Core Practical: Investigate the effect of light intensity on the rate of…

The idea

This core practical asks you to measure how changing light intensity alters the rate of photosynthesis in pondweed. You place a piece of submerged pondweed in a test tube of water with a light source at a set distance and count the oxygen bubbles released per minute, or collect the gas volume, as a measure of rate. You then move the lamp to a series of measured distances and repeat, keeping temperature and carbon dioxide supply constant so only light intensity changes. A control variable is anything you hold fixed; the independent variable is distance or light intensity; the dependent variable is bubble count or gas volume per unit time. Repeat readings and calculate a mean to improve reliability, and identify anomalies.

What earns the marks

• The independent variable is light intensity, changed by altering the distance between the lamp and the pondweed.
• The dependent variable is the rate of photosynthesis, measured by counting oxygen bubbles released per minute or collecting gas volume over time.
• Temperature and carbon dioxide concentration must be controlled, for example by using a heat shield or sodium hydrogencarbonate solution, so only light intensity varies.
• Repeats should be taken at each distance and a mean calculated, with anomalous results identified and excluded.
• Bubble counting is an indirect measure of oxygen production and therefore of photosynthetic rate.

Common mistakes and how to put them right

• Thinking the dependent variable is the number of bubbles rather than the rate (bubbles per unit time).

  Put it right by dividing the count by the time taken.

• Changing temperature or carbon dioxide supply between distances.

  Put it right by holding these constant so light intensity is the only variable altered.

• Assuming a single reading at each distance is reliable.

  Put it right by repeating and averaging to reduce random error.

Exam tips

• State clearly which variable you change, which you measure and which you keep constant before describing the method.
• Express rate as a quantity per unit time, for example bubbles per minute, not just a raw count.
• Explain why a heat shield or buffer solution is used, linking it to controlling a named variable.