Photosynthesis Completion Practical

This document discusses an experiment to observe photosynthesis in pondweed at different light intensities. It explains the factors that affect the rate of photosynthesis and provides a detailed method for conducting the experiment. The results and analysis of the experiment are also presented.

Photosynthesis Completion Practical:
Aim: To observe photosynthesis in a species of pondweed at different light intensities.
Introduction:
Plants are the most common autotrophs in the ecosystem as they are able to make their own food
by undergoing a process called photosynthesis. It is the process in which the plant transfers light
energy into chemical energy in the form of oxygen and glucose. This process takes place in the
chloroplastswhich are responsible for carrying out the reaction of photosynthesis (Khan
Academy, 2019) . Each chloroplastcontains a compartment called thylakoids. Each thylakoid
membrane contains chlorophyll that are green-colored pigments responsible for absorbing light
(Bassham and Lambers, 2019).
Chemical and word equation of photosynthesis:
Carbon dioxide + water → oxygen + glucose.
6CO2 + 6H2O > C6H12O6 + 6O2
(light energy)
Factors that affect the rate of photosynthesis are light intensity, temperature, the concentration of
carbon dioxide and water. Light is very crucial as it provides that energy required for plants to go
through photosynthesis. The rate of photosynthesis increases as the light intensity increases as
long as enough carbon dioxide and water are accessible. As aquatic plants photosynthesis, they
release bubbles of oxygen which is measured over a period of time then the rate of oxygen can
be determined.
In this experiment, the effect of light intensity on the rate of photosynthesis is investigated. A
torch is being used to determine the variety of light intensity alter the process of photosynthesis
in the pondweed.

Hypothesis:
If the distance of the torch increases then the amount of the oxygen bubbles will decrease.
Variables:
Independent variables:
- The distance between the light and the plant
Dependent variables:
- The number of bubbles formed over a limited period of time
Controlled:
- The amount of water in the beaker
- The length of the pondweed (hydrilla)
- The amount of hydrogen carbonate solution in the beaker (6%)
Variables that couldn’t be controlled:
- The temperature of the room

Photosynthesis Completion Practical:
Materials:
Glass funnel Ruler
800ml beaker Sodium hydrogen carbonate solution (6%)
Light source (torch) Pondweed
Scissors Stopwatch
Method:
1) Cut two x 5cm pieces of pond weed using ruler and scissors.
2) Fill a large beaker with room temperature 6% sodium hydrogen carbonate solution.
3) Set up the apparatus as shown in the diagram below. Be sure to insert the pieces of
pond week into the funnel with the cut ends positioned upwards.
4) Have the torch placed 15cm away from the beaker, aimed directly at the pieces of
pondweed. Allow the pondweed to acclimatize to this light intensity for 2 minutes before
counting any bubbles.
5) Record the number of bubbles produced by the pondweed in 4 minutes period.
6) Replicate steps 1-5 three more times, but with torch distances of 10cm, 5cm and 2
cm. Ensure that a 2-minute period of acclimatization occurs before commencing the 4-minute
count.
7) Collaborate with another group to obtain data for the purposes of a 2nd trial.
8) Record data in a table, including the conversion from the distance to intensity.

Safety:
The experiment had some safety and ethical risks and hazards such as the risk of breaking the
glass that was the beaker. It might fall and break if kept at the corner of the table that can lead to
cut. To avoid this, you must keep it at a safe place and if it breaks, ask for help from the teacher.
Another risk could be from the scissors while cutting the pondweed to its desired size.
Results:
Photosynthesis Completion Practical:
Light intensity
Distance ( Amount of bubbles produced in
cm) the funnel
Trial 1 Trial 2 Average
2cm 130 53 91.5
5cm 110 37 73.5
10cm 72 23 47.5
15cm 62 20 41

Rate of Photosynthesis
Distance Amount of bubbles produced in
(cm) the funnel
Trial 1 Trial 2 Average
2cm 32 13 22.5
5cm 27 9 18
10cm 18 6 12
15cm 15 5 10

Photosynthesis Completion Practical:
Analysis:
The data presented above represent the different rate of photosynthesis of the pondweed when
exposed to different light intensity. It clearly shows that the light intensity is directly
proportional to the rate of photosynthesis. The closer the distance of the torch the more light is
provided to the pondweed resulting in more oxygen bubbles produced in the funnel. In the first
trial, the distance of the torch was 2cm from pondweed which resulted in an average of 22.5

bubbles produced in 4 minutes. Gradually increasing the distance of the torch resulted in a
decrease in the number of bubbles. As reflected from the readings; 5cm torch away produced 18
bubbles, 10cm resulted in an average of 12 and 15 cm distance produced an average of 10
oxygen bubbles respectively. Exposure to a weaker light intensity produces fewer bubbles as
compared to higher light intensity.
This is because a plant cannot photosynthesize very quickly without enough light even if there
are all the other favorable conditions, that is water, carbohydrate, and a suitable temperature. The
light reaction of photosynthesis requires light to occur since the light photons excite the electrons
in the photosystem's pigments (chlorophyll) to activate the light reactions. Therefore, as more
light is introduced, more photosystems in the thylakoid membrane can be activated
Hence it is clear that the intensity of light has a huge impact on the rate of photosynthesis. The
more the light falls on the leaf of the plant the greater the number of chlorophyll molecules and
the more ATP is generated. Although, at a very high light intensity, chlorophyll may be damaged
which could lead to a decrease in the number of oxygen bubbles and the result could drop
steeply.
Evaluation:
Despite how carefully this experiment was performed there was still plenty of room for errors;
two of the main errors are random and systematic. Random errors are human errors that
fluctuate due to uncertainty in the measuring process or the variation in the quantity being
measured (Wang, et.al, 2016). A systematic error is one that results from a persistent issue and
leads to a consistent error in your measurements. (Johnson, 2018)
Some of the random errors that might have been encountered during this experiment include the
following. First, the cutting method of the plant might have differed among the two groups.
Cutting the plant in a cross-sectional method may produce a smaller surface area as compared to
one that is cut in a diagonal manner. This is because the cut is to allow for the escape of oxygen
from the stem in bubble form. This may, therefore, explain why the results for trial two being
much lower that of trial one.
Counting of the bubbles may be another random error that might have been encountered during
the experiment. The error of counting may occur as a result of confusion or disturbance while
counting the bubbles coming out of the stem through the cut since counting was done by people.

Those performing either trial 1 or trial 2 might have encountered this error due to the varying
results.
Systematic errors may be encounter as a result of varying measurements during the experiment.
The measurements of hydrogen carbonate solution and even the concentrations may have varied
slightly since both groups were working under different environments with a different set of
equipment. The measurement distance of the torch from the beaker may also not have been exact
for both trials hence varying results. Moreover, the amount of light produced by the torch of each
group. Both torches may have produced different amount of light which of course altered the rate
of photosynthesis during the experiment. For example, trial 1 participant might have had a torch
which much higher level of brightness hence obtaining a higher number of bubbles throughout
the experiment.
In addition, the amount of time taken before starting to count the bubbles after adjustment of the
torch distance also may have not been equaled for both trials. There is a likelihood that trial 2
participants did not allow enough time for the pondweed to adapt well to the changes in lighting
and hence obtaining lower results.
There are various factors that need to be controlled in a repeat experiment to ensure that the
results obtained are accurate. This include; the temperature of the laboratory, amount of lighting
and the size of pondweed being used. This is because temperature affects the rate of
photosynthesis and therefore may alter the results of the different groups. The amount of lighting
in the laboratory also alters the photosynthesis. It is therefore advisable to ensure that both
laboratories being used by the different groups have the same amount of lighting to get better
results. Regarding the size of pondweed being used, larger size means large surface area hence
more rate of photosynthesis as compared to one with the lesser surface area. However, such a
factor is hard to correct it and hence might have caused such differences in the results of the two
trials.
Conclusion:
It is concluded that more distance of the torch from the pondweed resulted in fewer bubbles in
the funnel. Therefore, the results support the hypothesis, if the distance of the torch increases
then the amount of the oxygen bubbles will decrease. However, the errors encounter could still
limit the conclusion obtained. More trials for each light intensity would be of much benefit in

ensuring that the conclusion is verified as two trials only might not be enough. Accuracy of the
results can also be improved further by ensuring that the random and systemic errors encountered
are reduced during the experiments. This will make the results accurate and hence leading to a
better conclusion.

Bibliography
Gu, L., Han, J., Wood, J. D., Chang, C. Y. Y., & Sun, Y. (2019). Sun‐induced chlorophyll
fluorescence and its importance for modeling photosynthesis from the side of light
reactions. New Phytologist.
Hans L., and James A. B. (2019). Photosynthesis.
https://www.britannica.com/science/photosynthesis accessed on May 9, 2019
Lanoue, J., Leonardos, E. D., & Grodzinski, B. (2018). Effects of light quality and intensity on
diurnal patterns and rates of photo-assimilate translocation and transpiration in tomato
leaves. Frontiers in plant science, 9.
Roston, R. L., Jouhet, J., Yu, F., & Gao, H. (2018). Structure and Function of
Chloroplasts. Frontiers in Plant Science, 9, 165
Wang, Z., Yi, D., Duan, X., Yao, J., & Gu, D. (2016). Measurement data modeling and
parameter estimation. CRC Press.