Three classic investigations you can do in a school lab or a kitchen — strawberry DNA extraction, the photosynthesis leaf-disk assay, and osmosis in potato tissue. Each one comes with a complete, worked lab-report example written the way the IB expects: a focused research question, controlled variables, real-looking example data, processed results, a conclusion, and an honest evaluation.
✍️ How to write the report 🍓 DNA extraction 🌿 Photosynthesis 🥔 Osmosis
In the IB Diploma, your practical work is assessed through one Internal Assessment (IA): a single scientific investigation of your own, written up in at most 3,000 words and marked out of 24 across four criteria — Research Design, Data Analysis, Conclusion, and Evaluation (6 marks each). The three investigations below are models: each one shows the exact structure a strong IA report follows.
One sentence that names the independent variable (what you change) and the dependent variable (what you measure, with units). Add a short background section with the science theory and cited sources, then a hypothesis — a prediction with a scientific reason, not a guess.
Make a variables table: independent, dependent, and every controlled variable — with how you control it and why it matters. Write the method so another student could repeat it exactly: numbered steps, quantities, at least 5 repeats or 5 levels where possible, and a note on safety, ethics, and waste.
Present raw data in a table with units and measurement uncertainties in the headings. Then process it: show one full sample calculation, calculate means, and draw a graph with a title, labelled axes with units, and error bars when you have repeats. Percentage change is your friend when starting amounts differ.
State directly what the data shows — quote processed values, name the trend, and say whether the hypothesis is supported. Then compare with the accepted science: a literature value, a textbook expectation, or the underlying theory.
Name real limitations and the direction of their effect on your results (not “human error” — say which error: heat loss, evaporation, reaction time). For each one, give a realistic improvement. Finish with an extension: the next investigation your results suggest.
Why a strawberry? Strawberries are octoploid — each cell carries eight copies of every chromosome — so they are packed with DNA. This practical makes an invisible molecule visible, and it teaches the logic of every DNA extraction used in real laboratories: break the cells open, free the DNA from its proteins, and precipitate it where you can collect it.
Research question: How does the temperature of the ethanol (ice-cold, ≈ 0 °C, versus room temperature, ≈ 22 °C) affect the mass of DNA that can be extracted from 30 g of strawberry tissue?
Hypothesis: Ice-cold ethanol will yield more DNA. DNA is insoluble in ethanol, and cold ethanol lowers its solubility further while slowing DNase enzymes that break DNA down — so more DNA should precipitate and be collectable.
| Variable | Detail |
|---|---|
| Independent | Ethanol temperature: ice-cold (≈ 0 °C) vs room temperature (≈ 22 °C) |
| Dependent | Mass of spooled DNA collected (g, ± 0.01 g) |
| Controlled | Same strawberry batch and mass (30 g); same extraction-buffer recipe and volume (50 cm³); same mashing time (2 min); same ethanol volume (20 cm³) and contact time (3 min); same spooling technique and time |
Materials: ripe strawberries, zip-seal bag, extraction buffer (100 cm³ water + 2 teaspoons washing-up liquid + 1 teaspoon table salt — made up fresh), coffee filter or muslin cloth, funnel, two beakers/test tubes, ice-cold ethanol (95%) and room-temperature ethanol, wooden skewer or glass rod, balance (± 0.01 g), ice bath, thermometer.
Safety: Ethanol is flammable — keep away from all flames and do not use Bunsen burners in this practical. Never eat or drink laboratory materials, even food-based ones. Wash hands afterwards; check for strawberry or latex allergies before starting.
| Ethanol | Trial 1 (g) | Trial 2 (g) | Trial 3 (g) | Mean (g) |
|---|---|---|---|---|
| Ice-cold (≈ 0 °C) | 0.45 | 0.39 | 0.42 | 0.42 |
| Room temp. (≈ 22 °C) | 0.20 | 0.16 | 0.18 | 0.18 |
Sample calculation: mean for ice-cold ethanol = (0.45 + 0.39 + 0.42) ÷ 3 = 0.42 g. Ice-cold ethanol yielded 0.42 ÷ 0.18 ≈ 2.3× more DNA than room-temperature ethanol.
Sample conclusion: The hypothesis is supported. Cold ethanol precipitated more than twice as much DNA as room-temperature ethanol, because low temperature reduces DNA solubility in ethanol and slows DNase activity, so a larger fraction of the DNA survived to be spooled.
Sample evaluation: (1) The spooled material is wet and traps buffer, so masses overestimate pure DNA — improvement: rinse the spool in fresh cold ethanol and dry to constant mass. (2) Spooling is subjective — some DNA is always left behind; improvement: standardise spooling time and use the same operator, or centrifuge and weigh the pellet instead. Extension: compare fruits — kiwi and banana are also DNA-rich; which fruit yields the most DNA per gram?
The idea: Leaf disks are made to sink by pulling the air out of them with a syringe (vacuum infiltration) and replacing it with sodium hydrogencarbonate solution, which supplies CO₂. Under light, photosynthesis releases oxygen into the leaf tissue, the disks become buoyant, and they float. The faster they float, the faster the photosynthesis.
Research question: How does light intensity (lamp distance 10–50 cm) affect the rate of photosynthesis of spinach leaf disks, as measured by ET₅₀?
Background in one line: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. ET₅₀ is the time taken for 50% of the disks to float; because a shorter ET₅₀ means a faster rate, the rate is proportional to 1 ÷ ET₅₀.
| Variable | Detail |
|---|---|
| Independent | Lamp distance: 10, 20, 30, 40, 50 cm (light intensity ∝ 1 ÷ distance²) |
| Dependent | ET₅₀ — time for 50% of 10 disks to float (min, ± 0.1 min); rate = 1 ÷ ET₅₀ |
| Controlled | Same leaf and disk diameter; 10 disks per trial; same hydrogencarbonate concentration (0.2%); same solution volume and container; LED lamp (minimal heat); same room temperature; 3 trials per distance |
Materials: fresh spinach leaves, cork borer or hole punch, 10 cm³ syringe (no needle), 0.2% sodium hydrogencarbonate solution with one drop of washing-up liquid (a surfactant that helps the solution enter the leaf), clear cups, LED lamp, ruler, stopwatch.
Safety: Take care with hot lamps — an LED avoids most heating. Wipe up spills (slip hazard). Wash hands after handling leaf material and solutions.
| Distance (cm) | ET₅₀ trial 1 | ET₅₀ trial 2 | ET₅₀ trial 3 | Mean ET₅₀ (min) | Rate 1 ÷ ET₅₀ (min⁻¹) |
|---|---|---|---|---|---|
| 10 | 4.5 | 4.0 | 4.1 | 4.2 | 0.238 |
| 20 | 8.2 | 7.5 | 7.7 | 7.8 | 0.128 |
| 30 | 13.1 | 12.0 | 12.4 | 12.5 | 0.080 |
| 40 | 20.3 | 19.0 | 19.5 | 19.6 | 0.051 |
| 50 | 29.5 | 27.6 | 28.1 | 28.4 | 0.035 |
Sample calculation: at 10 cm, rate = 1 ÷ 4.2 min = 0.238 min⁻¹. Graph rate (y-axis) against light intensity or 1 ÷ distance² (x-axis): the curve rises steeply at first, then begins to level off — at high light another factor (CO₂ supply or enzyme capacity) becomes limiting.
Sample conclusion: As light intensity increased, ET₅₀ fell and the calculated rate rose from 0.035 to 0.238 min⁻¹ — about seven times faster. This matches theory: more light energy drives faster photolysis and ATP/NADPH production, until a different factor limits the rate.
Sample evaluation: (1) Disks differ in thickness and chlorophyll content — improvement: take all disks from one leaf region and randomise allocation. (2) The floating endpoint is judged by eye — improvement: define “floating” as touching the surface and have one timer for all trials. (3) Even LED lamps warm the solution slightly at 10 cm, which would raise enzyme rates and exaggerate the trend — improvement: add a water heat-shield or monitor solution temperature. Extension: repeat with different hydrogencarbonate concentrations to test CO₂ as a limiting factor.
The idea: Potato cylinders are left in sucrose solutions of different concentrations. Water moves by osmosis: into the tissue from weak solutions (mass increases), out of the tissue into strong solutions (mass decreases). The concentration at which the mass does not change is isotonic with the cells — a neat way to estimate the solute concentration, and even the solute potential, of living tissue.
Research question: How does sucrose concentration (0.0–1.0 mol dm⁻³) affect the percentage change in mass of potato cylinders after 24 hours — and at which concentration is the solution isotonic to the tissue?
Hypothesis: Percentage change in mass will fall as sucrose concentration rises, crossing zero near the tissue’s own concentration (about 0.3 mol dm⁻³ for potato), because the water-potential gradient reverses at that point.
| Variable | Detail |
|---|---|
| Independent | Sucrose concentration: 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm⁻³ |
| Dependent | Percentage change in mass after 24 h (%, from masses in g, ± 0.01 g) |
| Controlled | Same potato (variety and tuber); cylinders cut with the same cork borer and trimmed to 30 mm; same solution volume (20 cm³) fully covering each cylinder; containers covered; same temperature and duration; same blotting technique; 3 cylinders per concentration (use 5 in your own IA) |
Materials: large potato, cork borer, scalpel and ruler, white tile, balance (± 0.01 g), 6 boiling tubes or cups, sucrose solutions 0.0–1.0 mol dm⁻³, paper towels, cling film, marker.
Safety: Cut away from your body with the scalpel and use a tile; hand in sharps immediately after use. Do not eat any laboratory material. Wipe up sucrose spills.
| Sucrose (mol dm⁻³) | % change, trial 1 | Trial 2 | Trial 3 | Mean % change |
|---|---|---|---|---|
| 0.0 | +12.4 | +10.9 | +11.2 | +11.5 |
| 0.2 | +5.8 | +4.6 | +5.2 | +5.2 |
| 0.4 | −3.1 | −4.4 | −3.9 | −3.8 |
| 0.6 | −9.0 | −10.2 | −9.6 | −9.6 |
| 0.8 | −13.5 | −14.8 | −14.0 | −14.1 |
| 1.0 | −17.2 | −18.5 | −18.0 | −17.9 |
Sample calculation: a cylinder of 2.05 g ending at 1.85 g: % change = (1.85 − 2.05) ÷ 2.05 × 100 = −9.8%. Plotting mean % change against concentration, the line crosses zero between 0.2 (+5.2%) and 0.4 (−3.8 mol dm⁻³): interpolating, zero change ≈ 0.2 + 0.2 × (5.2 ÷ 9.0) ≈ 0.32 mol dm⁻³. At this point the tissue’s solute potential can be estimated: Ψs = −iCRT = −(1 × 0.32 × 0.0831 × 293) ≈ −7.8 bar ≈ −0.78 MPa.
Sample conclusion: The hypothesis is supported. Mass change fell steadily from +11.5% in distilled water to −17.9% in 1.0 mol dm⁻³ sucrose, and the isotonic point, ≈ 0.32 mol dm⁻³, matches the expected solute concentration of potato tissue. Below it, solutions had a higher water potential than the cells, so water entered by osmosis; above it, water left.
Sample evaluation: (1) Blotting is the largest error source — under-blotting inflates final mass; improvement: standardise blotting (same towel, same seconds, same pressure). (2) Cylinders vary in skin and damaged surface cells, which leak solutes; improvement: rinse cylinders before the start and use more replicates (5+). (3) Evaporation over 24 h concentrates the solutions slightly, shifting results downward; covers were used, and a balance check of solution volumes would quantify it. Extension: compare tissues — sweet potato, carrot, and apple have different solute stores.
Pick one investigation, make the research question your own, and follow the five steps. Then check your science in the Lessons, practise terms in the Quizzes, and explore the molecules you met here in the Molecule Explorer.
References & further reading: OpenStax, Biology 2e (photosynthesis, osmosis and water potential); IB Biology Guide — Internal Assessment criteria (Research Design, Data Analysis, Conclusion, Evaluation); standard school protocols for the leaf-disk assay and strawberry DNA extraction. Example data on this page are illustrative teaching values — your report must use your own measurements.