Investigating the Rate of Photosynthesis (WJEC GCSE Biology)
Revision Note
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 Tips and Tricks
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
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