Phloem loading

different mechanisims of phloem loading

1. Topic: phloem loading Presented by: N asira 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. Phloem loading

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.

8. 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

9. Mechanisms for phloem loading

10. Apoplastic vs. symplastic loading

11. 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

12. 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 .

13. Apoplastic and symplastic loading

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

15. 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 .

16. 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.

17. Polymer trapping model

18. 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.

19. 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 )

20. 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

21. 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

22. 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

23. References L. Taiz , E. Zeiger Plant Physiology , 5 th 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/metadc30441/m1/21/med_res http://bio1903.nicerweb.com/Locked/media/ch36/36_17SucroseLoading.jpg