Investigating the Rate of Photosynthesis (CIE IGCSE Biology)

• The plants usually used are Elodea or Cabomba - types of pondweed
• As photosynthesis occurs, oxygen gas produced is released 
• As the plant is in water, the oxygen released can be seen as bubbles leaving the cut end of the pondweed
• The number of bubbles produced over a minute can be counted to record the rate
• The more bubbles produced per minute, the faster the rate of photosynthesis
• A more accurate version of this experiment is to collect the oxygen released in a test tube inverted over the top of the pondweed over a longer period of time and then measure the volume of oxygen collected
• This practical can be used in the following ways:

Investigating the effect of changing light intensity

• This can be done by moving a lamp different distances away from the beaker containing the pondweed

 

Investigating the effect of changing light intensity on the rate of photosynthesis

 

Investigating the effect of changing temperature

• This can be done by changing the temperature of the water in the beaker

 

Investigating the effect of changing temperature on the rate of photosynthesis

Investigating the effect of changing carbon dioxide concentration

• This can be done by dissolving different amounts of sodium hydrogen carbonate in the water in the beaker 

Investigating the effect of changing carbon dioxide concentration on the rate of photosynthesis

• Care must be taken when investigating a condition to keep all other variables constant in order to ensure a fair test
• For example, when investigating changing light intensity, a glass tank should be placed in between the lamp and the beaker to absorb heat from the lamp and so avoid changing the temperature of the water as well as the light intensity