7-2-transport-mechanisms AS water-and-mineral-ion-transport-in-plants

Water & mineral ion transport: pathways & mechanisms

• Plant roots have root hairs to increase the surface area for absorption of water and mineral ions from the soil

  • Root hair cells take in mineral ions from the soil; this can occur either by diffusion or by active transport depending on the mineral concentrations in the soil

  • The uptake of minerals lowers the water potential of the root hair cells

  • The uptake of water then occurs by osmosis

• After the uptake of water and dissolved mineral ions into the root hair cells, the solution needs to move across the plant root and into the xylem vessels

• There are two pathways that water (and dissolved solutes) can take to move across the root cortex:

  • Apoplast (also known as apoplastic)

  • Symplast (also known as symplastic)

The apoplast pathway

• Most water travels via the apoplast pathway, which involves the series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem

  • The cellulose cell walls form a network of microscopic channels and pores that allow water to move freely between cells

  • Their structure provides a continuous, unbroken route for water and dissolved minerals through the plant

• Water in the apoplast pathway moves by diffusion, as it does not cross any partially permeable membranes

• The movement of water through the apoplast pathway occurs more rapidly than in the symplast pathway

• When the water reaches the endodermis of the root, its progress is blocked by a waterproof, waxy band of suberin within the cell walls

  • This band is called the Casparian strip

• When the water and dissolved minerals reach the Casparian strip they move into the symplast pathway.

  • The presence of this strip is not fully understood, but it is thought that forcing water into the symplast pathway, and therefore across cell membranes, may help the plant control which mineral ions reach the xylem

• Within the xylem vessels, the cell walls are strengthened and waterproofed by lignin

  • This lignin prevents water from leaking out of the vessels and helps maintain the continuous column of water under tension, allowing efficient upward transport

Symplast pathway

• A smaller volume of water travels via the symplast pathway, which involves the cytoplasm, plasmodesmata, and vacuoles of the cells

• The water moves by osmosis into the cells and vacuoles, and by diffusion between cells through the plasmodesmata

• The movement of water in the symplast pathway is slower than the apoplast pathway