Translocation

• phloem transports organic substances (sucrose and amino acids) to all cells in a plant
• companion cells provide ATP required for active transport of organic substances
• sieve tube elements (living cells, no nucleus, few organelles) are perforated ends for continuous flow of sugar solution
• the source is where organic substances is created (leaves during photosynthesis) and the sink is the site where the organic substances are used up (respiring tissues)
• The Mass Flow Hypothesis is a theory that explains the movement of water and solutes from high to low concentration across a selectively permeable membrane.
• summary of mass flow hypothesis:

  1. In SOURCE (photosynthesising leaf) sucrose lowers water potential so water enter via osmosis
  2. higher hydrostatic pressure in source so solution forced towards sink cell via phloem
  3. In SINK (respiring tissues) sucrose is used up so has a high water potential so water leaves via osmosis
  4. lower hydrostatic pressure in the sink
• (1) SUCROSE TRANSFERRED FROM PHOTOSYNTHESISING TISSUE TO SIEVE TUBE ELEMENTS

  1. photosynthesis in leaves create sucrose = high concentration of sucrose so diffuses down conc gradient into companion cells via facilitated diffusion

  2. Sucrose is co-transported with H+ into sieve tube elements by companion cells
• (2) MASS FLOW OF SUCROSE THROUGH SIEVE TUBE ELEMENTS

  
  

  1.Increase of sucrose in sieve tube elements lowers water potential so water enters STE from surrounding xylem (has high wp) via osmosis

  
  

  2. increase in water volume in STE increases hydrostatic pressure

  
  

  3. causes mass flow of liquid to respiring cells/ sink
• (3) TRANSPORT OF SUCROSE INTO THE SINK CELLS

  
  

  1. sucrose is actively transported from STE into sink cells causing lowered water potential so water moves from STE into sink via osmosis (or some returns to the xylem)

  
  

  2. results in STE having decrease in hydrostatic pressure due to water loss .

  
  

  3.Sucrose in used up in respiration at sink or stored as insoluble starch

  
  

  movement of soluble organic substances is due to the difference in hydrostatic pressure between the source and the sink end of sieve tube element
• investigating translocation:

  tracers involves radioactively labelling carbon dioxide and measuring the amount of carbon dioxide in the leaf absorbed for photosynthesis produce sugars

  then thin slices cut and placed on x -ray film that turns black when exposed to radioactive material - section containing radioactive sugar highlights where phloem is
• investigating translocation:

  ring of bark containing phloem causes tree trunk to swell above removed section

  liquid in swelling contains sugar as cannot be transported in phloem; proving phloem transports sugars