translocation

describe the process of translocation:

• at the source, solutes like sucrose are actively loaded into sieve tube elements from companion cells

• this decreases ψ in sieve tube elements so water osmoses into the sieve tube elements from xylem and companion cells

• this increases hydrostatic pressure in the sieve tube elements at the source

• at the sink, solutes are actively removed from the sieve tube elements at the sink

• this increases ψ in sieve tube elements so water osmoses out of the phloem

• this decreases the hydrostatic pressure at the sink

• this creates a pressure gradient, pushing solutes from the source to areas of lower pressure at the sink by mass transport

• at the sink, solutes are actively loaded from the sieve tube element to companion cells, before diffusing into sink cells to be used (e.g. in respiration)

how is rate of translocation affected by the sucrose conc at the source?

the higher the sucrose conc at the source, the higher the rate of translocation

give 5 pieces of supporting evidence for the mass flow hypothesis:

• sucrose is delivered at the same rate everywhere rather than faster to regions w/ lower sucrose conc (at a higher pressure gradient, it should travel faster and vice versa)

• companion cells have many mitochondria, suggesting translocation is an active process

• glucose conc is higher in leaves than in sinks

• flow of sucrose in phloem occurs in daylight but not at night

• increase in sucrose conc followed by similar increase in mass flow rate

• metabolic poisons/lack of O₂ inhibits phloem loading

give 2 pieces of evidence against the mass flow hypothesis:

• sieve plates seem to hinder the mass flow of sucrose

• different solutes move at different speeds in the phloem (the size of the molecules should not make a difference for the pressure gradient)

name 3 experimental pieces of evidence for the mass flow hypothesis:

• ringing

• autoradiography

• aphids

what is ringing? how does it demonstrate translocation?

• removal of a ring of protective layer and the phloem around the circumference of the stem

• stem immediately above ring swells due to the accumulation of sugars - there is a higher conc of sugars above the below the ring

• suggests there is a downward flow of sugars

what is autoradiography? how does it demonstrate translocation?

• sucrose actively labelled w/ radioactive ¹⁴C isotope

• movement of sucrose can then be tracked w/ radiography

• autoradiograph of plant - wherever film turns black, ¹⁴C (and ∴ sucrose) present

• suggests movement of sucrose from source to sink

give 2 limitations of using autoradiography to demonstrate translocation:

• does not show mechanisms by which sucrose travels

• plant dies between images - timeline must be made using multiple plants

how can we use aphids to demonstrate translocation?

• aphids have mouthpieces called stylets which they insert into the phloem

• stylets cut - sap flows out quicker nearer leaves than further down the stem

• suggests pressure gradient down phloem

OR:

• expose top of phloem to ¹⁴C

• rate =- distance from starter colony/time for radioactivity to travel from starter colony