Transport Systems in Organisms

Small organisms can move materials by diffusion alone, but larger organisms have a low surface area to volume ratio and their innermost cells are far from any exchange surface. To supply every cell, they evolve transport systems that carry substances quickly over long distances. Plants and animals have solved this problem in strikingly different ways.

Transport in Plants: Xylem and Phloem

Plants have two separate transport tissues, xylem and phloem, arranged together in vascular bundles.

Xylem transports water and dissolved minerals upward from the roots to the leaves. Mature xylem vessels are dead, hollow tubes with walls reinforced by lignin, forming continuous pipes with no end walls to obstruct flow. Water movement is driven mainly by transpiration, the evaporation of water from the le