MYP Biology: Transport in Plants FAQ

In MYP 4-5 Biology you need to explain how xylem transports water and phloem transports sugars, understand transpiration and translocation, and link tissue structure to function in leaves, stems and roots. You'll also be expected to interpret data from practical investigations such as potometer experiments.

Xylem carries water and dissolved minerals upward from roots to leaves in one direction only, using dead, hollow cells reinforced with lignin. Phloem transports sugars made in photosynthesis both up and down the plant, through living cells called sieve tubes supported by companion cells that provide energy.

Transpiration is the loss of water vapour from leaves, mostly through stomata, which creates a pull that draws more water up through the xylem from the roots. This process, called the transpiration stream, also helps cool the plant and keeps cells turgid for structural support.

Translocation is the movement of sugars, mainly sucrose, from source tissues like leaves where photosynthesis happens to sink tissues like roots, fruits or growing shoots where energy is needed or stored. It happens in the phloem, using energy from companion cells to load and unload sugars actively.

Transpiration speeds up in warm, dry, windy, bright conditions and slows down in cool, humid, still, dark ones, because these factors change how quickly water vapour diffuses out through stomata. Humidity has the strongest effect: high humidity outside the leaf reduces the diffusion gradient almost to zero.

Your teacher will usually assess this topic against Criterion A: Knowing and Understanding and Criterion C: Communicating, often through a data-based question on a transpiration experiment or an extended response comparing xylem and phloem. If your class designs its own investigation, Criterion B: Inquiring and Investigating also applies.

The error I see most is students writing that phloem "only moves sugars downward" — it actually moves them toward whichever tissue is acting as the sink, up or down. Another frequent slip is calling xylem cells living; they're dead, hollow tubes, and that's exactly what makes them efficient.

Try linking sound to function: "xylem" starts like "xylophone" — think dead, wooden tubes, and wood is xylem tissue. "Phloem" sounds like "flow" — phloem is where sugars flow in either direction around the living plant. Small memory hooks like this stop the two terms blurring together mid-exam.

The classic MYP practical uses a potometer to measure water uptake as a proxy for transpiration rate, changing one variable at a time — light, wind or humidity. Some schools also use celery stalks or white carnation stems in coloured dye to show xylem vessels clearly under a microscope.

Most students find the vocabulary tricky at first — xylem, phloem, translocation and transpiration all sound similar — but the underlying ideas are logical once structure is linked to function. In my experience, students who struggle usually haven't practised applying the concepts to unfamiliar data, not the theory itself.

This MYP unit builds the foundation for DP Biology's topic on transport in plants (leaf and stem structure, water potential, translocation via mass flow); according to the IB, first exams for the current DP Biology guide were 2025. Students who master xylem/phloem function and practical data skills now find that DP content noticeably less daunting.

Look for resources that pair clear diagrams of xylem and phloem with practice data-response questions on transpiration experiments, since that's where most marks are actually lost. On RevisionPrep, MYP Biology Revision Notes and Topical Worksheets cover this unit with structure-function summaries and exam-style practical questions.