What substances need transporting in plants?

Oxygen? Carbon dioxide? Water? Minerals? Nutrients? Sugars,amino acids. Hormones?

Transport in Transport in multicellular plantsmulticellular plants

Carbon dioxideCarbon dioxide Needed for photosynthesis in Needed for photosynthesis in

all green parts of the plant, all green parts of the plant, mainly leaves.mainly leaves.

It It diffusesdiffuses from the air into from the air into leaves down the concentration leaves down the concentration gradient(high conc in air low gradient(high conc in air low conc in leaves)conc in leaves)

The large surface area/volume The large surface area/volume ratio of leaves helps thisratio of leaves helps this

It is It is NOT NOT moved by the plant moved by the plant transport systemtransport system

CO2

OxygenOxygen All plant cells need it for All plant cells need it for

respirationrespiration Photosynthesising cells produce Photosynthesising cells produce

their own suppliestheir own supplies For other cells such as roots it For other cells such as roots it

must must diffuse diffuse in from air in the soilin from air in the soil Excess oxygenExcess oxygen from leaves from leaves

diffuse outdiffuse out Plants have much Plants have much lower energylower energy

needs than animals ,so need needs than animals ,so need oxygen much oxygen much less rapidlyless rapidly

Oxygen is Oxygen is NOTNOT transported around transported around the plant.the plant.

Organic nutrientsOrganic nutrients Photosynthetic cells make their own Photosynthetic cells make their own

nutrientsnutrients These include These include glucoseglucose & & amino acidsamino acids These need to be transported to other These need to be transported to other

parts of the plantparts of the plant This involves the This involves the phloemphloem Inorganic ions and waterInorganic ions and water Ions such as Ions such as

magnesium,potassium,nitrates etc are magnesium,potassium,nitrates etc are needed by cellsneeded by cells

These including water are taken up by the These including water are taken up by the roots and transported in the roots and transported in the xylemxylem

The plant transport The plant transport systemsystem

Only WATER, MINERAL IONS and Only WATER, MINERAL IONS and ORGANIC NUTRIENTS need ORGANIC NUTRIENTS need transportingtransporting

Because of the Because of the LOW ACTIVITYLOW ACTIVITY of of plants these are not needed rapidlyplants these are not needed rapidly

The plant transport system is much The plant transport system is much simplersimpler than animals than animals

Fluids move much more Fluids move much more slowlyslowly There is no There is no pumppump to move the fluids to move the fluids The cells transporting substances The cells transporting substances

are known as are known as vascular tissuevascular tissue There are two tissues consisting of There are two tissues consisting of

systems of tubes called systems of tubes called xylem & xylem & phloemphloem

WATER WATER TRANSPORTTRANSPORT

In a plant water moves In a plant water moves down a water potential down a water potential gradientgradient

From From HIGH water HIGH water potential (potential (ψ)ψ) in the in the soilsoil to to

LOW water potential LOW water potential ((ψ)ψ) in the in the air air around the around the leavesleaves

Because of this there is a Because of this there is a constant flow of waterconstant flow of water through the plantthrough the plant

Soil to Root Soil to Root HairHair

Water is absorbed by the rootsWater is absorbed by the roots MostMost of the root is of the root is

impermeableimpermeable to water to water Only the Only the root hairroot hair cells have cells have

thin permeable cell wallsthin permeable cell walls These are found in a region These are found in a region

behind the root tipbehind the root tip They grow out between the soil They grow out between the soil

particles and provide a particles and provide a large large surface area.surface area.

They are very They are very delicatedelicate and and easily damaged,often lasting easily damaged,often lasting only a few days before being only a few days before being replacedreplaced

MycorhizasMycorhizas Some trees have fungi Some trees have fungi

growing in their rootsgrowing in their roots These associations are These associations are

called mycorhizascalled mycorhizas The fungi form a mass The fungi form a mass

of fine threads which of fine threads which help absorb nutrients help absorb nutrients especially phosphatesespecially phosphates

Some trees growing on Some trees growing on poor soils cannot poor soils cannot survive without the survive without the fungifungi

Water enters the root Water enters the root hair cell by hair cell by osmosisosmosis

The root hair cell The root hair cell contains dissolved contains dissolved nutrients and minerals, nutrients and minerals, this gives it a this gives it a low water low water potentialpotential

These minerals have These minerals have been pumped into the been pumped into the cells by active cells by active transporttransport

Soil waterSoil water has has higher higher water potential water potential

Water enters the cell by Water enters the cell by osmosis from a high WP osmosis from a high WP to a low WPto a low WP

Water Transport : From root hair Water Transport : From root hair to xylemto xylem

Water enters the root Water enters the root hair cellshair cells

It then passes through It then passes through the the cortexcortex and into the and into the stelestele

This consists of the This consists of the endodermis,pericycleendodermis,pericycle and into the and into the xylemxylem

Water potential in the Water potential in the xylem is lower than in xylem is lower than in the root hair cellsthe root hair cells

Water moves down the Water moves down the water potential gradientwater potential gradient

Root SectionRoot Section

Ranunculus: t.s young root

Tissue plan

Apoplast & SymplastApoplast & Symplast There are There are three routesthree routes for the water for the water

through the cortex cellsthrough the cortex cells1. 1. Apoplast pathwayApoplast pathway The water moves through the The water moves through the cell wallscell walls These are made of cellulose fibres and can These are made of cellulose fibres and can

soak up watersoak up water It moves between cells via the intercellular It moves between cells via the intercellular

spaces or via touching cell walls.spaces or via touching cell walls. It does not pass through any membranes, It does not pass through any membranes,

meaning mineral ions can be transportedmeaning mineral ions can be transported

2. 2. Symplast pathwaySymplast pathway Here the water enters the cellsHere the water enters the cells It passes through the It passes through the cytoplasmcytoplasm It passes from cell to cell through the It passes from cell to cell through the

cytoplasm in cytoplasm in plasmodesmataplasmodesmata It is thought that this pathway is It is thought that this pathway is most most

often usedoften used3. 3. Vacuolar pathwayVacuolar pathway This is similar to the symplast but also This is similar to the symplast but also

includes the vacuolesincludes the vacuoles ApoplastApoplast may be used when a lot of may be used when a lot of

water is being lost from the plant and water is being lost from the plant and flow through the root needs to be rapidflow through the root needs to be rapid

The endodermisThe endodermis When the water reaches When the water reaches

these cells it must follow these cells it must follow the the symplastsymplast pathway pathway

This is because the cell This is because the cell walls contain a thick walls contain a thick waterproof band called waterproof band called a a casparian stripcasparian strip..

This is made of a waxy This is made of a waxy substance called substance called suberinsuberin

This blocks the flow of This blocks the flow of water by the apoplast water by the apoplast routeroute

It is thought that this may allow control of It is thought that this may allow control of the the mineralsminerals entering the xylem as they entering the xylem as they must pass through cell membranesmust pass through cell membranes

Endodermis cells pump minerals into the Endodermis cells pump minerals into the xylem by active transportxylem by active transport

This lowers the This lowers the ψψ and helps move water and helps move water in to the xylem by osmosisin to the xylem by osmosis

It may also help generate It may also help generate root pressureroot pressure Casparian strip also blocks water return Casparian strip also blocks water return

out of the xylemout of the xylem

Apoplast & SymplastApoplast & Symplast

How does water move up How does water move up the stem?the stem?

1.1. Root pressureRoot pressure

2.2. Transpiration streamTranspiration stream

3.3. Capillary actionCapillary action

Water transport: From root to Water transport: From root to leafleaf

Water passes Water passes from the roots from the roots into the stem into the stem and then into and then into the leavesthe leaves

It does this It does this through the through the XYLEM TISSUEXYLEM TISSUE

Xylem TissueXylem Tissue Xylem tissue is a Xylem tissue is a group group

of cellsof cells that that work work togethertogether to to transporttransport water & minerals and water & minerals and supportsupport the plant the plant

It contains several It contains several different types of cellsdifferent types of cells

Xylem vessel Xylem vessel elementselements

TracheidsTracheids (Sclerenchyma) fibres(Sclerenchyma) fibres Parenchyma cellsParenchyma cells

Xylem vessel Xylem vessel elementselements

Xylem vessels transport Xylem vessels transport waterwater

They are made up of many They are made up of many elongated vessel elements.elongated vessel elements.

These form a hollow pipeThese form a hollow pipe These These beganbegan life as a life as a

normal plant cellnormal plant cell But a substance called But a substance called

ligninlignin has been laid down has been laid down in the wallsin the walls

LignificationLignification Lignin is a complex Lignin is a complex

carbohydratecarbohydrate It is It is hard,stronghard,strong and and

impermeable impermeable to waterto water It can be stained red in It can be stained red in

microscope slidesmicroscope slides This This thickeningthickening of the cell of the cell

wall with lignin wall with lignin killskills the the cellcell

This leaves a hollow space, This leaves a hollow space, a a lumenlumen, through which , through which water can passwater can pass

Types of lignificationTypes of lignification

Thickening Thickening starts off as starts off as circularcircular or or spiralspiral,,

then then reticulatereticulate

and finally and finally pittedpitted

Types of Types of thickeningthickening

PitsPits Pits are not openPits are not open They have the original They have the original

cell wall in themcell wall in them This is fully permeable to This is fully permeable to

waterwater Pits allow water to Pits allow water to enterenter

and and leaveleave the xylem the xylem They can also be used to They can also be used to

bypass blockagesbypass blockages

TracheidTracheid Also dead , hollow cells Also dead , hollow cells

with lignified wallswith lignified walls Their ends are not Their ends are not

completely open and completely open and tapertaper

Water passes between Water passes between cells via pitscells via pits

They are the main They are the main water conducting tissue water conducting tissue in more primitive plantsin more primitive plants

More modern More modern plants(angiosperms) plants(angiosperms) make more use of make more use of xylem vesselsxylem vessels

FibresFibres

Small dead Small dead elongated cellselongated cells

Lignified wallsLignified walls Small lumenSmall lumen Main role is Main role is

supportingsupporting the the plantplant

Stem Vascular StructureStem Vascular Structure

Vascular BundleVascular Bundle

Leaf Section(dicot TS)Leaf Section(dicot TS)

Leaf sectionLeaf section

Transpiration StreamTranspiration Stream To understand this we To understand this we

first need to consider:first need to consider: Movement of water: Movement of water:

Leaf to airLeaf to air The mesophyll cells of the leaf The mesophyll cells of the leaf

have have wet cell wallswet cell walls Water Water evaporatesevaporates into the into the air air

spacesspaces saturating them saturating them Water will move by Water will move by osmosisosmosis

from the xylem vessels (via from the xylem vessels (via pits) across the cells of the leaf pits) across the cells of the leaf to replace water lostto replace water lost

If the air If the air outsideoutside the leaf has a the leaf has a lowerlower water potential than water potential than insideinside, water vapour will , water vapour will diffuse diffuse outout of the leaf through of the leaf through the the stomatastomata

This loss of water by This loss of water by evaporation from the aerial evaporation from the aerial parts of plants is called parts of plants is called TRANSPIRATIONTRANSPIRATION

Transpiration: the price Transpiration: the price plants pay for plants pay for

photosynthesis photosynthesis Carbon dioxide is needed for Carbon dioxide is needed for

photosynthesisphotosynthesis Stomata must be open so this Stomata must be open so this

can diffuse into the leafcan diffuse into the leaf This means water is lost through This means water is lost through

the open stomatathe open stomata If water is being lost too quickly If water is being lost too quickly

the stomata may be closedthe stomata may be closed Leaves also Leaves also wiltwilt giving less giving less

surface area to lose watersurface area to lose water Transpiration can also be Transpiration can also be

important in important in coolingcooling plant leaves plant leaves as evaporation use heatas evaporation use heat

Transpiration StreamTranspiration Stream : root xylem : root xylem to leaf xylemto leaf xylem

As water evaporates from the leaf cells water As water evaporates from the leaf cells water replaces it from the leaf xylem vessels by replaces it from the leaf xylem vessels by osmosisosmosis

Removing water from the xylem Removing water from the xylem lowers the lowers the hydrostatic pressurehydrostatic pressure

This is now lower than the pressure in the This is now lower than the pressure in the roots causing water to move up the xylem roots causing water to move up the xylem (similar to being sucked up a straw)(similar to being sucked up a straw)

If you suck too hard on a straw the pressure If you suck too hard on a straw the pressure causes the walls to collapsecauses the walls to collapse

To prevent this To prevent this xylem vessels walls are xylem vessels walls are strongstrong, lignified., lignified.

The transpiration stream is a The transpiration stream is a passive passive processprocess, relying on evaporation of water from , relying on evaporation of water from the leaves.the leaves.

Mass FlowMass Flow This movement of water is This movement of water is

called called mass flowmass flow The water molecules move all The water molecules move all

together together as a massas a mass It can do this because the It can do this because the

water molecules are water molecules are held held togethertogether by H bonds by H bonds

This is called This is called COHESIONCOHESION They are also attracted to the They are also attracted to the

lignin in the walls of the lignin in the walls of the xylemxylem

This is This is ADHESIONADHESION

If the column of water broke an air If the column of water broke an air bubble would form and this would bubble would form and this would stop the mass flowstop the mass flow

The The small diametersmall diameter of the xylem of the xylem vessels makes this less likely to vessels makes this less likely to happenhappen

Capillary actionCapillary action

If thin tubes are If thin tubes are placed in water placed in water the water will the water will move some way move some way up the tubeup the tube

This is This is capillary capillary action action due to due to adhesion & adhesion & cohesioncohesion

Root PressureRoot Pressure Hydrostatic pressure at the Hydrostatic pressure at the

root end of the xylem can be root end of the xylem can be increasedincreased

Cells surrounding the xylem Cells surrounding the xylem in the roots pump mineral in the roots pump mineral ions into the xylem by active ions into the xylem by active transporttransport

This lowers the water This lowers the water potential and more water potential and more water enters the xylem from the enters the xylem from the cells by osmosiscells by osmosis

Hydrostatic pressure Hydrostatic pressure increasesincreases

This is not essential though This is not essential though and only helpsand only helps

Measuring the rate of Measuring the rate of transpirationtranspiration

The rate of transpiration The rate of transpiration variesvaries This will depend on:This will depend on: TemperatureTemperature: : The The higherhigher the temperature the the temperature the fasterfaster the the

rate of rate of evaporationevaporation inside the leaves and the inside the leaves and the faster the rate of faster the rate of diffusiondiffusion out of the stomata out of the stomata

Water potential gradient between leaf Water potential gradient between leaf and airand air

The steeper the gradient the faster the water The steeper the gradient the faster the water vapour will diffuse out of the leafvapour will diffuse out of the leaf

Moving air Moving air will remove water vapour from around the will remove water vapour from around the leaf. Keeping the gradient steep and speeding up leaf. Keeping the gradient steep and speeding up diffusiondiffusion

Dry air Dry air will mean a steeper gradientwill mean a steeper gradient Number of stomataNumber of stomata The more stomata the quicker water vapour will be lostThe more stomata the quicker water vapour will be lost Position of stomataPosition of stomata Stomata on lower surface are not exposed to the Stomata on lower surface are not exposed to the

heating effect of the sun.So less water lostheating effect of the sun.So less water lost LightLight Stomata open in the light.Stomata open in the light. Soil water availability Soil water availability Low levels of soil water may mean wilting,plant stress Low levels of soil water may mean wilting,plant stress

and stomatal closureand stomatal closure Waxy cuticleWaxy cuticle Its presence reduces water evaporation from leaf Its presence reduces water evaporation from leaf

surfacesurface

Factors affecting the rate of Factors affecting the rate of transpirationtranspiration

Factor affecting

transpiration rate

How it affects water loss

Using the Using the potometerpotometer

1.1. Label the diagram fullyLabel the diagram fully

2.2. The potometer does not measure The potometer does not measure the accurate uptake of water,it can the accurate uptake of water,it can only compare water uptake.Why?only compare water uptake.Why?

3.3. Why should the leafy shot stem be Why should the leafy shot stem be cut at an angle under water and cut at an angle under water and placed in the potometer?placed in the potometer?

4.4. What must the potometer be to What must the potometer be to ensure it works correctly?ensure it works correctly?

5.5. Explain how you can estimate Explain how you can estimate water uptake using the potometerwater uptake using the potometer

6.6. Explain how you can change the Explain how you can change the following conditions when using following conditions when using the potometer: humidity, light the potometer: humidity, light intensity, air currents, intensity, air currents, temperature.temperature.

7.7. Explain the role of the reservoir. Explain the role of the reservoir.

8.8. Plot a graph of your results and Plot a graph of your results and explain them.explain them.

condition

Movement of meniscus each minute(mm)

1 2 3 4 5

Using the potometerUsing the potometer Fill the apparatus and syringe totally with water by placing Fill the apparatus and syringe totally with water by placing

it in a bowl – air bubbles will block the water flowit in a bowl – air bubbles will block the water flow Cut a shoot under water – this ensures the xylem does not Cut a shoot under water – this ensures the xylem does not

get blocked with air bubblesget blocked with air bubbles Place the shoot in the apparatus under waterPlace the shoot in the apparatus under water Ensure it is a tight fit and seal with vaseline – any leaks Ensure it is a tight fit and seal with vaseline – any leaks

will let air into the apparatus.will let air into the apparatus. Set up on a standSet up on a stand As water evaporates from the leaves water will be taken up As water evaporates from the leaves water will be taken up

from the apparatus to replace it.from the apparatus to replace it. This will not give real total of water uptake as a small This will not give real total of water uptake as a small

amount of the water taken up will be used for amount of the water taken up will be used for photosynthesisphotosynthesis

Measure the movement of the water level in a set time.Measure the movement of the water level in a set time. The water level can be reset by adding water from the The water level can be reset by adding water from the

syringe.syringe. Try moving air, lower temperature, higher humidity to see Try moving air, lower temperature, higher humidity to see

their effect on transpiration rate.their effect on transpiration rate.

XerophytesXerophytes Plants adapted to living in very dry areas.Plants adapted to living in very dry areas. Explain using examples how xerophytic Explain using examples how xerophytic

plants reduce water loss by transpirationplants reduce water loss by transpiration Include:Include: Leaves reduced to spinesLeaves reduced to spines Waxy cuticleWaxy cuticle Position of stomataPosition of stomata Sunken stomataSunken stomata Hairy leavesHairy leaves Rolled leavesRolled leaves

Phloem TissuePhloem Tissue This tissue transports This tissue transports

ASSIMILATESASSIMILATES These are substances These are substances

the plants make the plants make themselves and include themselves and include sugars and amino acidssugars and amino acids

Phloem tissue consists Phloem tissue consists of 2 main types of cellsof 2 main types of cells

Sieve elementsSieve elements Companion cellsCompanion cells There are also There are also

parenchyma cells and parenchyma cells and fibresfibres

Sieve ElementsSieve Elements Sieve elements are the Sieve elements are the

single cells which join single cells which join up to form sieve tubesup to form sieve tubes

These are These are living cellsliving cells They have aThey have a Cellulose cell wallCellulose cell wall Cell membraneCell membrane Only a Only a thin liningthin lining of of

cytoplasm containingcytoplasm containing Endoplasmic reticulum Endoplasmic reticulum

& mitochondria& mitochondria NoNo nucleus,chloroplasts nucleus,chloroplasts

or ribosomesor ribosomes

Sieve platesSieve plates

The ends of the The ends of the cells are not cells are not completely open completely open but have a but have a sieve sieve plateplate containing containing open open sieve poressieve pores so materials are so materials are free to move free to move throughthrough

Companion cellsCompanion cells

These are next to the sieve elementsThese are next to the sieve elements They have the “normal plant cell” They have the “normal plant cell”

structurestructure However they have more mitochondria However they have more mitochondria

and ribosomes than normaland ribosomes than normal Smaller vacuole Smaller vacuole There are many plasomdesmata There are many plasomdesmata

between the cells so materials can between the cells so materials can easily pass between the cellseasily pass between the cells

MicroscopyMicroscopy Both light and electron Both light and electron

microscopy show microscopy show protein fibresprotein fibres in the in the phloemphloem

It It was thoughtwas thought these these had a role to playhad a role to play

It has now been shown It has now been shown they are only a they are only a response to damageresponse to damage by the cells when the by the cells when the sections are takensections are taken

TranslocationTranslocation The The transporttransport of of soluble soluble

organic moleculesorganic molecules within the within the plant (assimilates)plant (assimilates)

Takes place through the Takes place through the sieve sieve tubestubes

It takes place by It takes place by mass flowmass flow Due to Due to hydrostatic pressure hydrostatic pressure

differencesdifferences at either end of the at either end of the phloemphloem

But this is an But this is an active processactive process , not , not passive as in transpirationpassive as in transpiration

Assimilates are actively Assimilates are actively loadedloaded into the phloem and into the phloem and unloaded unloaded out out of itof it

Sources and SinksSources and Sinks

The area where assimilates The area where assimilates are being made is called are being made is called the the SOURCESOURCE

It is most often the It is most often the leaf leaf where many organic where many organic molecules are made during molecules are made during photosynthesisphotosynthesis

However it could be a However it could be a storage organstorage organ such as a such as a potato tuber where sugars potato tuber where sugars are being are being loadedloaded into the into the phloemphloem

Assimilates are transported from the source Assimilates are transported from the source to the to the SINKSINK

This is where the assimilates are This is where the assimilates are unloadedunloaded and are and are being usedbeing used by the plant by the plant

They might include:They might include: A growing point of the root or stemA growing point of the root or stem A storage organ in the root, building up A storage organ in the root, building up

starchstarch A developing fruit or flowerA developing fruit or flower A nectary in a flowerA nectary in a flower As sinks could be above or below the As sinks could be above or below the

sources sources materials can move in both materials can move in both directionsdirections in phloem in phloem

How translocation occursHow translocation occurs

Loading of assimilatesLoading of assimilates The most widespread assimilate The most widespread assimilate

which is loaded into the phloem which is loaded into the phloem is is sucrosesucrose

This is formed from triose This is formed from triose sugars(C3) made in the sugars(C3) made in the mesophyll leaf cells in mesophyll leaf cells in photosynthesisphotosynthesis

This is a disaccharide that is the This is a disaccharide that is the transport carbohydratetransport carbohydrate for for plantsplants

It moves across the leaves to It moves across the leaves to the phloem via apoplast or the phloem via apoplast or symplastsymplast

This varies between speciesThis varies between species

When it reaches the When it reaches the companion companion cellscells it is moved into it is moved into them(loaded) by them(loaded) by active active transporttransport

A A H+ protein pumpH+ protein pump moves moves hydrogen ions out of the hydrogen ions out of the companion cell by companion cell by active active transporttransport

This is This is against their against their concentration gradientconcentration gradient and and requires requires energyenergy from from ATPATP

This is provided by the many This is provided by the many mitochondriamitochondria in the companion in the companion cellscells

This means a high concentration This means a high concentration of H+ outside the cellof H+ outside the cell

Another protein in the Another protein in the membrane allows the H+ ions membrane allows the H+ ions back into the cell down their back into the cell down their concentration gradientconcentration gradient

This is linked to bringing This is linked to bringing sucrosesucrose into the cell into the cell

This protein is a This protein is a co-transporterco-transporter proteinprotein

Mass FlowMass Flow Sucrose can then Sucrose can then diffusediffuse into into

the sieve tube via thethe sieve tube via the plasmodesmataplasmodesmata

This This lowers the water lowers the water potentialpotential of the sieve tube of the sieve tube

Water enters them by Water enters them by osmosisosmosis from the from the xylemxylem raising the raising the hydrostatic hydrostatic pressurepressure

The sucrose solution is pushed The sucrose solution is pushed down the sieve tubesdown the sieve tubes

At the sink sucrose diffuses At the sink sucrose diffuses out of the phloem and water out of the phloem and water followsfollows

This lowers the hydrostatic This lowers the hydrostatic pressurepressure

Unloading at the sinkUnloading at the sink The mechanism of this is unclearThe mechanism of this is unclear sucrose may just sucrose may just diffuse outdiffuse out into the into the

cells that need itcells that need it The sucrose may be The sucrose may be convertedconverted to to

glucose & fructoseglucose & fructose by the enzyme by the enzyme invertaseinvertase, and be used for , and be used for respirationrespiration

or changed to or changed to starchstarch and stored and stored In both cases its concentration will In both cases its concentration will

decrease maintaining a concentration decrease maintaining a concentration gradientgradient

In some cases it is possible that sucrose In some cases it is possible that sucrose is unloaded by is unloaded by active transportactive transport

Evidence for the mechanism of Evidence for the mechanism of translocationtranslocation

Sources of evidence come from:Sources of evidence come from: Light and electron microscopyLight and electron microscopy Composition of phloem sapComposition of phloem sap Speed of flow of sap and hydrostatic Speed of flow of sap and hydrostatic

pressurepressure Potential difference readings from Potential difference readings from

inside and outside cellsinside and outside cells

Phloem sapPhloem sap This is not easy to collect.This is not easy to collect. As soon as you cur phloem As soon as you cur phloem

tissue it rapidly blocks the tissue it rapidly blocks the sieve pores with the protein sieve pores with the protein plugplug

Within hours this is replaced Within hours this is replaced by by callosecallose a polysaccharide a polysaccharide similar to cellulosesimilar to cellulose

This “clotting” mechanism This “clotting” mechanism stops loss of sap and entry stops loss of sap and entry of microbes.of microbes.

Castor oil sap can be Castor oil sap can be collectedcollected

SoluteSolute Concentration Concentration (m/l)(m/l)

SucroseSucrose 250250

K+K+ 8080

Amino acidsAmino acids 4040

Cl-Cl- 1515

phosphatephosphate 1010

Mg2+Mg2+ 55

Na+Na+ 22

ATPATP 0.50.5

nitratenitrate 00

auxinsauxins tracestraces

Using Using AphidsAphids

Aphids can place their Aphids can place their proboscis into the phloem proboscis into the phloem sieve tubesieve tube

The proboscis is so fine that The proboscis is so fine that it does not activate the it does not activate the plants protective plants protective mechanismmechanism

They can then be They can then be anaesthetised and heads anaesthetised and heads removedremoved

Samples of sap will then Samples of sap will then exude from the cut end of exude from the cut end of the styletthe stylet

Evidence Evidence forfor Translocation Translocation theorytheory

Phloem sap has a high pH about 8.0 , Phloem sap has a high pH about 8.0 , suggesting a low concentration of H+ ionssuggesting a low concentration of H+ ions

This would be the case if they are being This would be the case if they are being removed by active transport.removed by active transport.

There is a -150mv potential across the There is a -150mv potential across the membrane, again supporting the removal membrane, again supporting the removal of H+ ions, making the inside –ve.of H+ ions, making the inside –ve.

ATP is present in the sap which would ATP is present in the sap which would support active transportsupport active transport

Flow rate of sap is 10,000x faster than it Flow rate of sap is 10,000x faster than it would be for diffusion and match readings would be for diffusion and match readings of hydrostatic pressure differences at of hydrostatic pressure differences at source & sinksource & sink

Phloem sap is under pressure and will leak Phloem sap is under pressure and will leak out if cutout if cut

Evidence againstEvidence against

Why do the sieve pores still exist? Why do the sieve pores still exist? They obstruct mass flow and you They obstruct mass flow and you would think evolution would have would think evolution would have removed them!removed them!

Phloem transports to several Phloem transports to several different sinks at one time, not to the different sinks at one time, not to the one with the lowest hydrostatic one with the lowest hydrostatic pressure! pressure!

The differences between The differences between sieve elements and xylem sieve elements and xylem

vesselsvessels

Do exercise SAQ 10.9 p147