Photosynthesis " Limiting Factor

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Photosynthesis – Limiting Factor (Light Wavelength)

Nick Gourlay


The purpose of this experiment is to design a procedure to determine how much the rate of photosynthesis varies when different colours from the visible light spectrum are used as the light source.


As the light colours change from darker to lighter colours (for example from blue to white), the rate of photosynthesis will vary, in that it will not show any change in the middle of the colour spectrum (since green is reflected by chlorophyll), but will with the rest of the colours (white, red, and blue) because chlorophyll absorbs all colours and reflects green.


  • Elodea (water plant)
  • Graduated cylinders
  • Test tubes
  • Water
  • Thermometer
  • Lamp
  • Light filters

Experimental Design

The design of this experiment is one which allows us to observe the rate of photosynthesis as it is exposed to different colours of light. For photosynthesis to occur the presence of light is needed, (white light permits plants to absorb all colours of the spectrum except for green, which is reflected). In order to observe the impact of each colour on photosynthesis, we must have a similar mass of plant placed in water (in different test tubes) and use different light filters, (white as a control, and red, green, blue), in order to tests their effects on the rate of photosynthesis.

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  1. Measure out 100mL water and place in test tube
  2. Cut a length of Elodea and add to the test tube
  3. Put test tube in front of the lamp and leave for significant period of time (5 mins)
  4. Record qualitative  observations
  5. Repeat steps 1-4 with varying light filters between the lamp and the elodea.


The results gained were purely qualitative as it was difficult to measure any form of quantitative measurement. The only possible indicator available was the production of air bubbles. We observed whether photosynthesis was occurring through the production of air ...

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