Transport in Plants

• plants need transport systems due to:

  • metabolic demands - need o2 and glucose and removal of waste
  • size - some plants are very large and live for a long time
  • surface area to volume ratio is complicated due to trunks and stems
• dicotyledonous plant - make seeds that contain two cotyledons (organs that act as food stores and form first two leaves)
• vascular bundle in stem of herbaceous plant:

  • phloem on outside
  • xylem on inside
  • vascular bundle located in ring around centre parenchyma for strength and support
• vascular bundle in root of herbaceous plant:

  • xylem X shape in middle
  • phloems around xylem
  • located in middle to held plant withstand tugging strains
• vascular bundle in dicot leaf:

  • xylem on top
  • phloem below
  • located in middle of main vein of leaf (midrib) to carry nutrients throughout plant and provide support
• Xylem:

  • non-living tissue
  • transport of water and mineral ions
  • materials flow from root to shoot
  • made up of dead cells, xylem parenchyma and xylem fibres
• Xylem parenchyma:

  • packs around xylem vessels
  • store food
  • contain tannin deposits
• Xylem fibres:

  • long cells with lignified secondary walls
  • Lignin can be laid down in the walls of xylem vessels to form rings, spirals or solid tubes with bordered pits (unlignified)
• Phloem:

  • living tissue that transports food in the form of organic solutes around the plant from the leaves where they are made
  • supplies the cells with the sugars and amino acids needed for cellular respiration and the synthesis of other molecules
  • main vessels are sieve tube elements, with sieve plates and companion cells
• Sieve tube elements:

  • made up of many cells joined end to end to form a long, hollow structure
  • they are unlignified
• sieve plates:

  • perforated areas between the sieve tube elements
  • allow phloem contents through
  • as the large pores appear in these cell walls, the vacuole and other organelles break down
  • phloem becomes filled with phloem sap and mature phloem cells have no nucleus
• Companion cells:

  • linked to the sieve tube elements by plasmodesmata
  • maintain nucleus and all organelles
  • active cells
  • function as support for the sieve tube cells
• Root hair cells:

  • their microscopic size means they can penetrate easily between soil particles
  • each microscopic hair has a large SA:V ratio and there are thousands on each root tip
  • each hair has a thin surface layer through which diffusion and osmosis can take place quickly
  • the conc of solutes in the cytoplasm of root hair cells maintains a water potential gradient between soil and water
• Movement of water across the root:

  • symplast
  • apoplast