Investigating the Link Between Wavelength of Light and Rate of Photosynthesis
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I have been asked to investigate the link between wavelength of light
and rate of photosynthesis.
I predict that the order of best absorption in a plat to produce more
bubbles will be blue, yellow, orange, red and finally green. I predict
this because blue has the shortest wavelength which produces the most
energy and there is slightly higher absorption in the blue region by
the plant. The red has the largest wavelength in the visible spectrum
which produces the least energy. The reason why green is at the bottom
of the list of absorption in a plant is because green is reflected off
the plant and not absorbed as much as the others. It is reflected of
the plant to be seen as green in our eyes.
The spectral quality or colour of light is associated with its
wavelength. Blue light has a shorter wavelength and higher frequency
than red light. A simple table that indicates the wavelengths of
colours in the visible spectrum is shown below.
Colour of light
In photosynthetic plants the pigments are very important. They absorb
light energy and enable it to be converted into chemical energy which
is used by the plants to make glucose and oxygen from carbon dioxide
and water. Plants appear to be different colours because of the
dominant pigments they contain. These pigments absorb some colours of
light and reflect others, for example, the green chlorophylls absorb
light from the blue-violet and the red regions of the visible spectrum
and reflect green light. This is why plants which contain mostly
chlorophylls appear green. Other pigments found in green plants, the
yellow, orange and red carotenoids which absorb light only from the
blue-violet region of the spectrum, are mostly masked by the more
We can see how different wavelengths of light affect photosynthesis by
looking at action spectra. An action spectrum relates the rate of
photosynthesis to the wavelength of light being received by a plant.
For green plants, including algae, the action spectrum shows that most
photosynthetic activity takes place in blue-violet and orange-red
lights since these are the colours which are mostly absorbed by the
main chlorophylls and the carotenoids. Photosynthetic activity is
lowest in green light since green light is hardly absorbed at all by
these pigments. The relative absorption of light of different
wavelengths by pigments can be shown in absorption spectra. Action and
absorption spectra correspond quite closely. Wavelengths of light
which are more readily absorbed by photosynthetic pigments cause
higher levels of photosynthesis.
Some plants live in conditions where the spectral quality of light may
be different to that received by plants living on the land. Algae
which live in surface waters tend to be green and contain more or less
the same pigments as land plants since they exist under similar light
conditions. Algae living lower in the water receive more blue light
than red because red light has a relatively long wavelength and cannot
penetrate water as well as blue light which has a shorter wavelength
and more energy than red light.
Brown algae, which may be found deeper in the water than green algae,
have combinations of pigments which enable them to photosynthesise
using less of the red light utilised by green plants. Red algae, which
tend to be found at even greater depths, contain more pigments to
absorb the blue light which penetrates deeper into the water.
It is the combination of pigments in a plant which determines which
wavelengths of light can be utilised in photosynthesis. No plant
absorbs light with equal effectiveness across the visible spectrum.
This is why different colours of light may affect photosynthesis and
the subsequent growth of plants.
The equipment that I used was:
· Ruler - measure the distance between the lamp and the plant
· Timer - measure time
· Sodium hydrogen carbonate
Firstly I collected the equipment and set it up. We then filled the
beaker with 400ml of 2% sodium hydrogen carbonate. To this we added
the elodea and placed the beaker 50 cm away from the lamp. We then
placed a coloured filter and placed it in front of the lamp. We left
the plant there for 2 minutes (acclimatise to the conditions), and
then placed the syringe over the plant. We decided that we would
measure the volume of gas evolved from the plant during a 5-minute
period. We then repeated using the same colour to ensure that our
results are accurate. Once that was completed we changed the colour of
the filters, we will use the following colours: Violet, blue, green,
orange, yellow and red.
We tried to keep the water well away from the electricity. We also
tried to be careful when using the blade to cut the plant.
We tried to pop all the bubbles as soon as they formed. We also used
the same plant and same quality of chemicals. We also kept the light
the same distance from the plant throughout the experiment.
From the results that I have gathered I can state that certain
wavelengths affect the rate of photosynthesis more than others. My
results were not exactly the same as the graph on page two, but
several different things could cause this. Human error may have caused
the results to be inaccurate. Some white light could have been used by
the elodea, causing the rate of photosynthesis to go up slightly. The
heat being produced from the light may have also affected the rate of
photosynthesis. Since temperature affects the movement of molecules
and the reactions worked by enzymes, the heated molecules and enzymes
may be the cause of the increasing rate of photosynthesis, because
photosynthesis works using enzymes.
My results fit my hypothesis quite well, as the order of colours in
the hypothesis fit the results and the graph quite well, except for
one colour, blue. This could have happened because of impractical
mistakes made in practical.
In the table, the results show that I have consistent results.
Although they are consistent I do not think they are very accurate. In
hypothesis I said that blue would be the most absorbed as it has the
most energy to produce more oxygen. But as seen in the graph and
results table, it only has the second highest bubble rate. Another
strange result that I had was that green did not have the lowest
bubble rate. I would have expected green to have the lowest bubble
rate as the plant was also green, meaning that the green light should
have been reflected back by the plant. However this could be because
of a number of reasons, the shade of green that the filter was could
have been a different shade to that of the plant.
I could do more experiments for this investigation, as there are two
more things that can be varied to change the release of oxygen in the
plant. The two other factors are the amount of carbon dioxide added
and water added. I could also vary the intensity of light on the plant
to change the amount of oxygen released. This coursework
As a conclusion to this experiment I think my results were good enough
to say that they fitted my hypothesis and that with a little more
precaution I could have proved my hypothesis completely correct.