An Experiment to Investigate the Effect of Light Intensity on the Rate of Photosynthesis

An Experiment to Investigate the Effect of Light Intensity on the Rate of Photosynthesis

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An Experiment to Investigate the Effect of Light Intensity on the Rate of Photosynthesis


Photosynthetic takes place in the chloroplasts of green plant cells;
it can produce simple sugars using carbon dioxide and water causing
the release of sugar and oxygen.

The chemical equation of photosynthesis is:

[IMAGE]6CO2 + 6H20 C6 H12 O6 + 6O2

it has been proven many times that plants need light to be able to
photosynthesize , so you can say that without light the plant would
neither photosynthesize or survive. The reason light intensity is
being used compared to whether or not a plant needs light. Is because
the experiment wants to show that the rates of photosynthesis will
vary according to how much light from a light bulb will be trapped in
the chloroplasts, in the leaf. The more energy trapped the more
efficient a chemical reaction can take place and the speed of
photosynthesis will increase. There are many things which can affect
the photosynthesis of a plant such as light intensity, temperature and
carbon dioxide levels. There will be a maximum level of photosynthesis
during the experiment it is called a limiting factor. This factor will
prevent the rate of photosynthesis from rising above a certain level,
even if the conditions are improved to meet the best requirements for


Input - Light intensityis to be varied by increasing and decreasing
the distance from the light source to the plant

Output - The rate of photosynthesisis to be measured by counting the
bubbles of oxygen produced by the plant every two minutes, and
therefore finding the rate of photosynthesis

Control - The amount of water available to the Elodea will stay the
same level in the 400 cm3 beaker. The colour of the lamp will stay the
same (yellow) as to plants Chlorophyll easily absorbs blue light,
however it does not easily absorb green or yellow light, rather it
reflects it, this decrease's the rate of photosynthesis. This can
easily be controlled, simply by using the same lamp throughout the

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experiment. Finally the temperature will stay the same which was at
room temperature 21 0c. I will check the temperature frequently, in
case the heat given off from the lamp should slightly raise the


Apparatus used

Desk lamp

Elodea pond weed


Test tube




Cold water from tap

Stop watch


The apparatus was set up as below:

Elodea that was kept well illuminated in a lab was taken for this
experiment. The stem of the Elodea was cut with a scalpel at an angle,
and then a 400 cm3 beaker was filled with tap water. A funnel was the
placed at the bottom of the beaker with the pond weed inside the
funnel with the cut end placed up the tube of the funnel. A test tube
filled with water was then placed over the top the top of the funnel,
a thermometer was also placed in the water while the experiment was
taking place so that the temperature could be checked to see how hot
the water was. Then the lab blinds were pulled down so that no light
could come from the windows and the lights were turned off. A desk
lamp was set up facing the Elodea, a meter ruler laid along side the
lamp so that the distance could be measured. A stop clock was set and
the numbers of bubbles were recorded down. Each distance was done
twice so you had some results that you could compare against each
other. Then every two minutes the lamp was moved back 5 cm each time
and then left for two minutes and once again the results were recorded


I predict that the higher the light intensity increases, so will
photosynthesis. The lower the light intensity the slower the rate of
photosynthesis and eventually it will decrease in l level. Though eventually how ever high the height intensity is the
rate of photosynthesis will level off.


Distance (cm)

Number of bubbles counted (every two minutes)























My graph shows that the closer the distances of the light source the
higher the rate of photosynthesis. This is because photosynthesis is a
reaction and reactions need energy and it this gets from light. The
more light there is the more energy the reaction gets and this speed's
up the whole process of photosynthesis. This can be seen on the graph
as the highest point this is where the Elodea is photosynthesizing at
a high rate and it starts to slowly drop as the distance is
increasing. The graph goes from a slow decent to all of a sudden it
drops and keeps on falling until the results end at a distance of 20
cm and a number of 153 bubbles in those two minutes. The second
results follow the same pattern with a slight difference by of one or
two bubbles every two minutes. The only considerably difference is the
last point on the graph of the second results and the number of
bubbles was 148 there is quite a difference between the 148 and 153.
My graph has no anomalies, and it closely followed to what I had
predicted that the closer the distance of the light the higher the
rate of photosynthesis would be and thus produce more bubbles in the
time recorded. The only thing which did not fit my prediction was that
the graph did not level off, however I believe this did not happen as
I did not have enough time to record any more results. If I did have
more time and had extra distances then I believe that the graph would
have leveled off and come out of such a steep fall. I can say that the
increase in distance thus making the light intensity stronger
increased the rate of photosynthesis and when further away made the
rate of photosynthesis almost come to its limiting factor the
straitening of the graph.


I am happy how my experiment turned out, I do think though there were
many points at which the accuracy was not perfect in the experiment.
For instance working out the distance between the lamp and the beaker
holding the Elodea, as the lamp had a cover a

round the bulb which meant it was hard to measure the exact distance
as the cover restricted your measurement. If I repeated the experiment
I would use a lamp with out a cover around it. Also I would make the
time you counted the bubbles in longer possibly by another minute and
a half, as two minutes are not a long enough time to fully see
photosynthesis at its best. I would also give my self a longer time to
do the experiment in so I could get an even more reliable graph with a
limiting factor which could back up my evidence of photosynthesis. I
would be more prepare next time so that I could get ready the
apparatus more quickly and get on with the experiment, I would finally
have left a gap of time between when the lamp was moved to a new
distance and the recording of the bubbles began so that the Elodea
could adapt to the new changes before running straight on with the
next set of the results. I am happy with the experiment as I proved
that light intensity has an effect on the rate of photosynthesis. I
have a reliable set of results and graph with no anomalies my
experiment had a low error percentage.
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