Transport in plants and transpiration

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Plant vascular systems

  • Flowering plants have two transport (vascular) systems: xylem and phloem
  • In roots, xylem and phloem are found in a central vascular cylinder or stele; in stems, they are found in radially arranged vascular bundles that branch into the leaves.
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Xylem tissue

  • In xylem, water and ions are transported in vessels that are well adapted for this function - no cross walls or cytoplasm (to impede movement), lignified (to prevent collapse) but with pores (to allow water to move out laterally).
  • Water and ions are absorbed by root hairs, which have a large surface area, and are passed across the root cortex by the apoplast and symplast pathways.
  • At the endodermis that lines the stele, water and ions are forced to enter the cytoplasm (symplast pathway). From here ions are pumped into the xylem so that water follows osmotically, creating root pressure. 
  • Xylem carries water and ions from the roots through the stem to the leaves - it is one-way flow.
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  • Water is lost from the leaves by the process of transpiration, whereby water evaporates from the mesophyll surface and diffuses out, mainly through open stomata.
  • Transpiration is influenced by internal factors (e.g. stomatal density and cuticle thickness) and external factors (e.g. temperature, air movement, humidity and light).
  • Transpiration creates a negative pressure (tension) in the leaf xylem and is the major force drawing water up, supported by the combined forces of cohesion and adhesion, and to a lesser extent by the root pressure.
  • Xerophytes have adaptations that reduce transpiration
  • In all cases water moves down a water potential gradient, i.e. from a region of higher water potential to a region of lower (more negative) water potential.
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Phloem tissue

  • In phloem, organic solutes (mainly sucrose and amino acids) are transported in sieve tubes. These have reduced cytoplasm and cross walls with pores, the sieve plates, to facilitate movement. 
  • Translocation of organic solutes is an active process that relies on the metabolism of adjacent companion cells.
  • Phloem carries organic solutes from the 'source' of sugar (mature leaves) to the 'sinks' (roots and growth areas).
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