9.4 Translocation Flashcards
What is the definition of translocation?
Movement of dissolved substances from the source to the sink.
Why isn’t glucose transported during translocation?
Because glucose will be used up on its journey to the sink.
What is transported instead of glucose during translocation?
Sucrose
What is a “source”?
Origin of the glucose
What is a “sink”?
The destination of glucose
Do the sinks have higher or lower pressure than the sources? Why?
Sinks have lower pressure. The source has higher pressure because of solute accumulation in the phloem
What are the key sources of assimilates in a plant?
-Green leaves and green stems
-Storage organs that unload their stores at the start of a growth period
-Food stores in seeds when they germinate
What are the key sinks in a plant?
-Roots that are actively growing and absorbing mineral ions
-Meristems that are actively dividing
-Any parts of the plant that are laying down food stores
Why does water move from xylem to the phloem? (Describe concentrations)
-Because the water potential in the xylem is higher than in the phloem
-The build up of sucrose in the companion cells and sieve-tube elements reduces the water potential in the phloem
-So the water moves down the concentration gradient by diffusion.
How does water exit the xylem?
By diffusion through the non-lignified pits
How does water transport assimilates in the phloem?
-The water flowing into the phloem leads to a build-up of hydrostatic pressure, and so pushes the assimilates down the tubes of the sieve elements
-Water moves by mass flow to areas of lower pressure (i.e the sinks) because of turgor pressure
What is the name of the method describing how water travels down the phloem?
Mass flow
Why is water in the phloem forced to move down to an area of lower pressure?
Because of the turgor pressure caused by solute accumulation in source phloem
What is the apoplast route in translocation?
The sucrose travelling through cell walls to the companion cells and sieve elements, done by diffusion.
How is the concentration gradient of sucrose maintained for the apoplast pathway to be followed?
By removal of sucrose into phloem vessels
How is sucrose moved into the cytoplasm of the companion cells in the apoplast pathway? (Simple answer, terms only)
In an active process called CO-TRANSPORTATION
Describe and explain how sucrose enters the cytoplasm of companion cells through the apoplast pathway.
1- Hydrogen ions are actively pumped out of the companion cell into the surrounding tissue, using ATP
2- Hydrogen ions return to the companion cell down a concentration gradient via a CO-TRANSPORT PROTEIN (and SUCROSE is CO-TRANSPORTED)
3- This increases the sucrose concentration in the companion cells and sieve tube elements through the plasmodesmata between the 2 linked cells
How do companion cells support co-transport of sucrose?
-They have many mitochondria to supply the ATP needed for transport pumps
-They have many infoldings of their cell membranes to give an increased surface area (more points of contact for sucrose)
What is the main mechanism for phloem unloading sucrose?
Diffusion of sucrose into surrounding cells
How is a low concentration of sucrose maintained in sink cells?
-Sucrose is converted into starch or glucose
-Sucrose is stored in the cell vacuole
How does water leave the phloem and re-enter the xylem? (Long answer)
-Loss of the solutes in the phloem (into the sinks) leads to an increase in water potential
-So water diffuses out of some of the sieve tube elements and into the xylem transpiration stream.
