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moving assimilatesmoving assimilates
overviewoverview
structure -what cell types are involved?
distribution - where found in plants
function - how does phloem work?
functionalityfunctionality
The phloem in higher plants forms the conduit through which photoassimilates as well as a host of other matter (organic and inorganic) moves.
Phloem transport can occur because these materials are transported in a watery fluid.
conduit = system of open tubes
the system
Phloem forms a highly specialized tissue, that is always found in close proximity to the xylem.
Xylem and phloem together, form vascular bundles (stems and leaves) or strands in roots.
Four sieve elements, forming a sieve tube.
in stems
xylem and phloem forms vascular bundles
phloem
in leaves
xylem
phloem
in roots
The xylem and phloem form alternating strands in roots, shown here in Zea mays root.
xylemvessel
xylemvessel
phloem
10 10 µµmm
Sieve tubeSieve tube
Sieve tubeSieve tubeSieve tubeSieve tube
Sieve tubeSieve tube
companioncompanioncellscells
the sieve plate: stops the sieve plate: stops organelles from moving organelles from moving out of specific cellsout of specific cells
high rate of metabolism
The sieve tube takes up carbohydrate at the source from the companion cell, transports it, and unloads via companion cells and associated parenchyma, in sinks. The companion cells are also responsible for the maintenance of the sieve tubes.
Companion cells have a high metabolic rate. This micrograph shows high mitochondrial and ER activity in the companion cell, but not in the adjacent sieve tube
SS
CCCC
The hyperactive nurse, mitochondrial activity
The image to the left shows high mitochondrial and ER activity associated with a companion cell (left) and much less in the two glowing sieve elements.
The image was made using a confocal microscope and a ER-mitochondrion selective dye.
Protein and membrane maintenance of the enucleate sieve elements is carried out by the companion cells
how does phloem work?
sugar enters sieve tubes,water follows by osmosis –region of HIGH TURGORHIGH TURGORPRESSUREPRESSURE
SINKSINKSOURCE
sugar enters sieve tubes,water follows by osmosis
Sugar flows to regions of LOW LOW TURGOR TURGOR PRESSUREPRESSURE
leafleafbudbud
productionproduction
collectioncollection
transporttransport& export& export
SE
pm
pvm
sm
BS
BS
CC
assimilate movement out of a source (leaf) requires three stages:-production, collection, and transport
integration within the whole plant system
sinks -- above and below ground
budsbudsflowersflowers
young leavesyoung leaves
rootsroots
storage organsstorage organsNB... the SOURCE is a photosynthetically-active organ
redirection
Assimilates can be redirected on demand as a result of changes in metabolic requirements, and the flow (which is pressure-driven) changes.
Insects can cause massive long-term redirection of assimilate flow. Aphids are a good example.
stealing assimilatestealing assimilate
large aphid colonies such as this one, can redirect assimilate flow, by forming a significant local sink. The actively growing parts of the plant are thus deprived of energy needed for growth.
Aphids penetrate between and through cells, in search of the phloem. The stylet tips are open in this successful feeder
summary:
phloem transport is pressure-driven
Loading transport
unloading