Need for transport systems in plants

• Exchange site


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

  • Roots (for water and minerals)
• 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 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
• Mass transport in plants

  1. Mass flow is the bulk movement of materials, involving a source of force

  2. Diffusion is still involved but only at specific exchange sites at start and end of route

  3. Helps bring substances quickly between exchange sites, maintain diffusion gradients, and keep cell 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
• Reason plants don't need oxygen/CO2 transport system

  • They have adaptations giving high surface area: volume ratio for gas absorption and diffusion
  • Leaves and stems have chloroplasts producing oxygen and using up carbon dioxide
  • Plant tissues have low metabolic rate so low demand for oxygen