Transport in Plants: Water Transport and Translocation — Study Notes

A mature oak can lose hundreds of litres of water out of its leaves on a warm afternoon, yet nowhere inside that tree is anything beating like a heart to push replacement water back up from the soil. So what actually drives water — and food — through a plant that owns no pump at all?

Two Separate One-Way Highways, With Different Jobs

A plant stem contains two distinct transport tissues, and confusing them is a common exam slip. The xylem carries only water and dissolved minerals, and travels in a single fixed direction — upward, from root to leaf, replacing what keeps evaporating away above. The phloem carries dissolved sugar (mainly sucrose) and amino acids, but obeys no single fixed direction: it flows from wherever sugar is currently being released (a source) toward wherever it's currently required (a sink), so the same tissue can carry food upward on one day and downward on another. Water only ever needs one direction, since it's constantly being topped up at the leaves; food needs a flexible route because a growing plant's sugar demand keeps shifting between shoot and root. This flexible, source-to-sink movement of food through the phloem has its own name: translocation.

Worked Example — Tracking Translocation in a Sprouting Bean Seed

1. A bean seed keeps a store of food packed inside its cotyledons before it ever starts to grow.
2. Once the seed sprouts, its tiny embryo shoot and root cannot yet build their own sugar, because no true leaves have opened up to catch light.
3. Locate the source and the sink: the cotyledon, giving up its stored sugar, is the source; the growing embryo, spending that sugar to build new cells, is the sink.
4. Sucrose therefore moves from the cotyledon up toward the growing tip — the direction here just happens to be upward, purely because that's where the sink sits right now.

Notice the reasoning never leaned on a rule like "phloem goes down" — only on pinning down the source and the sink first, every single time.

Xylem's fixed one-way route from soil to stomata, plus worked examples on transpiration rate and on wilting, get the full treatment — with an audio walkthrough and a practice worksheet — in the complete lesson below.