different mechanisims of phloem loading

1. Topic: phloem loading
Presented by: Nasira Bashir
roll# 01

2. CONTENT
• Phloem
• Phloem loading
• Munch hypothesis
• SE-CC complex
• Mechanisms for phloem loading
• Apoplastic loading
• Symplastic loading
• Passive symplastic loading
• Summary/conclusion
• References

3. Phloem
Vascular tissue
Transportation of photosynthates from source to
sink
Composed of many cells
Sclerenchyma
Parenchyma
Sieve element
Companion cells
laticifers

4. Internal structure of phloem

5. The movement of the sugar at the source is referred as phloem
loading
OR
It is a process whereby carbohydrates (sugars) enter the sieve
tube at source.

6. Munch hypothesis
• A high concentration of sugar develops in
phloem cell near the source.
• Phloem loading result in lowered water
potential compared with adjacent xylem cells
causing water to move from xylem to phloem
by osmosis
• This influx of water creates a high turgor
pressure near the source and lower turgor
pressure near sink

7. • This causing the movement of water and sugar
from the source to sink.
• Removal of sucrose at sink increases water
potential causing water to move out of the
sieve tube at the sink.
• Solutes move to sink cell and water goes back
to xylem.

9. • Sieve element and companion cells are
considered functional units.
• Numerous plasmodesmata connect sieve
element to companion cells and the two
cells are referred as sieve
element/companion cell complex
Sieve element/companion
cell complex

10. Mechanisms for phloem
loading
• Source cell release sugar into the apoplast, from
there it is actively loaded into the SE/CC complex.
Apoplastic
loading
• Sugar diffuses via plasmodesmata from source
cells to the SE/CC complex.
Symplastic
loading

11. Apoplastic vs. symplastic loading

12. Apoplastic loading
• Sugar moves through plasmodesmata from
mesophyll cells up to companion cells where it
moves into the apoplast and is actively loaded
into ordinary companion cells.
• Ordinary companion cells with cell wall
ingrowths have very few plasmodesmata and
load sucrose from the apoplast

13. • Active uptake of sucrose from the apoplast by
a companion cell takes place.
• The plasma membrane H+-ATPase pumps
protons out of the cell, creating a proton
gradient.
• The energy of this gradient drives the uptake
of sucrose via an H+/sucrose sympoter.

15. Apoplastic and symplastic loading

16. Symplastic loading
• Intermediate companion cells are
characteristic of symplasmic loader
• Many plasmodesmata that connect them
photosynthesizing cells.

17. Polymer trapping model
• This model explains symplastic loading in
plants with intermediary cells
• Sucrose, synthesized in the mesophyll, diffuses
from the bundle sheath cells into the
intermediary cells through the abundant
plasmodesmata
• In the intermediary cells, raffinose is
synthesized from sucrose and galaticnol, thus
maintaining the diffusion gradient for sucose.

18. Polymer trapping model
• Because of large size, raffinose is not able to
diffuse back into the mesophyll
• As a result the concentration of transport
sugar rises in the intermediary cells and sieve
elements.

19. Polymer trapping model

20. Passive symplastic loading
• Tran locate sucrose and have ordinary companion
cells
• Possess abundant connections b/w the sieve
element-companion cell complex and
surrounding cells.
• Species with passive symplastic loading are
characterized by high overall sugar
concentrations in the source leaves, which
maintain a concentration gradient b/w the
mesophyll and SE/CC complex.

21. • The high sugar concentration give rise to the
high turgor pressures in the sieve elements of
the source leaves, generating the driving force
for long-distance transport.
• Example apple ( Malus domestica )
• willow( Salix babylonica)

22. Patterns in apoplastic, symplastic loading and
passive symplastic loading
Features Apoplastic loading Symplastic polymer
trapping
Passive symplastic
lo
Transport sugar sucrose raffinose and
sytachyose
sucrose
Characteristic
companion cells
ordinary intermediary ordinary
Number and
conductivity of
plasmodesmata
connecting the SE-
CC complex to
surrounding cells
low high high
Dependence on
active carriers in
SE-CC complex
Transporters driven Independent of
transporters
Independent of
transporters

23. Overall
concentration of
sugar in source
leaves
low low high
Cell types in which
driving force for
long-distance
transport is
generated
SE-CC complex intermediary cells mesophyll
Growth habit mainly herbaceous Herbs and woody
species
mainly trees

24. Conclusion
• Phloem loading is very important
phenomenon for the movement of sugars at
source.
• Loading of sieve tubes from the cell walls
requires energy which is derived by the proton
gradient.
• Different mechanisms are involved in phloem
loading

25. References
• L. Taiz, E. Zeiger Plant Physiology, 5th Edition, Sinauer
Associates, Sunderland, U.S.A
• http://biologyforums.com/gallery/33_25_07_11_12_
58_57.jpeg
• http://digital.library.unt.edu/ark:/67531/metadc304
41/m1/21/med_res
• http://bio1903.nicerweb.com/Locked/media/ch36/3
6_17SucroseLoading.jpg