Transport in Plants

• Transport in Plants


  The need for a transport system in Plants
• Transport over short distances in small organisms

  1. Diffusion

  2. Active transport

  3. Osmosis
• Amoeba


  • Gets nutrients from surroundings largely by diffusion due to large surface area to volume ratio
• As the size of an organism increases


  Its surface area to volume ratio decreases
• Diffusion alone


  Does not suffice for transport in plants due to the distance between various tissues
• Transport system in plants

  Required to transport water and mineral salts from soil to aerial parts and to transport photosynthetic products to the rest of the plant
• Conducting tissue in flowering plants

  • Xylem
  • Phloem
• Xylem


  • Conduction and support
  • Different types: vessel elements, tracheids, parenchyma and fibres
• Xylem vessels


  • Occur mainly in Angiosperms (flowering plants)
  • Formed by end to end fusion of cells with degenerated cross walls and dead cell contents
  • Walls develop a deposit of lignin, a tough waterproof substance
  • Pits are areas in the xylem wall where lignin is not deposited, allowing lateral water transport
• Formation of xylem


  No details provided
• Xylem structure and function

  • Vessels have no contents allowing unimpeded water and mineral transport
  • Vessels have no cross walls enabling continuous flow
  • Cell walls thickened with lignin to prevent collapse
  • Pits allow lateral water transport between vessels
  • Large lumen allows large volume of water transport
• Movement of water through flowering plant

  1. Water enters through roots, enters xylem vessels and is transported to aerial parts

  2. Transpiration: loss of water vapour from plant
• Transpiration stream


  The flow of water through the plant, from the roots to the leaves
• Pathways followed by water through the plant

  • Apoplast
  • Symplast
• Apoplast


  A system of connected cell walls, in leaves water evaporates from mesophyll cell walls into intercellular air spaces and escapes through stomata, creating tension that draws water through cell walls by mass flow
• Symplast


  Pathway linking cytoplasm of one cell to the next via plasmodesmata
• Mass flow


  Bulk transport of materials between two points due to pressure differences
• Transpiration


  Loss of water as vapour from a plant, in leaves water evaporates from mesophyll cell walls and diffuses out through stomata
• External factors affecting rate of transpiration

  • Temperature
  • Humidity
  • Air movement
  • Light intensity
• Temperature


  Has the greatest effect, higher temperature increases evaporation rate
• Humidity


  Lower humidity increases transpiration rate due to greater water potential gradient
• Air movement


  Sweeps away saturated air layer around leaf, increasing transpiration rate
• Light intensity


  Stomata only open in presence of light, increasing transpiration rate
• Internal plant factors affecting transpiration rate

  • Leaf surface area to volume ratio
  • Cuticle thickness
  • Stomata number
• Leaf surface area to volume ratio

  Larger ratio increases transpiration rate
• Cuticle


  Thicker cuticle reduces transpiration rate
• Stomata


  Greater number increases transpiration rate
• Functions of transpiration


  • Evaporative cooling
  • Creates tension that pulls water from soil to aerial parts, distributing dissolved salts
• Potometer


  Device used to measure rate of transpiration
• Dicot root structure


  Root hairs increase surface area for water and mineral absorption
• Endodermis


  Waterproof Casparian strips force water to bypass through cytoplasm, allowing plant control over entry
• Cohesion-tension theory


  Explains how water moves up xylem, evaporation creates tension that pulls water columns
• Cohesion


  Water molecules stick together due to hydrogen bonding, preventing water columns from breaking
• Adhesion


  Water molecules stick to xylem walls, also preventing water columns from breaking
• Evidence of water movement in xylem

  • Air bubble forms when xylem pierced
  • Trunk diameter minimum during high transpiration
• Dendrometer


  Device used to measure tree trunk size
• Phloem


  Translocates organic solutes, consists of living sieve tube elements and companion cells
• Sieve tube elements


  Formed by end to end fusion, have no nucleus or ribosomes so no protein synthesis
• Companion cells


  Metabolically active, essential for sieve tube element survival
• Translocation


  Movement of soluble organic substances (assimilates) through phloem