Implementing¶
Part of Module 1: Development of practical skills in biology.
Implementation turns a design into data. It means using apparatus correctly, measuring in the right units and recording as you go, and a well-designed practical can still fail if it is carried out badly.
What You Need to Learn¶
Further detail: AS Biology A (H020) and A Level Biology A (H420).
How to use apparatus correctly and safely, how to make and record accurate quantitative and qualitative observations with suitable units and precision, and how to present raw data and drawings in a scientific format.
Core Idea¶
Practical work should generate observations that can be used. Careful handling, sensible measurement and clear recording all matter.
Quantitative observations are numbers with units: time, length, volume, temperature, mass and pH. A unit error makes later calculations meaningless. Qualitative observations are descriptions, such as the colour change in a Benedict's test, the appearance of stained cells or the presence of colonies on agar, and they can be biologically significant before any number is calculated.
Recording happens as the data are collected, never reconstructed later from memory.
What Good Implementation Looks Like¶
Apparatus is used as intended: a microscope is focused correctly, a meniscus is read at eye level, a colorimeter is used consistently, and a potometer is assembled without leaks.
Raw data go into a table with headings and units. The precision is sensible for the measurement, so if small changes are expected the instrument must be able to detect them. Observation is kept apart from inference, so what was seen is separate from what it is thought to mean.
Worked example: recording raw data well
A student times how long a colour takes to disappear when amylase digests starch at different temperatures, using a stopwatch reading to 0.1 s.
| Temperature / °C | Time for colour to disappear / s | Mean time / s | ||
|---|---|---|---|---|
| Run 1 | Run 2 | Run 3 | ||
| 20 | 142.3 | 139.8 | 141.1 | 141.1 |
| 30 | 71.4 | 70.2 | 72.9 | 71.5 |
| 40 | 38.5 | 39.1 | 37.9 | 38.5 |
The table has a heading and unit for every column. The raw readings are kept, and the mean is shown separately, so the original measurements can be checked. Every time is given to the same number of decimal places, matching the stopwatch. A table that showed only the means would hide whether the repeats agreed.
Applied Contexts¶
- In 2.1.2 Biological molecules, implementation includes carrying out biochemical tests accurately and recording colour changes clearly.
- In 2.1.5 Biological membranes, it includes measuring timing, solution conditions and visible changes consistently.
- In 3.1.1 Exchange surfaces, it includes histology observations and spirometer-style data handling.
- In 4.2.1 Biodiversity, it includes consistent use of quadrats, pooters, sweep nets and pitfall traps.
Apparatus-Specific Implementation¶
| Technique | What makes it work | What spoils it |
|---|---|---|
| Microscopy | Clean slide preparation, a suitable stain, recalibration whenever magnification changes | Starting on high power, an unclean slide, forgetting to recalibrate |
| Colorimetry | A correct blank, a suitable filter, a clean cuvette, the same cuvette orientation each time | A wrong blank, bubbles in the light path, fingerprints on the cuvette |
| Potometer | Cutting and assembling underwater, airtight seals, letting the shoot acclimatise before timing | Air in the xylem, leaks, timing straight away |
| Microbiology | Aseptic technique: sterile tools, containers opened as little as possible, plates inverted for incubation to reduce contamination from condensation | Open plates, flaming forgotten, plates left upright |
| Dissection and fieldwork | Labelled sketches, correctly headed tables and explicit units, recorded as part of the work | Notes written up later, missing units |
Exam technique
For a method question about technique, give the action and its reason: "invert the plates, so that condensation does not drip onto the agar and contaminate it". The reason is what shows understanding.
Common Weaknesses¶
- Missing units, or inconsistent units across a data table.
- Recording only processed values, such as means, and not the original measurements.
- Mixing observation with conclusion. Writing "enzyme denatured" when only a change in rate or colour was observed claims more than the data show.
- Drawings or tables that omit labels, scale or headings.
Common Confusions¶
- Quantitative and qualitative: a measured length is quantitative, and "the solution turned blue-black" is qualitative. Both are observations.
- Accuracy and precision: accuracy is closeness to the true value, and precision is the fineness of the measurement or the closeness of repeats to each other.
- Raw and processed data: raw data are the original readings, and processed data are what is calculated from them, such as a mean or a rate.
Check Yourself¶
- Distinguish between quantitative and qualitative observations, and give an example of each from a food test.
- State two features of a well-constructed table of raw data.
- Explain why agar plates are incubated upside down.
- A student records "the enzyme was denatured" in the results column after seeing no colour change. Explain what is wrong with this.
- A student gives a stopwatch time as 142 s in one row and 71.4 s in another. Explain why this is a problem.
- Explain why a colorimeter is zeroed with a blank before taking readings.
Answers
- Quantitative observations are numbers with units, such as the time for a colour to disappear. Qualitative observations are descriptions, such as the solution turning from blue to brick-red in a Benedict's test.
- Any two of: headings with units for every column; raw readings recorded and not only means; a consistent number of decimal places; repeats shown in separate columns.
- Condensation forms on the lid as the plate warms. Inverted plates stop the droplets falling onto the agar and carrying in contamination.
- "Denatured" is an interpretation. The result column should record only what was observed, such as the time taken or the colour, and the inference belongs in the conclusion.
- The values are recorded to different precision, although the same stopwatch was used, so the precision looks inconsistent and data could be misread or wrongly processed.
- The blank sets the reading for the solvent and cuvette alone to zero, so later readings measure only the absorbance of the substance of interest.
Key Terms¶
- Raw data: the original measurements or observations recorded during the practical.
- Quantitative observation: an observation recorded as a number, often with a unit.
- Qualitative observation: a descriptive observation such as colour, appearance or presence of growth.
- Unit: the standard quantity used to express a measurement, such as seconds, centimetres or degrees Celsius.
- Precision: the level of detail or fineness of measurement an instrument can provide.
- Scientific drawing: a clear biological drawing that uses labels and scale carefully enough to communicate what was observed.