translocation Flashcards
what is translocation
the bidirectional transport of assimilates through a plant, occurs in the phloem
what does translocation require unlike xylem
energy
what is the source
the part of the plant that loads assimilates into the phloem
what is the sink
the part of the plant that removes assimilates from the phloem
what is the hypothesis of how sap moves through phloem vessels
the pressure flow hypothesis
what is the pressure flow hypothesis
-sap moves due to differences in hydrostatic pressure in the phloem vessels
evidence of the pressure flow hypothesis
-aphid style experiments
-tree ringing
-radioactively labelling carbon
what is glucose turned into and how is this used
-sucrose
-for transport around plant
-or temporarily stored as starch grains within a stroma matrix
how is sucrose moved into sieve tubes
through a process called active loading
steps of active loading
-ATP used to pump H+ ions out of companion cell into leaf
-forms a concentration gradient
-H+ ions diffuse back in through cotransporter proteins
-proteins only allow movement of H+ ions into the cell if accompanied by sucrose molecules
-under process called cotransport
what is co transport also known as
-secondary active transport
what does secondary active transport result from
-the active transport of H+ out of the cell which moves sucrose against its concentration gradient
what happens as the concentration of sucrose in the companion cell increases
-it can diffuse through plasmodesmata into sieve tube
what occurs due to the diffusion of sucrose into sieve tube
-lowers the water potential so water from xylem moves in and increases hydrostatic pressure
why is there a continuous flow from source to sink in the sieve tube
-a sink removes sugar from phloem, increasing water potential
-water leaves the sieve tubes by osmosis
-keeps the hydrostatic pressure low so that sap continuously flows from source to sink
what happens to sucrose wherever it is needed for growth or into storage organs like the tuber cell
-it is actively unloaded from phloem
-where it is converted into starch
-process known as a sink
what happens as water enters the phloem
-the hydrostatic pressure increases
-this forces the phloem sap through the vessels towards regions of lower pressure
-under physical process called mass flow
why is it beneficial for the carbohydrate in the plant to be transported as sucrose
-soluble so can be transported in sap
-metabolically (relatively) inactive so not removed during transport
