Core Practicals

Edexcel GCSE Biology has eight core practicals that you must know in detail. There is no separate practical exam — instead, questions about core practicals appear within Paper 1 and Paper 2.

You can be asked to describe the method, identify variables, explain results, evaluate the investigation, or suggest improvements. Examiners expect precise, detailed answers about these experiments.

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Overview of the Eight Core Practicals

CP	Title	Paper
CP1	Investigate biological specimens using microscopes	Paper 1
CP2	Investigate the effect of antiseptics or antibiotics on bacterial growth	Paper 1
CP3	Investigate the effect of pH on enzyme activity	Paper 1
CP4	Investigate the germination and growth of organisms	Paper 1
CP5	Investigate the effect of light intensity on the rate of photosynthesis	Paper 2
CP6	Investigate the rate of reaction of amylase using different methods	Paper 2
CP7	Investigate the effect of exercise on breathing rate and pulse rate	Paper 2
CP8	Investigate the vitamin C content of fruit juice	Paper 2

Exam tip: Core practical questions can appear in any section of the relevant paper. They are not always flagged with "core practical" in the question — you need to recognise them from the context.

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CP1 — Investigate Biological Specimens Using Microscopes

Purpose: To observe and draw biological specimens at different magnifications.

Method:

1. Place a thin specimen on a glass slide and add a drop of water or stain (e.g., iodine for plant cells, methylene blue for animal cells).
2. Lower a coverslip at an angle to avoid air bubbles.
3. Place the slide on the microscope stage and clip it in place.
4. Start with the lowest power objective lens.
5. Focus using the coarse focus knob, then the fine focus knob.
6. Increase magnification by rotating to a higher power objective lens and refocusing with the fine focus only.
7. Draw what you observe — drawings should be clear, in pencil, with labels and a title.

Key Variables:

• Independent variable: Magnification used
• Dependent variable: Detail observed in the specimen
• Control variables: Same specimen, same stain, same light intensity

Magnification Calculation:

• Magnification = Image size ÷ Actual size (M = I ÷ A)
• To find actual size: Actual size = Image size ÷ Magnification

Drawing Rules:

• Use a sharp pencil with clear, continuous lines (no sketchy lines)
• Include a title, labels, and magnification
• Drawing should be proportional to the specimen
• No shading

Exam tip: You may be asked to calculate the actual size of a cell from a micrograph. Make sure both measurements are in the same units before dividing. Remember: 1 mm = 1000 μm.

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CP2 — Investigate the Effect of Antiseptics or Antibiotics on Bacterial Growth

Purpose: To compare the effectiveness of different antiseptics or antibiotics at killing or inhibiting bacterial growth.

Method:

1. Sterilise all equipment (autoclave or Bunsen burner) to prevent contamination.
2. Pour sterile nutrient agar into a Petri dish and allow it to set.
3. Spread a bacterial culture evenly over the agar surface using a sterile spreader.
4. Place paper discs soaked in different antiseptics/antibiotics on the agar.
5. Include a control disc soaked in sterile water.
6. Tape the lid on (do not seal completely — allow air in to prevent anaerobic pathogens growing).
7. Incubate at 25°C in school labs (not 37°C, to reduce the risk of growing harmful pathogens).
8. After incubation, measure the diameter of the clear zone (zone of inhibition) around each disc.

Key Variables:

• Independent variable: Type of antiseptic/antibiotic
• Dependent variable: Diameter of the zone of inhibition (mm)
• Control variables: Volume of antiseptic on each disc, type of bacteria, incubation temperature, incubation time, size of paper disc

Calculating the Area of the Zone of Inhibition:

• Measure the diameter of the clear zone in mm.
• Area = π × r² (where r = diameter ÷ 2)
• A larger clear zone means the antiseptic/antibiotic is more effective.

Exam tip: You must know why the maximum incubation temperature in school labs is 25°C — it reduces the risk of growing harmful human pathogens, which thrive at 37°C (body temperature).

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CP3 — Investigate the Effect of pH on Enzyme Activity

Purpose: To determine how pH affects the rate of an enzyme-catalysed reaction.

Method:

1. Set up a water bath at a constant temperature (e.g., 35°C).
2. Add amylase solution and starch solution to separate test tubes and place them in the water bath to equilibrate.
3. Add a buffer solution of a specific pH to the amylase.
4. Mix the amylase with the starch and start a timer.
5. Every 30 seconds, use a dropping pipette to place a drop of the mixture onto a spotting tile containing iodine solution.
6. Iodine turns blue-black in the presence of starch. When the iodine stays orange-brown, all the starch has been broken down.
7. Record the time taken for the starch to be fully digested.
8. Repeat with different pH buffer solutions.

Key Variables:

• Independent variable: pH of the buffer solution
• Dependent variable: Time taken for starch to be completely digested
• Control variables: Temperature, concentration and volume of amylase, concentration and volume of starch

Expected Results:

• Amylase works fastest at its optimum pH (around pH 7).
• At very low or very high pH values, the enzyme is denatured — the active site changes shape and the substrate no longer fits.

Exam tip: Remember to say "the active site changes shape" rather than "the enzyme is destroyed." Denaturation is a change in shape, not destruction of the molecule.

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CP4 — Investigate the Germination and Growth of Organisms

Purpose: To investigate factors affecting seed germination or organism growth (e.g., the effect of light or gravity on seedling growth).

Method (example: effect of light on cress seedling growth):

1. Line several Petri dishes with damp cotton wool.
2. Place the same number of cress seeds in each dish.
3. Place each dish in a different light condition (e.g., full light, partial light, dark).
4. Keep all other conditions the same (temperature, water).
5. Measure the height of seedlings after a set period (e.g., 7 days).
6. Calculate the mean height for each condition.

Key Variables:

• Independent variable: Light condition (or whichever factor is being tested)
• Dependent variable: Height of seedlings / number of seeds germinated
• Control variables: Number of seeds, type of seeds, volume of water, temperature

Exam tip: This practical tests your ability to plan a fair test. Examiners may ask you to design a similar experiment — make sure you identify variables and explain how you would control them.

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CP5 — Investigate the Effect of Light Intensity on the Rate of Photosynthesis

Purpose: To determine how light intensity affects the rate of photosynthesis using an aquatic plant (e.g., Elodea / pondweed).

Method:

1. Place a piece of pondweed in a beaker of water with a small amount of sodium hydrogen carbonate (to provide CO₂).
2. Position a lamp at a known distance from the beaker.
3. Wait 2 minutes for the plant to acclimatise.
4. Count the number of oxygen bubbles produced per minute (or collect gas in a measuring cylinder over a set time).
5. Move the lamp to a different distance and repeat.
6. Repeat each distance at least 3 times and calculate a mean.

Key Variables:

• Independent variable: Distance of lamp from plant (used as a proxy for light intensity)
• Dependent variable: Number of oxygen bubbles per minute (rate of photosynthesis)
• Control variables: Temperature of water, concentration of sodium hydrogen carbonate, length/mass of pondweed, colour of light

Key Relationship:

• Light intensity is inversely proportional to the square of the distance: Light intensity ∝ 1/d²
• As the lamp moves closer, light intensity increases and the rate of photosynthesis increases — up to a point where another factor (CO₂ or temperature) becomes limiting.