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Biology SL
Biology SL
Sample Internal Assessment
Sample Internal Assessment

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Research question
The simulation program water weed

Investigating the effect of different light intensities on water weed

Investigating the effect of different light intensities on water weed Reading Time
21 mins Read
Investigating the effect of different light intensities on water weed Word Count
4,012 Words
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Word count: 4,012

Research question

Does the amount of oxygen (O2) produced by water weed (number of bubbles) fluctuate (increase/decrease) with changing light intensities?


Independent variable – the light intensity (varying from a scale of 1 – 10, unknown units)


Dependent variable - The number of bubbles produced over a known period of time (O2 production)

Control variables

Type and age of water weed
The water weed used in this experiment was unknown. It could not be controlled and it was assumed that it was healthy and would give consistent results.
Weight (amount) of pond weed
This could not be controlled. It was assumed that the experiment provided the optimum amount for best results.
Temperature of the CO2 solution
This was unknown and could not be controlled. It was assumed that the temperature remained constant and was at an optimum level.
Time of bubble collection
This should always be the same and of a reasonable length to provide stat data, A time of 30 seconds was pre-programmed into the simulation. The speed could be at normal time or x 5 so the faster simulation speed was used to quicken data collection. It was assumed that this did not influence the results given. The results could be doubled up to 1 minute (see evaluation)
External light intensities
These could not be controlled and no data was provided. It was assumed that the simulation was carried out in dark conditions to provide optimum results.
Sodium carbonate concentration

This could be controlled using an arbitary scale of 1 – 10. The simulation gave no indication of the solution composition or Molarity so it was assumed that CO2 was in excess. The level used in this experiment was 10 to ensure that the CO2 was in excess and would not influence the results.

Temperature of the room
This was unknown and could not be controlled. It was assumed that it was optimum for the experiment.
Size of bubbles
From observations during the simulation these appeared to be of a regular size and were released from the plant at regular intervals.
Figure 1 - Table On How Is It Being Controlled?

It is possible to show how oxygen evolves during photosynthesis using aquatic plants. Theoretically, an aquatic plant will undergo photosynthesis and release bubbles of oxygen-containing gas when it is submerged in a solution containing a source of carbon dioxide [CO2] and exposed to light of the proper intensity. These bubbles may be counted, and the bubbling rate can be used to estimate the speed of photosynthesis. The rate of bubble generation should rise when the light intensity is raised. Reduce the light's intensity, and the bubbling should slow down. The bubbling should stop if the light source is completely removed or if you relocate into an area where the energy levels are too low for photosynthesis.

Figure 2 - The Equation For Photosynthesis Is

The process of photosynthesis requires light. In the absence of light, green plant cells cannot photosynthesise. Up until a certain level of light intensity [typically 38% -(1)] above which the rate does not increase because the light saturation point has been reached and another factor (CO2 concentration or temperature) is limiting, photosynthesis will increase as light intensity increases.


In this experiment, the amount of light is altered by lowering the amount of light. The quantity of oxygen bubbles should decrease as photosynthesis proceeds more slowly.


As photons are absorbed by pigments in photosystems, which is how plants receive light energy, I believe that if the light intensity is very low, the number of oxygen bubbles created will likewise be very low. This energy is what propels the photosynthetic process. Water won't undergo photolysis at low light levels, hence O2 won't be produced as a byproduct. More photolysis takes place to replace the high energy electron as the light intensity rises because more electrons are activated in the photosystem's reaction centre. This releases more oxygen, which is seen as more bubbles.


The number of bubbles may plateau at the highest light levels because photosynthesis can be constrained by a number of variables, including light intensity, temperature, and carbon dioxide content. Even with high light intensities, the rate of photosynthesis is unlikely to keep growing since, for instance, carbon dioxide concentrations might not be at their ideal levels.


Additionally, photosynthesis requires the presence of CO2, which the simulation delivers as a solution of CO2 with adjustable units 1–10.


Although there is an excess of CO2, which is necessary for photosynthesis, the outcome will not be affected. In this simulation, decreasing CO2 levels might be investigated.


During photosynthesis, chlorophyll absorbs light from the blue-green spectrum. These wavelengths are present in normal light, so using a standard light bulb (white light) without filters is adequate. Using this simulation, the use of coloured light could be investigated.

The simulation program water weed

The experimenter can change the light's colour, carbon dioxide content, and intensity using this simulation.


The virtual setup is depicted in the screen capture below.

Figure 3 - Water Weed Simulation


The parameters were set on the simulation as follows:

  • CO2 level =10
  • Light filter white
  • Speed x5

Data was collected from light levels 1 -10. 5 repeats were carried out at each light level.

Figure 4 - Table On The Raw Data Collected Is Shown In Tables In The Appendix
Figure 5 - Show The Number Of Bubbles Produced Against Light Levels (With Standard Deviation)
Figure 6 - Show The Number Of Bubbles Produced Against Light Levels (Showing 95% Confidence Intervals)


An increase in oxygen (bubble) generation from plant photosynthesis occurs when light levels rise. This outcome indicates that my theory was true.


Unit 10 had light levels that were roughly twice as high as light level 5. This meant that photosynthesis levels at light level 10 were twice as high as those at light level 5. This also shows that there is a direct, positive association between light intensity and the quantity of bubbles created, with the number of bubbles at a light intensity of 4 units being double that seen with a light intensity of 2 units.


In order to evaluate the experiment's dependability, standard deviation was determined. The results are more reliable when the standard deviation is lower since it depicts how the data vary from the mean. The graph illustrates that the standard deviation and 95% confidence intervals grow with light level, indicating that the data become less dependable as light intensity rises. The standard deviation was, however, minimal compared to the mean for all light intensities, being, at most, less than 8% of the mean value (for light level 2).


The results of this experiment support the hypothesis that because there are few photons striking the photosystem and few electrons being excited to higher energy levels, photolysis does not occur frequently at low light levels. There is no photolysis, no water splitting, and little to no oxygen bubble emission on the part of the plant.


As the light intensity rises, more photons are absorbed by the pigments and more electrons need to be replaced in the photosystem's primary reaction centre, which accelerates photolysis and produces more oxygen bubbles (2).


My prediction that at a particular light intensity the number of bubbles will plateau because another factor would be limiting was only partially accurate. This did not occur, however it is possible that this was because the experiment was stopped before the light saturation point was reached.


No intermediate values could be assessed
The simulation does not allow assessment of any values outside the pre-programmed values. This is an area that could be explored in the laboratory. It would be useful if this could be changed.
Maximum light level of simulation did not reach light saturation point, therefore other limiting factors could not be determined.
Unlike a related practical carried out in the laboratory this simulation did not have maximum or minimum light levels that influenced photosynthesis. This could not be controlled and would add more to results if it could be explored.
Units of light and CO2 concentration were in units of 1 -10. There was no indication of how this is related to a laboratory situation and what the solutions and the light levels actually were. These variables cannot be controlled and may not provide sufficient data to comment fully or provide the variation in an experiment that would normally be carried out. No trial runs can be carried out.
Provide more detail about light units, solutions etc. Allow fine adjustments of variables. Provide more information.
Figure 7 - Table On Evaluation