Translocation

Question 1

Keywords

Answer 1

•Translocation: The transport of assimilates (sugars - sucrose and amino acids) throughout the plant in the phloem. Sources load assimilates into the phloem, sinks remove them. Requires energy – active.
•Sink: A part of the plant where assimilates are removed from the phloem.
- roots receiving sucrose from photosynthesis and storing them as starch.
- meristems receiving sucrose and using for respiration and growth.
•Source: A part of the plant that loads assimilates into the phloem.
- leaves photosynthesise, sucrose made is moved to phloem.
- Roots/seeds store energy as starch, which is converted to sucrose and transported to sinks
(meristems) during growth seasons.
Enzymes maintain a concentration gradient from source to sink by breaking down assimilates at sink (sucrose broken down into glucose/fructose for use in respiration).

Question 2

Active loading of sucrose:

Answer 2

• Active transport of H+ ions out of companion cells into surrounding tissue cells, using energy from
  ATP, increases H+ concentration outside of companion cells (& decreasing H+ concentration inside). - Creates H+ concentration gradient.
• Facilitated diffusion of H+ ions, accompanied by sucrose molecules, back into companion cells as they both bind to cotransporter proteins at the same time – cotransport.
• Sucrose moves against concentration gradient (secondary active transport).
• Conc of sucrose in companion cell increases, sucrose in companion cells diffuses through
  plasmodesmata into sieve tubes.

Active process needed as sucrose conc is higher in companion cells than surrounding tissue.

Question 3

Movement of sucrose in phloem:

Answer 3

• At the source sucrose is actively loaded into the sieve tubes, reducing the water potential in the
  phloem (makes it more negative).
• So water osmoses into the sieve tube element from companion cells and xylem, increasing hydrostatic pressure.
• At the sink sucrose is removed from the sieve tubes by surrounding cells as it is used for metabolic
  processes, this increases water potential in the sieve tube element.
• So water osmoses out of the sieve tube down water potential gradient at the sink, reducing
  hydrostatic pressure.
• This results in sap (water with dissolved assimilates) moves by mass flow from high to low
  hydrostatic pressure (down gradient) in phloem, from source to sink.
• This can be up or down tube, depending on location of sources/sinks – bidirectional flow. - Note sap in one sieve tubes all flows in same direction