Water Transport in the Xylem (AQA A Level Biology)

• Within a plant, mineral ions and organic compounds (e.g. sucrose) are transported while dissolved in water. The dissolved mineral ions are transported in the xylem tissue and the dissolved organic compounds are transported in the phloem tissue
• The plant roots are responsible for the uptake of water and mineral ions and can have root hairs to increase the surface area for absorption of the substances
• The uptake of water is a passive process and occurs by osmosis
• The uptake of minerals can be passive or active and occurs by diffusion or active transport respectively
• Plants must take in a constant supply of water and dissolved minerals to compensate for the continuous loss of water via transpiration in the leaves, and so that they can photosynthesise and produce proteins
• There are two pathways that water (and the dissolved solutes) can take to move across the cortex (and molecules can change between routes at any time):
  • Apoplast (also known as apoplastic)
  • Symplast (also known as symplastic)

Apoplast pathway

• Most water travels via the apoplast pathway (when transpiration rates are high), which is the series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem
• The water moves by diffusion (as it is not crossing a partially permeable membrane)
• The water can move from cell wall to cell wall directly or through the spaces between cells
• The movement of water through the apoplast pathway occurs more rapidly than the symplast pathway
• When the water reaches the endodermis the presence of a thick, waterproof, waxy band of suberin within the cell wall blocks the apoplast pathway
• This band is called the Casparian strip and forms an impassable barrier for the water
• When the water and dissolved minerals reach the Casparian strip they must take 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 through cell membranes, may help the plant control which mineral ions reach the xylem

Symplast pathway

• A smaller volume of water travels via the symplast pathway, which involves the cytoplasm and plasmodesmata, and vacuole of the cells
• The water moves by osmosis into the cell (across the partially permeable cell surface membrane), possibly into the vacuole (through the tonoplast by osmosis) and between cells through the plasmodesmata
• The movement of water in the symplast pathway is slower than the apoplast pathway

Water (and any dissolved substances) can travel from a high water potential (soil) to a low water potential (xylem) via the apoplast or symplast pathways. As the plant ages the apoplast pathway can be blocked by the presence of the Casparian strip