Investigating the Rate of Photosynthesis

The rate of photosynthesis can be measured by the rate at which a product is made (oxygen) or a reactant is used up (carbon dioxide). A common method uses an aquatic plant such as pondweed (Elodea or Cabomba), because the oxygen it releases can be collected.In a photosynthometer, a cut piece of pondweed is placed in water containing sodium hydrogencarbonate (a source of carbon dioxide). The gas released collects in a capillary tube. After a set time, the gas bubble is drawn along the tube with a syringe and its length is measured against a scale. The volume of gas = πr² × length. Counting bubbles is quicker but less accurate, because bubbles vary in size.To investigate one factor, change only that independent variable and keep the others constant: the same length of the same plant, the same sodium hydrogencarbonate concentration, and a water bath to keep the temperature constant when a lamp is close. Let the plant acclimatise to each new condition before measuring, and repeat each reading.Worked example: In 5 minutes, a pondweed produces a gas bubble 20 mm long in a capillary tube of radius 0.5 mm. Volume = 3.14 × 0.5² × 20 = 15.7 mm³. Rate = 15.7 / 5 = 3.1 mm³ min⁻¹.The measured rate is lower than the true rate of photosynthesis, because some oxygen is used by the plant in respiration and some dissolves in the water.