Designing a Fair Respiration Trial with Limited Equipment
A teacher has 30 students, 6 respirometers, and one 45-minute period. Students want to test whether germinating peas respire faster than dry (dormant) peas by measuring oxygen consumption. Each respirometer holds a fixed volume and uses a KOH packet to absorb CO2, so any gas-volume change is attributable to O2 uptake. The teacher can run two trials per setup. Room temperature varies from 21 to 25 °C during the period, and one lab group's thermometer reads 2 °C higher than the others. Students propose measuring the fluid movement after 10 minutes and comparing germinating vs. dry peas directly.
1. Identify the variables that must be controlled and explain why room-temperature variation threatens the comparison.
Read the worked answer
Independent variable: state of the peas (germinating vs. dry). Dependent variable: O2 consumed (fluid displacement per unit time). Controlled variables: pea mass or number, respirometer volume, KOH quantity, time interval, temperature, and equilibration time before reading. Temperature matters because gas volume expands with heat (Charles's law) and because respiration rate is temperature-sensitive; a 4 °C drift across the period could produce fluid movement unrelated to O2 uptake. The faulty thermometer means one group's recorded temperature is unreliable, so temperature cannot be treated as a controlled constant for that group. Solution: equilibrate all respirometers in a single water bath or the same location, record temperature at start and end, and treat temperature as a measured covariate rather than assuming it is constant.
2. The teacher can only run two trials. Propose an efficient design that still supports a defensible conclusion, and state a limitation.
Read the worked answer
Use paired or matched comparisons: each trial includes both germinating and dry peas of equal mass, with replicates across the 6 respirometers (e.g., 3 germinating, 3 dry per trial), yielding 6 measurements per condition across two trials. Randomly assign respirometers to conditions to avoid position bias. Compute means and a simple measure of variability (range or SD) rather than comparing single readings. Limitation: with only two time points/trials, statistical power is low and confounds such as residual temperature differences cannot be fully separated; conclusions should be stated cautiously ("consistent with higher respiration in germinating peas") and not generalized beyond the conditions tested. This is a design-and-inference judgment, not a memorized fact.