An Experiment to Show the Relation Between Light Intensity and Photosynthesis

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An Experiment to Show the Relation Between Light Intensity and Photosynthesis


Photosynthesis takes places in the leaves of plants, mostly in the
palisade layer of the leaf. Photosynthesis is the plants way of
creating energy for itself(sugar), for growth, repair, storage and
energy production.

The test will be to find out the relationship between light intensity
(voltage) and the rate of photosynthesis. Photosynthesis is:

Carbon dioxide + water + sunlight ---> oxygen + carbohydrate + water


6CO2 + 12H2O + sunlight ---> 6O 2 + C6 H12O 6 + 6H2 O

Oxygen is a product of photosynthesis; therefore we will be measuring
the amount or rate of this product being produced.

Plan and Pulmonary test

Text Box: Prior to the official test I conducted a pulmonary test.
This was so that I could identify any initial problems and work out
how to resolve them. Some problems I encountered where; that the plant
clearly took a few minutes to adjust to its optimum rate of
photosynthesis at the specific light intensity. In order to combat
this problem in the official test I would leave the plant subjected to
the required light intensity for 3 minutes prior to measuring and
recording the volume of oxygen produced. To allow it to
photosynthesise at its optimum rate.

To make it a fair test the plant was sealed off in a box and no other
light was allowed to enter the box, as this could affect the plants
rate of photosynthesis. The wattage was controlled by a transformer
and the pond weed was placed inside the box directly under the light
bulb and surrounded by foil. To give the plant Carbon Dioxide we added
sodium hydrogen carbonate powder (this bubbles to produce Co2
bubbles).To attempt to control the temperature the test tube
containing the pond weed (Canadian) was placed in a water bath- the
water would absorb some of the heat. The plant was subjected to
different wattages of light and the amount of oxygen (rate of

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MLA Citation:
"An Experiment to Show the Relation Between Light Intensity and Photosynthesis." 26 Mar 2017

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photosynthesis) was measured.

How the experiment was carried out:

I took a 10cm long sample of Elodea Caledensis (pondweed) and placed
this inside a 50cm3 syringe. I attached a capillary tube to the end of
the syringe with a piece of rubber tubing. I filled the syringe up
with water and with a heaped spoonful of sodium hydrogen carbonate.
Then I put the plunger into the syringe and pushed any oxygen inside
the syringe out, making sure to leave a few bubbles at the end of the
capillary tube closest to the syringe. I then put the syringe in a
retort stand so that it could be held up. I placed the end of the
capillary tube inside a beaker full of water standing on a bench. I
placed a transformer on the bench and plugged it in, connected it to
the lamp and placed the lamp adjacent to the syringe containing the
pondweed. I then set the transformer to the highest wattage (12watts)
and then left it for 3 minutes prior to the test. I then began
recording the length of water displaced (the length that the bubble
had travelled) over the period of a minute and recorded the amount to
work out the volume using this formula: π r 2 h or pi X radius (0.5) 2
X h.


I predict that the light intensity (controllable by volts) will be
directly in proportion to the rate of photosynthesis. Until the light
intensity is no longer a limiting factor and there is another limiting
factor(s) i.e. Carbon dioxide. I predict that the rate of
photosynthesis will increase in proportion to the light intensity as
long as there are no other limiting factors. This is because the plant
seeks to perform photosynthesis in order to stay alive; the plant will
photosynthesise at the optimum rate in the conditions provided. I
believe that above a certain light intensity the plant will no longer
be able to increase its rate of photosynthesis as there is another
limiting factor. i.e. light intensity will no longer be a limiting
factor and there will be another factor limiting the rate of
photosynthesis e.g. temperature or There will also be a point at
where the temperature of the water becomes so hot that the chloroplast
cells would de-nature. I predict that if I where to make a graph of my
results with voltage along the x axis and amount of oxygen produced
per minute along the y axis, I predict that the graph would look like


The Equipment I will use

* 1 retort stand

* 1 12 watt bulb (with variable intensity)

* 1 lux meter (light intensity meter)

* 1 piece of pondweed (elodea canadensis )

* 1 beaker with water

* 1 capillary tube

* 1 cardboard box

* Sodium hydrogen carbonate, a surplus amount (heaped spoonful)

* 50cm3 syringe.


The results showed that the amount of oxygen released, or rate of
photosynthesis is directly in proportion to the amount of light or
wattage. Below certain wattage, in this case, 6, the rate of
photosynthesis extremely drops. At this time I would imagine that the
plant is struggling to photosynthesise and so has slowed down the rate
of photosynthesis.


The conclusion I made is that plants will photosynthesise in relation
to the wattage of light up to a certain point until another factor
e.g. temperature or carbon dioxide is limiting the rate of
photosynthesis. This theory is called ‘limiting factors’. In this test
the point where light intensity is no longer controlling the rate of
photosynthesis is obvious. It is at the point where the gradient of
the graph plateauÂ’s out. At this point

I would think that the temperature was a limiting factor. In ideal
conditions of temperature, Co2 and water would be in plentiful amounts
therefore the plant would continue to quicken the rate of
photosynthesis. From these results, I am able to say that an increase
in light intensity does certainly increase the rate of photosynthesis.

The gradual decrease in the rate of increase of the rate of
photosynthesis (the swallowing of the curve) can be attributed to the
other factors limiting the rate of photosynthesis. As light intensity
increases, the photosynthetic rate is being limited by certain
factors, such as carbon dioxide and temperature. These factors do not
immediately limit the rate of photosynthesis, but rather gradually. As
light intensity increases further, so the rate of photosynthesis is
being limited by other factors more

and more, until the rate of photosynthesis is constant, and so is
almost certainly limited in full by another factor.

distance bubbles travelled





volume of bubbles(mm3) average





































If there was any way I could have made the test any fairer it would
have been to prevent temperature from becoming a variable, I would
have done this probably by placing the entire experiment in a huge
test tube this test tube would have a flow of water running through it
to remove heat and prevent temperature from changing. This test tube
would also be acting as the beaker.

I could make this experiment more accurate by questioning weather all
of the oxygen is contributing to the down force pushing bubbles down.
Some of the oxygen may be dissolved in the water and I could find a
more accurate way of measuring the distance travelled by the bubbles.
Also I could have positioned a volt meter in between the transformer
and the lamp to get an exact measurement of the voltage used.

Also temperature was a variable so using the above method of a large
water bath test tube could prevent this. I did not encounter any
anomalous results but my graphs do not show the plateau, or flatting
out that I described. This is because the light was still a limiting
factor and the higher voltages where not capable and therefore could
not be tested.

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