Investigating the Effect of Different Coloured Light on the Rate of Photosynthesis
In this piece of coursework I will be investigating the affect of
different coloured light on the rate of photosynthesis.
Green plants make their own food by a process called photosynthesis.
Photosynthesis occurs only in the presence of. Photosynthesis takes
place mainly in leaves and depends on an important green pigment
called chlorophyll, which is found in chloroplasts. To obtain the most
sunlight as possible, leaves have a large surface area and the more
sunlight the plant receives, the better it can photosynthesize.
Chloroplasts are found in palisade cells in large numbers and to allow
as much light to get in as possible, the cells are arranged like a
fence. This helps the energy entering the surface of the leaf to
travel a long way
through the palisade cells.
Photosynthesis can be defined as the production of simple sugars from
and water causing the release of sugar and oxygen.
Plants use the suns energy to join together water and carbon molecules
to make the glucose, which is sent around the plant to provide food.
Cells in the root or stem can use the glucose to make energy, if the
plant does not need to use all the glucose immediately then it is
stored which is difficult because glucose is hard to store in water.
Plants solve this problem by joining hundreds of glucose molecules
together to make starch. Starch does not dissolve in water very well
so it makes a better food store. The chemical equation for
photosynthesis can be expressed as:
6CO + 6H O + light ®C H O + 6O
Carbon dioxide +water + energy ®Glucose + oxygen
The fact that all plants need light in order to photosynthesise has
been proven many times in experiments, and so it is possible to say
that without light, the plant would die. The reason that light
intensity does affect the rate of photosynthesis
is because as light,
and therefore energy, falls on the chloroplasts in a leaf, the
chlorophyll, which then makes the energy available for chemical
reactions in the plant, traps it. Therefore, as the amount of
sunlight, or in this case light from a bulb, falls on the plant, more
energy is absorbed, so more energy is available for thechemical
reactions, and so more photosynthesis takes place in a given time.
There are many factors, which affect the rate of photosynthesis,
including light intensity, temperature and carbon dioxide
concentration. The maximum rate of photosynthesis will be constrained
by a limiting factor. This factor will prevent the rate of
photosynthesis from rising above a certain level, even if the other
conditions needed for photosynthesis are improved. It will therefore
be necessary to control these factors throughout the experiment so as
not to let them affect the integrity of my investigation into the
effect of light intensity.
Key Factors: CO2 is vital in photosynthesis because the plant takes in
CO2 from the air and joins with water molecules to make glucose. The
CO2 comes in through the stomata pores on the surface of the leaf and
only 0.03 % of the air around is CO2 so it's pretty scarce.
I predict that the light filters coloured green, yellow and orange
will produce the least amount of bubbles because the light will be
transmitted. Whereas placing red, clear/white and blue light filters
in front of the elodea will result in the greatest amount of bubbles
because the light is absorbed. Certain colours of light can limit the
rate of photosynthesis depending on how well it is absorbed into the
plants chlorophyll to photosynthesize. Also the wavelength can change
the rate of photosynthesis. If the lamp supplying heat for the plant
were placed twice as far away, I predict that there would be half as
many bubbles. Also if it were moved twice as far closer then there
would be twice as many bubbles. This is backed up with knowledge and
scientific understanding. This means that to keep my investigation
fair, I will have to keep the distance from the lamp to the elodea the
For my experiment I chose as accurate equipment as possible to give
myself the most accurate results. The equipment I used was-:
*Sodium hydrogen carbonate powder
*Light filters- red, green, yellow, orange, clear/white
The boiling tube was filled with water and the elodea placed in. Next
I used to clamp to keep the boiling tube in place. I placed a
thermometer into the boiling tube to measure the temperature to keep
the experiment fair. I held a light filter with forceps in front of
the lamp to make the light going onto the pondweed a different colour.
For each coloured light filter I counted the amount of bubbles
produced, in a minute. It was important to keep the experiment the
same each time to ensure it was fair test for example: The lamp stayed
the same distance from the beaker, we used the same plant each time
and the plastic sheets were all the same size. The experiment was
repeated three times and the results were averaged to ensure they were
regular and as expected. Below is a demonstration of how I will set up
the apparatus. Results were recorded each time and patterns observed.
The variables for this experiment include:
* Size of the pondweed.
*Amount of water.
*Distance of lamp.
* Size of the boiling tube.
* Transparency of the light filters.
*Time spent counting.
Changing either of the variables would have had effects on the end
results; we kept ours all the same each time to ensure a fair test.
These are results are secondary results. This is because in our
experiment, the elodea was not producing any bubbles. This may be
because the elodea had died, or because it was just too small.
This is a table showing one person's results after doing the
Colour of filter
Number of bubbles
30, 35, 41
1, 3, 2
The next results table below is of a class result.
The wavelength for each of the light filters.
As predicted, the results conclude that using sheets with colours near
the red and blue end of the spectrum produce a higher amount of
bubbles than those near green. Thereby proving that photosynthesis is
increased with certain colours of light.
In observation of the results, I have seen how the rate of
photosynthesis in the elodea has been affected by the various factors.
In reference to the prediction, I was correct in that the red,
clear/white coloured filters produced the highest rate of
photosynthesis, whereas the sheets, which were green and yellow,
resulted in the least bubbles. Which I did not predict was that the
orange filter would also make the elodea produce a lot of bubbles. You
can also get this evidence from my graph. I feel that we had taken
enough measurements to be sure of a fair test as the experiment was
repeated several times so. Each plastic coloured filter we used had
the same time, and variables as the others so we obtained precise
results for every test. We did not find anything, which stood out too
much from the pattern except that the red filter, when used resulted
more bubbles generally than the blue sheet. This shows that
chlorophyll absorbs red light more easily than blue. The Elodea
produced more bubbles with sheets at each end of the spectrum because
the chlorophyll in the plant absorbs all the colours but transmits
green. When the light is absorbed the plant converts it into energy to
photosynthesize. The more light energy it receives the better and
faster it can do this so when the sheets near the blue and red parts
of the spectrum are held in front of the Elodea it absorbs the light
and can photosynthesize better. If plastic sheets are held up which
are have a colour near the green part of the spectrum then the light
will be transmitted and the plant will not be able to photosynthesize
as well. In this experiment we have covered the main colours of the
visible spectrum and they are sufficient to produce the results that
we are looking for.
If I were to repeat the experiment then there are a number of ways I
could improve it. For example to get around the problem of the heat
from the lamp producing extra bubbles then a broad glass panel could
be positioned in the middle to avert any heat reaching the Elodea. To
advance the accurateness of counting the bubbles, you I could only
count the ones, which are a certain size, and only the ones coming
from the very end of the Elodea. If there were plenty of people
counting the bubbles and the results averaged then that would be a
more accurate way of obtaining the information required. To extend the
investigation I could alter certain variables for example the kind of
plant that I'm using to count the bubbles from. I could try a complete
species of plant and see if the results are comparable for every type.
I could also use dissimilar chemicals in the water each time to see
which chemicals result in the maximum rate of photosynthesis.