The Required Practicals (PAGs)
OCR Gateway Biology assesses your practical work entirely within the two written papers — there is no separate practical exam. Across the course you carry out a set of required practical activities (often called PAGs — Practical Activity Groups), and questions about them can appear on either paper. You can be asked to describe a method, identify variables, explain or interpret results, evaluate the investigation, suggest improvements, or do a calculation from practical data.
By the end of this lesson you should know the aim, key apparatus and method, and the variables for each of the standard required practicals, recognise the most common exam question on each, and be fluent in the magnification and zone-of-inhibition (πr2) calculations that arise from them.
Exam Tip: Practical questions are not always flagged as "practical" — they appear as ordinary structured or 6-mark questions. Recognise them from the context (apparatus, a method, a results table) and apply your working-scientifically vocabulary.
The Language of Working Scientifically
Almost every practical question rewards the same handful of ideas, so learn them as a checklist you can apply to any investigation:
- Independent variable (IV): the one thing you deliberately change.
- Dependent variable (DV): the thing you measure.
- Control variables (CV): everything you keep the same to make it a fair test.
- Repeats and a mean: repeating each measurement and taking a mean reduces the effect of random error and improves reliability (repeatability).
- Anomalies: results that don't fit the pattern; identify them and exclude them from the mean.
- Resolution: the smallest change an instrument can measure (e.g. a balance reading to 0.01 g).
- Accuracy / precision / validity: accurate = close to the true value; precise = readings close together; valid = actually measures what it claims to, with controlled variables.
Exam Tip: "Suggest one improvement to this method" is one of the most frequent practical questions. Strong answers name a specific control variable the method missed, add repeats and a mean, or use equipment of higher resolution — never just "be more careful".
The Required Practicals — Aim, Method, Variables and the Common Question
The list below covers the kinds of required practicals you meet in OCR Gateway Biology. Treat them as the standard set; always check your specification and your school's practical record for the exact activities you completed.
Using a light microscope (B1)
- Aim: observe and draw cells (e.g. onion epidermis or cheek cells) at different magnifications.
- Method: prepare a thin specimen on a slide; add a stain (iodine for plant cells, methylene blue for animal cells); lower the coverslip slowly at an angle to avoid air bubbles; start on the lowest-power objective; focus with the coarse then fine focus.
- IV/DV/CV: IV = magnification; DV = detail/structures observed; CV = same specimen, stain and lighting.
- Common question: calculate the actual size or magnification of a cell, and draw a clear labelled diagram with a magnification stated.
Testing for biological molecules — food tests (B1/B6)
- Aim: identify starch, reducing sugars, proteins and lipids in food samples.
- Method: iodine → blue-black for starch; Benedict's solution heated in a water bath (~80 °C) → brick-red precipitate for reducing sugar; Biuret reagent → violet/purple for protein; the emulsion test (ethanol then water) → cloudy white for lipid.
- CV: equal sample and reagent volumes; same temperature; use a distilled-water control.
- Common question: describe how to test a sample for two named molecules, stating the positive result for each.
Investigating enzymes — effect of pH or temperature (B1)
- Aim: find how pH (or temperature) affects the rate at which amylase digests starch.
- Method: mix amylase, starch and a buffer at a set pH in a water bath; every 30 s test a drop on a spotting tile with iodine; record the time for the iodine to stay orange-brown (all starch digested); calculate rate as time1.
- IV/DV/CV: IV = pH (or temperature); DV = time for starch to be digested; CV = temperature (if varying pH), volumes and concentrations.
- Common question: explain the shape of the rate curve in terms of denaturation and the active site changing shape.
Investigating osmosis in plant tissue (B2)
- Aim: find how sugar/salt concentration affects the mass of potato tissue.
- Method: cut potato cylinders of equal size, blot and weigh, immerse each in a different sugar concentration for ~30 min, blot and re-weigh; calculate percentage change in mass.
- IV/DV/CV: IV = solution concentration; DV = % change in mass; CV = surface area/size, temperature, time, blotting method.
- Common question: explain why mass increases or decreases using water potential and the partially permeable membrane, and a percentage-change calculation.
Investigating photosynthesis (B1)
- Aim: find how light intensity affects the rate of photosynthesis in pondweed (Elodea).
- Method: pondweed in water with sodium hydrogencarbonate (CO₂ source); a lamp at a known distance; a glass tank/heat shield of water to keep temperature constant; count O₂ bubbles per minute (or collect gas).
- IV/DV/CV: IV = lamp distance (proxy for intensity); DV = bubbles per minute; CV = temperature, CO₂ concentration, mass of pondweed.
- Common question: explain why the rate levels off (another factor — CO₂ or temperature — becomes limiting) and apply the inverse-square idea that intensity ∝d21.
Sampling using quadrats and transects (B4)
- Aim: estimate the abundance or distribution of organisms in a habitat.
- Method: for abundance, place quadrats at random coordinates (random number generator) and count or estimate % cover; for distribution along an environmental gradient, place quadrats at intervals along a transect (a line/tape).
- IV/DV/CV: IV = location/distance along transect; DV = number or % cover; CV = quadrat size, time of day, sampling method.
- Common question: estimate a total population (mean per quadrat × total area) and explain why sampling must be random to be representative.
Investigating reaction time (B3)
- Aim: measure human reaction time and the effect of a factor (e.g. caffeine, practice) on it.
- Method: the ruler-drop test — a partner drops a ruler between the subject's fingers; the catch distance converts to reaction time; repeat and take a mean.
- IV/DV/CV: IV = the factor tested; DV = catch distance/reaction time; CV = same person, hand, ruler, instructions.
- Common question: describe how to make it a fair test and how repeats improve reliability.
Investigating antibiotics/antiseptics on microbial growth (B6)
- Aim: compare how well different antiseptics or antibiotics inhibit bacterial growth.
- Method: use aseptic technique; spread bacteria on sterile agar; add paper discs soaked in each substance plus a sterile-water control; tape the lid (don't seal fully); incubate at 25 °C in school labs (not 37 °C, to avoid culturing human pathogens); measure the clear zone of inhibition.
- IV/DV/CV: IV = antiseptic/antibiotic; DV = area of the zone of inhibition; CV = volume on each disc, bacterium, incubation temperature and time, disc size.
- Common question: calculate the area of the zone of inhibition (πr2) and explain why 25 °C is used.
Investigating decay/decomposition (B4)
- Aim: find how a factor (temperature, pH, oxygen) affects the rate of decay.
- Method: e.g. measure the time for fresh milk to clot/curdle at different temperatures, or pH change as decomposers act; repeat and take a mean.
- IV/DV/CV: IV = the factor tested; DV = time for decay/measured change; CV = volume, microbe source, other conditions.
- Common question: explain results in terms of microorganism/enzyme activity and link to food preservation.
Investigating plant responses — tropisms (B3)