Bulk Flow Functions in long-distance Transport

This type of transport is usually along the vertical axis of the plant from root to leaves and vice versa. Vascular tissues are involved in this type of transport, as diffusion would be too slow. Bulk flow (movement due to pressure differences) moves water and solutes through xylem vessels and sieve tubes. Transpiration reduces pressure in the leaf xylem; this creates a tension, which pulls sap up through the xylem from the roots. Hydrostatic pressure develops at one end of the sieve tubes in the phloem; this forces the sap to the other end of the tube.

Absorption of water and minerals by roots: Water and minerals enter through the root epidermis, cross the cortex, pass into the stele, and are carried upward in the xylem.

Active accumulation of mineral ions: The cells cannot get enough mineral ions from the soil by diffusion alone. The soils solution is too dilute. Active transport of these ions must occur. Specific carrier proteins 'in the plasma membrane attract and carry their specific mineral into the cell. H+ is pumped out of the cell causing a change in pH and a voltage across the membrane. This helps drive the anions and cations into the cell. Water and minerals cross the cortex either by symplast, which is the living continuum of cytoplasm, connected by plasmodesmata or by apoplast, which is nonliving matrix of cell walls. At the endodermis the casparian strip blocks the apoplastic route. This is a ring of suberin around each endodermal cell. Here, water and minerals must enter the stele through the cells of the endodermis. Water and minerals enter the stele via symplast, but xylem is part of the apoplast. Transfer cells selectively pump ions out of the symplast into the apoplast so they may enter the xylem. This action requires energy.

The ascent of xylem sap depends mainly on transpiration and the physical properties of water. The shoot depends on the efficient delivery of its water supply. Xylem carries sap containing dissolved mineral and nutrients from the roots to the leaves. Water is pulled up through the xylem by the force of transpiration. Water transported up from roots must replace that lost through transpiration. The connection between the xylem gives a continuous column of water to form between roots and leaves. This column acts like a thread that moves upward by the pull of transpiration. Transpiration pulls the xylem sap upward, and cohesion of water transmits the upward pull along the entire length of xylem. The forces responsible for the ascent of sap through xylem are Transpiration, Adhesion, Cohesion, and Tension (TACT).

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