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Function of Phloem
 transports organic substances (sucrose) made by
plant during photosynthesis
 mostly away from leaves to storage organs
 transport referred to as translocation
 Solutes move up and down phloem unlike
movement in xylem
Structure of Phloem
Structure of Phloem
Structure of Phloem
 Made from cells called sieve tube elements
 Elements arranged end to end and fused together to
form sieve tubes giving it its tubular appearance and
conducting capacity
 end walls of sieve tube elements are partially
perforated due to enlarged plasmodesmata forming
sieve plates
 Sieve plates separate one sieve tube element from the
Structure of Phloem
 sieve plates allow for the flow of solution from one
element to the next
 sieve elements are living cells with active cytoplasm
and their walls contain no lignin unlike xylem
Companion Cells
Companion Cells
Companion Cells
 cells closely associated with sieve tube elements.
 formed from cell division of a parent cell which
formed sieve tube element
 Many plasmodesmata connect the cytoplasm of the
sieve tube element and the companion cell.
 have all the cellular organelles
 very metabolically active (contain many
 have a very intimate relationship with the sieve tube
elements assisting them in metabolic processes
Phloem Loading
 Photosynthesis produces sugars
 Sugars converted into sucrose for transport in
 Sucrose-relatively inactive, highly soluble, unused in
 Easily converted back into glucose and fructose
Phloem Loading
 Sucrose dissolves into water of mesophyll cell,
moves across leaf by apoplast or symplast pathways
 Loading-substances made in the leaves by
photosynthesising cells (source) must get to the
phloem tissue before it is translocated to where they
are needed (sinks)
Phloem Loading
 sugars produced in the leaves are of a lower
concentration (0.5%) compared to the higher
concentration (30%) found in companion cells and sieve
tube elements
 Thus, sucrose moves against a concentration to get into
the sieve tube elements
 Movement occurs by active transport
 active loading of amino acids, sucrose, phosphates,
potassium and ammonium ions into the companion cells
are thought to be carried out by specific carrier proteins in
the cell membranes of the companion cells
Phloem Loading
 Uses energy produced in mitochondrion of
companion cell
 H+ ions pumped out of companion cell by active
transport-excess H+ ions outside cell
 H+ ions diffuse rapidly back into cell down conc
gradient through carrier protein along with sucrose
 Sucrose mol carried through this co-transporter into
companion cell against conc gradient
Phloem Loading
 Active transport into companion cells result in a very
negative solute potential in them
 Hence, water enters companion cells by osmosis
 Sucrose moves from companion cell into sieve tube
through plasmodesmata connecting them
Phloem Unloading
 Sucrose unloaded into any tissue where needed
 Sucrose leaves sieve tubes thru plasmodesmata by
facilitated diffusion
 Solute potential inside sieve tubes becomes less
negative, maintaining the pressure gradient between
source (where sugar is loaded) and sink (sugar
 Unloading may also occur thru cell surface
membrane of sieve tube into the cell walls
Phloem Unloading
 Sucrose then converted into glucose and fructose by
the tissue
 This decreases its conc in the phloem thus
maintaining a conc gradient from the phloem into
the tissue
Mass Flow
 phloem tubes transport organic substances made in
the leaves of the plant
 substances move from where they are made (leaves)source
 substances are moved to the roots, storage organs
and other parts of the plant to where they are
Radioactive Tracers
 There are three basic pieces of experimental evidence
which suggest this:
1. Using radioactive tracers: by labelling CO2 with 14C
(radioactive isotope) to be used by the plant for the
production of organic substances by photosynthesis.
It is seen that the organic substances produced are
transported in the phloem tissue because the
radioactive organic carbon compounds blacken
photographic film in the areas that are thought to be
phloem in the stem of that plant
Radioactive Tracers
 Using sap-sucking insects- these insects eg. aphids,
insert their tubular mouthparts into phloem tissue
for a replenishing supply of sugars and amino acids.
However, if we were to put the insect on anaesthetics
and remove it but leaving its mouth part immersed
in the phloem tissue it would show that fluid oozes
out due to the pressure in the sieve tubes.
Ringing Experiment
 Using ringing experiments- phloem tissues are
usually just found under the bark of a tree, thus if
the tree is ringed, ie, tissue is removed then the
phloem would be as well. However, the plant
continues to survive for some time because the
xylem lies much deeper in side but over time the
plant dies because where the ring is made, the area
just above swells as substances needed by the root
accumulates there as they are not translocated and
roots are starved of nutrients and die.
Ringing Experiment
Mass Flow
 The proposed mechanism of transport of organic
substances down the phloem tubes is called the
 Firstly, organic substance (sucrose) is actively uploaded
into the phloem tubes by active transport via the
companion cells.
 This creates a very negative solute potential/low water
potential in the sieve tube elements of the phloem tissue.
 water from the xylem vessels rushes into the sieve tube
elements to create a mass flow of solution or a large
hydrostatic pressure there.
Mass Flow
 This pushes the organic substances in solution down
the phloem tissues to where it is needed in the sinks
eg roots where there is a high water potential/less
negative solute potential.
 This maintains the pressure gradient which allows
for the flow of solution down the phloem tissues into
the sinks from the sources.
Evidence supporting the Mass
Flow Hypothesis
1. There is a flow of sol’n from phloem when it is cut
or punctured by the stylet of an aphid
 There is some evidence of conc gradients of sucrosewith high conc in the leaves and lower conc in roots
 Some reasearchers have observed mass flow in
microscopic sections of living sieve elements
 Viruses or growth chemicals applied to leaves are
only translocated downwards to roots when the leaf
is illuminated.
Evidence Against the Mass Flow
1. Sugars have been observed to move at different
speeds and in different directions in the same
vascular bundles. The theory suggests that all
materials being transported travel at the same speed
2. theory offers no explanation for the existence of
sieve plates which act as a series of barriers
impeding flow
3. Theory does not explain why sieve tubes should be
living as opposed to the dead xylem