Transport in Plants

Two tissues, two jobs, two mechanisms

Xylem carries water and dissolved mineral ions upwards only. A mature xylem vessel is dead: the end walls have gone, the cytoplasm has broken down and the wall is thickened with lignin. The result is a continuous hollow tube of low resistance that can withstand the tension inside it without collapsing. Phloem carries sucrose and amino acids from source to sink, in either direction. A sieve tube element is living but has almost no organelles; it depends on its companion cell, which is packed with mitochondria and carries out active loading of sucrose.

How water gets to the top of a tree

Water evaporates from the wet cell walls of mesophyll cells and diffuses out through the stomata — this is transpiration. Loss of water lowers the water potential of those cells so they draw water from the xylem, putting the water column under tension. Because water molecules attract each other by hydrogen bonding, the column has cohesion and does not break; adhesion to the lignified walls holds it in place. This is the cohesion–tension theory: water is pulled from the top, not pushed from the bottom. Across the root, water travels through the cell walls by the apoplast pathway until the Casparian strip of suberin in the endodermis blocks it and forces it into the symplast, which allows the plant to control which ions enter.

Translocation

At a source such as a photosynthesising leaf, companion cells use ATP to pump hydrogen ions out and then bring sucrose in by co-transport. Sucrose passes into the sieve tube, lowering its water potential, so water follows from the xylem by osmosis and raises the hydrostatic pressure. At a sink, sucrose is removed, water leaves and pressure falls. Sap therefore flows down a pressure gradient — mass flow. Evidence includes ringing experiments, aphid stylet studies and the observation that translocation stops when respiration is inhibited.