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LECTURE 7:SUGAR TRANSPORT

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Pressure Flow Hypothesis Water in the adjacent xylem

moves into the phloem byOSMOSIS.

As osmotic pressure (potential)builds up the phloem sap willmove to areas of lowerpressure.

At the sink osmotic pressure must be reduced. Again activetransport is necessary to move the sucrose out of the phloemsap and into the cells which will use the sugar – converting itinto energy, starch, cellulose, or other substances

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XYLEM PHLOEM

Source (left): http://files.lcbiology.webnode.com/200000125-09bd00ab86/osmosisss.jpg

Water potential ( = W) = P + SP = pressure/turgor potentialS = solute/osmotic potential

OSMOSIS

HOW FAR HAVE WE GONE?1. INTRODUCTION

PLANT FUNCTION: CARBOHYDRATE2. CARBOHYDRATE PRODUCTION

Lecture 2: Light Reaction (NADPH Synthesis) Lecture 3: Light Reaction (ATP Synthesis) Lecture 4: Dark Reaction (C3 plants) Lecture 5: Dark Reaction (C4 & CAM plants)

3. CARBOHYDRATE USE Lecture 6: Respiration

4. CARBOHYDRATE TRANSPORT Lecture 7: Sugar Transport

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LECTURE OUTCOMEStudents, after mastering materials of the presentlecture, should be able:1. To explain the pathway of sugar transport in plants2. To explain physical structure of transport pathway

of sugars3. To explain evidence supporting the transport

pathway of sugars4. To explain water potential as the driving force of

sugar transport5. To explain the mechanism of sugar transport in the

pathway

LECTURE FLOW

1. Phloem Structure2. Evidence of Phloem3. Direction of Flow4. Mechanism of Transport

– Phloem Loading– Pressure-Flow Hypothesis– Phloem Unloading

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QUESTIONS1. What is the function of the phloem sieve elements?2. What is the difference between sieve tube elements and

sieve cells3. What is the evidence that phloem is the transport pathway of

photosynthate?4. In what direction does phloem transport substances

throughout the plant?5. Why is the downward movement of sugars prevented when a

complete ring of bark is removed6. What is the most abundant compound found in Phloem?7. Are leaves sink or source?8. What are anatomical and developmental determinants of the

direction of source-sink translocation?9. What does the factor cause flow in the phloem?10. What is the effect of chilling of a source leaf petiole on

the rate of translocation in sugar beet ?

1. Phloem StructureBark of a woody plant contains phloem but not xylem

Where is theposition of phloem?

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View ofsieve platepores

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Sieve tube elements• Tubular cells with end

wall pores and• lateral sieve areas• Membrane bound• Have some organelles• Have adjacent

companion cells

Sieve tubeelements

orSieve cells

Sieve element featuresliving, membrane-bound cells

(compare to tracheary elementsof xylem)lack some structures and organelles in most

living cells - no nuclei, vacuole, Golgi,ribosomes, microtubules, microfilamentsassociated with companion cells that have full

set of structures and organelleshave sieve areas or pores that interconnect

adjacent sieve elementsSieve tube elements in angiosperm are called

sieve cells in gymnosperms

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“Girdling” a woody plant causes swelling of stem abovethe point of damage, indicating a blockage of phloemtransport.

2. Evidence of PhloemA classic experiment - girdling

Evidence 1

a) Mason and Maskell (1928) demonstrated thatremoving a complete ring of bark, while leaving thewood (xylem) intact, prevented downward movementof sugars.

b) When a strip ofbark was retainedbetween upper andlower stem parts,sugars floweddownwards indirect proportion tothe width of theremaining bark

barkstrip

Evidence 2

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Radioactivelabeling with14CO2 can tracemovement ofsugars in thephloem, and fromsource leavesto sinks throughoutthe plant.

More experimental evidence that phloem is the transporttissue for carbohydrates.

Evidence 3

The phloem is the vascular system for moving (translocating)sugars produced in photosynthesis (photosynthate) and othersubstances throughout the plant.

0.5M

Evidence 4

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Phloem Sap Composition– Sugars primarily (sucrose, many species)– Very concentrated– Cations: K, Ca, Mg, Na in dec.

concentration– Anions: P, Cl, S, and NO3-* in some

species– Amounts of constituents varymore sugars, e.g. when trees breaking dormancy,

cotton boll formation, tomatoes & grapes buildingsolidsK+ conc. ’s because K+ important to

translocation of sugars.More PGR’s at certain times

In what direction does phloem transportsubstances throughout the plant?From an area of carbohydrate supply to anarea of carbohydrate demand.

Source → SinkSource - produces more carbohydrates than

required for its own needsSink - produces less carbohydrates than it

requires

3. Direction of Flow

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1. Leaves: typically sources

(supply roots,meristems, fruits), but

when just forming, arebriefly sinks andnutrients enter viaphloem rather thanxylem.

2. Roots: typically sinks (fed by phloem).– May become sources when trees leaf out in the spring

– Then assimilated nutrients supply leaves rather than nutrients fromsoil.

Sugar derived from CO2 and light energy is loaded with H2O into the phloem

of mature leaves to establish a region of high pressure. Growing tips(represented by developing leaves) utilize sugars to create a region of lowpressure, causing bulk flow from source to sink tissues.

3. Fruits, or storageorgans: are sinks asthey develop.– If seed germinates, or

the storage organsbecome sources whenthe bulb sends outshoots in the spring.

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The direction of phloem translocation within the plant can beexplained by source-sink relationships.

Distribution ofradioactivity from asingle labeled sourceleaf in an intact sugarbeet plant (Betavulgaris) . This wasdetermined 1 weekafter 14CO2 wassupplied for 4 hours toa single source leaf(arrow).

The degree of radioactive labeling is indicated by the intensity of shadingof the leaves. Leaves are numbered according to their age; the youngest,newly emerged leaf is designated 1

1. Proximity - sinks tend to besupplied by closer sources

2. Vascular connections maycause distinct source-sinkpatterns that counterproximity

3. Source-sink relationshipsmay shift duringdevelopment

Anatomical and developmental determinants of thedirection of source-sink translocation.

Young leaf is completely dependent on carbohydrates from other sources. It is astrong sink.

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As the leaf grows it increasingly provides for its own carbohydrateneeds.

Mature leaf is largely a carbohydrateexporter (source)

Transfer cells

4. Mechanism of Phloem Transport

• What is the mechanismof phloem transport?

• What causes flow?, andWhat’s the source ofenergy?

Velocities ≈ 1 m hour-1, much faster than diffusion

Osmosis *

*

Activetransport*

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Sugars are moved fromphotosynthetic cells and actively(energy) loaded into companion &sieve cells.

Theconcentrating ofsugars in sievecells drives theosmotic uptakeof water.

Sucrosetransport acrossplasmamembrane

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Phloem loading uses a proton/sucrose symport.

Sieve plate pore

Cell wall betweensieve elements

Companion cell

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However, translocation rates recover with time despite the fact thatATP production and utilization are still largely inhibited by chilling

14CO2 was supplied to a source leaf, and a 2-cm portion of its petiole waschilled to 10C. Translocation was monitored by the arrival of 14C at a sink leaf.(1 dm [decimeter] = 0.1 meter) (After Geiger and Sovonick 1975.)

The energyrequirement fortranslocation in thepath is small

The chilling of asource leaf petiolepartially reducesthe rate oftranslocation insugar beet.

= W = water potentialP = pressure potential (turgor pressure)S = solute (osmotic) potential

W = P + S

PRESSURE-FLOW MECHANISM

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Plasmolysis

High water potential approaches zero or less negativeHigh water potential more negative

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The bags are immersed in two interconnected troughs filled with water. Assoon as the bag A is immersed in water, water from the trough rushes into themembranous bag A. On the contrary, the bag B as it is filled with only water

The pressure-flow model of phloemtranslocation

1. Flow is driven by a gradient of pressure,YP.2. At source end of pathway

• Active transport of sugars into sieve cells• Ψs and Ψw decrease• Water flows into sieve cells and turgor increases

3. At sink end of pathway• Unloading (active transport again) of sugars• Ψs and Ψw increase• Water flows out of sieve cells and turgor

decreases

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• Phloem solution moves along a gradient of pressuregenerated by a solute concentration (osmotic potential, s)difference between source and sink ends of the pathway

1. Sugar is actively loaded into the sieve tube at thesource

2. With the increased concentration of sugar, thewater potential is decreased, and water from thexylem enters the sieve tube by osmosis.

3. Sugar is removed (unloaded) at the sink, and thesugar concentration falls; as a result, the waterpotential is increased, and water leaves the sievetube.

4. With the movement of water into the sieve tube atsource and out at the sink, the sugar moleculesare carried passively by the water along theconcentration gradient between source and sink.

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5. Note that the sieve tube between source andsink is bounded by a differentially permeablemembrane, the plasma membrane.Consequently, water enters and leaves the sievetube not only at the source and sink, but all alongthe pathway.

6. Evidence indicate that few if any of the originalwater molecules entering the sieve tube at thesource end up in the sink, because theyexchanged with other water molecules that enterthe sieve tube from the phloem apoplast alongthe pathway

GOODBYE

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1. Phloem Structure

The primary phloem is toward the outside of the stem. Both theprimary phloern and the primary xylem are surrounded by a bundlesheath of thick-walled sclerenchyma cells, which isolate thevascular tissue from the ground tissue. (©J.N.A. Lott/BiologicalPhoto Service.)

Transverse sectionof a vascularbundle of trefoil, aclover (Trifolium).(130x

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FIGURE 10.2 Transversesection of a 3-year-old stemof an ash (Fraxinusexcelsior) tree. (27x). Thenumbers 1, 2, and 3 indicategrowth rings in thesecondary xylem. The oldsecondary phloem has beencrushed by expansion of thexylem. Only the most recent(innermost) layer ofsecondary phloem isfunctional. (© P.Gates/Biological PhotoService.)