I keep mixing up xylem and phloem in the exam. Is there a way to stop doing that?

Yes, and it is not a mnemonic. Learn one fact about each and derive the rest, because the two tissues differ in almost every way once you have the starting point.

The one fact each

Xylem carries water, upwards only, through dead cells. Phloem carries dissolved sugars, in both directions, through living cells.

Everything else follows from those, and if you can rebuild the table rather than recall it, you will not mix them up under pressure.

Deriving the rest

Why is xylem dead and phloem alive? Xylem is a pipe. Water is pulled up it by evaporation from the leaves, so no energy is needed along the way — the cells can afford to die, lose their contents and become hollow tubes. Phloem moves sugar actively, which costs ATP, so its cells must stay alive. That is also why phloem has companion cells: the sieve tube cells lose most of their organelles, and the companion cell supplies the energy.

Why is xylem one-directional? Water enters at the roots and leaves at the leaves. There is nowhere else for it to go. Sugar is made in the leaves but needed everywhere — growing tips, storage organs, roots — so phloem must be able to carry it up or down depending on where the demand is.

Why is xylem strengthened with lignin? The column of water is under tension as it is pulled upward. Without rigid walls the vessels would collapse inwards. Lignin also happens to be what makes wood woody, which is a useful association.

The vocabulary examiners want

Two word pairs carry marks and are worth using precisely.

• Movement in xylem is transpiration, driven by evaporation. It is a physical process and needs no energy from the plant.
• Movement in phloem is translocation, and it is active. Sugar is loaded at a source (usually a leaf) and unloaded at a sink (anywhere using or storing it).

A leaf is a source in summer. A potato tuber is a sink while it fills and a source in spring when it sprouts. Questions like that are testing whether you understand the terms or just memorised an example.

Where marks go missing

Questions about a ringed tree — bark removed in a circle — are common and catch people out. Removing bark removes the phloem but leaves the xylem intact, so water still reaches the leaves and the tree stays alive for a while, but sugar cannot get down to the roots, which eventually starve.

If you can explain that outcome, you have genuinely understood the difference, and that is the question examiners use to find out.