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Photosynthesis and Limiting Factors

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Photosynthesis and Limiting Factors

Plan for the experiment, which will evaluate how the rate of
photosynthesis in a water plant is affected when the intensity of a
light source is varied.


To investigate how light intensity affects the rate of photosynthesis.

Background Information:

Plants need carbon dioxide, water, light and chlorophyll in order to
make food; and starch and oxygen are produced, if one of these is
missing then starch cannot be produced. Photosynthesis occurs rapidly
or slowly, depending on the circumstances and this will determine, how
much food is made in a certain period of time. Carbon dioxide and
water are the raw materials of photosynthesis and they react to
produce starch and oxygen, which are the products. These reactions
need energy, which comes from the light and it is the chlorophyll,
which enables the plant to use light energy for this process. Glucose
is then formed and later turned in to starch.


In order to make a prediction for my experiment I will have to
research photosynthesis and it's limiting factors. A limiting factor
is something that will either slow a process down or if the amount of
it is increased the rate of the process will speed up. There are five
limiting factors for the rate of photosynthesis, which are: light,
carbon dioxide, water, temperature and chlorophyll.

Experiments show that the more carbon dioxide there is in the air the
faster the plant photosynthesises. Raising the temperature up to 40
degrees will increase the rate of photosynthesis. However,
temperatures beyond this can destroy the enzymes that are needed for
the chemical reactions. I will be controlling all of the following
variables: levels of oxygen, levels of water and the temperature. I
will be investigating light as my uncontrolled variable.

The rate of photosynthesis can be measured in two ways:

1) The rate of production of products (eg glucose, starch, oxygen)

2) The rate of disappearance of the reactants (eg carbon dioxide,

In order for my investigation to be fair I have decided to repeat the
experiment three times, and then take an average. This will enable me
to get more reliable and varied results. It is also important for me
to take correct measurements for my experiment and this, will be
achieved by me closely following my method.


We are able to find out the effect of light on photosynthesis by using
a water plant the produces bubble's when a source of light is shone
upon it. I think that the brightness of the light will effect the
speed of photosynthesis and as a result more bubbles will be produced.
I think that the closer the lamp is to the plant, the faster the rate
of photosynthesis. This will be because the light coming from the lamp
will be more intense which will mean that more bubbles will be given

Equipment list:

· pond weed

· beaker

· paperclip

· metre rule

· lamp

· water

· stopwatch

· straw



GCSE Biology Coursework

Photosynthesis and limiting Factors

Ilona Beiboer


1) Darken the room and black out the windows

2) Set up your equipment by filling the beaker with water and placing
the pond weed in the beaker with a paperclip attached to the bottom
(this will weigh the plant down).

3) Cut the top of the plant off to allow oxygen bubbles to escape.

4) Carbonate the water by blowing into the beaker with a straw (this
will ensure that there is a good supply of carbon dioxide for the pond

5) Place a metre rule on the table to one side of the beaker. Position
the beaker at a certain distance away from the lamp and it is this,
distance that is the variable, which we are measuring.

6) Time a minute on the stopwatch and count how many bubbles appear
during this time.

7) Repeat this three times and then take an average.

8) Move the lamp 20cm closer and repeat the experiment.

9) Record your results

Throughout the experiment I will be changing the distance of the lamp
from the pond weed which will allow me to assess how the light
intensity affects the rate of photosynthesis. I will have to ensure
that this experiment proves to be a fair test, so we can produce the
most accurate results. In order to do this I will have to keep certain
things such as: the size of the pond weed, the distances of the lamp
from the beaker and the time spent recording and the amount of carbon
dioxide which is blown through the straw into the water constant

I have decided to start my experiment with the lamp being placed 100cm
away from the beaker with the pond weed. I will then move the lamp
20cm closer and time how many bubbles come off the water plant by
using the stop watch. I think that this range of measurements will be
sufficient for me to obtain the necessary evidence.

Risk Assessment:

1) Make sure that the surface area you are working on is completely
clear to avoid accidents.

2) Be careful not to spill any of the water on your surface area.

3) Make sure that the water does not make contact with the lamp or
electrical socket.

Results Table:































See graph for results


My results clearly show that light intensity increases the rate of
photosynthesis and that the closer the lamp to the beaker, the quicker
the rate of photosynthesis. The results shown in the table above, all

demonstrate this, and proves that my prediction was correct. I believe
that my results were accurate and the repetition of all of the stages
of my experiment meant that I was able to record reliable evidence.

The graph also helps to determine that there is a steady and constant
raise in the number of bubbles given off each minute at each separate
distance. This gradual raise in my findings also tell me that only the
variable (he light intensity) changed during the experiment, whereas
the carbon dioxide levels, water levels and the size of the pond weed
remained equal throughout the experiment which allowed this
investigation to be a fair test.


I feel that this investigation went successfully and I was able to
obtain clear and relevant information that proves that light intensity
speeds up the rate of photosynthesis, provided that all other limiting
factors are kept constant. My measurements were as reliable as I could
have made them and the range of numbers that I used between 0- 100 cm
proved suitable for this investigation.

There are other experiments, which we can do to investigate further
the rate of photosynthesis. These include the levels of carbon
dioxide, the amount of water and the temperature as each of these can
effect the rate and will prevent photosynthesis happening if it is
removed. I could also vary the angle of the head of the lamp when
shining upon the water plant or the size of the lamp.

However there are some improvements that I could make if I was to
repeat this experiment again, which would allow me to support my
conclusions further. If I was, to do this for a second time I would
extend the range of numbers that I used and try to establish the point
at which photosynthesis stops. I could also place another beaker full
of water between the lamp and water plant, which would prevent the
pond weed heating up and would act as a heat shield. In order to make
this investigation even more accurate you could also have two people
recording the time and counting the number of bubbles so that no
mistakes will occur.

In conclusion I feel that this experiment has successfully proved my
initial thoughts and predictions and has enabled me to gain a wider
understanding of the properties of photosynthesis and the affect that
the light intensity has upon the rate of photosynthesis. This
investigation has also given me ideas for other experiments, which
could prove alternative aspects of photosynthesis.

How to Cite this Page

MLA Citation:
"Photosynthesis and Limiting Factors." 20 Apr 2014

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