This document summarizes an experiment that investigated the relationship between the rate of photosynthesis of an aquatic plant and the amount of light given to it. The experiment found that increasing the brightness of light by placing the plant closer to the light source increased the rate of photosynthesis, as measured by the number of oxygen bubbles produced over time. Specifically, the rate of photosynthesis doubled when the distance to the light was halved. The experiment supported the hypothesis that greater light intensity leads to higher photosynthetic rates.

1. Mina Yamanaka
IB Biology SL Year1
The Affect of Bright of Light to the Rate of Photosynthesis
Introduction:
Photosynthesis is the process of converting light energy to chemical energy and storing
it in the bonds of sugar. This process takes place in plants and some aquatic plants
such as elodea. Plants need only light energy, CO2, and H2O to make sugar. The
process of photosynthesis happens in the chloroplast, specifically using chlorophyll. The
overall chemical reaction involved in photosynthesis is:
6CO2 + 6H2O (+ light energy) C6H12O6 + 6O2.
There are two parts to photosynthesis. The light reaction occurs in the thylakoid
membrane and turns light energy into chemical energy. Therefore light is essential for
plants to conduct this chemical reaction. Chlorophyll and several other pigments such
as beta-carotene are organized in clusters in the thylakoid membrane and are involved
in the light reaction. Each of these differently colored pigments can absorb a slightly
different color of light and pass its energy to the central chlorophyll molecule to do
photosynthesis.
(http://biology.clc.uc.edu/courses/bio104/photosyn.htm)
Figure 1: Image of photosynthesis

2. DESIGN
Research question: What is the relationship between the rate of photosynthesis of an
aquatic plant and the amount of light given to it?
Hypothesis: If the amount of light given to the elodea plant is increased, then the rate
of photosynthesis will be accelerated and vice versa.
Variables:
Variable Type How
Brightness of light Independent Distance of lamp
closer/farther to the plant
Rate of photosynthesis Dependent Counting the number of
bubbles created within a
given time
0.25% sodium hydrogen Controlled Add exactly the same
carbonate solution amount of substance in the
(concentration) same amount of water
0.25% sodium hydrogen Controlled Pour the same amount of
carbonate solution water into the test tube
(Amount)
Time Controlled Five minutes for each trial
Temperature Controlled Maintained at the same
temperature through trials
Surrounding condition Controlled Dark room
Materials:
- 1000ml Beaker - Stirring rod
- Test tube - Thermometer
- Two Elodea plants taken from the same - Timer
piece - Knife
- Lamp - Ruler
- Sodium bicarbonate - Electronic balance
- Masking Tape - Graduated cylinder
 Figure 2: experimental set up
Procedure:

3. 1. Prepare 0.25% sodium hydrogen carbonate solution by mixing 0.25g of baking soda
weighed by electronic balance and 100ml water measured by graduated cylinder.
2. Take a big piece of elodea plant from the fish tank. Get two branches similar in
appearance from the piece.
3. Set the lamp, light on
4. Fill the 1000ml beaker with tap water and also put the thermometer in. Place it in
front of the lamp.
5. Set the timer to 5 minutes.
6. Slightly cut the end of the elodea by a knife.Place it in the test tube and cover them
completely with the baking soda solution. Put the test tubein the 1000ml beaker.
7. Immediately start the timer. Observe and record the number of oxygen bubbles that
Elodea generates in five minutes.
8. Place the beaker further and mark the location with masking tape. Repeat steps 4
through 7.
9. Record all data, 3 trials for each 3different distance.
DATA COLLECTION
Table 1: Number of bubblesvs Distance of Elodea form the Light bulb
Number of bubbles in 5 minutes
Trials
Distance from the light bulb
(cm) 1st 2nd 3rd Average
10 209 125 80 138
24.5 23 16 46 28.3
36.5 5 8 2 5
 Table 1: As shown in the table above, the more number of bubbles are observed when the plant is
exposed to a brighter light.
Sample calculation:
In order to calculate average number of oxygen bubbles generated within five minutes,
add all trials and divide it by the number of trials which in this case is 3:
= = 138
Table 2: Qualitative Observation of the Oxygen Bubbles

4. Oxygen Bubbles from Elodea plant
Distance from Interval of bubbles generated Size of the bubbles
the light bulb
10cm Short; constant and very Tiny bubbles
rapidly generated
24.5cm Longer; constant but slowly Medium sized bubbles
generated
36.5cm Very long; bubbles generated Medium sized bubbles
very slowly
 Table 2: the table above indicates the fact that the quality of bubbles varies when generated under
different brightness of the light.
Table 3: Rate of Photosynthesis vs Brightness of the Light by Distance
Number of bubbles per second
Trials
Distance from the light bulb
(cm) 1st 2nd 3rd Average Uncertainty
10 0.70 0.41 0.30
0.46 ±0.2
24.5 0.08 0.05 0.15
0.09 ±0.05
36.5 0.02 0.03 0.01
0.02 ±0.01
 Table 3: the table above shows the data in table 1 converted into the rate of photosynthesis by
calculation. The number of uncertainty of distance 10cm being too big is noticeable.
Sample calculation
In order to calculate the rate of photosynthesis per second, the number of oxygen
bubbles isdivided by the time took to get that number of bubbles in seconds
Rate of photosynthesis = number of oxygen bubbles = 138 = 0.46 bubbles / sec
Time in seconds 300
Sample calculation

5. For it is inevitable for the values of bubbles counted
to have some errors, uncertainty needs to be calculated.
The uncertainty can be calculated by this formula
=
= 0.2
Graph 1: Rate of Photosynthesis vs Distance of the Light bulb from the Plant
 Figure 3: the graph above clearly displays that the rate of photosynthesis declines, as the elodea plant
gets further away to the lamp. In other word, the rate oh photosynthesis declines as the brightness given
to the plant is decreased.
Evaluation

6. Weakness point What How to improve
Temperature The target temperature could Use Temperature controlled bath
not be maintained at all time
Bubbles The size of the bubbles Repeat the trialover and over until the same
generated were not always the size of bubbles show up
same
Elodea Fatigue of the elodea plant Conduct the lab not in row but give a certain
might have caused inaccuracy amount of break to the plants
of the data
Observation Some bubbles might have Person timing should be more concentrating
missed during the counting on it and let the other person be responsible
process for counting the bubbles
Conclusion:
The experiment has verified the hypothesis, “If the amount of light given to the
elodea plant is increased, then the rate of photosynthesis will be increased and vice
versa”. According to figure 3, it is obvious that the rate of photosynthesis increased as
the distance between the elodea plant and the lamp was closer together, which is the
same as the brighter the light is given to the plant, the bigger the rate of photosynthesis
becomes.This is because the light energy is directly used when the plant is converting
the carbon dioxide and water into sugar, so the more intense the brightness of the light
helps to go through the chemical process rapidly and efficiently, andthe reverse is also
true.
Through this investigation, I realized the importance of the existence of plants in
the environment. The plants play a vital role of the cycle of gas in the atmosphere by
generating oxygen into the air and consuming the carbon dioxide that are in the air. If
there were no plant on the earth, organisms would die from the lack of oxygen and
choke them to death. We should work harder on preserving the existing plants as well
as increasing the number of plants so that there will always be plenty of oxygen to
breathe in!