Designing in Biology (DP IB Biology): Revision Note
Designing in Biology
This is where you turn your well-explored research question into a practical, step-by-step plan or methodology
Your goal is to design a valid procedure that allows you to collect sufficient, high-quality data to properly answer your research question
The design must be explained clearly, considering variables, measurements, and all safety aspects
Designing your methodology
Identify and justify the choice of variables
At the start of your design, you must clearly list and explain your choice of variables.
Independent variable (IV): The single variable you deliberately change to see its effect
Dependent variable (DV): The variable you measure to see how it is affected by the change in the IV
Controlled variables (CVs): All other factors that could plausibly affect the outcome. You must explain how you will keep these constant to ensure the investigation produces reliable and valid results
Justify the range and quantity of measurements
It is not enough to just state your measurements; you must justify them
Range of the independent variable:
You should plan to collect data across a suitable range
A minimum of five different values for the independent variable is recommended to establish a clear trend
Justify your choice
For an enzyme experiment, you might state: "A temperature range of 10°C to 50°C was chosen because temperatures below this result in a reaction that is too slow to measure, while temperatures above 50°C may cause the amylase to denature."
Quantity of measurements:
You must repeat the experiment for each value of the independent variable to ensure the results are reliable
A minimum of three trials (replicates) is recommended in biology, and five is even better due to the inherent variability of biological systems
Repeating trials allows you to calculate the mean average, which:
Reduces the effect of random error
Helps you to identify and discard any anomalous results
Design and explain a valid methodology
This is the detailed, step-by-step procedure of your experiment
It must be a logical sequence of instructions that is clear enough for another chemist to follow and replicate your experiment exactly
Include precise details of the apparatus used
For example, "measure 5 cm3 of the 1% starch solution using a 10 cm3 graduated pipette", rather than "add some solution"
Creativity in design can be shown in how you solve a measurement problem
For example, using a colorimeter to measure the disappearance of starch with iodine is more creative and objective than timing a colour change by eye
When writing your report, a good way to structure your methodology is:
Materials and apparatus:
A bulleted list of all chemicals (with concentrations) and equipment (with sizes and precision)
Safety, ethical, and environmental:
A brief risk assessment, identifying key hazards and outlining specific precautions and waste disposal methods
Procedure:
The numbered, step-by-step instructions
Develop investigations using different approaches
While hands-on laboratory work is most common, your investigation could be based on other sources of data:
Databases
Using an established biological database (e.g., GenBank (opens in a new tab) for DNA sequences or the IUCN Red List (opens in a new tab) for conservation status) to find data, which you then process and analyse to find a trend
Simulations
Using a simulation (e.g. from PhET (opens in a new tab)) to collect data for a process that would take too long or be too complex for a school lab, such as population dynamics or evolution
Surveys
Designing and using a questionnaire to gather data on human physiology or ecological awareness
Pilot methodologies
A pilot study is a small-scale trial run of your experiment
This is extremely useful to check that your planned methodology works
For example, a quick pilot test of an enzyme experiment can help you determine if the reaction is too fast or too slow
This allows you to adjust the enzyme or substrate concentrations before starting your main experiment
Worked Example
Designing an enzyme investigation
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?"
Identifying and justifying variables:
Independent variable:
The pH of the buffer solution in which the reaction occurs (ranging from pH 4 to pH 10)
Dependent variable:
The rate of reaction, determined by measuring the time taken for the enzyme to break down a protein (e.g., casein in milk), causing the solution to turn from cloudy to clear
The rate is calculated as 1/time
Control variables:
The temperature must be kept constant using a water bath
The concentrations of the enzyme and the substrate (milk) must be the same for all trials
The total volume of the solution must also be constant
Justifying the methodology:
A range of seven different pH buffers will be used (e.g., pH 4, 5, 6, 7, 8, 9, 10) to establish the optimal pH for the enzyme
This range was chosen because it covers conditions from acidic to alkaline and brackets the known optimal pH of trypsin
A water bath will be used to maintain a constant temperature of 37°C for all solutions
This is crucial because temperature also affects enzyme activity, and it must be kept constant to ensure that only pH affects the reaction rate
Worked Example
Designing an osmosis investigation
Research question:
"What is the effect of sucrose concentration (from 0.0 M to 1.0 M) on the percentage change in mass of potato (Solanum tuberosum) cylinders after 24 hours?"
Identifying and justifying variables:
Independent variable:
The concentration of the sucrose solution (ranging from 0.0 mol dm-3 to 1.0 mol dm-3)
Dependent variable:
The percentage change in mass of the potato cylinders after 24 hours
Percentage change is used instead of absolute change to account for slight differences in the initial mass of the cylinders
Control variables:
The species of potato must be the same
The cylinders must be cut to the same dimensions (length and diameter)
The volume of the sucrose solution must be constant
The temperature and time of immersion must be the same for all samples
Justifying the methodology:
Six different sucrose concentrations (0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M) will be used
This range includes pure water (0.0 M) and concentrations expected to be hypertonic, which should be sufficient to find the isotonic point
The potato cylinders will be patted dry with a paper towel before the initial and final mass are measured
This is to remove excess surface water that is not part of the potato tissue, which would otherwise cause a systematic error in the mass readings
Examiner Tips and Tricks
Detail is key
A vague method cannot be replicated and will not score well
Instead of "Cut up a potato," write:
Use a size 5 cork borer to cut three cylinders of potato, and use a scalpel and ruler to trim each cylinder to a length of 3.0 cm
Safety is not an afterthought
Your design must include a dedicated safety section
Be specific
Instead of "Handle chemicals with care," write:
Wear safety goggles and a lab coat at all times
Iodine solution is an irritant; handle with care
Dispose of all biological and chemical waste in the designated containers
Justify your choices
For top marks, you must explain why you chose a specific piece of apparatus, a certain range of values, or a particular organism
This shows the assessor you are thinking like a biologist
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