Download Phloem tissue is composed of

TRANSLOCATION
Translocation is the transport of organic assimilates
(e.g. sugars). It takes place in phloem tissue.
Phloem tissue is composed of:
Sieve elements – the tubes
through which the assimilates flow.
Companion cells – assist
in the flow of assimilates.
Other types of cell are
present (parenchyma & fibres).
Each sieve element has a
companion cell.
Lots of sieve elements join
to form a sieve tube.
Both sieve elements and companion
cells are biochemically active – living!
Companion cells have a
full suite of organelles
(they tend to have more
mitochondria and ribosomes),
Sieve tubes have less
organelles and reduced
cytoplasm (which is only
found at the edge of sieve tubes).
Plasmodesmata (pores) linking the
two cell types.
Running through the phloem is a
sap, it contains mostly sucrose
with salts (potassium/chloride &
phosphate)
At the end of each sieve element
is a plate with pore – sieve pore
How does translocation take place?
Similar to xylem vessels – by mass flow
– e.g. differences in pressure.
ATP is needed to– the plant has
to invest energy – it is an active process.
The pressure difference is
created by active loading of
sucrose into the sieve element.
This decreases the water potential in
the sieve element and therefore water
follows down the water potential gradient
When sucrose needs to be removed –
it is taken from the cell and
water again follows by osmosis.
This system creates
high
hydrostatic pressure in
the leaf………
………………low hydrostatic
pressure in the roots.
leading to a flow of
water that takes solutes
with it.
SOURCE – an area
that loads assimilates
into the phloem. A
region of P.
SINK – an area that
removes assimilates
from the phloem.
A region of R.
LOADING SUCROSE
Photosynthesis
produces sugars
(3C-triose)
These are
converted to
Sucrose
Sucrose then
moves from P
cells to the
companion cells
via the Apoplast
and Symplast
pathways.
COMPANION CELL
SIEVE
ELEMENT
H+
H+
S
S
H+ are pumped out of
the companion cell. ATP rqd an active process.
A protein in the CCell wall acts
as co-transporter, H+ flow down
the conc. gradient, taking
sucrose into the CCell.
Sucrose diffuses through
Plasmadesmata and enters
sieve element. Water follows.
RESULT: A HIGH HYDROSTATIC
PRESSURE IN THE LEAF (SOURCE)
UNLOADING SUCROSE
Sucrose moves out of sieve
element via diffusion
Sucrose is quickly broken down by
the enzyme INVERTASE.
SUCROSE to Glucose & Fructose
This mechanism creates a conc.
grad., down which sucrose flows
out of the phloem and into the
CCell.
Evidence for mechanism of phloem
transport:
The rate of mass flow is 10,000 times faster
than it would be if diffusion alone was responsible.
Phloem sap has a relatively high pH (pH 8) –
e.g. H+ are being removed, making the cytoplasm slightly
alkaline.
There is an electrical difference across
the membrane of the companion cell (-150mV)
– you would expect this if H+ were being redistributed.
Mitochondria and ATP are present in large quantities;
this would be expected if H+ were being
transported out of the companion cell.
A common question……………
‘Compare & contrast sieve elements & xylem
vessels’
Find 8 facts!
Comparing sieve elements and xylem vessels
SIMILARITIES
Liquid moves by mass flow along pressure gradient in both P &
X. Cells are stacked end to end in both P & X.
DIFFERENCES
Xylem transports water and mineral ions.
Phloem transports sugars/ions and amino acid in solution.
Xylem vessels are strengthened – lignified (sieve elements are
not).
Lignified cells eventually die.
All cells in the phloem are alive.
Xylem vessels are continuous columns, sieve elements have
plates.
Sieve plates provide support; xylems vessels do not need this,
they have fibres.
Sieve elements can heal small holes, xylem vessels can not.