Required practicals: biology
Biology · GCSE · The School
What the questions actually ask
Almost every required-practical question is one of four: name the variable you changed, name the one you measured, name two you kept the same, or say why a result is unreliable and what you would change. Learn each practical as that set of four answers rather than as a story of what you did in the lab, and the questions stop being about memory.
The ones you must know
Microscopy and cell measurement. Osmosis in potato cylinders. Enzyme action and pH, using iodine to follow starch. Food tests — Benedict's for sugar, iodine for starch, biuret for protein, ethanol emulsion for fat. Photosynthesis rate against light intensity. Reaction time. Field sampling with quadrats and a transect. That is the list, and each has a variable structure you can state in one line.
Control variables win marks
Naming what you kept the same is where the easy marks are, and it is where answers are thinnest. For the enzyme practical: same volume of starch, same concentration of amylase, same temperature, same time between samples. Four control variables, four possible marks, and they are the same kind of answer every time — so this is the most efficient thing you can revise on this topic.
PRACTICAL: how light intensity affects the rate of photosynthesis in pondweed. INDEPENDENT variable: distance of the lamp, which sets light intensity. DEPENDENT variable: bubbles of oxygen per minute. CONTROL variables: same pondweed, same temperature, same carbon dioxide concentration in the water, same time to settle before counting. WHY A WATER BATH: the lamp warms the water, and temperature also affects the rate, so without it you are changing two things at once and the result means nothing. IMPROVEMENT: count for longer, or repeat and take a mean, because a single count of bubbles is easily wrong. Now write the same five lines for osmosis in potato — the shape does not change.