Bubble Counting with Elodea
Four groups of students investigate how light intensity affects photosynthesis in Elodea. Each group places a sprig 10, 20, 30 and 40 cm from a 60 W incandescent lamp and counts bubbles emerging from the cut stem for one minute immediately after moving the lamp. Group results (bubbles per minute): 10 cm - 44, 38, 41; 20 cm - 27, 24, 29; 30 cm - 11, 15, 9; 40 cm - 2, 0, 1. A thermometer in the beaker reads 22 °C at 40 cm and 31 °C at 10 cm. The group at 10 cm also records that bubbles started slowly, then sped up after about 40 seconds. The class concludes: 'Light intensity is directly proportional to the rate of photosynthesis.' Their teacher wants them to present the data as a rate per minute with a measure of spread, and to justify any change to the method before the investigation is written up.
1. Test the conclusion 'directly proportional' against the class means. Show the arithmetic.
Read the worked answer
Means: 10 cm = 41.0; 20 cm = 26.7; 30 cm = 11.7; 40 cm = 1.0 bubbles per minute.
Proportionality would mean rate ∝ intensity, and for a lamp intensity falls as the inverse square of distance, I ∝ 1/d².
- 30 → 40 cm: intensity falls by 40²/30² = 1.78 times. The rate falls from 11.7 to 1.0, about 12-fold. Far steeper than proportional.
- 10 → 20 cm: intensity falls 4-fold. The rate falls only about 1.5-fold (41.0 → 26.7). Far shallower than proportional.
- 20 → 30 cm: intensity falls 2.25-fold, rate falls 2.3-fold. This pair alone looks proportional, which is why one pair is not enough.
So the data are not proportional overall; they are closer to a saturating curve at high light (limited by another factor) and near the detection limit at low light.
Two further reasons to reject the claim:
- The 10 cm point is confounded - 31 °C versus 22 °C raises enzyme-controlled rates and lowers oxygen solubility, so the high value is not a clean light effect.
- Counts of 0–2 per minute are at the resolution limit of the method; proportionality cannot be tested where the instrument is nearly blind.
Defensible conclusion: within this range, increasing light intensity increases the rate of oxygen release, but not in direct proportion.
2. Identify the main threats to validity and reliability, and justify the changes you would make before the write-up.
Read the worked answer
Validity (are we measuring what we claim?)
- Temperature co-varies with distance. Moving the lamp closer heats the beaker (22 °C → 31 °C). Temperature affects both photosynthetic enzymes and oxygen solubility, so distance is confounded. Fix: a heat shield or water bath between lamp and beaker, with temperature monitored at every distance and readings accepted only within about ±1 °C. A LED lamp removes most of the heat load.
- Steady state not reached. Counting began immediately, yet the group noted bubbles accelerating after about 40 s. Early counts are therefore systematically low, and the lag may vary between trials. Fix: allow at least two minutes to equilibrate after each move and discard that period.
- Proxy measurement. Bubbles vary in size and may include other gases; the rate of gas release is net oxygen, not gross photosynthesis. Fix: state the assumption explicitly, or collect gas in an inverted capillary tube and measure the length of the column, which is far less subjective.
Reliability (would we get the same answer?)
- Three counts at each distance are repeats within one group, and order is not randomised - the sequence always runs from near to far, so warming and plant fatigue track the independent variable. Fix: randomise the order of distances, repeat with a second sprig, and report mean ± range.
Presentation: calculate rate per minute for each repeat, plot mean against measured light intensity (or 1/d²), not against distance, add error bars, and flag the 10 cm point as temperature-affected.
3. A student writes: 'At 40 cm there is no photosynthesis.' Evaluate this statement and describe a test that would settle it.
Read the worked answer
The statement over-reads the data. Zero or near-zero bubbles per minute means the net release of oxygen is below the detection limit of bubble counting, not that the plant has stopped photosynthesising.
Bubble counting measures net gas exchange: gross oxygen production minus oxygen consumed by respiration. At low light, photosynthesis may still be occurring at a rate close to or barely above respiration, so net release is nil even though the light reactions are running. The plant may also be photorespiring or the cut stem may be healing over, reducing gas escape.
There is also a resolution problem: with single-bubble counting you cannot distinguish 0 from 1 bubble per minute, so the method cannot tell 'very slow' from 'stopped'.
A test that would settle it:
- Increase sensitivity - use more Elodea biomass or a capillary tube/volumeter to measure gas volume rather than counting bubbles.
- Find the light compensation point properly: measure at several close distances around 40 cm with a light meter, so the intensity is quantified rather than inferred.
- Compare with a dark control; if the plant releases more gas in light than in darkness at the same temperature, photosynthesis is still occurring.
The exam-answer habit to build: distinguish no detectable net effect from no process, and name the measurement limitation that creates the ambiguity.