The need for plant transport

• Exchange site


  The location within an organism where the exchange of substances with the surrounding environment occurs
• Exchange site in plants

  • Roots (for water and minerals)
• Substances entering/leaving an organism


  Substances are said to not have entered or left an organism until it crosses the cell surface membrane
• Small organisms like Chlamydomonas


  • Able to exchange substances directly with the environment due to their large surface area: volume ratio
  • Diffusion or transport distance is very small so essential nutrients/molecules can reach necessary parts efficiently
  • Tend to have lower levels of activity and smaller metabolic demands
• Larger organisms


  • Require specialised mass transport systems due to:
  • Increasing transport distances
  • Decreasing surface area: volume ratio
  • Increasing levels of activity
• Transport of substances in plants

  1. Roots take in water and mineral ions

  2. Leaves produce glucose by photosynthesis

  3. Glucose transported as sucrose
• Diffusion


  Not viable method for transporting substances long distances in large organisms as it wouldn't be fast enough to meet metabolic requirements
• Surface area: volume ratio

  As organism size increases, surface area increases less rapidly than volume, so the ratio decreases
• Single-celled organisms


  • Have high surface area: volume ratio allowing exchange via diffusion
  • Large surface area for absorption/secretion
  • Short diffusion distances to all organelles
• Larger organisms


  • Have lower surface area: volume ratio
  • Less surface area for absorption/secretion
  • Longer diffusion distances
• Plant adaptations to increase surface area: volume ratio

  • Branching body shape
  • Flat, thin leaves
  • Root hairs
• Plant cells and tissues have a much lower metabolic rate than animal cells, so their demand for oxygen is reduced
• Mass flow


  The bulk movement of materials, directed movement involving a source of force
• Mass transport systems in plants

  • Still involve some diffusion but only at specific exchange sites at start and end of route
  • Help bring substances quickly between exchange sites
  • Maintain diffusion gradients at exchange sites and between cells and fluid surroundings
  • Ensure effective cell activity by keeping immediate fluid environment within suitable metabolic range
• Mass transport systems in flowering plants

  • Xylem transports water and mineral ions
  • Phloem transports sucrose and other nutrients
• Plants have no specialised transport system for oxygen and carbon dioxide as they have adaptations for high surface area: volume ratio and low metabolic demand