Define the absorption of Minerals and water.

Absorption of water and minerals by roots:

Water and mineral enter through 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.

A Proton Pump:

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 in one of 2 ways: Via SYMPLAST which is the living continuum of cytoplasm connected by PLASMODESMATA. Via APOPLAST which is nonliving matrix of cell walls. At the endodermis the apoplastic route is blocked by the CASPARIAN STRIP. 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.

Water transported up from the roots must replace water lost by transpiration. Xylem sap rises against gravity, driven by a gradient of water potential. Water flows from an area of high potential to an area of low potential. Water Potential is expressed in units of pressure: 1 bar is the pressure needed to push up a column of water 10 meters. 1 megapascal = 10 bars. Pure water has a potential of 0. Addition of pressure increases water potential. Addition of solutes decreases it. When transpiration is low, ions pumped into the stele decrease water potential and cause water uptake by the stele. This uptake force is called root pressure. It Cannot keep pace with transpiration, and can only force water up a few meters.