talk 7 -translocation in the phloem

8/12/2019 Talk 7 -Translocation in the Phloem

1/31

Translocation in the

Phloem


2/31

Phloem transport

A highly specialized process forredistributing: Photosynthesis products

Other organic compounds (metabolites,hormones) some mineral nutrients

Redistributed from SOURCE SINK


3/31

Phloem transport: Sources andsinks

Source: Any exporting region that produces

photosynthate above and beyond that of

its own needs Sink:

any non-photosynthetic organ or an organthat does not produce enoughphotosynthate to meets its own needs


4/31

How the growing parts of the plant are providedwith sugar to synthesize new cells

Photosynthesis

New growth

Translocation

A system of vascular tissueruns through all higherplants.

It evolved as a response to

the increase in the size ofplants, which caused anprogressing separation ofroots and leaves in space.

Thephloem is the tissuethat translocatesassimilates from matureleaves to growing or storageorgans and roots.


5/31

Sources and sinksDirection of transport throughphloem is determined by relativelocations of areas of supply, sourcesand areas where utilization ofphotosynthate takes place, sinks.

Source: any transporting organcapable of mobilizing organic

compounds or producingphotosynthate in excess of its ownneeds, e.g., mature leaf, storageorgan during exporting phase ofdevelopment.

Sink: non photosynthetic organsand organs that do not produceenough photoassimilate to meet theirown requiements, e.g., roots, tubers,develpoping fruits, immature leaves.

Photosynthesis provides a

sugar source

New growth is a

sugar sink

Translocation


6/31

Exactly what istransported in

phloem?


7/31

What is transported in phloem?


8/31

Carbohydrates transportedin phloem are allnonreducingsugars. This is because they are

less reactive

Reducing sugars, such asGlucose, MannoseandFructosecontain anexposed aldehyde orketone group Too chemically reactive to

be transported in the

phloem

Sugars that are not generallyin phloem


9/31

Sugars that are in phloem(polymers)

The most commontransported sugar is sucrose. Made up from glucose &

Fructose

This is a reducing sugar The ketone or aldehyde group

is combined with a similargroup on another sugar

Or the ketone or aldehyde

group is reduced to an alcohol D-Mannitol

Most of the other mobilesugars transported containSucrose bound to varying

numbers of Galactoseunits


10/31

Remember Sucrose? Sucrose The osmotic effect of a substance

is tied to the number of particlesin solution, so a millilitre ofsucrose solution with the sameosmolarity as glucose will behave twice the number carbon

atoms and therefore about twicethe energy. Thus, for the same osmolarity,

twice the energy can betransported per ml.

As a non-reducing sugar, sucroseis less reactiveand more likely tosurvive the journey in the phloem.

Invertase (sucrase) is the onlyenzyme that will touch it and

this is unlikely to be present inthe phloem sieve tubes.


11/31

Other compounds Water!!!!!!!!!

Nitrogenis found in the phloem mainly in: amino acids(Glutamic acid)

Amides(Glutamine)

Proteins (see later)


12/31

Phloem Structure The main components of phloem

are sieve elements companion cells.

Sieve elements have no nucleusand only a sparse collection ofother organelles . Companion cellprovides energy

so-named because end walls areperforated - allows cytoplasmic

connections between vertically-stacked cells.

conducts sugars and aminoacids - from the leaves, to the

rest of the plant


13/31

Phloem transport requiresspecialized, living cells

Sieve tubes elements jointo form continuous tube Poresin sieve plate

between sieve tubeelements are open channels

for transport Each sieve tube element is

associated with one ormore companion cells. Many plasmodesmata

penetrate walls between sievetube elements and companioncells

Close relationship, have aready exchange of solutes

between the two cells


14/31

Phloem transport requiresspecialized, living cells

Companion cells: Role in transport of

photosynthesis products fromproducing cells in mature leavesto sieve plates of the small vein

of the leaf

Synthesis of the variousproteins used in the phloem

Contain many, manymitochondria for cellularrespiration to provide thecellular energy required foractive transport


15/31

The mechanism of

phloem transportThe Pressure-Flow Model


16/31

The Pressure-Flow ModelTranslocation is thought to move

at 1 meter per hour Diffusion too slow for this

speed

The flow is driven by anosmotically generated

pressure gradient betweenthe sourceand the sink. Source

Sugars (red dots) is activelyloaded into the sieve element-

companion cell complex Called phloem loading

Sink Sugars are unloaded

Called phloem unloading

Th P


17/31

The Pressure-Flow Model

yw= ys+ yp+ yg

In source tissue, energy drivenphloem loading leads to a buildup of

sugars Makes low (-ve)solute potential Causes a steep dropin water

potential In response to this new water

potential gradient, water enters

sieve elements from xylem Thus phlem turgor pressure

increases

In sink tissue, phloem unloadingleadsto lower sugar conc. Makes a higher (+ve)solute

potential Water potential increases Water leaves phloem and enters

sink sieve elements and xylem Thus phloem turgor pressure

decreases


18/31

The Pressure-Flow Model So, the translocation pathway

has cross walls Allow water to move from xylem to

phloem and back again

If absent-pressure differencefrom source to sink would quickly

equilibrate Water is moving in the phloem

by Bulk Flow No membranes are crossed from

one sieve tube to another Solutes are moving at the same

rate as the water

Water movement is driven bypressure gradientand NOTwater potential gradient


19/31

Phloem Loading:Where do the solutes come from?

Triose phosphate formedfrom photosynthesis duringthe day is moved fromchloroplast to cytosol

At night, this compound,

together with glucose fromstored starch, is converted tosucrose Both these steps occur in a

mesophyll cell

Sucrose then moves from themesophyll cell via the smallestveins in the leaf to near thesieve elements Known as short distance pathway

only moves two or three cells

Phl L d


20/31


In a process called sieveelement loading, sugars aretransported into the sieveelements and companion cells

Sugars become moreconcentrated in sieve elementsand companion cells than inmesophyll cells

Once in the sieve element/companion cell complex sugarsare transported away from thesource tissue called export Translocation to the sink tissue

is called long distance transport

Phl L di


21/31


Movement is via either apoplastor symplast

Via apoplastic pathwayrequires

Active transport against itschemical potential gradient

Involves a sucrose-H+

symporter The energy dissipated by protons

moving back into the cell iscoupled to the uptake of sucrose


22/31

Symplastic phloem loading Depends on plant species

Dependant on species that transport sugars other than sucrose

Requires the presence of open plasmodesmata betweendifferent cells in the pathway

Dependant on plant species with intermediary companion

cells


23/31

Symplastic phloem loading Sucrose, synthesized in mesophyll, diffuses into

intermediary cells Here Raffinoseis synthesized. Due to larger size,

can NOTdiffuse back into the mesophyll Raffinose and sucrose are able to diffuse into sieve

element


24/31

Phloem unloading Three steps

(1) Sieve element unloading: Transported sugars leave the sieve elements of

sink tissue

(2) Short distance transport: After sieve element unloading, sugars

transported to cells in the sink by means of a

short distance pathway (3) storage and metabolism:

Sugars are stored or metabolized in sink cells


25/31

Phloem unloading Also can occur by symplastic or apoplatic pathways Varies greatly from growing vegetative organs (root tips

and young leaves) to storage tissue (roots and stems) toreproductive organs

Symplastic: Appears to be a completely symplastic pathway in young

dicot leaves Again, moves through open plasmodesmata


26/31

Phloem unloading Apoplastic: three types

(1) [B] One step, transport from the sieve element-companion cell complex to successive sink cells,occurs in the apoplast.

Once sugars are taken back into the symplast of

adjoining cells transport is symplastic


27/31

Phloem unloading Apoplastic: three types

(2) [A] involves an apoplastic step close to the sieveelement companion cell.

(3) [B] involves an apoplastic step father from thesieve element companion cell

Both involve movement through the plant cell wall


28/31

Summary

Pathway of translocation: Sugars and other organic materials are

conducted throughout the plant in the phloemby means of sieve elements Sieve elements display a variety of structuraladaptations that make the well suited fortransport

Patterns of translocation: Materials are translocated in the phloem fromsources(usually mature leaves) to sinks(roots,immature leaves)


29/31

Summary

Materials translocated in phloem: Translocated solutes are mainly carbohydrates

Sucrose is the most common translocated sugar

Phloem also contains: Amino acids, proteins, inorganic ions, and planthormones

Rate of translocation: Movement in the phloem is rapid, well in excess

of rates of diffusion Average velocity is 1 meter per hour


30/31

General diagram of translocation

Physiological process of

loading sucrose into the

phloem

Physiological process

of unloading sucrose

from the phloem into

the sink

Pressure-flow

Phloem and xylem arecoupled in an osmotic system

that transports sucrose and

circulates water.

The pressure-flow process


31/31

Pressure flow schematic

The pressure-flow process

Build-up of pressure at thesource and release of pressure atthe sink causes source-to-sinkflow.

At the source phloem loadingcauses high solute concentrations.

decreases, so water flows intothe cells increasing hydrostaticpressure.

At the sink is lower outside

the cell due to unloading ofsucrose. Osmotic loss of waterreleases hydrostatic pressure.Xylem vessels recycle water fromthe sink to the source.

talk 7 -translocation in the phloem

Documents