Method, variables, results and the specific exam questions each practical generates — everything you need to know.
Required practicals appear in every GCSE Biology exam. Questions about them can include describing methods, identifying variables, explaining results, suggesting improvements, and evaluating reliability. You don't need to have performed the practical — but you do need to know it in enough detail to answer questions about someone else doing it. This guide covers all the required practicals for AQA GCSE Biology.
For every practical: know the independent variable (what you change), the dependent variable (what you measure), and at least three control variables (what you keep the same). Exam questions award separate marks for each of these.
Prepare a slide of a plant or animal cell (e.g. onion epidermis or cheek cells). Stain with iodine (plant) or methylene blue (animal) to make structures visible. View under a light microscope, starting with the lowest magnification objective lens and increasing.
Key skill: Calculate magnification using: magnification = image size ÷ actual size. Rearrange for actual size or image size as required.
Common exam question: An image of a cell is 30 mm wide. The actual cell is 0.03 mm wide. What is the magnification? Answer: 30 ÷ 0.03 = ×1000.
Cut cylinders of potato (or another plant tissue) of equal length and mass. Place each cylinder into a different concentration of sucrose solution (e.g. 0%, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M). Leave for 20–30 minutes. Remove, blot dry, and measure the change in mass or length.
Independent variable: Concentration of sucrose solution.
Dependent variable: Change in mass (or length) of potato cylinder.
Control variables: Initial size of potato cylinders, temperature, time in solution, same variety of potato.
Results: In dilute solutions the potato gains mass (water enters by osmosis). In concentrated solutions it loses mass (water leaves). The concentration at which mass doesn't change is the potato's own cell sap concentration.
Investigate the effect of pH (or temperature) on the rate of an enzyme-catalysed reaction. A common version uses amylase and starch: set up several test tubes at different pH values (using buffer solutions) each containing starch solution and amylase. Sample each tube regularly by placing drops on a spotting tile with iodine solution — a blue-black colour indicates starch is still present. Record the time until starch is fully digested (iodine stays orange-brown).
Independent variable: pH (or temperature).
Dependent variable: Time for starch to be fully digested (or rate = 1/time).
Results: There is an optimum pH at which rate is fastest. Either side of the optimum, the enzyme's active site shape is altered and rate decreases. At extreme pH, the enzyme denatures.
Place aquatic pondweed (Elodea) in a beaker of water. Illuminate with a lamp at different distances. Count the number of oxygen bubbles produced per minute at each distance. Closer lamp = higher light intensity = faster photosynthesis = more bubbles per minute.
Independent variable: Distance of lamp from pondweed (used as a proxy for light intensity — note that light intensity is inversely proportional to distance squared).
Dependent variable: Number of bubbles per minute (or volume of gas collected).
Control variables: Temperature (use a water bath or large beaker of water as a heat shield), CO₂ concentration (add sodium hydrogen carbonate to the water), same piece of pondweed.
One person holds a ruler vertically. A second person places their hand at the bottom of the ruler, ready to catch it. Without warning, the first person releases the ruler. The second person catches it as quickly as possible. The distance the ruler falls is recorded and converted to reaction time using the equation: t = √(2d/g) where d is distance in metres and g = 10 m/s².
This practical links to: The nervous system — reflex arcs vs conscious responses. Exam questions often ask why this test measures a voluntary response, not a reflex.
Improving reliability: Repeat multiple times and calculate a mean. Remove anomalous results. Ensure the catcher cannot see the releaser's hand to prevent anticipation.
Measure resting heart rate and breathing rate. Perform a set period of exercise (e.g. stepping up and down for 3 minutes). Immediately measure heart rate and breathing rate again. Record how long it takes for both to return to resting levels (recovery time). Repeat at different exercise intensities or durations.
Results: Both heart rate and breathing rate increase during exercise because muscles need more oxygen and glucose and produce more CO₂. Recovery time varies by fitness level — fitter individuals recover faster.
Whenever an exam question asks you to identify variables in a required practical, structure your answer like this: "The independent variable is [X], which I will change by [method]. The dependent variable is [Y], which I will measure by [method]. Control variables include [A], [B] and [C], which I will keep constant by [method]." Writing it this way ensures you hit every mark available.
Prepare an agar plate. Using aseptic technique, inoculate the plate with bacteria (using an inoculating loop that has been sterilised by flaming). Seal the plate with adhesive tape (but not completely airtight — to prevent anaerobic growth). Incubate at 25°C (not 37°C — this is the school safety rule to prevent growth of pathogens that thrive at body temperature). Observe bacterial colonies after 24–48 hours.
Aseptic technique: Flame the inoculating loop before and after use. Work near a Bunsen burner to create an upward air current that keeps microorganisms away from the work area. Never fully seal agar plates.
Exam link: This practical connects directly to antibiotic testing — you can place paper discs soaked in different antibiotics on an inoculated plate and measure the clear zone (inhibition zone) around each disc to compare antibiotic effectiveness.
The full list of required practicals for AQA GCSE Biology, including assessment criteria, is available on the AQA GCSE Biology specification page.
PaperPlus generates Biology questions including required practical scenarios for AQA, Edexcel and OCR — with full mark schemes. Free for all students.
Start Practising Free →