Practical - effect of light intensity on photosynthesis in pondweed.
Greg Foot explains the effect of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis
Investigating photosynthesis
The effect of light intensity on photosynthesis can be investigated in water plants. Use Cabomba or Elodea, which are sold in aquarium shops.
The plants will release bubbles of oxygen - a product of photosynthesis - which can be counted.
A lamp with an LED bulb is set up at different distances from the plant in a beaker of water:
- an LED bulb is best as this will not raise the temperature of the water
- sodium hydrogencarbonate - formula NaHCO3 - is added to the water to supply carbon dioxide - a reactant in photosynthesis - to the plant
- the light intensity is related to distance - it will decrease as the distance away from the bulb increases - so light intensity for the investigation can be varied by changing the distance from the lamp to the plant.
The bubbles produced over one minute periods are recorded.
Aim
To investigate the effect of light intensity on the rate of photosynthesis.
Method
- Set up a boiling tube containing 45 cm³ of sodium hydrogencarbonate solution (1%). Allow the tube to stand for a few minutes and shake to disperse any air bubbles that might form.
- Cut a piece of the pondweed, Cabomba. The pondweed should be 8 cm long.
- Use forceps to place the pondweed in the boiling tube carefully. The pondweed should be cut end uppermost. Make sure that you don't damage the pondweed, or cause the liquid to overflow.
- Position the boiling tube so that the pondweed is 10 cm away from the light source. Allow the boiling tube to stand for five minutes. Count the number of bubbles emerging from the cut end of the stems in one minute. Repeat the count five times and record your results.
- Calculate the average number of bubbles produced per minute. Repeat the experiment at different distances away from the light source.
Variables
- Independent variable - distance from the light source/light intensity.
- Dependent variable - the number of bubbles produced per minute.
- Control variables - concentration of sodium hydrogencarbonate solution, temperature, using the same piece of Cabomba pondweed each time.
Risks
Care must be taken when using water near electrical equipment. Ensure that your hands are dry when handling the lamp.
Extension activities
The volume of oxygen produced could be measured by collecting the gas produced in a gas syringe.
The changes in the oxygen concentration in the water could be measured using datalogging equipment.
You could investigate the effect of different The length of a single wave, measured from one wave peak to the next. of light on photosynthesis.
Use coloured acetate filters to investigate the effects of the blue, green and red parts of the The different types of electromagnetic radiation, arranged in order of frequency or wavelength. on photosynthesis.