This cheat sheet covers the main skills and methods used in GCSE Biology required practicals. It helps students plan investigations, identify variables, collect valid data, and write clear conclusions. The reference is useful before practical lessons, during revision, and when preparing for exam questions about experimental method.
Key Facts
- The independent variable is the factor you change, the dependent variable is the factor you measure, and control variables are kept the same.
- A valid experiment tests only the effect of the independent variable on the dependent variable by controlling all other important factors.
- Magnification is calculated using magnification = image size / real size, with image size and real size in the same units.
- A mean is calculated using mean = sum of values / number of values, but anomalous results should be identified before using them.
- Percentage change is calculated using percentage change = (change / original value) x 100.
- In osmosis, water moves through a partially permeable membrane from a dilute solution to a more concentrated solution.
- Enzyme rate can be estimated using rate = 1 / time or rate = amount of product formed / time.
- Benedict's test gives a brick-red precipitate for reducing sugars, iodine turns blue-black for starch, Biuret turns purple for protein, and ethanol emulsion turns cloudy white for lipids.
Vocabulary
- Independent variable
- The factor deliberately changed by the student during an investigation.
- Dependent variable
- The factor measured or observed to see the effect of changing the independent variable.
- Control variable
- A factor kept the same so the test is fair and the results are valid.
- Resolution
- The ability to distinguish two close points as separate points in an image.
- Osmosis
- The net movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated solution.
- Anomalous result
- A result that does not fit the pattern of the other results and may be caused by error.
Common Mistakes to Avoid
- Mixing up the independent and dependent variables is wrong because the independent variable is what you change, while the dependent variable is what you measure.
- Forgetting to keep control variables constant is wrong because other factors could affect the result and make the experiment invalid.
- Using different units in magnification calculations is wrong because magnification = image size / real size only works when both measurements use the same unit.
- Including an anomalous result in a mean without checking it is wrong because it can distort the average and hide the real pattern.
- Heating food test samples too strongly is wrong because some reagents and biological molecules can be damaged, giving unreliable color changes.
Practice Questions
- 1 A microscope image of a cell is 24 mm wide, and the real cell is 0.03 mm wide. Calculate the magnification.
- 2 A potato cylinder has an initial mass of 4.80 g and a final mass of 5.28 g after being placed in sucrose solution. Calculate the percentage change in mass.
- 3 In an enzyme practical, a reaction takes 40 seconds for the cross to disappear. Calculate the rate using rate = 1 / time.
- 4 Explain why a student should repeat each required practical measurement and calculate a mean instead of using only one result.