Planning¶

Part of Module 1: Development of practical skills in biology.

A good plan matches a biological question to a method that could genuinely answer it. It shows an understanding of the biology being tested and of the practical limits on measuring it, and it is specific enough that someone else could follow it and collect usable data.

What You Need to Learn¶

Further detail: AS Biology A (H020) and A Level Biology A (H420).

How to turn a question into a plan: choosing and justifying the variables, the apparatus and the measurements, recognising what a method can and cannot show, and planning sampling and repeat readings so that the data are valid and reliable.

Core Idea¶

A plan starts with a clear question and a clear dependent variable: the thing measured, such as enzyme activity, transpiration rate, a biodiversity count, membrane permeability or a length under the microscope. The independent variable is the factor deliberately changed. Controlled variables are every other factor that could change the result, and each one has to be kept constant or managed.

Apparatus is chosen to suit the scale and precision of the measurement. A light microscope suits whole cells and tissues, and a colorimeter suits a change in absorbance. A method is valid only if it measures what the question is about. If it gives only an indirect estimate, the plan should say so from the start.

Worked example: a complete plan for an enzyme investigation

Question: how does temperature affect the rate at which catalase breaks down hydrogen peroxide?

Item	Decision	Reason
Independent variable	Temperature: 10, 20, 30, 40 and 50 °C, set in water baths	A range either side of the expected optimum shows the shape of the curve
Dependent variable	Volume of oxygen collected in 60 s in a gas syringe, giving a rate in cm³ s⁻¹	Oxygen is a product, so its volume tracks the rate directly
Controlled variables	pH (buffer), volume and concentration of enzyme and of substrate, and a 5-minute wait for each tube to reach the target temperature	Each one changes enzyme shape or collision frequency if it varies
Repeats	Three runs at every temperature, and a mean taken	Allows an anomalous result to be spotted and reduces the effect of random error
Safety	Hydrogen peroxide is an irritant, so wear eye protection	Part of a plan that can be carried out

The plan also predicts a pattern: the rate rises with temperature, then falls above the optimum as the enzyme denatures. Predicting the outcome, with a reason, shows the biology behind the method.

What Good Planning Looks Like¶

The method follows a logical sequence: set up, vary one factor, control the others, and record the result in a repeatable way.

The measurements match the biology. An enzyme investigation needs a valid rate measure, a transpiration investigation needs a way to track water movement, and biodiversity work needs a sampling method that fits the habitat.

Each control is justified biologically, not just listed. Temperature is controlled in enzyme work because enzyme shape and collision frequency change with it. Light intensity is controlled in plant investigations because it affects photosynthesis and stomatal behaviour.

The expected outcome is plausible and explained, even if the exact numbers are unknown.

Exam technique

In a planning question, give each variable together with how it is changed or measured and why. "Control the temperature" is incomplete, and "keep the tubes in a water bath at 30 °C, because temperature changes enzyme activity" is a full point. Always state how many repeats and what will be calculated from them.

Common Planning Weaknesses¶

Naming a variable without saying how it is measured. "Measure the rate" needs the measurement: volume of gas per minute, or time for a colour to disappear.
Apparatus that is too imprecise. A beaker cannot measure a 1 cm³ change in volume.
Controls that do not fit the topic. The controls in microscopy are different from those in field sampling.
A method that produces observations but not usable data. "Watch what happens" gives no values to process.

Applied Examples¶

In 2.1.1 Cell structure, planning includes choosing the right microscope and deciding how magnification will be measured.
In 2.1.4 Enzymes, it includes selecting a sensible dependent variable for enzyme rate and deciding how pH or temperature will be controlled.
In 3.1.3 Transport in plants, it includes deciding how a potometer reading will stand in for transpiration.
In 4.2.1 Biodiversity, it includes choosing between random, systematic, stratified and opportunistic sampling.

PAG-Linked Planning Patterns¶

Microscopy: decide on slide preparation, the stain, and how the structures will be measured once seen. "Look at cells" is not a plan.
Sampling: the strategy has to be justified. Random sampling reduces selection bias. Stratified sampling protects coverage across visibly different sub-habitats. A systematic transect suits a question about change along a gradient.
Enzyme rate: define rate, fix the pH, use equal concentrations, and allow the tubes to reach the target temperature before the reaction starts.
Potometer: state that bubble movement estimates water uptake and does not measure transpiration directly.
Response investigations: heart-rate and tropism work need repeated measurements over time, a defined baseline and a clear control condition.

Common Confusions¶

Independent and dependent variable: the independent variable is what you change, and the dependent variable is what you measure. The dependent variable depends on the other.
Control group and controlled variable: a control group is a comparison treatment with the independent variable left out or unchanged. A controlled variable is any factor kept the same across all treatments.
Valid and reliable: a valid method measures what the question asks. Reliable results are those that repeat closely. A method can be reliable and still invalid.

Check Yourself¶

Define independent variable, dependent variable and controlled variable.
A student plans to investigate the effect of light intensity on the rate of photosynthesis in pondweed by counting bubbles. State the dependent variable and two variables that must be controlled, with a reason for each.
Explain why a plan should include repeats at each value of the independent variable.
A student's plan says "measure the plant's water loss" using a potometer. Explain why this wording is not valid.
Suggest a sampling strategy to investigate how the number of daisies changes from a path into a lawn, and justify it.
Explain the difference between a valid method and a reliable set of results.

Answers

The independent variable is the factor deliberately changed. The dependent variable is the factor measured to see the effect. A controlled variable is any other factor kept constant.
The dependent variable is the number of bubbles per minute (the rate of oxygen release). Any two of: temperature, because it changes enzyme activity; carbon dioxide concentration, because it is a raw material; the pondweed's size or mass, because it sets the amount of photosynthetic tissue.
Repeats show whether results are consistent, allow anomalous results to be identified, and let a mean be calculated, which reduces the effect of random error.
A potometer measures the uptake of water, not the loss. Some of the water taken up is used in the plant, so uptake is only an estimate of transpiration.
A systematic transect with quadrats at regular intervals from the path into the lawn. The question is about change along a gradient, and a transect samples that gradient evenly.
A valid method actually measures what the question asks. Reliable results are consistent when repeated. A method can give consistent results and still not answer the question.

Key Terms¶

Biological variable: a factor in a biological system that can change and be measured or controlled in an investigation.
Independent variable: the factor that is deliberately changed by the investigator.
Dependent variable: the measured outcome used to judge the effect of the independent variable.
Controlled variable: a factor kept the same so it does not distort the result.
Apparatus choice: selection of equipment that matches the scale, precision and type of measurement required.
Validity: the extent to which the method actually tests the biological question being asked.