Transport in Plants P2

What is xylem/phloem responsible for

• xylem mass transfer of water (one direction) roots to ariel parts
  

• phloem mass transfer of sugars from leaves (bidirectional)

Explain the Transport of Water in Xylem

• Transpiration - Cohesion - Tension Mechanism

1. evaporation of water from leaves (high ψ → low ψ in atmosphere)

2. loosing water decreases cell’s ψ

3. the xylem sap has a high ψ so it moves into mesophyll cells

4. water leaves the xylem and tension is created in xylem

5. tension generates a transpiration stream

6. water moves up the xylem by the forces of cohesion and adhesion

7. tension is transmitted all the way down to the root

8. forces water to move into the vascular bundle at the root by osmosis, pulling up water

Root Pressure - defentition

• pressure generated at the roots which causes an upward movement of water through xylem

• depends on active transport

Explain Root Pressure and proof of root hair pressure

• active transport lowers the ψ of the root hair cell

• water in soil hence enters

• root hair cells absorb water so their ψ increases forcing uptake of water from roots all the way to xylem

• guttation - humid nights (no transpiration) still see droplets on plant leaves

Uptake of mineral Salts by root

ION	FUNCTION
k+	voltage gates k+ channels
nitrogen in nitrates	nucleic acids and proteins
phosphorus in phosphate	nucleic acids and ATP
magnesium	chlorophyll structure

• recall they pass through apoplast then symplast vacuolar (due to casparian strip)

• then taken up with xylem

Translocation of Organic Solutes

• translocation is the long-distance transport of organic solutes within a plant

• in phloem

• from source to sink

• transports auxins, amino acids and vitamins, and sucrose

• Pressure-Flow model

Pressure Flow Model - Phloem Unloading

1. Phloem Unloading

• mesophyll cells produce sucrose and this moves toward the companion cells via symplast/apoplast pathway

• some cells called transfer cells have infoldings increasing the sa of plasma membrane - more uptake of sucrose

• transfer cells have mitochondria → atp

• proton pump - pumps out h+ ions, then as the h+ ions move back into the cell it allows the cotransport of sucrose with h+ ions via secondary active transport

• sucrose in transfer cells moves into sieve tube elements through plasmodesmata

• accumulation of sucrose creates a negative ψ

• water enters via osmosis

• increases hydrostatic pressure - force for bulk of materials to move to sink

Phloem Unloading

• sucrose then leaves the sieve-tube element cells at the sink → transfer cells

  • at sink they are metabolized or stored

• now since sucrose conc has decreased, ψ increase so water moves back into xylem

• this decreases hydrostatic pressure - so sink always has a lower pressure then source facilitating movement

Define a Source and Sink

• source: tissues that produce and expore sugars

• sinks: tissues that import and utilize sugars

Sources and Sinks in Spring and Early Summer

sources: mature leaves photosynthesizing are producing sugars

sinks: growing tissues like young leaves and flowers are the sink - need energy to grow

active growth phase

Sources and Sinks - Mid to late summer

• Sources; leaves remain primary source

• sinks: fruits and leaves - accumulate sugars

fruit development

Autumn - Sources and Sinks

• sources: rate of photosynthesis drops as temps drop

  • sinks: storage organs like tubers and roots act as sinks storing sugars and starch for future use

Winter

dormant phase

• sources: storage organs as they provide sugars for metabolic functions

• sinks: v few sinks are present

Spring Restart

• stored carbs are remobilized to support new shoot growth just before the leaves grow in

• emerging shoots are the sinks

• sources remain storage organs

→ shift of source-sink relationships ensures efficient energy use and resource allocation