Required Practicals
AQA GCSE Biology has 10 required practicals that you must know in detail. Questions on required practicals appear on every exam paper and can be worth significant marks. You need to know the method, variables, expected results, potential sources of error, and how to improve accuracy and reliability for each one.
Why Required Practicals Matter
- AQA must assess required practicals in the written exams — they will appear.
- Questions may ask you to describe the method, identify variables, explain results, suggest improvements, or evaluate the experiment.
- At least 15% of the total marks across both papers are allocated to practical-based questions.
- You do not need to have physically carried out every practical, but you need to write about them as if you have.
Exam Tip: When answering required practical questions, always use precise scientific vocabulary. Say "independent variable" not "the thing I changed". Say "repeat the experiment three times and calculate a mean" not "do it again to make sure".
Required Practical 1: Microscopy
Specification reference: 4.1.1
Aim: Use a light microscope to observe, draw, and label a selection of plant and animal cells.
Method
- Place a thin layer of onion epidermis (or cheek cells) on a clean glass slide.
- Add a drop of iodine solution (for plant cells) or methylene blue (for animal cells) to stain the cells.
- Carefully lower a coverslip at an angle using a mounted needle to avoid trapping air bubbles.
- Place the slide on the microscope stage and clip it in place.
- Start with the lowest power objective lens (x4).
- Use the coarse focus to bring the image roughly into focus.
- Switch to a higher power objective lens (x10, then x40).
- Use the fine focus to sharpen the image.
- Draw what you observe — large, clear, with ruled label lines, no shading.
Variables
| Variable | Detail |
|---|
| Independent variable | Type of cell observed (e.g., onion cell, cheek cell) |
| Dependent variable | Structures visible under the microscope |
| Control variables | Magnification used, staining technique, slide preparation method |
Expected Results
- Onion epidermal cells: regular, rectangular cells with visible cell walls, nuclei (stained dark by iodine), and cytoplasm. No chloroplasts visible (onion epidermis is not green).
- Cheek cells: irregular shape, visible nucleus (stained blue by methylene blue), cell membrane, and cytoplasm.
Common Exam Questions
- "Describe how you would prepare a slide of plant cells" — give step-by-step method with staining.
- "Why is a stain used?" — to make colourless cell structures visible.
- "Why start with the lowest power objective lens?" — to locate the specimen easily and get a wide field of view.
- "Calculate the magnification" — use magnification = image size / actual size.
Sources of Error and Improvements
- Air bubbles under the coverslip distort the image — lower the coverslip at an angle.
- Too much stain makes the image too dark — use only one or two drops.
- Cells overlapping makes observation difficult — use a very thin layer of tissue.
- Moving the slide while focusing — keep the slide clipped securely.
Required Practical 2: Testing for Biological Molecules
Specification reference: 4.2.2
Aim: Use qualitative reagents to test for a range of carbohydrates, lipids, and proteins.
Food Tests Summary
| Substance Tested | Reagent | Method | Positive Result | Negative Result |
|---|
| Reducing sugars (e.g., glucose) | Benedict's solution | Add Benedict's to sample, heat in water bath at 75°C for 5 minutes | Colour change: blue → green → yellow → orange → brick red | Stays blue |
| Starch | Iodine solution | Add a few drops of iodine to the sample | Colour change: brown-orange → blue-black | Stays brown-orange |
| Proteins | Biuret reagent (NaOH + CuSO₄) | Add sodium hydroxide, then copper sulfate solution, and mix | Colour change: blue → purple/lilac | Stays blue |
| Lipids (fats/oils) | Ethanol (emulsion test) | Dissolve sample in ethanol, pour into water | White cloudy emulsion forms | Solution stays clear |
Variables
| Variable | Detail |
|---|
| Independent variable | Type of food sample tested |
| Dependent variable | Colour change observed |
| Control variables | Volume of reagent, volume of sample, temperature of water bath (for Benedict's), time |
Expected Results
Each test gives a specific colour change. For Benedict's test, the intensity of the colour change indicates the concentration of reducing sugar — a brick red precipitate indicates a high concentration, while green indicates a low concentration.
Common Exam Questions
- "Describe how you would test a food sample for protein" — add NaOH then CuSO₄; a purple colour indicates protein is present.
- "A student tested a sample and it turned blue-black with iodine. What does this show?" — the sample contains starch.
- "Why must the Benedict's test be heated?" — the reaction requires energy; heating increases the rate of the reaction between Benedict's solution and reducing sugars.
Sources of Error and Improvements
- Cross-contamination between samples — use clean test tubes and pipettes for each test.
- Inconsistent heating for Benedict's — use a thermostatically controlled water bath, not a Bunsen burner.
- Subjective colour judgement — use a colour chart for comparison; for quantitative Benedict's test, use a colorimeter.
Exam Tip: You must know all four food tests, their reagents, and their results. A very common question asks you to complete a table of food tests — learn this table by heart.
Required Practical 3: Osmosis in Plant Tissue
Specification reference: 4.1.3
Aim: Investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue (potato cylinders).
Method
- Cut potato cylinders of equal length (e.g., 3 cm) using a cork borer — ensures they all have the same diameter.
- Blot each cylinder dry with a paper towel and record its initial mass.
- Place each cylinder in a different concentration of sucrose solution (e.g., 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M).
- Leave for a set time (e.g., 30 minutes or overnight).
- Remove each cylinder, blot dry, and record the final mass.
- Calculate the percentage change in mass: ((final mass - initial mass) / initial mass) x 100.
graph LR
A["Cut potato cylinders<br/>(cork borer, 3 cm)"] --> B["Blot dry &<br/>record mass"]
B --> C["Place in sucrose<br/>solutions (0.0–1.0 M)"]
C --> D["Leave for<br/>set time"]
D --> E["Remove, blot dry,<br/>record final mass"]
E --> F["Calculate %<br/>change in mass"]
style A fill:#27ae60,color:#fff
style F fill:#e67e22,color:#fff
Variables
| Variable | Detail |
|---|
| Independent variable | Concentration of sucrose solution |
| Dependent variable | Percentage change in mass of potato cylinder |
| Control variables | Length and diameter of potato cylinder, volume of solution, temperature, time left in solution, type of potato |
Expected Results
- In dilute solutions (low concentration): water enters potato cells by osmosis → potato gains mass → cells become turgid.
- In concentrated solutions (high concentration): water leaves potato cells by osmosis → potato loses mass → cells become plasmolysed.
- At the concentration where there is no change in mass, the solution has the same water potential as the potato cells — this is the isotonic point.
Common Exam Questions
- "Explain why the potato cylinder in 1.0 M sucrose lost mass" — the sucrose solution has a lower water potential than the potato cells, so water moves out of the cells by osmosis down the water potential gradient.
- "Why do we calculate percentage change rather than actual change?" — to allow fair comparison between cylinders that may have had slightly different starting masses.
- "Why must the potato cylinders be blotted dry before weighing?" — to remove excess surface water, which would affect the mass measurement.
Sources of Error and Improvements
- Potato cylinders of unequal size — use a cork borer and ruler to ensure consistency.
- Not blotting consistently — blot each cylinder the same number of times.
- Temperature variation — keep all solutions at the same temperature (e.g., in the same room/water bath).
- Not enough repeats — repeat three times at each concentration and calculate a mean to improve reliability.
Required Practical 4: Food Tests (Quantitative)
Specification reference: 4.2.2
Aim: Use quantitative reagents to test for sugars, estimating the concentration of glucose solutions using semi-quantitative Benedict's test.
Method
- Prepare a range of known glucose concentrations (e.g., 0%, 0.5%, 1.0%, 2.0%, 4.0%).
- Add equal volumes of each glucose solution to separate test tubes.
- Add equal volumes of Benedict's solution to each test tube.
- Heat all test tubes in a water bath at 75°C for 5 minutes.
- Observe the colour change in each tube and create a colour reference chart.
- Test an unknown glucose solution using the same method and compare its colour to the chart to estimate its concentration.
Variables
| Variable | Detail |
|---|
| Independent variable | Concentration of glucose solution |
| Dependent variable | Colour change (intensity) of Benedict's solution |
| Control variables | Volume of glucose solution, volume of Benedict's solution, temperature, time heated |
Expected Results
- 0% glucose: stays blue.
- 0.5% glucose: green.
- 1.0% glucose: yellow.
- 2.0% glucose: orange.
- 4.0% glucose: brick red.
The unknown sample is compared to this colour gradient to estimate its glucose concentration.
Sources of Error and Improvements
- Subjective colour comparison — use a colorimeter to measure the absorbance of each sample for a more objective, quantitative result.
- Inconsistent heating — use a thermostatically controlled water bath.
- Colour chart fading — prepare fresh standards each time.
Exam Tip: The difference between Required Practical 2 (qualitative) and Required Practical 4 (quantitative) is that qualitative tells you if a substance is present, while quantitative estimates how much. AQA frequently tests whether you understand this distinction.
Required Practical 5: Enzymes and pH
Specification reference: 4.2.2
Aim: Investigate the effect of pH on the rate of reaction of amylase enzyme.
Method
- Set up a spotting tile with drops of iodine solution in each well.
- Add amylase solution and starch solution to a test tube.
- Add a buffer solution of a specific pH (e.g., pH 2, 4, 6, 7, 8, 10).
- Start a timer.
- Every 30 seconds, take a sample using a pipette and add it to a well of iodine on the spotting tile.
- Record the time taken for the iodine to stop turning blue-black — this indicates all the starch has been digested by amylase.
- Repeat for each pH value.
graph TD
A["Mix amylase + starch<br/>+ buffer solution"] --> B["Start timer"]
B --> C["Sample every 30 seconds<br/>onto iodine on spotting tile"]
C --> D{"Iodine stays<br/>brown-orange?"}
D -- "No (blue-black)" --> C
D -- "Yes" --> E["Record time:<br/>all starch digested"]
style A fill:#4a90d9,color:#fff
style E fill:#27ae60,color:#fff
Variables
| Variable | Detail |
|---|
| Independent variable | pH of the buffer solution |
| Dependent variable | Time taken for starch to be completely digested (iodine stays brown-orange) |
| Control variables | Temperature, concentration and volume of amylase, concentration and volume of starch, sampling interval |
Expected Results
- Amylase works fastest at its optimum pH (approximately pH 7 for salivary amylase).
- At pH values above or below the optimum, the enzyme's active site changes shape (denaturation at extreme pH values), and the rate of reaction decreases.
- At very low pH (e.g., pH 2) or very high pH (e.g., pH 10), the enzyme is denatured and the starch is not digested — iodine remains blue-black.
Sources of Error and Improvements
- Temperature fluctuations — carry out the experiment in a thermostatically controlled water bath to keep temperature constant.
- Inconsistent sampling — always take samples at exactly the same time intervals.
- Amylase contamination — use a clean pipette for each sample.
- Difficulty judging exact endpoint — the iodine test is somewhat subjective; using a colorimeter to measure starch concentration would give more precise, continuous data.
Required Practical 6: Photosynthesis Rate
Specification reference: 4.4.1
Aim: Investigate the effect of light intensity on the rate of photosynthesis using an aquatic plant (e.g., Elodea/pondweed).