Required practicals — AQA A-Level Biology
The 12 required practicals on this course, as named in the specification.
Required practical 1: Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.
You choose one named independent variable, temperature, pH, substrate concentration, enzyme concentration or inhibitor concentration, and measure a rate. The common systems are catalase from potato, celery or yeast acting on hydrogen peroxide, with the volume of oxygen collected in a gas syringe per unit time; amylase acting on starch, taking samples into iodine in potassium iodide solution at fixed intervals and timing how long the blue-black colour takes to disappear; and trypsin clearing a milk suspension, timed or followed with a colorimeter. Enzyme and substrate must be brought separately to the test temperature in a water bath before mixing, the pH held with a buffer, and every other volume and concentration kept the same, so that only the chosen variable changes. Timing starts at mixing.
Go to Many proteins are enzymesRequired practical 2: Preparation of stained squashes of cells from plant root tips; set-up and use of an optical microscope to identify the stages of mitosis in these stained squashes and calculation of a mitotic index.
Cut the last few millimetres from a growing root tip, as the meristem contains dividing cells. Warm in 1 mol dm⁻³ hydrochloric acid at 60°C to hydrolyse the middle lamella, separating cells. Rinse, place on a slide, add a DNA stain (e.g. toluidine blue), cover with a coverslip and press firmly without twisting to squash cells into a single layer. Set up an optical microscope: start on the lowest objective lens, focus using the coarse knob, then switch to high power. Identify mitotic stages by chromosome appearance (e.g. chromosomes at equator in metaphase). Mitotic index = (cells in mitosis / total cells) × 100.
Go to All cells arise from other cellsRequired practical 3: Production of a dilution series of a solute to produce a calibration curve with which to identify the water potential of plant tissue.
Produce a dilution series of a solute, calculating volumes of stock solution and distilled water so every tube has the same final volume. Cut potato cylinders with a cork borer for equal diameter and length, blot them and record their initial mass. Place one cylinder in each concentration for the same time at a constant temperature. Remove them, blot gently to remove surface liquid, and reweigh. Calculate the percentage change in mass for each cylinder, as starting masses vary, and plot this against concentration. Where the line crosses zero, there is no net osmosis. The water potential of that solution equals the water potential of the potato cells. Read off the concentration and convert it to a water potential.
Go to Transport across cell membranesRequired practical 4: Investigation into the effect of a named variable on the permeability of cell-surface membranes.
Beetroot is used because its vacuoles contain the red pigment betalain, which stays inside while the tonoplast and cell-surface membrane are intact and leaks out once they are damaged. Cut identical cylinders with a cork borer and wash them thoroughly to remove pigment released by the cells cut at the surface; without that step the first colour you measure is an artefact of cutting rather than an effect of your variable. Expose each cylinder to a different level of the named variable, usually temperature or ethanol concentration, for a fixed time, then remove the beetroot and measure the colour of the surrounding solution in a colorimeter, using a filter of the complementary colour and calibrating with distilled water as the blank. More pigment gives a higher absorbance, and so indicates greater permeability.
Go to Transport across cell membranesRequired practical 5: Dissection of animal or plant gas exchange system or mass transport system or of organ within such a system.
Dissection techniques and biological drawing conventions are assessable. When dissecting an organ like a heart or fish gills, safety is paramount. Use a scalpel or scissors on a dissecting board, cutting away from your body. Disinfect surfaces and wash hands afterwards to minimise infection risk. Before cutting a heart, orientate it by locating the thicker left ventricle wall. Record observations using biological drawings. These must feature continuous, unbroken lines with no shading or sketchy strokes. Draw structures in proportion, include a title, and use straight, ruled label lines that do not cross or have arrowheads. Include a magnification or scale bar where appropriate to indicate size.
Go to Mass transport in animalsRequired practical 6: Use of aseptic techniques to investigate the effect of antimicrobial substances on microbial growth.
Grow a bacterial lawn on sterile nutrient agar to test antimicrobial substances. Aseptic technique keeps unwanted microorganisms out. Sterilise a wire loop by flaming until red hot, or a glass spreader by dipping in alcohol and flaming, then cool. Work close to a lit Bunsen so the updraught carries spores away; lift the Petri dish lid at an angle; flame the neck of culture bottles. Spread the culture evenly, then place discs soaked in different antimicrobials, plus a sterile water control, well apart. Tape the lid at two points rather than sealing completely, invert the dish to prevent condensation dripping, and incubate at max 25 degrees Celsius. Measure the clear zone of inhibition around each disc and calculate its area. Repeating the plates and calculating a mean reduces the effect of random errors, improving repeatability.
Go to Genetic diversity and adaptationRequired practical 7: Use of chromatography to investigate the pigments isolated from leaves of different plants, eg, leaves from shade-tolerant and shade-intolerant plants or leaves of different colours.
Pigments are extracted by grinding leaf tissue (e.g., from shade-tolerant and shade-intolerant plants) with sand and a solvent. A pencil origin line is drawn on chromatography paper. The extract is spotted repeatedly onto the origin. The paper is suspended in a closed vessel with the solvent below the origin line, preventing the spot from washing off. Pigments separate based on their solubility in the solvent and attraction to the paper; more soluble pigments travel further. The run stops before the solvent reaches the top. Rf values (distance moved by pigment divided by distance moved by solvent front) are calculated to identify and compare pigments between different plant types, revealing adaptations to varying light intensities.
Go to Photosynthesis (A-level only)Required practical 8: Investigation into the effect of a named factor on the rate of dehydrogenase activity in extracts of chloroplasts.
Chloroplasts are extracted by grinding leaf tissue in ice-cold, buffered, isotonic solution, then filtering and centrifuging. Cold reduces enzyme activity and tissue damage, the buffer holds pH constant so enzymes are not denatured, and the isotonic solution prevents osmotic damage. The suspension is mixed with DCPIP, a blue dye acting as an artificial electron acceptor in place of NADP. In the light, electrons from photoionisation and photolysis reduce DCPIP, which turns from blue to colourless; the faster the colour loss, the greater the dehydrogenase activity and the faster the light-dependent reaction. Rate is the reciprocal of time to decolourise, or is calculated from colorimeter readings at set intervals. A named factor, such as light intensity or temperature, is varied while others are controlled.
Go to Photosynthesis (A-level only)Required practical 9: Investigation into the effect of a named variable on the rate of respiration of cultures of single-celled organisms.
The organism is usually yeast, and the named variable might be temperature, substrate concentration or the type of sugar supplied. Rate can be measured by the volume of carbon dioxide produced, collected in a gas syringe, or by the time taken for a redox indicator such as methylene blue or TTC to change colour as it is reduced by hydrogen from the respiring cells. In a respirometer, carbon dioxide is absorbed by potassium hydroxide or soda lime, so the liquid movement in the manometer measures oxygen uptake alone, not carbon dioxide release. A control tube containing boiled yeast corrects for pressure changes caused by temperature. Controlled variables include the volume and concentration of the yeast suspension, the volume of substrate, pH, and the time allowed for equilibration in the water bath before readings begin.
Go to Respiration (A-level only)Required practical 10: Investigation into the effect of an environmental variable on the movement of an animal using either a choice chamber or a maze.
A choice chamber offers contrasting conditions, like damp and dry, where animals are introduced and allowed to distribute. The independent variable is the environmental condition. The dependent variable is the number of animals per compartment, or in a maze, the time taken. Distribution is recorded at fixed intervals after a settling period, as counting immediately records placement, not response. Confounding variables must be controlled: species, size, temperature, and time of day. The chamber is rotated between runs so external light or draughts do not skew results. For categorical count data in a choice chamber, a chi-squared test compares observed and expected frequencies; for continuous data like time in a maze, a t-test is used.
Go to Survival and response (A-level only)Required practical 11: Production of a dilution series of a glucose solution and use of colorimetric techniques to produce a calibration curve with which to identify the concentration of glucose in an unknown ‘urine’ sample.
You make a dilution series from a glucose solution of known concentration, usually by serial dilution, mixing equal volumes of stock and distilled water repeatedly so that each tube is half the concentration of the one before, or by mixing measured volumes of stock and water to a fixed total. Each known concentration is then heated with an excess of Benedict's solution in a water bath, all tubes for the same time and at the same temperature, so that the amount of brick-red copper(I) oxide formed depends only on the reducing sugar present. The precipitate is removed by filtering or centrifuging and the remaining blue solution is read in a colorimeter using a red filter. The more glucose there was, the less blue remains, so absorbance falls as concentration rises.
Go to Control of blood glucose concentration (A-level only)Required practical 12: Investigation into the effect of a named environmental factor on the distribution of a given species.
This practical investigates how an environmental factor affects species distribution. Choose a measurable factor: light intensity, soil moisture, pH, or temperature. Where the factor changes along a gradient, use a belt transect by running a tape measure along the gradient and placing quadrats at regular intervals to capture the pattern. At each quadrat, record species abundance (e.g., percentage cover or frequency) and measure the environmental factor, taking repeat readings to calculate a mean. If comparing two uniform areas without a gradient, sample randomly within each area using coordinates generated by a random number calculator to avoid bias.
Go to Populations in ecosystems (A-level only)
Practise the questions examiners set on these practicals.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge