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Transport in Vascular Plants II HKDSE Biology

Transport in Vascular Plants II HKDSE Biology. Organic nutrients are translocated through the phloem. Concept : Organic nutrients are translocated through the phloem Translocation : Is the transport of organic nutrients in the plant. Movement from Sugar Sources to Sugar Sinks.

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Transport in Vascular Plants II HKDSE Biology

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  1. Transport in Vascular Plants II HKDSE Biology

  2. Organic nutrients are translocated through the phloem • Concept : Organicnutrients are translocated through the phloem • Translocation: Is the transport of organicnutrients in the plant

  3. Movement from Sugar Sources to Sugar Sinks • Phloem sap • Is an aqueous solution that is mostly sucrose • Travels from a sugar source to a sugar sink Sucrose is the most commonly found carbohydrate in Phloem. It has a concentration range of 0.3 to 1 M. Some amino acids and proteins can be present in Phloem Sap.

  4. Source and sink Source: Any exporting organ Sinks: Any non-photosynthetic organ that does not produce enough photosynthetic products to support their own growth and development

  5. Source and sink Source: Any exporting organ • Storage organ during export phase • Mature leaves Sinks: Any non-photosynthetic organ that does not produce enough photosynthetic products to support their own growth and development • Apical regions • Immature fruits • Developing storage organs

  6. Translocation patterns shift during development The Developmental Stage of an organ

  7. Translocation patterns shift during development The Developmental Stage of an organ

  8. Transition of a leaf from sink to source youngleaf Entire young leaf functions as a sink. – 14CO2 labeled sugar accumulates throughout leaf

  9. Mature leaf Tip of leaf begins to lose label, indicates unloading of labeled sugar, now acting as a source.

  10. Transition to Source Transition to Source nearly complete as leaf reaches full size and maturity.

  11. Dicot Vascular Bundle (Primary tissue)

  12. Dicot Woody Stem with Secondary Phloem

  13. Companion Cell and Sieve Tube Electron Micrograph of ordinary Companion Cells and Mature Sieve tube elements Companion Cell Sieve Tube Elements

  14. Sieve Tube Elements (STE) and Open Sieve Plates

  15. Sieve Tube Member / Elements (STE) and Sieve Plates

  16. Sieve tube elements • At Maturity • Cells Lack: • Nucleus • Vacuole/Tonoplast • Cells Have: • Plasma membrane • Sieve plate

  17. Sieve tube elements • At Maturity • Cells Lack: • Nucleus • Vacuole/Tonoplast • Cells Have: • Plasma membrane • Sieve plate

  18. Companion Cells (CC) • “Life support” for Sieve tube members • Numerous plasmodesmata between CC and STE • Function in • Critical Metabolic functions for sieve tube element (e.g. Protein synthesis)

  19. Development of Companion cell and Sieve tube member Primary phloem develops by longitudinal division and subsequent elongation of meristematic cells. The cells do often divide unequally. The bigger daughter cell differentiates into a sieve element, the smaller after one or two further longitudinal divisions into two to four companion cells.

  20. Why Phloem need to be Livingwhile xylem isDEAD? Living ? DEAD?

  21. Why Phloem need to be Livingwhile xylem isDEAD? Living DEAD

  22. Why Phloem need to be living? • Sugar must be loaded into sieve-tube members before being transported to sinks Sugar must be activelyloaded into the phloem sieve tube

  23. High H+ concentration Cotransporter H+ Proton pump S Key ATP Sucrose H+ H+ Apoplast S Low H+ concentration Symplast In many plants, phloem loading needs active transport • Proton pumping and co-transport of sucrose and H+ enable the cells to accumulate sucrose active transport of sucrose into companion cells and sieve-tube members. Proton pumps generate an H+ gradient, which drives sucrose accumulation with the help of a co-transport protein that couples sucrose transport to the diffusion of H+ back into the cell.

  24. the cross section of a leaf photosynthesis occurs in the chloroplasts of the palisade mesophyll cells.

  25. Phloem loading Sugars from photosynthesis in mesophyll cells is passed to phloem cells.

  26. Companion cells: Transfer cells with cell wall ingrowths Transfer Cell Sieve tube element Wall Ingrowths Plasmodesma -living connection between CC and sieve tube element Parenchyma Cell

  27. Transfer cells Transfer cells, adjacent to the phloem have highly folded cell membranes. This increased surface area is due to infoldings in the cell wall, they also have many mitochondria to drive active transport -- to pump organic molecules into the transfer cells and to load them into the adjacent sieve tube elements.

  28. Phloem loading – a summary

  29. Studies of phloem transport:Rates of Movement • Velocity: • Measured velocities far exceed those explained by diffusion (500 to 1500 mm/h). A pressure gradient drives translocation • Mass Transfer Rate: • Mass of transport material passing through a single point over a given time • Range from 1 – 15 g h-1 cm-2 of sieve elements.

  30. Phloem sap is under a Positive pressure while xylem sap have a negative pressure

  31. Aphids and Phloem Research

  32. Aphids, honeydew and Ants - mutualism Ants often found nursing and milking aphids

  33. EXPERIMENT To test the pressure flow hypothesis,researchers used aphids that feed on phloem sap. An aphid probes with a hypodermic-like mouthpart called a stylet that penetrates a sieve-tube member. As sieve-tube pressure force-feeds aphids, they can be severed from their stylets, which serve as taps exuding sap for hours. Researchers measured the flow and sugar concentration of sap from stylets at different points between a source and sink. 25 m Sieve-tubemember Sieve- Tube member Sap droplet Sap droplet Stylet Aphid feeding Stylet in sieve-tube member Severed stylet exuding sap RESULTS The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration. CONCLUSION The results of such experiments support the pressure flow hypothesis. why phloem sap always flows from source to sink… • The pressure flow hypothesis explains why phloem sap always flows from source to sink • Experiments have built a strong case for pressure flow as the mechanism of translocation in angiosperms

  34. Pressure flow hypothesis (Münch, 1930)

  35. Pressure Flow: The Mechanism of Translocation in Angiosperms Loading of sugar (green dots) into the sieve tube at the source reduces water potential inside the sieve-tube members. This causes the tube to take up water by osmosis. Vessel (xylem) Sieve tube (phloem) Source cell (leaf) 1 • In studying angiosperms • Researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure H2O Sucrose 1 H2O This uptake of water generates a positive pressure that forces the sap to flow along the tube. 2 2 The pressure is relieved by the unloading of sugar and the consequent loss of water from the tubeat the sink. 3 Transpiration stream Pressure flow In the case of leaf-to-root translocation, xylem recycles water from sink to source. 4 Sink cell (storage root) 4 3 Sucrose H2O

  36. http://www.dbbe.fcen.uba.ar/materias/botanica/Phloem%20Translocation.htmhttp://www.dbbe.fcen.uba.ar/materias/botanica/Phloem%20Translocation.htm http://www.biologie.uni-hamburg.de/b-online/e06/06d.htm http://plantphys.info/plant_physiology/

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