AQA A-Level Biology required practicals: A complete guide for 2026

AQA A-Level Biology (7402) has 12 required practicals every student must complete during the course. These practicals develop the scientific skills tested in the written exams, and they feed into the separate practical endorsement.

The written exams do not test whether you physically did the practical. They test whether you understand the method, can identify variables, interpret data, and spot weaknesses. Around 15% of the marks across the written papers come from practical-skills questions.

This guide lists every required practical, breaks each one down into method, expected results, and common mistakes, and gives you a checklist for revision.

Required practicals are doubly important. First, around 15% of the marks on the written exams come from practical-skills questions. AQA can ask you to describe a method, identify variables, suggest improvements, or interpret unfamiliar data.

Second, your teacher assesses you against the Common Practical Assessment Criteria (CPAC) across the 12 practicals. CPAC has five strands covering procedures, investigative approaches, equipment use, observation, and reporting. You need to demonstrate every strand at least twice.

The outcome is the practical endorsement, reported as a separate pass or fail on your final certificate. It does not affect your overall A-Level grade, but universities can see it, particularly for science degrees.

The table below lists all 12 practicals in the official numbering used on the AQA 7402 specification. Practicals 1 to 6 sit in Year 1 / AS content; practicals 7 to 12 sit in Year 2 / A2 content. Wording follows the AQA specification.

For each required practical, you should know the method from memory, be able to identify the variables, predict the expected results, and explain the underlying science. The breakdowns below cover the key points for each.

An open practical: The 'named variable' can be temperature, pH, enzyme concentration or substrate concentration. Many schools use temperature or pH with amylase and starch (timing the disappearance of starch with iodine) or catalase with hydrogen peroxide (collecting O₂ over time).

The independent variable is whatever you change; the dependent variable is rate (1/time for a colour change, or gas volume per unit time). Control variables include the other factors above plus total volume and concentration of substrate.

Expect rate to rise to an optimum (pH 7 or around 40°C for many enzymes) then fall as denaturation kicks in. Common exam questions: Why use a buffer (keeps pH constant during the run), why use a water bath (keeps temperature constant), and why repeat for a mean. Watch out for the difference between inhibition (competitive vs non-competitive) and denaturation in extended answers.

Use the growing tip of a root (commonly onion, garlic or broad bean) to observe mitosis. Cut a few millimetres of root tip, soften in warm dilute hydrochloric acid, stain with acetic orcein or toluidine blue, and squash on a slide under a coverslip. Observe under a light microscope.

Count cells in each stage of mitosis (prophase, metaphase, anaphase, telophase) and calculate the mitotic index: Number of cells in mitosis divided by total number of cells, multiplied by 100.

A high mitotic index indicates rapid cell division, which is why this technique is also used to study cancerous tissue. Exam questions often ask you to identify stages from a micrograph, calculate a mitotic index from data, or explain why HCl is used (separates cells by hydrolysing the middle lamella).

You produce a dilution series of a solute (typically sucrose) of known water potentials, then place equal-sized pieces of plant tissue (commonly potato cylinders) into each concentration for a fixed time.

Measure mass or length change of each cylinder. Cylinders gain mass in solutions with higher water potential than the cell (water moves in by osmosis) and lose mass in solutions with lower water potential (water moves out). Plot percentage change in mass against solution water potential. The point where the line crosses zero is the water potential of the tissue.

Exam questions often give you a graph and ask you to read off the water potential of an unknown tissue, identify why mass is preferred over length, or explain why a calibration curve is needed (you cannot directly measure water potential).

Beetroot cells contain a red pigment (betalain) in their vacuoles. If the cell membrane is damaged, the pigment leaks out. You cut beetroot cylinders of equal size, rinse to remove pigment from cut surfaces, then place them in test tubes at different temperatures (or in different solvent concentrations such as ethanol). Measure the colour intensity of the surrounding water with a colorimeter.

As temperature rises, more pigment leaks because the phospholipid bilayer becomes more fluid and membrane proteins denature. Above around 50°C, leakage rises sharply. Ethanol increases leakage by disrupting the phospholipid tails.

Common exam questions ask you to explain the molecular reason for increased leakage, identify control variables (size of cylinders, time, volume of water), and describe how to use a colorimeter correctly (zero with distilled water, use the same filter).

Common dissections at A-Level include the mammalian heart, lungs, kidney, or a fish head (gills). The aim is to relate observable structures to functions covered in the specification.

For a heart, you identify the four chambers, the major vessels (aorta, vena cava, pulmonary artery, pulmonary vein), the valves and the thicker left ventricle wall (because it pumps blood at higher pressure around the body). For a kidney, you identify the cortex, medulla, pelvis and ureter, and relate the gross structure to filtration and reabsorption.

Common exam questions ask you to label a dissection diagram, explain why a particular structure has its specific features, or describe the safety precautions used during dissection.

You investigate the effect of antimicrobial substances (antibiotic discs, plant extracts, mouthwashes) on the growth of bacteria on an agar plate, using aseptic technique throughout.

Aseptic technique steps: Sterilise equipment by autoclaving or flaming, work near a Bunsen flame for an updraft of clean air, do not breathe over the plate, and tape the lid in two or three places (without sealing fully, so anaerobic pathogens cannot grow). Spread the bacterial culture evenly, then place discs of antimicrobial soaked in known concentrations onto the agar. Incubate (typically 25°C in a school lab) and measure the clear zone of inhibition around each disc.

Exam questions ask why each aseptic step is needed, how to calculate the area of inhibition (πr²), why the bacterial culture must be evenly spread, and how to set up a control disc (e.g. soaked in solvent only).

You use thin-layer or paper chromatography to separate the photosynthetic pigments in leaves of different plants. Grind leaves in a solvent (propanone) using sand, spot the extract onto a pencil line on the chromatography plate, then suspend the plate in a beaker with a small amount of solvent below the spot.

As the solvent rises, the pigments separate. Typical pigments separated: Chlorophyll a (blue-green), chlorophyll b (yellow-green), carotene (orange), xanthophyll (yellow). Calculate Rf values for each pigment (distance moved by pigment / distance moved by solvent front) and compare against reference values to identify them.

Common exam questions: Why use pencil (ink would dissolve), why the spot must be above the solvent (otherwise pigments dissolve into the solvent), calculate Rf, and explain why different pigments move different distances (differences in solubility in the solvent and binding to the paper).

You investigate the effect of a factor (light intensity, light wavelength, or temperature) on the rate of the light-dependent reaction by measuring dehydrogenase activity in extracted chloroplasts. DCPIP is added as a redox indicator: It is blue when oxidised and colourless when reduced. As the light-dependent reaction runs, dehydrogenase enzymes reduce DCPIP, and the solution decolourises.

Use a colorimeter to measure absorbance over time. The rate of decolourisation gives the rate of the light-dependent reaction. The independent variable might be light intensity (lamp distance, applying the inverse square law) or wavelength (coloured filters). Control variables include chloroplast concentration, DCPIP concentration, temperature and volume.

Common exam questions: Why DCPIP works as an indicator, how to use a colorimeter, why the chloroplasts are kept on ice during extraction (to prevent enzyme denaturation), and how to apply the inverse square law to light intensity.

You investigate the effect of a named variable (temperature, substrate concentration, type of respiratory substrate) on the rate of respiration in a culture of single-celled organisms, typically yeast.

A common set-up uses a respirometer with a yeast suspension, soda lime to absorb CO₂, and a manometer or capillary tube with a coloured marker. As yeast respires, oxygen is consumed and CO₂ is absorbed, so the gas volume drops and the marker moves towards the yeast. Measure the distance moved per unit time. Volume change is πr² × distance moved, where r is the capillary radius.

Alternatively, methylene blue can be used as a redox indicator: It is blue when oxidised and colourless when reduced by respiring yeast. Common exam questions: Why soda lime is needed, why a control tube is set up (typically with boiled yeast or glass beads), and how to relate gas volume change to a respiration rate.

You investigate how an environmental variable (light intensity, humidity, temperature) affects the movement of an animal using either a choice chamber or a maze. Common organisms: Woodlice, maggots or beetles in a choice chamber that lets you create different conditions in each quarter.

For a choice chamber, place 10 to 20 organisms in the centre and let them move freely for a fixed time. Count how many end up in each section. For a maze, time how long it takes the animal to reach a target.

The independent variable is the environmental factor (e.g. dark vs light, dry vs damp), the dependent variable is the number of organisms in each section or the time taken, and control variables include time allowed, number of organisms, age and species, temperature and starting position.

Common exam questions: Identify variables, explain the ethical considerations (return animals to their habitat, avoid harm), explain why organisms move towards their preferred conditions (taxis or kinesis), and apply statistical tests (chi-squared) to results.

You produce a serial dilution of a glucose solution of known concentrations, react each with Benedict's reagent (boiled for a fixed time), and measure the absorbance of the resulting solution in a colorimeter. The Benedict's reagent changes colour from blue through to brick red as more glucose is reduced.

Plot a calibration curve of absorbance against glucose concentration. You can then use the curve to determine the concentration of glucose in an unknown sample by measuring its absorbance and reading off the curve.

Common exam questions: Why a calibration curve is needed (Benedict's is not directly quantitative without one), why excess Benedict's reagent is used (so glucose is the limiting factor), how to filter out the precipitate before measuring absorbance, and how to use the curve to find an unknown concentration.

You investigate how an environmental factor (light, soil pH, soil moisture, distance from the sea) affects the distribution of a named species in a habitat. Two common methods.

For random sampling across a uniform habitat, place quadrats randomly using number coordinates from a random number generator. Count the species (or estimate percentage cover) inside each quadrat. Calculate a mean per quadrat and scale up to the whole area.

For a gradient in conditions (e.g. away from a hedge or up a sand dune), use a belt or line transect. Place quadrats at fixed intervals along the transect and measure species abundance plus the environmental factor at each point.

Common exam questions: Justify random sampling vs systematic sampling, explain the assumptions of the method, calculate a Spearman's rank or other correlation coefficient between abundance and an environmental factor, and evaluate sources of error (e.g. quadrat size too small).

Reading through methods is not revision. AQA tests whether you think like a scientist, not whether you can recite a recipe. The students who score well on practical-skills questions have worked through past paper questions for each practical.

For every practical, write the method from memory, then check against your notes. Pay attention to what you missed: Those gaps are the ones that cost marks. Practise drawing results tables with correct headings, units, and decimal places.

Make sure you can name the independent, dependent, and control variables for each practical. Then practise explaining how each control variable would be kept constant. Vague answers like 'keep the temperature the same' lose marks; specific answers like 'use a water bath at 25 degrees C' earn them.

Finally, practise interpreting graphs. AQA often uses graphs that plateau, dip, or peak. For each pattern, be ready to explain the biology. A photosynthesis graph plateaus because another factor becomes limiting; an enzyme rate dips beyond the optimum pH because the enzyme denatures.