Translocation

Question 1

Define source

Answer 1

A part of the plant that loads assimilates into the phloem (eg leaf makes sucrose from phsynsis products, root makes sucrose from stored starch)

Question 2

Define sink

Answer 2

A part of the plant that removes sucrose and other assimilates from the phloem (used in growth and storage in leaves, roots, buds, storage organs)

Question 3

Mechanism of translocation

Answer 3

Mass flow

Question 4

Define mass flow

Answer 4

High hydrostatic pressure at source (active loading, hydrogen ions), low hydrostatic pressure at sink (cells use sucrose, low water potential cell, water osmoses out of phloem)

Question 5

Evidence for phloem usage in mass flow

Answer 5

Radioactive labelled carbon (in CO2, in air) appears in sieve tubes
Ringing plant-removes phloem (stops translocation) kills plant

Question 6

Evidence for energy requirement of mass flow

Answer 6

Metabolic poisons stop translocation
Rate of flow too high for simple diffusion
Companion cells MANY mitochondria

Question 7

Evidence for translocation

Answer 7

pH higher in companion cells than surrounding (higher pH, less H+)
Sucrose concentration always higher in source than sink
Many plasmodesmata between sieve tube elements and companion cell

Question 8

Evidence against mass flow

Answer 8

Different solutes a move at different rates
Sucrose move to all parts at same rate, rather than faster to low concentration
Electron micrograph a show sieve plates blocked with protein
Role of sieve plates unclear

Question 9

Define translocation

Answer 9

Translocation is the movement of assimilates (mostly sucrose) around the plant. It occurs in the sieve tubes