Lab Report

Research question: Effect of light intensity on the rate of photosynthesis
Independent variable
Light intensity. I will be taking data at 10cm, 20cm, 30cm, 40cm, and 50cm away
from the light source.
Dependant variable
Rate of photosynthesis. I will be measuring this by counting the oxygen bubbles
released from the plant during each trial.
Controlled variables Details
room/water temp Temperature of room and water will be

measured using a thermometer. If temp
changes, the rate of photosynthesis will
change.
Size of plant/ number of leaves The amount of leaves will affect the
(40 leaves) amount of oxygen produced as
photosynthesis takes place in the
chloroplasts, and the chloroplasts are in
leaves.
Amount of water The amount of water must stay the

(230 ml) same, as water is a factor of
photosynthesis and can also affect the
rate, therefore will be kept the same,
and measured using a 250 ml
measuring beaker.
Time duration If the time of each trial differs, the rate

(2 minutes) of photosynthesis cannot be accurately
calculated, therefore will be controlled
using a chronometer.
Strength/colour of light source The same light source must be used

throughout the entire experiment, as a
different light source may produce a
higher/lower intensity of light.
Introduction:
Photosynthesis is the process of using light energy to make food for plants. The
plant takes in water from its roots and carbon dioxide from its stoma, as well as
energy from the sun. The chloroplasts located in the leaves then produce glucose for
the plant, and the stoma release oxygen as a by product.
The formula for photosynthesis is 6CO2 + 6H2O C6H12O6 + 6O2
Hypothesis
When a plant containing chlorophyll is exposed to light, it will start to carry out
photosynthesis releasing oxygen gas according to the general equation:
6CO2 + 6H2O C6H12O6 + 6O2
When the elodea plant carries out photosynthesis bubbles of oxygen are visible and
can be counted to determine a rate of photosynthesis. According to BBC Bitesize
“increasing the light intensity increases the rate of photosynthesis, until some other
factor – a limiting factor – becomes in short supply.” (BBC, n.d.).
As the light source is moved closer to the Elodea plant, the light intensity increases,
therefore the rate of photosynthesis will increase until it reaches a maximum.
In this experiment, as the light source moves closer to the Elodea plant, the rate at
which oxygen bubbles are produced by photosynthesis increases until it reaches a
maximum and stays at the same level.
Materials
Elodea plant
Water
Light source
Small test tube
100 ml graduated cylinder
100 ml erlenmeyer
250 ml beaker
30 cm ruler
Chronometer
Cutter
Thermometer
Method
1. Cut elodea plant into a piece with 40 leaves

2. Pour water into beaker containing the elodea, and place in erlenmeyer
3. Place the test tube on top of funnel
4. Place light source 10 cm away from plant
5. Start chronometre
6. Count the number of oxygen bubbles released in two minutes
7. Repeat steps 4 - 6 with the light source 20cm, 30cm, 40cm, and 50 cm away
from the plant.
8. Repeat experiment 4 more times
Data analysis
The rate of photosynthesis decreases, as the light source is moved further away.
This means that the data is direct, because as the light intensity decreases, the rate
of photosynthesis decreases as well.
Conclusion
As displayed in my graph, there is a negative trend line, meaning that as the sunlight
goes further away from the plant, the amount of oxygen produced decreases. This is
because sunlight provides the plant with energy, which it then uses to take both
carbon dioxide and water from the world around them, and transform it into glucose,
also known as sugar. This process also makes oxygen, which the plant doesn’t
need, so it releases it. Therefore if the plant has more sunlight available, it has more
energy and is able to produce more oxygen. (Beth Winston, 2018)
My method was valid, as it was clear to follow, and the controlled variables were
properly established and therefore easy to control. One flaw that my method had,
was that I didn’t mention the levels of carbon dioxide in my controlled variables. This
would be very difficult to control, however should still be mentioned.
Weakness in the design How the Error May Have Suggested Solution to
of the experiment Affected the Outcome Reduce the Error
My eyes may not have This would have affected By having a second
been 100% accurate in the results, and caused person count at the same
counting the bubbles an inaccurate rate of time, or by filming the
photosynthesis procedure and recounting
calculation. afterwards, the results
would be far more
accurate.
Other light sources like This would have affected Closing all the
sunlight through a window the amount of light the blinds/curtains would
and the room lamp might plant was getting and prevent other light from
have affected the rate of would ruin the coming in.
photosynthesis. independent variable
The bubbles can be hard This can lead to a Use a magnifying glass to
to see/differentiate rom misscount. determine what is a
specs of dust bubble and what isn’t.

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Research question: Effect of light intensity on the rate of photosynthesis

Controlled variables Details

room/water temp Temperature of room and water will be

Amount of water The amount of water must stay the

Time duration If the time of each trial differs, the rate

Strength/colour of light source The same light source must be used

6CO2 + 6H2O C6H12O6 + 6O2

1. Cut elodea plant into a piece with 40 leaves

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