Rate of photosynthesis: measure and calculate rates of photosynthesis

The idea

A common method uses pondweed in water with a light source at a set distance. As the pondweed photosynthesises, oxygen bubbles are released; you count the bubbles in a fixed time, or collect the gas in a syringe and read the volume. The rate is then the change divided by the time, for example 30 bubbles ÷ 2 minutes = 15 bubbles per minute, or 4.8 cm³ ÷ 4 minutes = 1.2 cm³ per minute. To make the test valid, keep temperature and carbon dioxide concentration constant while changing light intensity, and allow the pondweed to settle before timing. Repeating readings and calculating a mean improves reliability. You should also be able to compare rates at different light intensities and explain the pattern using the idea that light is a limiting factor.

What earns the marks

• Select a suitable dependent variable, such as number of oxygen bubbles or volume of oxygen gas collected.
• Use the equation rate = change ÷ time and substitute measured values correctly.
• Give the rate with a correct compound unit, for example bubbles min⁻¹ or cm³ min⁻¹.
• Control key variables such as temperature, carbon dioxide concentration and the distance of the light source.
• Interpret a set of rate values to identify a trend, such as rate increasing with light intensity until another factor becomes limiting.

Common mistakes and how to put them right

• Forgetting to convert seconds to minutes when the rate is expected per minute.

  Put it right: Divide by 60 or time the experiment in minutes from the start.

• Using the distance from the lamp as if it were light intensity.

  Put it right: Light intensity is inversely proportional to the square of the distance, so halving the distance greatly increases intensity.

• Averaging rates without checking for anomalous results.

  Put it right: Identify and exclude anomalies before calculating a mean, and state why the anomaly is ignored.

Exam tips

• Show the substitution line in calculations so the examiner can see how your answer was obtained.
• Use the phrase 'rate of oxygen production' rather than 'amount of photosynthesis' when describing the dependent variable.
• When describing a graph, quote figures from the axes and state the trend before explaining it in terms of limiting factors.