Effect of Light Colour on the Rate of Photosynthesis

To what extent does the colour of light have on the rate of photosynthesis on Cabomba Aquatica plants?

Background knowledge:

Photosynthesis is a process that occurs in all plants including bacteria and algae. They utilize the energy provided by the sun to produce energy in the form of glucose and oxygen. Photosynthesis is vital for the survival of most life forms on earth. The process of photosynthesis includes light energy being absorbed by the plants and synthesised with carbon dioxide (CO₂) and water (H₂O) and gets converted into glucose (C₆H₁₂O₆) and oxygen (O₂).

Cabomba Aquatica plants are also known as fanwort plants and are native to South America. Cabomba plants survive in aquarium environments with hard water and minimal light. They are favoured by aquarists as oxygenating plants for fishtanks.

6CO₂ + 6H₂O  C₆H₁₂O_{6 +} 6O₂

Light is in the form of electromagnetic energy that travels in wave form. The range of electromagnetic radiation is known as the emission spectrum, a small strip of which is visible light. Visible light is light that is detected by the naked human eye. Visible light has a wavelength that ranges from 400-700 nanometres. The percentage of light absorbed by plants in photosynthesis differs for the colour of the light. Red and violet light are on opposite ends on the visible light spectrum. Red light has the longest wavelength of 700nm whereas violet light has the shorter wavelength of 400nm. Red light also possesses the lower frequency while violet light possesses the higher frequency. White light is a mixture of all seven colours of the spectrum. As visible light passes through a prism the light is refracted, each light refracting at different degrees. Red light refracts the least whilst violet light refracts the most.

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Light can also be absorbed by pigments. Different pigments absorb different wavelengths of light. The leaves of a plant are green because the chlorophyll within them absorb all other wavelengths of light and reflects green light. In the absorption spectrum it can be shown that red light and violet light are the most absorbed whereas green light is most commonly reflected.

There are three main limiting factors for the rate of photosynthesis; Light intensity, Temperature and Concentration of Carbon Dioxide. To increase one or decrease one would have a correlating effect on the rate of photosynthesis.

This investigation will be focusing on which wavelengths of visible light, colour of light, has the highest rate of photosynthesis. It is safe to assume that green would have to lowest rate of photosynthesis because it is reflected by the leaves of plants and red/violet would have the highest rate of photosynthesis.

Hypothesis:

I hypothesise that the rate of photosynthesis will be less for the green light whereas lights such as red and violet will have the fastest rate of photosynthesis.

Variables:

Preliminary results:

During my preliminary I attempted to measure different colours in order to gain an understanding on how the colour of light affects the rate of photosynthesis. I kept the light bulb at 5cm away from the test tube and placed a coloured filter in front of the light so that the colour of the filter shone on the plant.

I realised that the way in which I filtered the light wasn’t the best way at all and in order to get the best results I thought it best to wrap the filter around the test tub and allow the light to shine upon it ensuring most of the coloured light reaches the Cambodia leaf.

Apparatus:

• 100W light bulb
• Cabomba plant
• 1 metre ruler±0.5mm
• 1% sodium hydrogen carbonate (NaHCO₃)
• Stopwatch (±1seconds)
• 250cm³ beaker (±25cm³)
• Colour filters (x4)
• Boiling tube
• Safety goggles
• Thermometer (±0.5ºC)

Method:

• Ensure safety goggles are on to prevent damage to the eyes and for protection from Sodium Hydrogen Carbonate.
• Lab coats must be worn at all times during the experiment
• Hands must be washed before and after the experiment

1. Set up apparatus on working area as pictured above
2. Collect your sample of Cambodia plant into the boiling tube. Make sure it is big enough to fill about two thirds of the tube.
3. Place boiling tube into a beaker and fill the beaker with water, also to about two thirds in order to add the sodium hydrogen carbonate which will submerge the plant.
4. Fill boiling tube containing the Cambodia plant with 1% Sodium Hydrogen Carbonate. Fill it till it covers the plant.
5. Record temperature ensuring it is at room temperature of about 25 ºC
6. Plug in the 100W light bulb in and turn it on to its side so that it is facing the beaker. Place it 5cm away from the beaker.
7. Allow the beaker to acclimatise for 2 minutes and return to starting temperature before beginning net experiment
8. After 2 minutes count the number of bubbles that appeared with 30 second increments in 2 ½ minutes.
9. Record the number of bubbles given off
10. Change the colour of the filter and allow the plant to acclimatise for 2 minutes again before repeating the experiment.
11. Repeat the experiment again with a different wavelength of light

Data collection and Processing (Quantitative and Qualitative data):

Measures and average number of bubbles produced

Qualitative Data:

As the plant photosynthesised, there was a constant stream of bubbles being emitted from the stem. The uncertainty of 5% is due to the large number of bubbles being emitted at times and the factor of human error when counting the bubbles. The level of uncertainty for time was ±1 second.

Average rate of photosynthetic reaction

Standard deviation from the mean:

σ_x =

Interpretation of results:

Conclusion:

(http://blog.canacad.ac.jp/bio/BiologyIBHL1/1121.html)

The data table supports my hypothesis partially. The green light photosynthesised the least whilst yellow light photosynthesised and then red light. I hypothesised that red light would photosynthesis the most and would be the most efficient whilst green light would be the most insufficient and synthesise the least, the only anomaly was the effect of yellow light which indicates an inaccuracy and unreliability of the apparatus used. Violet light was also unusually low suggesting inaccuracy and error. Perhaps the yellow filter was in fact a mix of multiple wavelengths causing the strange results I gathered whilst white light was a mix of all wavelengths.

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Mean results would be plotted on a graph and a smooth curve of best fit would be drawn through the points with error bars are each point. Each bar represents the standard deviation from the mean for each wavelength of light. The presence of anomalies and outliers creates inaccuracy and unreliability within my results. The error bars where slightly large for some of the wavelengths of light indicating inaccuracy of results.

The colour of the leaf played a part in the rate of photosynthesis. Leaves may come in different colours, because the leaves used in this experiment were green, green light is more likely to be reflected. A different coloured leaf may produce different results and thus further investigation would be required. Looking at the image above which shows the literary value of wavelength and rate of photosynthesis it became evident to me that repeated investigations are important. While I was correct that green light would have as low photosynthetic rate, my data did not follow the trend that it should. Yellow light should have also been one of the lowest rates however in my experiment I found it to be one of the highest and blue light showed to have one of the lowest rates. This led me to conclude that my experiment was inaccurate.

Evaluation:   

There were numerous weaknesses within my experiment that caused unreliability and inaccuracy.

Firstly I performed my experiment at the same time as the rest of the students within the class meaning that the light intensity was mixed with those near to mine thus increasing the light intensity and affecting my results. This issue is a minor one yet it still adds to the unreliability and inaccuracy of my results.