Required Practical Reference¶
Practical work turns biological explanations into observations and measurements. This page brings the ten course investigations together so you can compare their methods, control choices and results. Follow each topic link for the science behind the investigation.
Investigation Map¶
| Investigation | Main teaching page | What to observe or measure | Important control choices |
|---|---|---|---|
| Microscopy | Cell Structure | Focus plant and animal cells, make labelled biological drawings and relate image size to actual size. | Prepare a thin specimen and use a clear magnification or scale indication. |
| Antimicrobial substances | Cell Structure | Compare clear zones around antimicrobial discs on a bacterial agar plate. | Use the same bacterial culture, disc size and incubation conditions, with an appropriate control disc. |
| Osmosis | Transport in Cells | Compare percentage changes in plant-tissue mass after immersion in different salt or sugar concentrations. | Keep tissue dimensions, solution volume, temperature and immersion time consistent; blot before weighing. |
| Food tests | Animal Tissues, Organs and Organ Systems | Use the colour changes in reducing-sugar, starch, protein and lipid tests to identify food molecules. | Use comparable sample volumes and known positive and negative controls. |
| Amylase and pH | Animal Tissues, Organs and Organ Systems | Sample an amylase–starch mixture into iodine at regular intervals and record when starch is no longer detected. | Use buffers to change pH while keeping temperature, enzyme concentration and starch concentration constant. |
| Photosynthesis and light | Photosynthesis | Compare oxygen output per unit time as the distance between a lamp and pondweed changes. | Keep temperature, carbon dioxide supply, measurement time and the plant sample consistent. |
| Reaction time | The Human Nervous System | Investigate one factor using repeated reaction-time measurements, such as the distance a ruler falls before being caught. | Keep the release position and response method consistent; avoid cues that let the participant anticipate release. |
| Seedling responses | Plant Hormones | Record changes in seedling length and direction of growth under different light or gravity conditions. | Use comparable seedlings and keep water, temperature and growth time consistent. |
| Field sampling | Organisation of an Ecosystem | Estimate the abundance of a common species and investigate how its distribution changes with an environmental factor. | Use consistent quadrat size and counting rules; choose random samples or a transect according to the question. |
| Temperature and decay | Organisation of an Ecosystem | Compare how quickly the pH of fresh milk changes at different temperatures. | Keep the milk source, volume, starting conditions and measurement intervals consistent. |
Recurring Method Patterns¶
State the independent variable you will change and the dependent variable you will measure. Control other factors that could affect that measurement. A control sample can show whether the treatment itself caused a result; a control variable is a condition kept constant. These two uses of “control” have different jobs.
Repeat measurements to judge their spread and calculate a mean where appropriate. An unexpected result should prompt a check of the method and recorded data, rather than automatic deletion. Repeats cannot remove a systematic error such as an incorrectly calibrated balance.
Some investigations give a qualitative result, such as a colour change. Others give a numerical measurement, such as mass or distance. Include units in tables, choose a graph suited to the data and explain any trend using the underlying biology. A line or curve can show a relationship between continuous variables; separate categories usually call for a bar chart.
Interpreting Results¶
For osmosis, calculate percentage mass change as the change in mass divided by the initial mass, multiplied by 100. This allows samples with slightly different starting masses to be compared. A concentration giving approximately zero mass change marks the point where there is no overall movement of water into or out of the tissue.
In an enzyme investigation, a shorter time to the same endpoint indicates a faster reaction. The reciprocal of that time can provide a relative rate. For pondweed, bubble counts are a useful estimate of oxygen output, but unequal bubble sizes limit the comparison; collected gas volume is a more direct measurement.
Field sampling estimates a population from a sample. More representative samples improve an estimate, while placing every quadrat in a convenient patch can introduce bias. A correlation between abundance and an environmental measurement does not, by itself, establish the cause of the distribution.
Practical Habits¶
Use the correct apparatus and follow the supervised method for handling biological material and reagents. In microscopy, begin with a low-power objective to locate the specimen before increasing magnification. In microbiology, aseptic technique reduces contamination and safe incubation conditions limit the growth of harmful organisms.
Evaluate the evidence by considering measurement precision, uncontrolled variables, sample size and the consistency of repeated results. An improvement should address a specific weakness in the method and explain how that change would improve the evidence.