Pondweed, Lamp Distance and a Hidden Variable
A student investigates how light intensity affects the rate of photosynthesis. She places a shoot of pondweed in a test tube of 0.5% sodium hydrogencarbonate solution and positions a bench lamp 10 cm away. After one minute she counts the bubbles released in one minute, then moves the lamp to 20 cm, 30 cm and 40 cm, counting immediately at each new distance. The test tube is not moved and the same pondweed is used throughout. Her counts are 34, 31, 19 and 11 bubbles per minute. A thermometer in the tube reads 31 °C at 10 cm, 27 °C at 20 cm, 24 °C at 30 cm and 21 °C at 40 cm. She concludes that light intensity is the only factor controlling the rate of photosynthesis in this experiment.
1. Identify the main weakness in the method and explain precisely how it undermines the conclusion.
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The lamp is a heat source as well as a light source. Moving it closer raises both light intensity and temperature, and the thermometer shows this: 31 °C at 10 cm against 21 °C at 40 cm. Two variables change together, so the experiment cannot separate their effects.
Temperature affects the rate of the enzyme-controlled reactions of photosynthesis, so the high count at 10 cm may be partly a temperature effect, and the low count at 40 cm partly a cooling effect. The apparent 'light intensity' trend is therefore exaggerated and the conclusion that light is the only factor is unsupported.
There is a second, smaller flaw: counting immediately after each move gives no time for the pondweed to settle to a steady rate, and a one-minute bubble count is a small sample that is easily affected by rate fluctuation.
2. Describe two specific changes to the method that would make the conclusion valid, including the variables that must be held constant.
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- Remove the heating effect. Place a clear water-filled tank (a heat filter) between the lamp and the tube, or use a low-heat LED lamp. Check that the water temperature in the tube stays the same at every distance, and record it.
- Stand the tube in a water bath at a fixed temperature (for example 25 °C) so any remaining heat is absorbed, and allow 2–3 minutes at each distance before counting.
Variables to keep constant: volume and concentration of sodium hydrogencarbonate solution, same pondweed shoot and same length, same starting time of day, and the same counting period. Count for three minutes at each distance and take a mean to reduce random error. Only the distance of the lamp (the independent variable) should change.
3. Assuming temperature is controlled, predict the shape of the graph of bubble rate against distance and explain why the rate stops rising at very high light intensities.
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As distance increases, the rate should fall in a curve, not a straight line, because light intensity spreads out and decreases roughly with the square of the distance (inverse square law). Doubling the distance should reduce intensity to about one quarter, so the fall is steep at first and then flattens at large distances. Plotted as rate against 1/distance², the points should be close to a straight line through the origin at low intensities.
At very small distances (very high intensity) the curve should plateau. This is light saturation: the light-dependent stage can no longer be speeded up because another factor becomes limiting - usually carbon dioxide concentration (fixed here by the 0.5% hydrogencarbonate) or temperature, which limits the enzyme-controlled reactions of the light-independent stage. Adding more light then produces no extra bubbles per minute.