Transport tissues in plants

• Xylem tissue


  Transports water
• Phloem tissue


  Transports dissolved solutes (e.g. sucrose)
• How are xylem and phloem arranged?

  In vascular bundles
• Cambium


  Layer of unspecialised cells that divide to give rise to more specialised cells, which, in turn, form the xylem and phloem
• Plasmodesmata


  Cytoplasmic bridges between plant cells allowing communication between cells
• Middle lamella


  First layer of the cell wall to form when the cell divides

  Mainly made of pectin - binds layers of cellulose together
• Xylem and phloem distribution in stems

  Arranged in vascular bundles - xylem on inside, phloem on outside (with sclerenchyma tissue as the outermost layer)
• Xylem and phloem distribution in roots


  Arranged in vascular bundles - xylem arranged in a cross-like structure, surrounded by phloem on the outside
• Xylem and phloem arrangement in leaves


  Arranged in vascular bundles - xylem found on top, phloem found underneath
• Xylem

  • First xylem found = protoxylem - can stretch and grow because walls are not fully lignified
  • extra cellulose microfibrils - increases tube strength, withstands compression forces from weight of plant pushing in
  • As stem ages and cells stop growing, lignin is laid down in cell walls
  • Cells become impermeable to water
  • Tissue strengthens but cells die = metaxylem - mature xylem vessels made of lignified tissue
  • End walls break down; xylem forms hollow tubes running from the roots to the tips of stems and leaves
• Protoxylem


  The first xylem formed, that can stretch and grow because the walls are not fully lignified
• Metaxylem


  Mature xylem made of lignified cell tissue
• Function of xylem

  • Water moves OUT of xylem into surrounding cells through unlignified areas or specialised pits (holes in the walls)
  • Lignified xylem vessels are strong and support the stems of larger plants
  • Support in smaller, non-woody plants given to in the form of three different types of cell:
  • Parenchyma
  • Sclerenchyma
  • Collenchyma
• Parenchyma


  Relatively unspecialised plant cells that act as packing in stems and roots to give support
• Sclerenchyma


  Plant cells that have thick lignified cell walls and an empty lumen with no living contents, that provide structural support in mature plant tissues
• Collenchyma


  Plant cells with areas of cellulose thickening that give mechanical strength and support to the tissues
• Role of parenchyma cells


  Act as packing in stems and roots to give support
• Role of sclerenchyma cells


  Provide structural support in mature plant tissues
• Role of collenchyma cells


  Give mechanical strength and support to the tissues
• What is the xylem vessel?

  A continuous tube with no living contents but with lignified cell walls
• Name three ways that show evidence for movement of water through the xylem

  • Placing the cut end of a shoot in eosin dye solution
  • Ringing experiments
  • Autoradiography
• How can shoots be used to show the movement of water through the xylem?

  If the cut end of a shoot is placed in eosin dye solution, the dye can be seen being carried into the transport system and through to the vascular tissue of the leaf
• How can ringing experiments be used to show the movement of water through the xylem?

  Ringing experiments involves removing/killing a complete ring of bark - destroys phloem cells but not xylem cells

  If eosin dye is placed in the water, can be observed that the upward movement of water through the plant is unaffected
• Autoradiography


  A technique for following transport around plants
• How can autoradiography be used to show the movement of water through the xylem?

  • Plant is given radioactively labelled version of substance being studied (e.g. water with a radioactive isotope of hydrogen)
  • Radioactive substance taken up by the plant in the same way as normal isotope
  • Substance tracked by placing plant against photographic film to produce an AUTORADIOGRAPH
  • labelled substance causes film to shadow - reveals areas of accumulation
  • radioactive label can be traced by examining each layer of plant separately
• Phloem

  • Contains phloem sieve tubes (aka sieve tube elements) - made of many cells joined together to form long tubes, run from highest shoots to the ends of the roots
  • Phloem cells DO NOT become lignified - cell contents remains living
  • Formation of sieve plates - perforated walls between phloem cells allowing phloem sap to flow
  • Mature phloem cells have no nucleus, but survive due to companion cells - support phloem vessels and actively load sucrose into phloem
• Phloem sieve tubes/sieve tube elements

  Made of many cells joined together to form long tubes that run from the highest shoots to the bottom of the roots
• Sieve tubes


  Perforated walls between phloem cells allowing phloem sap to flow
• Companion cells


  Support phloem vessels and actively load sucrose into phloem
• Features of COMPANION CELLS (found in phloem)

  • Infoldings in their cell membranes - increase the surface area over which they can transport sucrose
  • Have many mitochondria to supply the ATP needed for active transport
• How are the companion cells linked to the sieve tube elements?

  By many plasmodesmata