AQA A-level Biology required practicals
There are 12: here is what exam questions ask about each one and where marks are lost.
Try the free enzyme rates walkthrough1Measure how one variable (temperature, pH, enzyme or substrate concentration) changes the rate of an enzyme-controlled reaction
Exam questions ask: A product-time or colour-change data set; initial rate from a tangent at t = 0; rate against the independent variable.
Where marks go: Measuring rate from the final plateau instead of the initial gradient; no control of pH or temperature; confusing the dependent and independent variables.
2Prepare and stain a root-tip squash, observe mitosis with a light microscope and calculate a mitotic index
Exam questions ask: Counts of cells in interphase and each mitotic stage; mitotic index; sizes from a calibrated graticule.
Where marks go: Counting the same field twice; using the unstained or overlapping region; forgetting to calibrate the graticule for each objective.
3Make a dilution series of a solute and use plant tissue to find the water potential of the tissue from a calibration curve
Exam questions ask: Percentage change in mass for each concentration; the x-intercept (no change) as the solute concentration with the same water potential.
Where marks go: Plotting absolute rather than percentage change; not blotting consistently; reading the intercept without explaining what zero change means.
4Investigate how one variable (temperature or a solvent concentration) affects the permeability of plant-cell membranes
Exam questions ask: Absorbance or percentage transmission against the independent variable; a trend explained by membrane and protein structure.
Where marks go: Not rinsing cut discs; unequal disc surface areas; claiming the pigment is produced rather than leaked.
5Dissect an animal or plant gas-exchange or mass-transport system or organ
Exam questions ask: Annotated, scaled drawings; named structures and their functions.
Where marks go: Drawings shaded or sketchy instead of clear lines; no scale or magnification; confusing left and right sides of the heart.
6Use aseptic technique to investigate the effect of antimicrobial substances on microbial growth
Exam questions ask: Diameters or areas of the clear zones; area from πr²; comparison with the control.
Where marks go: Sealing the plate completely; incubating at 37 °C in a school laboratory; measuring diameter as radius.
7Separate photosynthetic pigments by chromatography
Exam questions ask: Distances moved by each pigment and by the solvent; Rf values; identification from known values.
Where marks go: Drawing the origin in ink; letting the spot dip into the solvent; measuring Rf to the end rather than the centre of the spot.
8Investigate the effect of a factor on dehydrogenase activity in chloroplast extracts
Exam questions ask: Time to decolourise or absorbance over time; rate against the independent variable; a control kept in the dark.
Where marks go: Explaining the colour change as oxygen production; letting the extract warm up; no dark control.
9Investigate the effect of a variable on the rate of respiration of cultures of single-celled organisms
Exam questions ask: Volume or colour change over time; rate per minute; comparison between conditions.
Where marks go: Not equilibrating the apparatus; leaks in the respirometer; forgetting the carbon dioxide absorber when measuring oxygen uptake.
10Investigate the effect of an environmental variable on the movement of an animal using a choice chamber or a maze
Exam questions ask: Counts on each side over time; a chi-squared test comparing observed and expected distributions.
Where marks go: Changing two variables at once; handling the animals roughly; concluding taxis from data that show only a change in speed or turning.
11Produce a dilution series of a glucose solution and use colorimetric techniques to find the glucose concentration of an unknown urine sample
Exam questions ask: Absorbance for each known concentration; the unknown read from the calibration curve.
Where marks go: Extrapolating beyond the calibration range; different heating times for different tubes; no blank to zero the colorimeter.
12Investigate the effect of a named environmental factor on the distribution of a given species
Exam questions ask: Percentage cover or frequency with the factor at each point; a correlation coefficient and its significance.
Where marks go: Choosing quadrat positions by eye; too few samples along the gradient; concluding causation from a correlation.