01 January 2013

#41 Effect of Light intensity on the rate of Photosynthesis

Plants need light energy to make the chemical energy needed to create carbohydrates. Increasing the light intensity will boost the speed of photosynthesis. However, at high light intensities the rate becomes constant.

  • Place a pond weed Elodea upside in a test tube containing water.
  • Place the tube in a beaker of fresh water at 25°C. This helps to maintain a constant temperature around the pond weed.
  • Place excess sodium bicarbonate (NaHCO3) in the water to give a constant saturated solution of CO2.
  • Place the lamp (the only light source) at distance from the plant.

  • Count the number of oxygen bubbles given off by the plant in 1 minute period. This is the rate of photosynthesis at that particular light intensity.
  • The gas should be checked to prove that it is indeed oxygen - relights a glowing splint.
  • Repeat at different light intensities by moving the lamp to different distances.
Photo from passmyexams.co.uk

  •  Graph the results placing light intensity on the x-axis.

  • Light energy absorbed by chlorophyll is converted to ATP and H+
  • At very low light levels the plant will be respiring only not photosynthesising.
  • As the light intensity increases, the rate of photosynthesis increases. However, the rate will not increase beyond a certain level of light intensity.
  • At high light intensities the rate becomes constant, even with further increases in light intensity, there are no increases in the rate.
  • The plant is unable to harvest the light at these high intensities and the chlorophyll system can be damaged by very intense light levels.

Additional sources:  skoool.ie


  1. Why is sodium bicarbonate used instead of water?

  2. Sodium Bicarbonate is used because it produces carbon dioxide- the concentration of carbon dioxide will affect the experiment, so if you put sodium bicarbonate it should maintain the concentration of carbon dioxide. idk tho