Transport in Vascular Plants II HKDSE Biology Organic

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 • 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.

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

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

Translocation patterns shift during development The Developmental Stage of an organ Young stem tips strong Sinks Inactive stem tips Young Fruits strong Sinks mature fruits Young leaves are strong Sinks mature leaves

Translocation patterns shift during development The Developmental Stage of an organ Young stem tips strong Sinks Inactive stem tips not strong sinks Young Fruits strong Sinks mature fruits not strong sinks Young leaves are strong Sinks mature leaves strong Sources

Transition of a leaf from sink to source young leaf Entire young leaf functions as a sink. – 14 CO labeled sugar 2 accumulates throughout leaf

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

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

Dicot Vascular Bundle (Primary tissue)

Dicot Woody Stem with Secondary Phloem

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

Sieve Tube Elements (STE) and Open Sieve Plates

Sieve Tube Member / Elements (STE) and Sieve Plates

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

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)

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.

Why Phloem need to be Living while xylem is DEAD? Living ? DEAD?

Why Phloem need to be Living while xylem is Living DEAD?

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

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 cotransport protein that couples sucrose transport to the diffusion of H+ back into the cell. High H+ concentration Cotransporter H+ Proton pump S Key ATP H+ Low H+ concentration Sucrose H+ S Apoplast Symplast

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

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

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

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.

Phloem loading – a summary

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.

• Phloem sap is under a Positive pressure while xylem sap have a negative pressure

Aphids and Phloem Research

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

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 EXPERIMENT To test the pressure flow hypothesis, researchers used aphids that feed on phloem sap. An aphid probes with a hypodermiclike 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 Sievetube member Sap droplet Aphid feeding RESULTS CONCLUSION Stylet in sieve-tube member Sieve. Tube member Sap droplet Severed stylet exuding sap The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration. The results of such experiments support the pressure flow hypothesis.

Pressure flow hypothesis (Münch, 1930)

Pressure Flow: The Mechanism of Translocation in Angiosperms Vessel (xylem) Sieve tube (phloem) • In studying angiosperms H 2 O Source cell (leaf) Sucrose 1 H 2 O 2 2 3 Pressure flow Transpiration stream – Researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure 1 4 Sink cell (storage root) 4 3 Sucrose H 2 O Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. This uptake of water generates a positive pressure that forces the sap to flow along the tube. The pressure is relieved by the unloading of sugar and the consequent loss of water from the tube at the sink. In the case of leaf-to-root translocation, xylem recycles water from sink to source.

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