Investigating the Rate of Photosynthesis

The rate of photosynthesis can be investigated by measuring the rate of oxygen production by an aquatic plant such as Elodea or Cabomba. A simple method counts bubbles, but bubbles vary in size, so a photosynthometer (Audus apparatus) is more accurate.A cut shoot is placed upside down in a test tube of water containing sodium hydrogencarbonate solution, which supplies carbon dioxide. The tube stands in a water bath to keep the temperature constant. Oxygen collects in the flared end of a capillary tube. After a set time, the gas bubble is drawn along the capillary with a syringe and its length is measured against a scale.The independent variable might be light intensity (lamp distance), carbon dioxide concentration (concentration of hydrogencarbonate) or temperature (water bath). Other variables are kept constant, and the plant is left to acclimatise for a few minutes at each new condition. A heat shield or LED lamp stops the lamp from warming the water. Repeat readings at each value and calculate a mean.The volume of gas is the volume of a cylinder:volume = πr²lWorked example: the capillary radius is 0.5 mm and the bubble is 12 mm long after 5 minutes. Volume = π × 0.5² × 12 = 9.42 mm³. Rate = 9.42 ÷ 5 = 1.88 mm³ min⁻¹.The gas collected is not pure oxygen, and some oxygen is used in respiration, so this measures the net rate of photosynthesis.