Paper 6 Skills: Alternative to Practical

01 What Paper 6 tests

Paper 6 (Alternative to Practical) is a written exam. You do not work in a lab, but you must show the same skills as Paper 5:

• Identify independent, dependent, and control variables.
• Complete or design results tables and graphs.
• Make or improve biological drawings and calculate magnification.
• Suggest errors, limitations, and improvements.
• Write a clear, numbered method for an investigation.
• Complete food tests, estimate populations with quadrats, and calculate rate or % change.

Theory notes explain what happens (e.g. osmosis in Chapter 3, enzymes in Chapter 5, photosynthesis in Chapter 6). This page explains how Cambridge tests investigation skills.

02 Independent, dependent & control variables

Independent variable (IV)

The factor you deliberately change. Only one IV in a fair test.

Dependent variable (DV)

The factor you measure. It depends on the IV.

Control variables

All other factors that could affect the result. Keep them constant.

Fair test

Only the IV changes; all control variables stay the same.

Exam-style scenario: Equal volumes of starch and amylase are mixed in water baths at 20°C, 30°C, 40°C, 50°C, and 60°C. Every 30 s a drop is tested with iodine until the blue-black colour disappears.

• IV: temperature of the water bath
• DV: time taken for starch to be digested
• Controls: volume and concentration of starch and amylase, pH, same testing method

Tip: Values listed with different numbers (20°C, 30°C…) are usually the IV. Words like “measured”, “recorded”, or “timed” point to the DV.

03 Control experiments, range & repeats

A control experiment is a comparison set-up where the factor being tested is absent or at a standard level, to show the effect is genuine.

• Photosynthesis: foil on a leaf (no light) or soda lime (no CO₂) — no starch made there (Chapter 6).
• Enzymes: boiled (denatured) enzyme with substrate — no reaction.

Range

Use at least five values of the IV, evenly spaced across a sensible range.

Repeats

Repeat each condition at least three times and calculate a mean.

Anomalies

Results that do not fit the pattern. Identify them and exclude from the mean with a reason.

Reliability

Consistent, repeatable results. Wide scatter means improve the method (e.g. thermostatic water bath).

State controls with quantities and units (e.g. 10 cm³, 30 minutes, 5 cm length), not vague “same amount”.

04 Results tables

A good Paper 6 table has:

• Clear column headings with units (e.g. Salt concentration / %, Change in mass / %)
• IV in the first column; DV (and repeats / mean) after
• Consistent decimal places; no units in every data cell

Percentage change = (final mass − initial mass) ÷ initial mass × 100. Blot potato cylinders dry the same way before each weighing.

05 Graphs

• Put the IV on the x-axis and the DV on the y-axis.
• Label both axes with quantity and unit.
• Choose an even scale that uses most of the grid.
• Plot points carefully; draw a best-fit line or smooth curve — do not join points with a ruler if the trend is curved.
• You may be asked to interpolate (read a value between points) or describe the trend.

From the sample osmosis table, 0% salt gives about +18% mass (water enters) and 2.0% salt about −15% (water leaves). The concentration where mean change is zero is the point matching the cells’ water potential — read it from the graph, do not guess.

06 Biological drawings & magnification

Paper 6 often asks you to draw a specimen or photomicrograph:

• Use a sharp, continuous outline — no sketchy lines.
• No shading or colouring in.
• Label with ruled horizontal lines that touch the structure; labels clear and outside the drawing.
• State magnification or include a scale bar if asked.

To find actual size from a photomicrograph: measure the image size (mm), convert to µm if needed (× 1000), then rearrange magnification. Full formula practice: Chapter 2.

07 Observations and food tests

Paper 6 often asks you to record colours or complete a results table. State the reagent, the condition (e.g. heat), and the positive colour. Full chemistry of the tests: Chapter 4.

A negative result is the original reagent colour (e.g. iodine stays brown-orange; Benedict’s stays blue). Do not invent a new colour.

08 Sampling with quadrats

To estimate how many plants grow in an area, you cannot count every individual. Use a quadrat (usually 0.25 m² or 1 m²).

• Random sampling: use random coordinates (or throw with eyes closed only if the question allows). Avoid choosing “typical-looking” patches — that is biased.
• Count the organisms (or % cover) in each quadrat.
• Calculate a mean per quadrat, then scale up: estimated number = mean × (total area ÷ quadrat area).
• More quadrats improve reliability.

Worked numbers: Mean of 6 plants per 0.25 m² quadrat. Field is 50 m². Number of quadrats that would fill the field = 50 ÷ 0.25 = 200. Estimate = 6 × 200 = 1200 plants.

09 Calculations, trends and conclusions

Typical Paper 6 calculations:

• Mean = sum of repeats ÷ number of repeats (exclude a named anomaly).
• Percentage change = (change ÷ original) × 100. Include the sign if mass is lost.
• Rate when you time a colour change or the disappearance of starch: rate = 1 / time. A shorter time is a faster rate.

Worked % change: Potato mass 8.0 g to 7.2 g. Change = −0.8 g. Percentage change = (−0.8 ÷ 8.0) × 100 = −10%.

When you describe a graph, say what happens to the DV as the IV increases, mention any peak or plateau, and use the data (with units). A conclusion links the pattern to the biology (e.g. “enzyme activity is highest at 40°C; above this the enzyme is denatured”) without inventing results that are not on the graph.

10 Errors, accuracy & improvements

Accuracy

How close a result is to the true value (better apparatus, careful technique).

Precision / reliability

How close repeats are to each other (more repeats, tighter method).

When asked for a source of error or improvement, be specific:

• Uneven blotting of potato → blot with the same number of tissues / same pressure.
• Temperature of water bath drifts → use a thermostatically controlled bath.
• Human reaction time with a stopwatch → use a data logger / light gate if appropriate, or time a larger change.
• Only two concentrations tested → use at least five values of the IV.

Do not write only “human error” — say what went wrong and how to fix it.

11 Writing a method

1. Aim — one sentence stating what you investigate.
2. Variables — IV, DV, and at least two controls with quantities.
3. Equipment — list apparatus with sizes where relevant.
4. Method — numbered steps; imperative verbs (Measure, Cut, Record, Repeat).
5. Safety — relevant hazards (iodine stain, cork borer, hot water).
6. Results table — headings and units ready.
7. Graph — state axes (IV on x, DV on y).

Do not put results or a conclusion inside the method.

12 Full worked example — osmosis in potato

Plan an investigation to find how salt concentration affects osmosis in potato tissue.

Independent variable

Concentration of salt solution (0%, 0.5%, 1.0%, 1.5%, 2.0%).

Dependent variable

Percentage change in mass of potato cylinders.

Control variables

• Same type of potato; cylinders cut to the same length and diameter
• Same volume of solution (e.g. 20 cm³) in each tube
• Same soaking time (e.g. 30 minutes)
• Same temperature; same blotting method before reweighing

Outline method

1. Cut five potato cylinders to 4 cm using a cork borer.
2. Prepare salt solutions of 0%, 0.5%, 1.0%, 1.5%, and 2.0%.
3. Weigh each cylinder and record the initial mass.
4. Place one cylinder in each solution for 30 minutes.
5. Remove, blot dry gently, reweigh, and calculate percentage change in mass.
6. Repeat the whole experiment three times and calculate means.
7. Plot mean % change in mass (y) against salt concentration (x).

Theory behind the mass changes: Chapter 3 · Diffusion and osmosis.

13 Second worked example — temperature and amylase

Plan an investigation to find how temperature affects the activity of amylase on starch.

Independent variable

Temperature (20°C, 30°C, 40°C, 50°C, 60°C) using a water bath.

Dependent variable

Time for the mixture to stop giving a blue-black colour with iodine / s; then rate = 1 / time.

Control variables

Equal volumes of starch and amylase (e.g. 5 cm³ of each); same concentrations; same pH (buffer if available); same drop size of iodine; same testing interval (e.g. every 30 s).

Outline method

1. Equilibrate separate tubes of starch and amylase in a 20°C water bath for 5 minutes.
2. Mix, start a stopwatch, and test a drop with iodine every 30 s until it stays orange-brown.
3. Record the time. Repeat at each temperature with fresh solutions.
4. Repeat the whole set three times; calculate mean times and rates.
5. Plot rate (y) against temperature (x). Expect a peak near the enzyme’s optimum.

Safety

Iodine stains skin and clothes; hot water baths can scald. Wear goggles; wipe spills.

Enzyme theory: Chapter 5.