transport in plants

• Exchange site


  The location within an organism where exchange of substances with the surrounding environment occurs
• Small organisms like the single-celled Chlamydomonas are able to exchange substances directly with the environment
• Small organisms


  • Have a large surface area: volume ratio
  • Have a short diffusion distance
• Larger organisms


  • Require specialised mass transport systems
  • Have increasing transport distances
  • Have decreasing surface area: volume ratios
  • Have increasing levels of activity
• Mass transport in plants

  1. Bulk movement of materials

  2. Directed movement

  3. Involves a source of force

  4. Maintains diffusion gradients at exchange sites

  5. Ensures effective cell activity
• Mass transport systems in flowering plants

  • Xylem
  • Phloem
• Plants have no specialised transport system for oxygen and carbon dioxide
• Xylem


  • Vascular tissue that carries dissolved minerals and water up the plant
  • Provides structural support
  • Stores food
• Phloem


  Transports organic compounds, particularly sucrose, from source to sink
• Xylem tissue


  • Found in vascular bundles
  • Location depends on organ (roots, stems, leaves)

  
• Phloem tissue


  • Found in vascular bundles
  • Location depends on organ (roots, stems, leaves)
• In roots and stems, xylem tissue is found on the inside, while in leaves, xylem is found above phloem tissue

  
• Cell types in xylem tissue

  • Tracheids
  • Vessel elements
  • Xylem parenchyma
  • Sclerenchyma cells
• Tracheids


  Long, narrow tapered cells with pits
• Vessel elements


  Large with thickened cell walls and no end plates when mature
• Cell types in phloem tissue

  • Sieve tube elements
  • Companion cells
  • Parenchyma
  • Strengthening fibres
• Sieve tube elements


  • Line up end to end to form a continuous tube
  • Main conducting cells of phloem
• Sieve tube element


  • Line up end to end to form a continuous tube
• Structure and function of phloem sieve tube elements

  1. Sieve tube elements

  2. Companion cells
• Companion cells


  • Associated with each sieve tube element
  • Control the metabolism of their associated sieve tube member
  • Play a role in loading and unloading of sugars into the phloem
• Mature xylem tissue is dead, so there is no evidence of organelles, and they have lignified cell walls, whereas sieve tube elements have no lignin, do have sieve plates, and their companion cells contain nuclei and dense cytoplasm
• Dicotyledonous (dicots) plants


  • Seeds contain two cotyledons (seed leaves)
  • Network of veins
  • Leaves have broad blades (leaf surface) and petioles (stalks)
  • Tap root with lateral branches
  • Herbaceous dicots have a relatively short life cycle (one growing season) and non-woody tissue
• Transport systems in plants

  • Vascular system with xylem (transports water and mineral ions from the roots to the rest of the plant) and phloem (transports substances from the source (eg. leaf) to the sink (eg.root))
  • Vascular bundles contain xylem and phloem arranged together
• Tissue plan diagrams of a dicotyledonous leaf, stem and root

  Diagrams showing the arrangement of tissues

  
• Calculating eyepiece graticule unit and actual width of plant stem

  1. Step 1: Calculate the number of divisions

  2. Step 2: Calculate the value of each division

  3. Step 3: Calculate the actual width
• When drawing tissue plan diagrams, read the instructions carefully, draw a large diagram, use a sharp pencil and do not shade, use clear continuous lines, ensure correct proportions, include magnification, and label all tissues and relevant structures
• Transpiration


  • Loss of water vapour from a plant to its environment by evaporation and diffusion
  • Provides a means of cooling the plant via evaporative cooling
  • Transpiration stream helps in the uptake of mineral ions
  • Turgor pressure provides support to leaves and stems
• Transpiration stream


  1. Movement of water from the roots to the leaves

  2. Driven by the gradient in water potential from the soil (high water potential) to the atmosphere (low water potential)
• Transpiration only occurs when there is a concentration gradient


  There is usually a lower concentration of water molecules in the air outside the leaf
• Apoplastic 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
  • Water moves by diffusion
• Symplastic pathway


  Water and dissolved solutes can move between living cells via the cytoplasm and plasmodesmata
• Passive process


  Osmosis (the diffusion of water from a higher (less negative) water potential to a lower (more negative) water potential)
• 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
• Pathways for water (and dissolved solutes) to move across the cortex

  • Apoplastic
  • Symplastic
• Apoplastic pathway


  • Most water travels via this pathway (when transpiration rates are high)
  • The series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem
  • Water moves by diffusion (as it is not crossing a partially permeable membrane)
  • Water can move from cell wall to cell wall directly or through the intercellular spaces
  • Movement of water occurs more rapidly than the symplastic pathway
• Casparian strip


  A thick, waterproof, waxy band of suberin within the cell wall that blocks the apoplastic pathway
• When the water and dissolved minerals reach the Casparian strip they must take the symplastic pathway
• Symplastic pathway


  • A smaller amount of water travels via this pathway, which is the cytoplasm and plasmodesmata or vacuole of the cells
  • 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
  • Movement of water 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