Specified Practical: Investigating Factors Affecting Photosynthesis
- The most common way to investigate the rate of photosynthesis is to study the effect of an environmental factor on oxygen release in an aquatic plant
- Aquatic plants are very useful for this as the oxygen gas forms bubbles as it is released from the plant; the bubbles can then either be counted or collected in a gas syringe
Investigating the effect of light intensity on the rate of photosynthesis
Equipment
- Large beaker (250 cm3)
- Funnel
- Boiling tube
- Sodium hydrogen carbonate
- Aquatic plant, e.g. Elodea (approx 8 cm length)
- Lamp/light source
- Metre ruler
- Water tank
- Stopwatch
Investigating the effect of light intensity on photosynthesis diagram
The effect of light intensity on the rate of photosynthesis can be investigated by measuring oxygen production in aquatic plants
Method
- Place a length of Elodea pondweed into a beaker of water
- Add sodium hydrogen carbonate to the water
- This allows the concentration of dissolved carbon dioxide to be controlled
- Place a glass funnel over the top of the Elodea
- Fill a test tube with water and quickly invert over the end of the funnel; the end of the funnel must be under the water after this step
- This means that air bubbles can be clearly seen and counted
- Use the ruler to place the lamp a set distance from the plant, e.g. 5 cm
- Place a glass tank filled with water between the lamp and the plant
- This prevents heat from the lamp from warming up the water in the beaker; this could affect the results
- Record the number of bubbles produced over 1 minute
- Repeat step 6 twice more
- Repeat steps 6-7 with the lamp at different distances from the plant, e.g. 10, 15, 20, 25 and 30 cm
Risk assessment
- A hazard in an experiment is something that could potentially harm you
- A risk is the harm that could occur as a result of the risk
- A control measure is the action taken to reduce the chance of the hazard causing a problem
Risk assessment for photosynthesis practical table
| Hazard | Risk | Control measure |
| Light bulb can become hot over time | Contact between skin and bulb could result in burns |
Avoid touching the bulb when moving the lamp Leave the bulb to cool down before touching it |
Variables
- Independent variable: light intensity
- Dependent variable: number of bubbles
- Control variables:
- Temperature
- Carbon dioxide concentration
- Other light sources, e.g. the classroom lights should be turned off and window blinds closed
- Size / type of pond weed
Results
- A graph of the distance from the lamp against the number of bubbles produced per minute can be drawn
- We would expect to see a decrease in bubble production as distance from the lamp increases; this is due to a decrease in light energy available at lower light intensities
Distance from lamp and bubbles produced per minute graph
A graph of distance from the lamp against number of bubbles per minute shows a decrease in bubble production as distance from the lamp increases
Examiner Tip
You are expected to be able to evaluate practical methods such as this, so make sure that you are aware of the following:
- Which variables you need to control and how, e.g. the use of sodium hydrogen carbonate and a water tank
- How to ensure that your results are reliable, e.g. by carrying out repeats at each light intensity
- How to improve on the suggested method, e.g. by measuring the volume of gas in a gas syringe rather than by counting bubbles to improve accuracy
- Hazards, risks, and corresponding control measures
