AQA A-level Biology required practicals
12 practicals. Check the reasoning behind your results.
Check your practical reasoning
Question 1 of 2
An enzyme reaction reaches the same endpoint at two pH values. Using the mean times, what is the rate at pH 7 divided by the rate at pH 5?
| pH | Time 1 / s | Time 2 / s | Time 3 / s |
|---|---|---|---|
| 5 | 42 | 44 | 43 |
| 7 | 24 | 25 | 26 |
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The 12 required practicals
Try the free enzyme rates walkthrough1Measure how one variable (temperature, pH, enzyme or substrate concentration) changes the rate of an enzyme-controlled reaction
What to practise
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
What to practise
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
What to practise
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
What to practise
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
What to practise
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
What to practise
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
What to practise
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
What to practise
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
What to practise
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
What to practise
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
What to practise
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
What to practise
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.