A-Level Biology: Practical Techniques and Calculations

Every A level biology paper contains questions set in a practical context, and many of them test the same core techniques that appear in the required practical activities: how to make up a dilution, how to read a chromatogram, why a respirometer contains potassium hydroxide, how to count dividing cells or calibrate a microscope. This material brings those techniques together and pairs each with the calculation that goes with it.

The solutions part covers 1 in 10 serial dilutions, the volumes of stock and water needed for a given concentration, and why a calibration curve of known concentrations is needed before a colorimeter can give the concentration of an unknown. Chromatography of photosynthetic pigments comes next: calculating Rf from distances measured from the origin, and the details of good technique — a pencil origin above the solvent level, a small concentrated spot, the solvent front marked before it evaporates. The DCPIP question checks that you know which way the colour changes when chloroplasts reduce it.

The respiration and cell-division techniques are handled quantitatively. For a respirometer you explain the role of potassium hydroxide or soda lime and calculate the volume of oxygen taken up from the movement of a coloured liquid with V = πr²l, spotting the common slips (radius not squared, diameter used). For a root-tip squash you explain what hydrochloric acid and the stain do and calculate a mitotic index. The microscopy item converts eyepiece units into micrometres after calibration against a stage micrometer.

The last items are about design and safety: identifying independent, dependent and control variables in a beetroot membrane-permeability investigation, and the steps of aseptic technique when culturing bacteria.

The material offers a multiple-choice quiz with explained answers, a flashcard deck of key formulae and reasons, a printable written-work sheet of eight open questions (method descriptions and multi-step calculations) answered by hand and checked, and an oral exam in which an examiner asks one question at a time about methods and gives feedback at the end.

The content is based on the practical skills and the apparatus and techniques listed in the DfE A level biology subject content, as practised through current exam-board required practicals (for example AQA required practicals, OCR practical activity groups). It is practice material and does not replace your own board's practical handbook.

• Carry out serial dilutions and use a calibration curve to find an unknown concentration
• Calculate Rf values and explain good chromatography technique
• Calculate the volume and rate of oxygen uptake from respirometer readings
• Prepare a root-tip squash and calculate a mitotic index
• Calibrate an eyepiece graticule and calculate actual size and magnification
• Identify variables and controls, and apply aseptic technique

Practice material written by Zestly, based on the practical skills and apparatus and techniques in the DfE A level biology subject content.