The Cell Cycle

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9

Slide 59

Sugar must be loaded into sieve-tube elements before being exposed to sinks.

Depending on the species, sugar may move by symplastic or both symplastic and apoplastic pathways.

Transfer cells are modified companion cells that enhance solute movement between the apoplast and symplast.

Phloem: Translocaton: source to sink

Slide 60

Loading of sucrose into phloem proton pump -- Cotransport of Sucrose

Mesophyll cell

Cell walls (apoplast)

Plasma membrane

Plasmodesmata

Companion (transfer) cell

Sieve-tube element

High H+ concentration

Cotransporter

Proton pump

Low H+ concentration

Key

Apoplast

Symplast

Mesophyll cell

Bundle- sheath cell

Phloem parenchyma cell

Sucrose

ATP

H+

H+

H+

S

S

Slide 61

In many plants, phloem loading requires active transport.

Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose.

At the sink, sugar molecules are transported from the phloem to sink tissues and are followed by water.

Slide 62

Loading of sucrose into phloem: Cotransport

High H+ concentration

Cotransporter

Proton pump

Low H+ concentration

Sucrose

H+

H+

H+

ATP

S

S

Slide 63

Bulk Flow by Positive Pressure: The Mechanism of Translocation in Angiosperms

In studying angiosperms, researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure.

The pressure flow hypothesis explains why phloem sap always flows from source to sink.

Slide 64

4

Bulk flow by positive pressure. Pressure Flow in a sieve tube

3

2

1

1

2

3

4

Vessel (xylem)

Sieve tube (phloem)

Source cell (leaf)

Loading of sugar

Uptake of water

Unloading of sugar

Water recycled

Sink cell (storage root)

Sucrose

H2O

H2O

Bulk flow by negative pressure

H2O

Sucrose

Bulk flow by positive pressure

Slide 65

Does phloem sap contain more sugar near sources than sinks?

Sap droplet

25 µm

Sieve- tube element

Stylet

Sap droplet

Aphid feeding

Stylet in sieve-tube element

Separated stylet exuding sap

EXPERIMENT

Slide 66

The Symplast is highly dynamic - Plasmodesmata - Continuously Changing Structures