The Effect of Light Intensity on the Rate of Photosynthesis

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The Effect of Light Intensity on the Rate of Photosynthesis


Green plants don't exude food from the soil they make their own, using
sunlight. This is called photosynthesis, which means "making through
light". This occurs in the cells of a green plant, which are exposed
to sunlight. Four things are needed to make this process work and this
gives two thing: the four things needed are light from the sun, carbon
dioxide diffuses from the air, water from the soil and chlorophyll in
the leaves. This gives out oxygen which is released into the
atmosphere and glucose which is used for biomass and energy. The word
equation for the process of photosynthesis is:


Carbon Dioxide + water Glucose + oxygen

The energy which is produced is used to build up smaller molecules
into larger molecules. Here are some uses of energy:

* Sugars into starch

* Sugars into cellulose- needed for cell walls

* Sugars, nitrates and other nutrients for protein- for growth and to
make to enzymes

* Sugars into lipids for storing in seeds

Factors affecting photosynthesis

Temperature, carbon dioxide concentration and light intensity can
interact to limit the rate of photosynthesis. Any one of them, at a
particular time, maybe the limiting factor. Carbon Dioxide can have an
effect on the plant because plants need carbon dioxide to
photosynthesize. Temperature has an affect but if the sunlight was low
then the temperature will be low so the carbon dioxide can be the
limiting factor.

The one we need to concentrate more on which is our aim is about light

Here is a diagram to show what I mean by this and also with it
explains what the diagram shows.


1) Here as the light intensity increases so does the rate of
photosynthesis. This means that the light is limiting the rate of

2) Rise in light intensity now has no effect. Light intensity is no
longer the limiting factor.

How to Cite this Page

MLA Citation:
"The Effect of Light Intensity on the Rate of Photosynthesis." 29 Mar 2017

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This means carbon dioxide or temperature
must be the limiting factor.


I predict that the more intense the light, the higher the rate of
photosynthesis. To photosynthesise, plants need light. It provides the
energy for the process to happen. Chlorophyll is an enzyme and it
speeds up the reaction. If a plant does not get enough of either of
these things, photosynthesis will not happen as quickly, if at all.
Therefore, I predict that when the light is not very intense we will
not see so many bubbles being produced. This is because the plant will
not have so much energy (derived from light) to activate
photosynthesis. All reactions require certain activation energy, and
if this is not reached the reaction will occur more slowly. I think
that as we move the lamp away (and therefore reduce the light
intensity) from the pondweed the number of bubbles produced will
decrease steadily. For instance, say at 10cm distance 50 bubbles are
counted, it is likely that at 20cm distance 25 bubbles will be
counted, as the lamp is twice the distance away. This means the rate
of photosynthesis is halved but we are measuring the light intensity
and so this will not mean the rate of photosynthesis is halved. I
think that if we move the lamp any further away than 50cm no bubbles
at all will be produced because there will simply not be enough light
for photosynthesis to work but we are only doing it till 17cm which
will give us enough results.

When I am doing my experiment I will measure the amount of oxygen made
which will be measured by the bubbles.

I will vary the distance apart from the beaker to the lamp. The
equipment I will be using are as follows: a large beaker, a healthy
pondweed plant, a lamp, water, a stopwatch, a pinch of sodium
bicarbonate which will give the plant more carbon dioxide.


I will vary the distance of the lamp to the beaker which will help me
to measure the light intensity using the equation: E=1/d2.

The things I will keep the same are: the water, the weed, the lamp,
the beaker and the place where I did the experiment the same which
will make it a fair test.

I will measure how many bubbles come up in 1 minute.

What I expect to happen is in the hypothesis above and also I have
showed my expected results in my graphs. I will repeat the experiment
twice. I will vary the distance because this will show us that the
further the lamp is the rate of photosynthesis will decrease. This
does not mean that if I move the lamp away from 5 cm to 10 cm that the
light intensity will be half. I will keep the weed the same because if
we use different weeds then it will not be a fair test because some
plants are better and quicker then others. We will keep the water the
same because then the temperature can differ and also we will have to
keep on adding sodium bicarbonate and we can't always get the exact
amount. What I expect to happen is explained in the hypothesis.

We will be able to do a trial experiment 1st which will show us that
the experiment is set up and it is working. We will do the experiment
twice which will help us work out averages and also if there are
anomalous results we will find out what went wrong and redo that bit
of the experiment again.

What we need to do in order to get the experiment working.

1) Set up the apparatus as shown in the diagram


2) When the weed has settled down and is producing bubbles then start
the clock and time it for 1 minute and see how many bubbles have came
up. Do this twice so we get an accurate result.

3) The change the distance of the lamp from the beaker and do this


Take care of the light bulb, which may get very hot. As we will be
using electricity (for the light bulb) and water at the same time,
special care must be taken.


Our aim is to find out how light intensity affects the rate of


As in plan- everything which was supposed to be in the method in the


Distance from lamp (cm)

Number of oxygen bubbles produced in 1 minute
































Here are Anisha's group's results

Distance from lamp (cm)

Number of oxygen bubbles produced in 1 minute



































Here is the light intensity for:

Our group Anisha's group

5cm= 0.04 2cm=0.25

7cm=0.02 4cm=0.06

9cm=0.02 6cm=0.07

11cm=0.006 8cm=0.016

13cm=0.006 10cm=0.001

15cm=-0.004 12cm=6.94



From my graphs and also my results table I can reach to a conclusion.
My results show that the light intensity does affect the rate of
photosynthesis. We compared our results with Anisha's group's results
and also I drew graphs of their group and this also clearly states
that the light intensity affects the rate of photosynthesis. My
experiment did work and also to make more carbon dioxide Mr Leaver
told us to add sodium bicarbonate. I also did some research and I
found that this experiment is used to find out if light intensity
affects the rate of photosynthesis. Also I used a book which sir gave
to us which explains the experiment. My experiment produced these
results because we had a stop watch and we only started the stopwatch
when the bubbles had started coming out. We had to change how we did
the experiment because the bubbles were coming out very rapidly and we
couldn't count them for 1 minute because we were bound to blink and so
then we would miss a few and loose count. We then asked Mr Leaver who
suggested to us that count them for 15 seconds and then times that by
4 which gave us what we would get after a minute. I can proof this
conclusion with my results. My conclusion shows that my prediction was
correct. I can prove this with my hypothesis and also I had done a
graph before the experiment which showed what I expected and when I
got my results I added the average to see if my prediction was correct
and it was.


My results were accurate and they were reliable. There were no
unexpected results and our results were suitable to get the results I
needed. If I did the experiment again I would measure the temperature
because an artificial light which was the lamp gives out 12 joules per
second of light energy and 48 joules per second heat energy because it
was a filament bulb. If we used a low energy bulb then it would have
gave 12 joules per second light energy and 3 joules per second heat
energy. Because we used a filament bulb we had more heat then light
and so the temperature can be a limiting factor in some cases. To make
it a fair test we could have used a low-energy bulb which is more
efficient and less heat is given out but they are too expensive. I had
enough results to reach a conclusion and my results prove that light
intensity is effects the rate of photosynthesis: the further the light
the slower the rate of photosynthesis.

The change I suggested could have made the results more reliable
because if a filament bulb which we were using gives out 4 times as
much heat then light then the limiting factor can be the temperature.
If we use the low energy bulb then that give 4 times less heat and
more light which we need. If we do the investigation again I would use
a low-energy bulb to get more accurate and reliable results. I have
carried out a fair trial.

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