Download Phloem - The Virtual Plant

moving assimilates
overview
distribution - where found in plants
structure -what cell types are involved?
function - how does phloem work?
functionality
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.
phloem
xylem
vessel
xylem
vessel
Sieve tube
Sieve tube
companion
cells
Sieve tube
the sieve plate: stops
organelles from moving
out of specific cells
Sieve tube
10 µm
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
S
CC
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?
SOURCE
SINK
sugar enters sieve tubes,
water follows by osmosis
bud
leaf
Sugar flows to
regions of LOW
TURGOR
PRESSURE
sugar enters sieve tubes,
water follows by osmosis –
region of HIGH TURGOR
PRESSURE
assimilate movement out of a source (leaf)
requires three stages:production, collection, and transport
pm
BS
CC
pvm
SE
BS
sm
transport
& export
collection
production
integration within the whole plant system
sinks -- above and below ground
buds
flowers
young leaves
roots
NB... the SOURCE is a
photosynthetically-active organ
storage organs
redirection
Assimilates can be redirected on demand as a result of changes
in metabolic requirements, and the flow (which is pressuredriven) changes.
Insects can cause massive long-term redirection
of assimilate flow. Aphids are a good example.
stealing 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
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transport
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