Transport In Plants

The concept of apoplast and symplast was introduced by Munch in 1930. In apopiast pathway, "water passes . from root hairs to xylem through the walls of intervening cells, without crossing any membrane or cytoplasm. It provides the least resistance to movement of water. It is interrupted by the preserice of impermeable lignosube1in Casparian strips in the walls of endodermal cells. In symplast pathway, water passess from cell to cell through their protoplasm facing plasmalemma (cell membrane), at least at one place. It is also called transmembrane pathway. It does not enter cell vacuoles. The cytoplasm of the adjacent cells are connected through bridges called plasmodesmata. Symplastic movement is aided by cytoplasmlc streaming of individual cells. It is, however, slower than apoplastic movements.

In roots, the most efficient region of water absorption is the root hair zone.' Root hairs function as tiny osmotic 'systems. Each root hair has a thin 'permeable cell wall, a semi-permeable cytoplasm and an osmotically active cell . sap present in the central vacuole. Because of the latter, a root hair cell has a water potential of -3 to -8 bars. Water potential of the soil water is - 0.1 to 0.3 bars. As a result water of the soil passes into the root hair cell. However, water does not pass into its vacuole. Instead it passes into apoplast and symplast of cortical, endodermal and pericycle cells and enter the xylem channels passively because of the very low water. potential due to . tension under which water is present in them, caused by transpiration in the aerial parts. A gradient of water potential exists between root hair cell, cortical cell, endodermal, pericycle' and xylem channel so that flow of water is not interrupted.

Endosmosis is the net movement of water molecules from a region of higher to lower concentration through a differentially permeable membrane. It is essentially a type of diffusion but through a partially permeable membrane. In terms of water potential water moves from area of high i.e., less negative to area of low i.e. more negative water potential.

Amount of imbibition depends upon:
(i) water potential gradient between the absorbent and the liquid imbibed, and
(ii) affinity between the adsorbant and the liquid.

The endodermal cells of the roots have many transport proteins embeded in their plasma membrane. They let some solutes to cross the membrane, but prevent others. Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and types of solutes that reach the xylem. The root endodermal cells because of the layer of suberin have the ability to actively transport ions in one direction only