IB Biology: Transport in Plants — FAQs

Common mistake: writing "water evaporates from the leaf so more is pulled up" without naming cohesion, adhesion, or the water potential gradient. That description alone scores zero on a 'explain' command term question.

Quick tip: whenever a question says explain transpiration, your answer needs four linked ideas in order: evaporation from mesophyll cells → lowered water potential in the leaf → tension pulling the water column → cohesion between water molecules holding the column continuous through the xylem.

According to the IB Biology guide (first assessment 2025), Structure 2.4 sits in the AHL-inclusive section, meaning HL papers can probe application questions — comparing xerophytic leaf structure across different biomes, for instance — that SL Paper 2 rarely does.

Worked example — a typical 6-mark exam response, in order:

1. Transpiration removes water from mesophyll cell surfaces (evaporation).
2. This lowers water potential at the leaf end of the xylem.
3. Water moves from xylem into mesophyll cells down the water potential gradient.
4. Tension is transmitted down the whole water column due to cohesion.
5. Adhesion between water molecules and xylem vessel walls resists gravity.
6. Root pressure adds a smaller contribution at the base.

Miss any one of these steps and examiners typically cap the answer at 3-4 marks out of 6.

Quick tip: examiners often ask you to explain why phloem transport is bidirectional while xylem isn't. The answer is that direction depends entirely on where the source and sink are at that moment — a leaf can be a source in summer and a sink for stored sugars during regrowth, but xylem only ever moves water from roots upward.

3 things to check before your next mock:

1. Have you named the correct tissue (xylem vs phloem) for each process?
2. Can you state whether the process is passive (transpiration) or requires energy (phloem loading)?
3. Do you know that stomata regulate transpiration rate via guard cell turgor, not xylem structure itself?

Common mistake: naming an adaptation ('rolled leaves') without explaining the mechanism ('traps humid air near stomata, reducing the concentration gradient for water vapour diffusion, which slows transpiration'). The IB mark scheme almost always rewards the mechanism over the label.

Quick tip: when a potometer question shows a bubble moving faster under a fan, don't just say "transpiration increased" — explain that increased air movement removes the humid boundary layer around the leaf, steepening the water vapour concentration gradient and increasing the rate of diffusion out of stomata.

The command terms to watch for: 'describe' wants observable trends and figures from the data; 'explain' demands the underlying mechanism (water potential, cohesion, active loading); 'compare' requires you to state similarities as well as differences — leaving out similarities is a very common way to drop a mark on comparison questions.

1. Direction confusion: writing that phloem "only moves sugars down from leaves to roots" — it's bidirectional, source to sink.
2. Vague mechanism: saying "transpiration pulls water up" without naming cohesion or tension.
3. Overstating root pressure: it matters in short plants but cohesion-tension dominates in tall trees — conflating the two loses marks on evaluate-style questions.

Quick tip: build a two-column revision sheet — one side lists the process step, the other lists the exact IB vocabulary term the mark scheme expects for it. Reciting this out loud before a mock genuinely changes how specific your written answers become.

For parents helping from outside the subject: the goal isn't re-teaching biology, it's checking your child can explain the mechanism out loud in under a minute — if they can't, that's the gap worth flagging before a topical worksheet session.