degree of freedom = n1 + n2 – 2
= 3 + 3 – 2
= 4
Level of significance = 5% = 0.05
0 2 4 6 8 10 12 14 16 18
0 1 0 2 0 3 0 4 0 5 0 6 0
NUMBER OF BUBBLES RELEASED
LIGHT INTENSITY/%
TABLE 6 – T test critical value table:
Therefore, critical value (tcrit) = 2.78
ð Because tcal > tcrit , null hypothesis rejected and alternate hypothesis accepted.
For light intensity 40% and 50%:
X1 = 17, X2 = 16 S1 = 0, S2 = 0.58 n1, n2 = 3
tcal = ("!#"")
%(($!)"&! &($")"
&"
tcal = (')#'+)
%((')"+ &('.)*)"
+
= 1 / √(0.3364/3)
= 2.98 tcrit = 2.78
ð Although tcal > tcrit , the difference between tcal and tcrit is extremely low, so there is a possibility that the difference is due to experimental error and limitations. Therefore, no hypothesis can be rejected for sure in this case.
xv) Analysis & Conclusion:
With the light intensity scale available, it is clear that with higher levels of light intensity, we have higher number of oxygen bubbles released from the pondweed. This, in turn, signifies that with a higher light intensity we had a higher rate of photolysis producing oxygen in the first place, and therefore a higher rate of photosynthesis.
It is also noticed that as light intensity is increased initially, there is a high increase in the rate of photosynthesis. However, in the final increases of light intensity, although there is an increase, there is not a significant change in the number of bubbles released and hence the rate of photosynthesis. This is observed as the graph of results (Figure 2) is steep initially and tends towards flattening over the last few data points. This occurs because the pondweed nears its maximum absorption capacity.
This is in accordance with the study alluded to in the introduction6, experiments in which provided the following results:
FIGURE 3 – Results of study conducted by Emil L. Smith6:
The different graph lines represent different concentrations of CO2. Meter candles is the unit used to measure light intensity. The results are similar to this experiment, since for all concentrations of CO2 an upward trend is observed which eventually stabilises and flattens.
The statistical testing, too, supports this conclusion since with the first two data points tested, the alternate hypothesis can be decisively accepted. However, with the last two data points tested, there is no clear indication of which hypothesis to accept as at this point, changing the light intensity makes very less of a difference to the number of
6 Smith, Emil L. “Limiting Factors In Photosynthesis: Light And Carbon Dioxide.” Journal of General Physiology, vol. 22, no. 1, 1938, pp. 21–35., doi:10.1085/jgp.22.1.21.
bubbles released, and therefore the null hypothesis too could be a possibility for the last two data points.
In conclusion, the alternate hypothesis supports all the above results and is accepted.
xvi) Evaluation:
Because the experiment was conducted over a simulator, it had certain limitations that could have been improved upon had the experiment took place in a physical laboratory.
The results of the experiment would vary if conducted physically, because results using the simulator were rather predictable in nature and differed by a maximum of one count in all the cases, therefore the averages were not realistic.
The light intensity scale did not have a unit and therefore could not be compared to research studies using standardised units such as the lux meter. Additionally, the only method to indicate the rate of photosynthesis available on the simulator was to count the number of bubbles released, which is a relatively inaccurate method. If conducted
physically, this could have been replaced by using a syringe to collect the oxygen released and record the volume, leading to more precise results.
There was a limitation as the CO2 concentration could not be recorded but rather set either high or low. The stopwatch was correct to 1 second, which could lead to relatively imprecise results. Similarly, bubbles can be counted correct only to whole numbers, which gives a relatively high error margin as it is possible to miss count or skip a bubble by accident. There was a maximum rate of bubble release as a limitation of the animated simulation as only one bubble could be released at a time, which could lead to inaccuracy.
Three trials at each light intensity level were repeated so as to minimise the effect of this error possibility.
A significant limitation was that higher light intensity levels beyond 50% could not be tested due to simulator limitations, and therefore the results lack the certainty of when the pondweed reaches its maximum absorption capacity and hence when the curve flattens.
Therefore, the optimum light intensity percentage could not be ascertained.
xvii) Works cited:
• Allot, Andrew and Mindorff, David. Biology: Course Companion. Oxford University Press, 2014 Edition.
• Allot, Andrew. Biology for the IB Diploma: Study Guide. Oxford University Press, 2014 Edition.
• “What happens during photosynthesis? - OCR 21C.” BBC Bitesize, BBC,
www.bbc.co.uk/bitesize/guides/z9pjrwx/revision/5, Last accessed 3 November 2020.
• Smith, Emil L. “Limiting Factors In Photosynthesis: Light And Carbon Dioxide.” Journal of General Physiology, vol. 22, no. 1, 1938, pp. 21–35., doi:10.1085/jgp.22.1.21.