Evaluating in Biology (DP IB Biology): Revision Note

Research question:

• "What is the effect of pH (from pH 4 to pH 10) on the rate of activity of the enzyme trypsin in breaking down casein protein?"

Weakness 1 (systematic error)

• The reaction endpoint is judged by eye (when the mixture “looks clear”)

• Impact:

  • Some runs will be stopped earlier/later than others

  • This consistently biases times (and therefore rates), especially at intermediate pH where clearing is gradual

  • It reduces accuracy and can flatten the true pH–activity curve

• Realistic improvement:

  • Use a colorimeter with fixed wavelength (e.g., ~400–440 nm) and set an objective absorbance threshold (e.g., stop timing when A ≤ 0.05 above a water blank)

  • Use matched cuvettes, a casein blank for zeroing, and record absorbance every 2–3 s or use a data logger to detect the threshold automatically

Weakness 2 (random error)

• Variability in mixing the enzyme and substrate or pH drift (e.g. CO₂ absorption, buffer not fully equilibrated to 37 °C)

• Impact:

  • This can cause trial-to-trial scatter in times

  • This increases standard deviation, making differences between adjacent pH values less clear

• Realistic improvement:

  • Pre-equilibrate separate tubes of buffer, casein, and trypsin at 37 °C for ≥10 min; cap tubes to limit gas exchange

  • Mix reactions in a consistent way (e.g., invert exactly 3× or use a small sterile stir bar at fixed speed) and start the timer at the same moment (enzyme addition)

  • Measure and confirm pH at 37 °C with a calibrated pH meter (pH can shift with temperature)

  • Increase replicates from 3→5 at each pH and report mean ± SD

Limitation

• Trypsin activity can decline over the course of the practical (room-temperature handling, repeated warming/cooling, autolysis)

• Impact:

  • Trypsin activity can decline over the course of the practical (room-temperature handling, repeated warming/cooling, autolysis)

  • If pH conditions are always run in the same order (e.g., 4→10), later conditions may appear slower for reasons unrelated to pH, confounding the conclusion

• Realistic improvement:

  • Prepare single-use aliquots of trypsin; keep on ice until needed; discard leftovers

  • Randomise or counter-balance the order of pH runs (e.g., 7, 4, 9, 6, 10, 5, 8) so any activity drift is spread evenly

  • Limit total session time; if long, split into blocks with fresh enzyme each block

Research question:

• "What is the isotonic point of potato tuber tissue in a sucrose solution?"

Weakness 1 (systematic error):

• The potato cylinders were blotted dry with paper towels before the final weighing

• Impact:

  • This method is not consistent.

  • Some cylinders may have been blotted more thoroughly than others, and some surface water may have remained

  • This would lead to a final mass that is inaccurately high, especially for cylinders that were in hypotonic solutions

  • This reduces the accuracy of the determined isotonic point

• Realistic Improvement:

  • A more consistent method, such as a "salad spinner" protocol, could be used

  • All cylinders could be placed in a spinner for a set time (e.g., 10 seconds) to remove excess surface water in a highly repeatable way

Weakness 2 (random error):

• Natural biological variability between the potato cylinders

• Impact:

  • Despite efforts to control their size, the cylinders will have had slight differences in their cellular structure and initial water potential

  • This inherent variability is the most likely cause of the scatter of data points around the line of best fit, as shown by the standard deviation error bars

• Realistic Improvement:

  • The impact of this random error could be further minimised by increasing the number of replicates from three to five for each sucrose concentration

  • This would make the calculated mean percentage change in mass a more reliable estimate

Limitation:

• The experiment was conducted at a single room temperature

• Impact:

  • The conclusion is only valid for the temperature at which the experiment was conducted (e.g. 21°C)

  • Temperature affects the kinetic energy of water molecules and can alter the permeability of cell membranes, so the isotonic point may be different at other temperatures

• Realistic Improvement:

  • The investigation could be extended by repeating the entire experiment in temperature-controlled water baths set at different temperatures (e.g. 10°C, 20°C, 30°C) to determine how temperature affects the isotonic point

Be specific

• Never blame "human error"

• Instead of saying "My measurements were wrong," identify a specific source of error, like "The parallax error when reading the volume in the measuring cylinder could have led to inconsistent volumes of solution being used."

Prioritise your evaluation

• Focus on the one or two most significant sources of error that had the biggest impact on your final result

• Discussing the biological variability of your samples is often a key point in a biology evaluation

Close the loop: Weakness → Impact → Improvement

• For every weakness you identify, you must explain its impact on your final result and then suggest a specific improvement to fix it

Evaluate your own data.

• Do not write a generic evaluation that could apply to any experiment

• Refer back to your own results, graphs, and observations

• For example, "The large standard deviation at 40°C, as shown by the error bars, suggests that the reaction became unpredictable at this temperature, reducing the reliability of the calculated mean."