Xylem carries water and mineral ions, mostly upwards from the roots to the leaves. Phloem carries sucrose and amino acids, to wherever the plant is growing or storing. Phloem movement is called translocation, and it can go up or down. Questions on this appear as comparison tables, tissue labelling on cross-sections and experiments that remove or block a tissue.
It follows on from water movement from roots to leaves and is part of plant transport.
How do the two tissues compare?
| Feature | Xylem | Phloem |
|---|---|---|
| What it carries | Water and mineral ions | Sucrose and amino acids |
| Direction | Upwards, roots to leaves | Up and down, from source to sink |
| Cells | Dead, hollow, no cross walls, walls strengthened with lignin | Living sieve tube cells with companion cells |
| Driving force | Evaporation and transpiration pull | Sucrose loaded at a source and removed at a sink |
Tip: the xylem is the plant’s water pipe, and because its cells are dead there is no pumping. The phloem is a living tissue, which is why companion cells matter.
What do “source” and “sink” mean?
A source is where sugar enters the phloem. A mature leaf doing photosynthesis is a source. A sink is where sugar is used or stored, such as a root, a flower, a fruit or a growing tip.
The direction of translocation depends on where the source and the sink are. That is why phloem moves sugar both up and down the plant.
Worked example
The data are invented for teaching. A ring of bark (which contains the phloem) was removed from around a woody stem. After two days, sugar was measured in the stem just above and just below the ring, and compared with an unringed control stem.
| Stem | Sugar above the ring (mg per g) | Sugar below the ring (mg per g) |
|---|---|---|
| Ringed | 14 | 6 |
| Control | 10 | 10 |
Explain the results, and state what you expect for the leaves above the ring.
Step 1, compare to control: in the control stem the sugar is the same above and below (10 and 10). In the ringed stem it is 4 mg per g higher above the ring (14 − 10) and 4 mg per g lower below it (10 − 6).
Step 2, explain: the ring removed the phloem, so sugar made in the leaves above could not be translocated down past the ring. It built up above and ran short below.
Step 3, leaves: the xylem lies deeper in the wood and was not removed, so water keeps reaching the leaves. They stay green and the plant does not wilt at once.
Check: the drop below and the rise above are each 4 mg per g, so the two readings agree that the ring stopped downward movement of sugar.
The mistake to watch for
The common mix-up is to attach the wrong substance or the wrong “direction” rule.
Mistaken answer: Phloem carries water up the plant and xylem carries food down.
This reverses the tissues. It also treats “food” as one thing, when it is sucrose and amino acids.
The correction is to anchor each tissue to its cargo: xylem for water and ions, phloem for sucrose and amino acids. Then add the direction: xylem upwards, phloem both ways.
Check yourself
Try these without the table, then open each answer.
1. Name the tissue that carries each: (a) water from the soil to the leaf, (b) sucrose from a leaf to a root.
Show answer
(a) Xylem. (b) Phloem.
2. Give two structural differences between a xylem vessel and a phloem sieve tube.
Show answer
Xylem vessels are dead, hollow and have no end walls, with lignin in their walls. Sieve tubes are living, have sieve plates at their ends and have companion cells beside them.
3. In the ring-of-bark example, the bark above the ring swells over several weeks. Explain this.
Show answer
Sugars from the leaves collect above the ring because they cannot move past it. The extra sugar supports extra growth in that region, so it swells.
Where this leads next
The next lesson explains why each tissue has the features it has: connect a structural feature with a transport role. Then check yourself with the plant transport practice set.
Comparison tables only help if you can reproduce them and apply them to an unfamiliar experiment. Online one-to-one Biology tuition gives you a teacher who can give you a new experiment and listen to how you reason about it.