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Flashcards · H420 · Topic 1

Development of practical skills in biology flashcards

23 cards on Topic 1, subtopics 1.1 to 1.2. Pick a subtopic or study them all.

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1.1 Practical skills assessed in a written examination

Control variable vs control experiment
A control variable is kept the same so it does not affect the result. A control experiment is a parallel set-up lacking the factor under test, such as boiled enzyme or water in place of the solution.
What should a plan for an investigation include?
The aim; the independent variable and its range; the dependent variable with how and in what unit it is measured; apparatus suited to the quantity; each control variable and how it is held constant; repeats; how results will be processed.
Precision vs accuracy
Precision: how close repeat readings are to each other (a small range). Accuracy: how close a reading is to the true value.
Uncertainty and percentage uncertainty
Uncertainty is the margin of error in a reading, commonly half the smallest division. Percentage uncertainty = uncertainty ÷ value × 100.
How do you combine uncertainties?
For a difference of two readings, add the absolute uncertainties. For a quotient such as a rate, add the percentage uncertainties.
An 18 mm length is read with a ruler to ±0.5 mm at each end. What is the percentage uncertainty?
Total uncertainty = 0.5 + 0.5 = ±1.0 mm. Percentage uncertainty = 1.0 ÷ 18 × 100 = 5.6 %.
Percentage error
(measured − accepted) ÷ accepted × 100. It shows how far a result is from an accepted value; it is not the same as uncertainty.
How do you find a rate from a curved graph?
Draw a tangent at the point and find its gradient: (y₂ − y₁) ÷ (x₂ − x₁), using two widely spaced points on the tangent. The initial rate is the gradient of the tangent at time zero.
Graph-drawing checklist
Independent variable on the x-axis; both axes labelled with quantity and unit; scale using more than half the grid in equal steps; points plotted accurately; a line or smooth curve of best fit, with points not joined.
Repeat times to an end point: 22, 24, 23, 35, 23 s. Find the mean and the rate.
Exclude the anomaly (35 s) and say so. Mean = (22 + 24 + 23 + 23) ÷ 4 = 23.0 s. Rate = 1 ÷ 23.0 = 0.0435 s⁻¹.
Which chart for which data?
Categories: bar chart. Continuous data in classes: histogram. Two variables, looking for a relationship: scattergram with line of best fit. Continuous independent variable: line graph.
Rule for significant figures in a calculated answer
The answer cannot be more precise than the least precise value used to calculate it. Round only at the end.

1.2 Practical skills assessed in the practical endorsement

What is the Practical Endorsement?
A minimum of 12 assessed practical activities covering PAG1 to PAG12, assessed by the teacher against the Common Practical Assessment Criteria and reported separately from the A level grade.
How do you calibrate an eyepiece graticule?
Line up the graticule with a stage micrometer, count the graticule divisions matching a known stage length, and divide: length per division = known length ÷ divisions. Repeat for each objective.
Using a colorimeter
Zero with a blank of solvent; choose a filter the sample absorbs strongly; make standards by serial dilution; plot absorbance against concentration; read unknowns from the calibration curve without extrapolating.
Serial dilution
Each solution is diluted by the same factor to make the next, for example 1 cm³ into 9 cm³ of diluent at each step (a tenfold dilution).
Volume of a bubble moving along a potometer capillary
Volume = πr² × distance moved. Example: r = 0.35 mm, 30 mm in 4.0 min: π × 0.35² × 30 = 11.5 mm³, a rate of about 2.9 mm³ per minute.
Rf value in chromatography
Rf = distance from the baseline to the centre of the spot ÷ distance from the baseline to the solvent front. It has no units and lies between 0 and 1.
Key steps of aseptic technique when inoculating agar
Disinfect the bench and wash hands; flame the loop and let it cool; open lids only slightly, near a flame; seal with two pieces of tape and incubate upside down; sterilise cultures before disposal.
52 colonies grow from 0.1 cm³ of a 10⁻³ dilution. What was the original concentration of bacteria?
52 ÷ 0.1 = 520 per cm³ in the dilution. 520 ÷ 10⁻³ = 5.2 × 10⁵ bacteria per cm³ in the original culture.
Capture–mark–recapture estimate and assumptions
N = (M × n) ÷ m, where M = number first marked, n = second sample, m = marked in the second sample. Assumes a closed population, lasting harmless marks and full mixing. Example: 30 × 25 ÷ 5 = 150.
Label vs annotation on a biological drawing
A label names a structure. An annotation describes or explains it.
Ethics when using human volunteers in practical work
Informed consent, the right to stop, no strenuous activity where a health condition makes it unsafe, and results kept anonymous.

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