AQA A Level Biology复习笔记3.6.1 Water Transport in the Xylem

• Within a plant mineral ions and organic compounds (eg. sucrose) are transported by being 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 diffusion of water from a higher (less negative) water potential to a lower (more negative) water potential
• 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):
  • Apoplastic
  • Symplastic

   

Apoplast pathway

• Most water travels via the apoplastic 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 intercellular spaces
• The movement of water through the apoplastic pathway occurs more rapidly than the symplastic pathway
• When the water reaches the endodermis the presence of a thick, waterproof, waxy band of suberin within the cell wall blocks the apoplastic 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 symplastic pathway. The presence of this strip is not fully understood but it is thought that this may help the plant control which mineral ions reach the xylem and generate root pressure
• As the plant ages the Casparian strip thickens (as more suberin is deposited) except in cells called the passage cells, allowing for further control of the mineral ions

Symplast pathway

• A smaller amount of water travels via the symplastic pathway, which is the cytoplasm and plasmodesmata or 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 symplastic pathway is slower than the apoplastic pathway

Water (and any dissolved substances) can travel from a high water potential (soil) to a low water potential (xylem) via the apoplastic or symplastic pathways. As the plant ages the apoplastic pathway can be blocked by the presence of the casparian strip helping the plant control which mineral ions can move into the xylem vessels