Transport in Plants

A small single-celled organism such as Amoeba can get everything it needs by diffusion straight across its cell membrane: its surface-area-to-volume ratio is so high that no specialised transport is needed.

A large multicellular plant is the opposite. Its roots are in the soil, picking up water and dissolved mineral ions. Its leaves are in the air, producing sugars by photosynthesis. The two are tens of centimetres or even tens of metres apart, far too far for diffusion alone to move substances at a useful rate. A plant therefore needs a transport system: a network of tubes that moves water, mineral ions and sugars between the parts of the plant that need or supply them.

In a plant this transport system is the vascular system, made of two types of tube running side by side through the roots, stem and leaves:

• The xylem moves water and mineral ions from the roots upwards to the rest of the plant.
• The phloem moves sucrose and amino acids from the leaves (where they are made) to wherever they are needed for growth or storage.