Mass Transport In Plants

• Define Xylem Vessels

  Hollow, thick-walled tubes that transport water in the stem and leaves of plants.

  
• Define transpiration

  Evapouration of water from a plant's surface
• How does temperature affect transpiration rate?

  Warmer molecules have more energy so evapourate from inside leaf faster.

  This increases the concentration gradient between inside and outside of leaf, making water diffuse out of leaf faster
• Describe how water moves across the cells of a leaf

  1. Water lost from mesophyll cells by evaporation from cell walls to leaf air spaces is replaced by water from xylem, via cell walls/cytoplasm.

  
  

  2. Cells' wp lowered so water moves into them from other cells by osmosis.

  
  

  3. Wp gradient established that pulls water up from xylem, across leaf mesophyll, into atmosphere.
• Outline the cohesion-tension theory

  1. Water evaporates from mesophyll cells due to heat from sun leading to transpiration.

  
  

  2. Water molecules form H-bonds with one another ---> cohesion.

  
  

  3. Water forms a continuous, unbroken column across mesophyll cells and down xylem.

  
  

  4. Column of water pulled up xylem as a result of transpiration => transpiration pull.

  
  

  => puts xylem under tension ---> negative pressure within xylem.

  
• Evidence for cohesion-tension theory

  1. Water flow decreases at night (as would be expected - less photosynthesis).

  
  

  - Diameter of tree trunk increases - therefore tension has decreased as decrease in negative pressure - xylem not pulled inwards as much.

  
  

  2. If xylem vessel broken and air enters it ---> tree can no longer draw up water ---> continuous column of water is broken ---> H20 molecules can no longer stick together.

  
  

  3. Water does not leak out when xylem vessel broken ---> but air drawn in. Suggests water not under pressure but that xylem is under negative pressure.

  
  

  4. Xylem vessels have no end walls ---> form a series of continuous, unbroken tubes from root to leaves ---> essential to cohesion-tension theory of water flow up the stem.

  
  

  => consistent with xylem being under tension.
• Define Translocation

  Process in which organic molecules (sucrose/amino acids) and some mineral ions are transported from one part of the plant to another.
• Define Source


  Part of the plant where sugars produced during photosynthesis
• Define Sink

  Parts of the plant where sugars used directly or stored for future use.
• Define Phloem

  The vascular tissue in plants which conducts sugars and other metabolic products downwards from the leaves.
• Define Mass Flow

  Bulk movement of a substance through a given channel/area in a specific area.
• What are the phloem made out of

  Sieve tube elements

  Companion cells ( provide energy needed for active transport of solutes)
• Describe the mass flow hypothesis

  1. Solutes actively transported from companion cells to sieve tubes at source

  2. This lowers water potential inside sieve tubes, so water enters tubes by osmosis from xylem + companion cells

  3. Creating high pressure inside sieve tubes at source end of phloem

  
  

  4. At sink, solutes are removed from phloem to be used up

  5. This increases water potential inside sieve tubes, so water leaves the tubes by osmosis

  6. Lowering pressure inside sieve tubes

  
  

  7. This creates a pressure gradient from source end to sink end

  8. Gradient pushes solutes along sieve tubes towards the sink

  9. At sink, solutes are used or stored
• Supporting evidence for mass flow

  1. If ring of bark ( including phloem not xylem) removed from woody stem, bulge forms above ring. Bulge has higher concentration of sugars than fluid from below, shows there is downward flow of sugars

  2. Radioactive tracer tracks movement fo organic substances in a plant

  3. Aphids pierce phloem to access sap, sap flows quicker nearer leaves than further down stem, shows a pressure gradient

  4. Metabolic inhibitor stops translocation
• Objections (of Mass Flow Hypothesis)

  Evidence against Mass Flow Hypothesis

  - Sugar travels to many sinks (not just one)

  - Sieve plates act as barriers so a lot of pressure is needed