Investigating the rate of photosynthesis
Biology · Key Stage 3 · The School
The basic method
Place a piece of pondweed in water, near a lamp. Count the gas bubbles it releases in a fixed time, such as one minute. Oxygen is a product of photosynthesis. So more bubbles per minute suggests a faster rate of photosynthesis under those conditions.
Controlling the variables
To test LIGHT INTENSITY alone, keep everything else the same. Use the same piece of pondweed, or an equal one. Keep the water temperature the same. Keep the carbon dioxide level the same. Change light intensity only by moving the lamp, to set, measured distances.
Why distance and light intensity need care
Light intensity falls with the SQUARE of the distance, not in a straight line. Double the distance from the lamp to the pondweed, and the light intensity drops to a quarter, not to a half. Record the exact distances used. Keep this pattern in mind when you read the results afterwards.
A strong write-up does more than report the bubble counts. It names what the method cannot control for. Bubbles come in different sizes, so counting them treats a big bubble and a tiny one as equal, even though they hold different amounts of gas. Some bubbles merge before they can be counted. Some escape from view unseen. A more accurate version of this test collects the gas in a tube and measures its actual VOLUME over time, instead of counting bubbles. That trades a fast, simple method for a slower, more exact one. Naming that trade-off, and saying why the simpler method was still a fair choice for a classroom test, is exactly what a strong evaluation does.