Example 1 — "Explain", not "describe"

Explain how the polarity of water contributes to its use as a transport medium in plants. [4]

Water is a polar molecule, so hydrogen bonds form between adjacent water molecules. ✓ These hydrogen bonds give water cohesion, so a column of water in the xylem is pulled upwards as a continuous thread without breaking. ✓ Water also adheres to the polar cellulose of the xylem walls, which helps support the column against gravity. ✓ Because water is polar it is also a solvent for polar and ionic solutes, so mineral ions dissolve and are carried with it. ✓

4/4.

Each tick is a distinct idea, and every one traces back to polarity. A very common 1-mark answer lists cohesion, adhesion and solvency without ever saying why water has them — that is a description, and the question said explain. The word "polar" or "hydrogen bond" must appear.

Example 2 — "Distinguish" must be written as a comparison

Distinguish between the structure of DNA and RNA. [3]

A weak answer that scores 1:

DNA is a double helix with deoxyribose and the bases A, T, G, C. RNA has ribose and uracil.

A full answer:

DNA is double-stranded, whereas RNA is single-stranded. ✓ DNA contains deoxyribose, whereas RNA contains ribose. ✓ DNA uses thymine as a base, whereas RNA uses uracil in its place. ✓

3/3.

The difference is entirely in the construction. Both answers contain roughly the same information, but distinguish requires the two sides set against each other. "Whereas", "while" and "in contrast" are the words that turn two facts into a comparison. Two separate paragraphs, one on DNA and one on RNA, routinely score a fraction of the marks available.

Example 3 — Data-based reasoning (Paper 1B style)

A student measured the rate of water loss from two plant species over 6 hours in identical conditions. Species X lost 4.2 cm³ and species Y lost 1.1 cm³. (a) Suggest one reason for the difference. [2] (b) State one variable that must be controlled. [1]

(a)

Species Y may have fewer or smaller stomata, or stomata that close for longer, ✓ so less water vapour diffuses out of the leaf, reducing transpiration. ✓

(b)

The surface area of leaf exposed on each plant (accept: humidity, air movement, temperature, light intensity). ✓

"Suggest" tells you this is not recall. The plants are not in the syllabus; you are being asked to reason from what you know about transpiration to an unfamiliar case. Note the two-part shape of a 2-mark suggest: a proposed cause, then its consequence. One without the other scores 1.

For (b), "the plants" or "the conditions" would score nothing — a controlled variable must be named specifically enough that another student could control it.

Example 4 — Reading a cladogram

A cladogram shows species P and Q at adjacent tips, with species R branching further left. A student concludes that P and Q are the most closely related. Evaluate this conclusion. [3]

The conclusion may be correct, but it does not follow from tip position alone. ✓ Relatedness is determined by the position of the most recent common ancestor, that is, the branch point at which two lineages diverge — not by how close the tips appear on the diagram. ✓ If P and Q share a more recent branch point than either shares with R, then the conclusion is supported; the student must cite the branch point rather than the tips. ✓

3/3.

"Evaluate" requires a judgement plus reasons on the relevant side. An answer that simply says "this is wrong" scores nothing without the explanation. Note that the honest answer here is conditional — the student's conclusion might be right for the wrong reason, and saying so is better biology than a flat verdict.

Example 5 — A full "explain" on speciation

Explain how allopatric speciation can occur. [5]

A single population becomes divided by a geographical barrier, such as a river forming or sea level rising, so the two groups are physically separated. ✓ Gene flow between them stops, because individuals can no longer interbreed. ✓ The two environments present different selection pressures, so different alleles are favoured in each. ✓ Over many generations the allele frequencies of the two populations diverge, and mutations arising in one are not shared with the other. ✓ Eventually the populations differ enough that they can no longer produce fertile offspring even if reunited, so they are separate species. ✓

5/5.

The marks are in the sequence. Isolation → no gene flow → different selection → divergence → reproductive incompatibility. Students who know the material often lose two marks by jumping from "a river separates them" straight to "they become different species", skipping the mechanism in the middle. On a 5-mark explain, count your distinct steps before you start writing.

Educerie · original worked examples written against the published IB syllabus structure for Biology Theme A, first assessment 2025. All scenarios are our own. Last reviewed 5 September 2026.