Photosynthesis Experiment With Different Light Intensities
Aim: This is an experiment to determine how much a plant is able to
photosynthesise (ie how much oxygen it produces) in a certain amount
of time with different light intensities.
Prediction: I predict that with a large amount of light, the plant
will photosynthesise well, but as the light is moved further away from
the plant (ie the light intensity decreases) the amount of oxygen
produced will be less than before, showing that the plant is unable to
photosynthesise as well as when the light intensity was greater.
Scientific Knowledge: I predicted that the further the plant was from
the light, the amount of photosynthesis
would diminish accordingly.
This is because for photosynthesis to occur light must be present, and
with increased light intensity the available energy is increased,
therefore the chemical reactions within the cell occur faster and
photosynthesis will occur at a faster rate. Similarly, with decreased
light the rate of photosynthesis must also decrease.
Safety: In this experiment
the scientist must be aware of and take the
necessary steps to avoid:
* Touching the rim or bulb in the lamp as they will rapidly become
hot and will burn.
* Cutting themselves on broken glass (depending on the type of jar
the Sodium Bicarbonate is kept in).
Fair test: In order to keep my experiment fair I will keep the
following factors the same:
* I will use the same wattage bulb in the lamp throughout the
experiment so that the light intensity is only changed when the
scientist changes the distance the lamp is from the plant.
* I will use the same plant throughout the experiment to rule out
any chance of using a plant half way through the experiment that
is better suited to photosynthesising with less light, as this
would render the results inaccurate.
* I will allow the plant the same amount of time at each distance so
that the only altered factor in the experiment that will change
the amount of photosynthesis will be the different light
* I will keep the plant in a water bath to stop the light heating up
I will keep these factors the same to ensure that the only altering
factor is the light intensity, and so if I do experience any changes
in the rate of photosynthesis I can safely assume that it is due to
the lack/gain in light intensity.
* To monitor the temperature of the water bath I shall use a
thermometer to make sure that it is not heating up.
* I shall monitor the time by using a stop clock.
Apparatus: In this experiment I shall use the following equipment:
* Beaker - to act as a water bath.
* Test tube - to hold the plant.
* Aquatic plant.
* Sodium Bicarbonate - to add extra Carbon Dioxide to the water to
help the plant photosynthesise.
* 100 watt light bulb.
* Meter ruler.
* Stop clock.
* Syringe (1mm² scale) - to collect and accurately read the amount
of oxygen produced.
* Clamp - to block off the end of the syringe.
Preliminary Experiment: Before the actual experiment is performed, a
test is necessary to ascertain details that are otherwise unknown (eg
distances, times etc).
Having done a preliminary experiment, I have obtained the following
results that will help me in the actual experiment:
After having tested distances from 5cm to 50 cm (with 5cm gaps in
between) I have decided that it would be best to use distances 10cm to
50cm (with 10 cm gaps in between), as I have found these to range from
good photosynthesis to hardly any photosynthesis, and I believe these
would produce interesting results when given more time.
During the testing for distances I was unable to give the plant much
time to photosynthesise, but afterwards I found time to test different
times ranging from 1 minute to 4 minutes, and I found that 2 minutes
was appropriate, as it gave enough time to measure the plants progress
at each distance, but also allows us to complete the experiment in one
Oxygen collection and calculation method
Out of the two methods available to us (either collecting the oxygen
in a syringe OR counting the bubbles), I found that the best method
(and the one we will be using) is to collect the oxygen in a syringe,
as it is much more accurate than counting bubbles, and will therefore
allow us to be more confident in our results, as we can believe them
to be reasonably more reliable than those which would be collected
from counting bubbles.
I have not needed to modify my plans for the experiment, as these
results are more of less the same as I expected.
Method: Firstly, the apparatus should be set up as in the diagram
above, ensuring that sodium bicarbonate has been added to the water,
and that all gas has been expelled from the syringe. (Note that the
plant should only be removed from the light where it is kept to where
the apparatus is set up when the experiment is completely ready to
begin, otherwise it may stop photosynthesizing).
The plant should be placed in front of the light (which should be set
up at the first distance), the syringe placed onto the top of the
plant and the timer started.
After the pre-determined time is complete, the amount of oxygen
collected should be recorded, the plant moved to the second distance
and the timer started again.
This should be repeated for the five different distances, and the
entire experiment should be repeated three to five times and the
averages taken to ensure accurate results.
Above is a graph showing the average results from the experiment.
Unfortunately, a scientist could not be completely confident with
these results, as they do not seem as accurate as was hoped. However,
my results do prove that (apart from any anomalous results which break
the trend), photosynthesis does in fact decrease as the light
intensity is less.
This happened because a plant is unable to photosynthesise properly if
it does not receive all the 'ingredients' it needs. As light is a very
important 'ingredient', it needs a lot of it, and therefore if it does
not get the right amount, or an amount it is not accustomed to it will
not photosynthesis as well as it could/would with a higher amount of
In my prediction I hypothesised that when the light intensity was
decreased, photosynthesis in the plant would decrease accordingly.
After having completed the experiment, I have found that, apart from
any anomalous results, the results did more or less match my
prediction. However, the amount of photosynthesis did not decrease by
equal amounts each time, but instead decreased randomly. This could
have been because the plant did not start photosynthesising straight
away, or it may not even usually photosynthesise at the same rate all
the time, but may instead photosynthesise in bursts, causing
fluctuations in the results.
· The circled point (the highest on the graph) must be anomalous, as
it should follow the trend and therefore be lower than the previous
points, but it is much higher. This is probably because it was only at
this point that the plant started photosynthesising fully, and
therefore gave a sharp 'burst' of oxygen, or it could simply be a
'bad' result (ie it was read incorrectly).
Unfortunately in this experiment there were a few sources of error, as
it is virtually impossible to complete an experiment without any.
· The accuracy of the time measurement.
To make the experiment completely accurate, a more 'advanced' method
of timing should be used, as using a stopwatch with just minutes and
seconds would not be accurate, and could allow extra bubbles into the
tube, which would harm the final results.
· The accuracy of how the oxygen was measured.
Although a syringe with a 1cm ³ was used, it would still not be
completely accurate, as the exact measurement would be hard to read.
· The average.
To get a more accurate average, the experiment could have been
replicated a few more times (maybe five), to give a better chance of
Also, between the different distances, it would have been a good idea
to give the plant thirty seconds to a minute for the plant to start
photosynthesizing properly, and to make sure that no extra oxygen from
the previous light intensity was carried forward before the new light
intensity measurement was commenced.
To extend my investigation I could try one of the following to get a
more comprehensive set of results:
· Investigate the effects of different coloured lights at different
on a plant.
· Investigate the rate of photosynthesis with a wider range of light
· I could use many different types of plants and investigate whether
the effects are the same with different species of plants.