Photosynthesis Rate vs Light Intensity in Elodea


LED full spectrum lamp
Boiling tube
Clamp & stand
Sodium bicarbonate (Baking Soda)
Elodea pond weed


Make up a 0.2% solution of the Sodium Bicarbonate and water to provide a source of Carbon Dioxide for the Elodea.

Fill the boiling tube with the solution and add a piece of Elodea such that there is 2cm of solution present above the Elodea. Clamp this in place making sure the that clamp obscures as little of the Elodea and solution as possible. Gently tap the tube to dislodge any gas introduced to the tube with the Elodea.

Set up the LED lamp at a 5cm distance from the side of the boiling tube. A normal lamp can be used, however the temperature increase caused by using an incandescent lightbulb will introduce an error into your experiment as temperature also affects the photosynthesis rate. For improved results ambient lighting should be kept constant or the experiment should be done using the LED lamp as the only source of light.

Wait for the Elodea to start producing bubbles. Start the stopclock and count the bubbles produced over a one minute period. Repeat measurement a further two times.

Move the lamp further away from the Elodea by between 5cm and 10cm depending on how many measurements you wish to take. Wait two or three minutes for the rate of bubble production to settle and then repeat the measurements of the bubble count over a minute.


Light intensity obeys and inverse square law – this means that if you double the distance you quarter the intensity. To analyse your results you should square the distance between the lamp and the Elodea and then take the inverse of this number to plot the x axis of your graph. By plotting this against the number of bubbles produced you will be able to determine the relationship between light intensity and photosynthesis rate.