CLARE BIOLOGY

• what does the phloem do?

  • transports assimilates from source to sink.
  • these are sucrose or amino acids
• why is sucrose transported in the phloem and not glucose?

  • it is less reactive than glucose
  • it is more efficient
• sink explanation:

  • where the assimilates are used in respiration -anywhere in the plant- or converted for storage e.g. starch in roots and therefore are in a low concentration
• source explanation:

  • where sugars are made in the plant or released from a carbohydrate source and are therefore in high concentration
• ACTIVE LOADING:

  • hydrogen ions in the companion cells are actively transported out through the proton pump into surrounding tissue
  • increasing the hydrogen ion concentration gradient
  • hydrogen ions then move back into the companion cell with a sucrose or an amino acid, using a co-transporter protein by facilitated diffusion
  • sucrose diffuses through the plasmodesmata into the sieve tube elements
• MASS FLOW HYPOTHESIS:

  • sucrose lowers the water potential of the sieve tube elements therefore water moves into the sieve tube elements from the xylem or surrounding tissue through osmosis
  • this causes an increase in the hydrostatic pressure inside the sieve tube elements at the source
  • at the sink, sucrose diffuses out of the sieve tube elements and the water potential of the elements increases
  • then water leaves the elements by osmosis
  • theres a decrease in hydrostatic pressure inside the elements at the sink
  • THEREFORE THE ASSIMILATES MOVE FROM SOURCE - SINK DOWN THE HYDROSTATIC PRESSURE GRADIENT BY MASS FLOW