Lesson 17: Required Practicals — Biology

The biology required practicals are not optional extras — they are examined directly, with questions worth significant marks asking you to describe methods, explain results, and evaluate techniques. This lesson walks through each one: microscopy, osmosis in potato, food tests, enzymes and pH, photosynthesis, and field sampling. For each, learn the method in order, the variables, how to process the data, and the common exam questions. Examiners want to see that you have actually done these — vague answers score poorly.

Practical 1 — Microscopy (Onion Cells)

Method: peel a thin layer of onion epidermis with tweezers; place on a slide with a drop of iodine stain; lower a coverslip at an angle to exclude air bubbles; start on the lowest magnification to locate the cells, focus with the coarse then fine wheel, then move to higher magnification; draw several cells large and clear with ruled label lines (cell wall, nucleus, cytoplasm). Calculate magnification or actual size using magnification = image size ÷ real size.

Variables and technique: the independent variable could be the magnification used; you control the stain and sample. Key skills: avoiding air bubbles (lower coverslip carefully), finding the sample (always start low power), and drawing scientifically — large, clear outlines, no shading, accurate labels. Iodine stains the nucleus yellow-brown, making it visible; without stain, cells are nearly transparent.

Exam questions: "a student's slide shows air bubbles — explain how to avoid this"; magnification calculations with unit conversions (1 mm = 1,000 µm); "why is a stain used?"; comparing electron and light microscopes (magnification and resolution). Higher-tier: explain why electron microscopes reveal more detail (shorter wavelength of electrons gives higher resolution).

Practical 2 — Osmosis in Potato

Method: cut equal-sized potato cylinders with a cork borer and trim to identical length; blot dry and weigh each (initial mass); place in boiling tubes with different sugar concentrations (e.g. 0.0–1.0 mol/dm³); leave for a set time (e.g. 24 hours, or 30 minutes in lesson time); remove, blot dry, reweigh (final mass); calculate percentage change = (change ÷ initial) × 100. Plot percentage change against concentration.

Variables: independent — sugar concentration; dependent — percentage mass change; controls — potato type, cylinder size, volume of solution, time, temperature. Using percentage change (not absolute change) allows fair comparison if initial masses differ slightly. The graph typically crosses zero: at low concentrations potato gains mass (water enters by osmosis), at high concentrations it loses mass; the intercept estimates the potato's internal concentration.

Exam questions focus on: explaining the results with osmosis (water moves from high to low water concentration across the partially permeable cell membrane); why percentage change is used; identifying the concentration matching the potato's cells; and evaluating (e.g. "the cylinders were different lengths — how does this affect validity?"). Repeats and means improve reliability; a water bath controls temperature.

Practical 3 — Food Tests

Tests to know: starch — add iodine, orange-brown to blue-black; reducing sugars (glucose) — add Benedict's solution, heat in a water bath, blue to brick-red precipitate; protein — add biuret solution (sodium hydroxide + copper sulphate), blue to purple/lilac; lipids — shake with ethanol, pour into water, white emulsion forms. For non-reducing sugars (sucrose): boil with dilute HCl first (hydrolysis), neutralise with sodium hydrogen carbonate, then Benedict's.

Method for an unknown food: grind with water, filter, then test the solution with each reagent in separate test tubes. Safety: wear goggles (Benedict's contains copper compounds; biuret contains sodium hydroxide), use a water bath not a Bunsen flame for heating (ethanol is flammable in the lipid test). Record results in a table with observations and conclusions for each test.

Exam questions: give a table of test results and ask which food groups are present; ask you to describe the test for a named nutrient; or present a student's flawed method and ask for improvements (e.g. "she heated the lipid test over a Bunsen — why is this dangerous and what should she do?"). Quantitative extension: colorimeters measure Benedict's colour intensity to estimate sugar concentration — linking to calibration curves.

Practical 4 — Enzymes and pH

Method: set up water baths or test tubes at different pH values (using buffer solutions, e.g. pH 5–9); add starch solution and amylase to each; start a timer; sample every 30 seconds, placing a drop on a spotting tile with iodine; record the time until the iodine stays orange-brown (no starch remains). The rate = 1 ÷ time. Plot rate against pH — expect a peak at the optimum (~pH 7 for amylase).

Variables: independent — pH; dependent — time for starch breakdown (rate); controls — temperature (water bath), amylase and starch concentrations and volumes, sampling interval. Continuous sampling with a spotting tile is the classic technique; an alternative is a colorimeter. Safety: buffers may irritate — wear goggles; water baths are safer than Bunsen burners.

Exam questions: explain the shape of the curve (below optimum, fewer successful collisions; above optimum, the enzyme denatures as bonds break and the active site changes shape); identify the optimum pH from data; suggest why the student used buffers not just acid/alkali drops (buffers hold pH constant); evaluate (e.g. temperature was not controlled — how would this affect results?). This practical links directly to Lesson 2's enzyme theory.

Practical 5 — Photosynthesis (Pondweed)

Method: place pondweed (Cabomba) in a boiling tube of water with sodium hydrogen carbonate (provides CO₂); illuminate with a lamp at measured distances; count oxygen bubbles per minute (or collect gas in an inverted capillary/upturned test tube and measure volume); move the lamp to different distances; plot rate against 1/distance² (light intensity ∝ 1/d²). Control temperature with a water bath (the lamp heats the tube) and use the same piece of pondweed.

Variables: independent — light intensity (distance); dependent — rate of oxygen production; controls — CO₂ concentration, temperature, pondweed amount, time. Inverse-square law: doubling the distance quarters the intensity — exam questions test this calculation ("the lamp moves from 10 cm to 20 cm — what happens to intensity?" → falls to a quarter).

Exam questions: explain why the rate plateaus at high light intensity (another factor — CO₂ or temperature — becomes limiting); describe how to measure the gas more accurately (collect in a graduated tube over time); suggest why counting bubbles is less reliable (bubbles vary in size). Limiting-factor graphs (rate vs light, CO₂, temperature) are a favourite — learn all three shapes and their explanations.

Practical 6 — Field Sampling (Quadrats and Transects)

Method (random quadrats): generate random coordinates (random number tables or a phone app); place a 1 m² quadrat; count the target species or estimate percentage cover; repeat at least 10 times; calculate the mean. Population estimate = mean per quadrat × (total area ÷ quadrat area). Method (transect): lay a tape from one habitat to another (e.g. path to woodland); place quadrats at regular intervals; record species at each — shows zonation along an environmental gradient.

Variables and validity: random sampling avoids bias (not choosing "nice" spots); large sample sizes improve reliability; measuring abiotic factors (light with a lux meter, soil pH, moisture) at each quadrat lets you correlate distribution with conditions. Safety/ethics: do not disturb wildlife, return organisms, wear gloves, beware of tides on rocky shores.

Exam questions: calculate estimated population sizes from quadrat data; explain why random sampling is used; suggest why a transect is better than random quadrats for studying change along a gradient; evaluate a student's method ("she placed quadrats where daisies looked densest — what is wrong and how does it affect the results?"). Data skills: means, modes, and presenting results as bar charts or kite diagrams.

Conclusion — Why This Matters

The biology required practicals test whether you can think like a biologist: observe carefully, measure precisely, control variables, and interpret honestly. For the exam, learn each practical as a method sequence with its variables, its data processing (percentage change, 1/time, Rf-like calculations where relevant), and its characteristic evaluation points. The common threads across all six: repeats and means excluding anomalies, controlling the key variables (temperature, pH, concentration, time), choosing the right measurement (mass change for osmosis, time for enzymes, counts for sampling), and explaining results with the underlying biology (osmosis, enzyme kinetics, limiting factors). When faced with an unfamiliar investigation in the exam, apply the template: variables → method → data processing → conclusion → evaluation. Examiners are not testing your memory of these exact experiments so much as your ability to do science — the practicals are simply the context. The student who can plan, execute and critique an investigation has learned something far more valuable than any single result: the method by which biology advances.

Worked example — enzymes and pH, full data treatment

Students tested amylase at five pH values, recording the time for starch to disappear: pH 5 — 240 s; pH 6 — 120 s; pH 7 — 60 s; pH 8 — 90 s; pH 9 — 300 s. Converting to rate (1 ÷ time): 0.0042, 0.0083, 0.0167, 0.0111 and 0.0033 s⁻¹. Plotted, the rates peak at pH 7 — the optimum for this amylase. Either side, the rate falls because the active site shape is distorted by the changed pH, and at extreme pH the enzyme is denatured: bonds break, the active site no longer fits the substrate, and the effect is permanent.

Evaluation: pH must be maintained with buffer solutions, not a one-off drop of acid; temperature must be controlled in a water bath because it is a second variable that changes rate; and sampling every 10 seconds limits time resolution — a colorimeter measuring starch concentration directly would give a continuous, objective rate curve.

Common mistakes in this set of practicals

• Not blotting potato cylinders before reweighing — surface solution inflates the final mass and wrecks the percentage change.
• Counting pondweed bubbles of different sizes as equal — measure the volume of gas collected in a syringe instead.
• Choosing "nice" quadrat positions. Placement must be random, or the sample is biased however carefully you count.

Exam Experience

Question 1. Describe how to prepare a microscope slide of onion epidermis so that the cells can be viewed clearly, avoiding air bubbles. [2 marks]

Question 2. A potato cylinder has an initial mass of 6.40 g. After 24 hours in a concentrated sugar solution its mass is 5.76 g. Calculate the percentage change in mass, and state the direction of water movement. [3 marks]

Question 3. A student investigates the effect of light intensity on the rate of photosynthesis using pondweed. Describe a method she could use, including how she would make the results valid and reliable. [6 marks]

Mark scheme

Q1: place a thin layer of epidermis/peel flat on a drop of water on the slide and add a stain such as iodine (1); lower the coverslip slowly at an angle (with a mounted needle) so air is pushed out rather than trapped (1).

Q2: change = 5.76 − 6.40 = −0.64 g; percentage change = (−0.64 ÷ 6.40) × 100 = −10.0% (1 method, 1 answer); mass fell, so water moved out of the cells by osmosis (1).

Q3: Level 3 (5–6 marks): a full, ordered method with control and reliability built in — pondweed in a beaker/funnel arrangement under water; a lamp at a set distance, with intensity changed by moving the lamp (relative intensity follows the inverse-square law) or using measured light levels; allow the plant to acclimatise at each intensity; count bubbles per minute over a fixed period, or better, collect gas in a graduated syringe/capillary and measure volume (bubbles vary in size); control variables: same piece of pondweed (or same length/mass), same water temperature (lamp heat shielded or LED used), same CO₂ availability (e.g. added sodium hydrogencarbonate); repeat at each distance and calculate a mean, identifying anomalies. Level 2 (3–4 marks): a workable method with some controls or repeats missing. Level 1 (1–2 marks): a brief outline lacking variables ("put weed near a lamp and count bubbles").