Form 5 · Common mistakes

Transport in Plants, common mistakes

The mistakes SPM students make on Transport in Plants, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Xylem carries food.	Students confuse the two transport tissues because both run in the same vascular bundle.	Xylem carries water and dissolved minerals; phloem carries dissolved food (sucrose) by translocation.
Transpiration pushes water up the stem.	The word 'stream' suggests something is pumping water upward.	Transpiration creates a pull (tension) at the leaves; water is drawn up, not pushed.
High humidity increases the rate of transpiration.	Learners assume more water in the air means more water lost.	High humidity lowers the rate, because the moist air reduces the diffusion gradient out of the leaf.
Phloem sieve tubes are dead like xylem vessels.	Both are tubular, so they look similar in diagrams.	Sieve tubes are living; only xylem vessels are dead and hollow.
Root pressure alone lifts water to the top of a tall tree.	Root pressure is real, so students over-credit it.	Root pressure only pushes water part of the way; the transpiration pull is the main force in a tall plant.
Minerals enter the root by osmosis, the same as water.	Water and minerals are absorbed at the same place, so they seem to enter the same way.	Water enters by osmosis; mineral ions enter by active transport against the concentration gradient, using energy.
Companion cells carry the sugar sap.	Companion cells sit right beside the sieve tube, so their role is misread.	Sap flows through the sieve tube elements; companion cells only supply energy for loading.
Translocation always moves sugar from root to leaf.	Students memorise one fixed direction.	Translocation moves sugar from any source to any sink, so the direction depends on the plant's needs.
A potometer measures transpiration directly.	It is introduced as a transpiration experiment.	A potometer measures water uptake by the shoot, used as an estimate of transpiration, since some water stays in the plant.
Guttation is caused by transpiration.	Both involve water leaving the leaf.	Guttation is caused by root pressure forcing water out of leaf tips when transpiration is low, such as at night.
Any plant can be used for phytoremediation.	The process sounds like it should work with ordinary plants.	Only hyperaccumulator species can absorb and tolerate high pollutant concentrations without being poisoned.
Lignin makes xylem vessels living and flexible.	Lignin is linked with strength, which gets muddled with being alive.	Lignin is a non-living material that strengthens the wall and prevents collapse; the vessel cells themselves are dead.

How to avoid these mistakes

Compare xylem and phloem in a table (structure, contents, direction).
Learn how light, temperature, humidity and wind change transpiration.
Practise a potometer experiment with variables and expected results.
Draw the cohesion-tension pathway from root hair to leaf stoma, labelling each force involved.
Compare xylem and phloem structure, contents and direction of flow in one table.
Explain the pressure-flow mechanism of translocation using the terms source, sink and companion cell.

Frequently asked questions

What is the difference between xylem and phloem?

Xylem is made of dead, hollow, lignified vessels that carry water and dissolved mineral salts upward from the roots to the leaves. Phloem is made of living sieve tubes with companion cells that carry sugars made in photosynthesis from the leaves to other parts of the plant, in a process called translocation. Xylem carries water; phloem carries food.

How does water move up a tall plant?

Water is lost from the leaves as vapour through the stomata by transpiration. This creates a pull, or tension, at the top of the xylem. Because water molecules stick together (cohesion) and to the xylem walls (adhesion), this pull draws a continuous column of water up the xylem from the roots, the transpiration stream.

Which factors affect the rate of transpiration?

Higher light intensity opens the stomata and speeds transpiration; higher temperature increases evaporation; moving air (wind) removes water vapour and speeds it up; higher humidity slows it down because the surrounding air already holds a lot of water vapour, reducing the gradient.