Chapter 10

Chapter 10 Chapter 10

Transport in multicellular plants

Transport in multicellular plants

Why do multicellular plants need a transport system?

Why do multicellular plants need a transport system?

like animals – need O2 and nutrients

differ from animals – nature of the nutrients needed, gases required, and RATE

like animals – need O2 and nutrients

differ from animals – nature of the nutrients needed, gases required, and RATE

Particular requirements of plant cells

Particular requirements of plant cells

CO2 - during daylight- photosynthesis

6 CO2 + 6 H2O C6H12O6 + 6 O2

CO2 - during daylight- photosynthesis

6 CO2 + 6 H2O C6H12O6 + 6 O2

Particular requirements of plant cellsParticular requirements of plant cells

Oxygen (O2) – Photosynthesizing cells make enough O2 for their own needs, but at a slower rate than animals

Oxygen (O2) – Photosynthesizing cells make enough O2 for their own needs, but at a slower rate than animals


Organic nutrients (containing carbon) – some cells make their own but other parts must be supplied

Organic nutrients (containing carbon) – some cells make their own but other parts must be supplied


Inorganic ions & H2O NH4+, NO3

-, H20, get from roots

Inorganic ions & H2O NH4+, NO3

-, H20, get from roots

Transport of H20Transport of H20

H20 is transported in xylem tissueH20 is transported in xylem tissue

Transport of H20, the ProcessTransport of H20, the Process

H20 uptake near root tips (root hairs)

H20 enters xylem

H20 moves up xylem

H20 from xylem to leaf cells

evaporation of H20 into leaf air spaces

transpiration of H20 vapor through open stoma into the atmosphere

H20 uptake near root tips (root hairs)

H20 enters xylem

H20 moves up xylem

H20 from xylem to leaf cells

evaporation of H20 into leaf air spaces

transpiration of H20 vapor through open stoma into the atmosphere

Overall Overall

H20 moves from high to low water potential ()

from soil to root from root to leaf

H20 moves from high to low water potential ()

from soil to root from root to leaf

From Soil to Root HairFrom Soil to Root Hair

remember Chapter 4, page 61? remember Chapter 4, page 61?

Root hairsRoot hairs

extensions of epidermisH20 moves into root hair with the

water potential () gradient

extensions of epidermisH20 moves into root hair with the

water potential () gradientSoilLow soluteHigh

H20

Root cytoplasmHigh solute (proteins, ions, sugars)Low

Root hairsRoot hairs

large # of root hairs – large surface area - increase rate of osmosis - root hairs constantly being

replaced

large # of root hairs – large surface area - increase rate of osmosis - root hairs constantly being

replaced

Mycorrhizas Mycorrhizas

fungi which function like root hairs, form a mutualistic symbiotic

relationship with the root - useful in poor soil that can’t

grow root hairs - absorb nutrients, especially

phosphate

fungi which function like root hairs, form a mutualistic symbiotic

relationship with the root - useful in poor soil that can’t

grow root hairs - absorb nutrients, especially

phosphate

MycorrhizaeMycorrhizae

Root Hair to XylemRoot Hair to Xylem

H2O movementH2O movement

- H2O moves into the root hair through epidermal cells

- H2O must move across cortical cells to the xylem tissue in the center of the root (Ψ gradient is responsible)

H2O movement can take two pathways

- which pathway depends on the type of plant and condition

- H2O moves into the root hair through epidermal cells

- H2O must move across cortical cells to the xylem tissue in the center of the root (Ψ gradient is responsible)

H2O movement can take two pathways

- which pathway depends on the type of plant and condition

Routes from cell to cell Routes from cell to cell Moving water & solutes between cells

transmembrane route repeated crossing of plasma membranes slowest route but offers more control

symplast route move from cell to cell within cytosol

apoplast route move through connected cell wall without crossing cell

membrane fastest route but never enter cell

Moving water & solutes between cells transmembrane route

repeated crossing of plasma membranes slowest route but offers more control

symplast route move from cell to cell within cytosol

apoplast route move through connected cell wall without crossing cell

membrane fastest route but never enter cell

Apoplast Apoplast

H2O moves through the cortex by traveling along criss-cross fibers of cellulose fibers located in cortical cell walls

- never enters the cells

H2O moves through the cortex by traveling along criss-cross fibers of cellulose fibers located in cortical cell walls

- never enters the cells

ApoplastApoplast

SymplastSymplast

H2O enters the cortical cells cytoplasm and vacuole

- moves to the next cell by interconnecting plasmodesmata channels

- enters the cells

H2O enters the cortical cells cytoplasm and vacuole

- moves to the next cell by interconnecting plasmodesmata channels

- enters the cells

SymplastSymplast

Water & mineral uptake by rootsWater & mineral uptake by roots

Mineral uptake by root hairs dilute solution in soil active transport pumps

this concentrates solutes (~100x) in root cells

Water uptake by root hairs flow from high H2O potential to low H2O

potential creates root pressure

Mineral uptake by root hairs dilute solution in soil active transport pumps

this concentrates solutes (~100x) in root cells

Water uptake by root hairs flow from high H2O potential to low H2O

potential creates root pressure

Route water takes through root

Route water takes through rootWater uptake by root hairs

a lot of flow can be through cell wall route

apoplasty

Water uptake by root hairsa lot of flow can be through cell wall

route apoplasty

H2O reaches the steleH2O reaches the stele

Once the H2O reaches the stele (center), apoplast pathway stops

- endodermal cells have a thick waterproof waxy band of suberin in the cell walls – Casparian strip

- can only pass into these cells by symplast or cytoplasm movment

Once the H2O reaches the stele (center), apoplast pathway stops

- endodermal cells have a thick waterproof waxy band of suberin in the cell walls – Casparian strip

- can only pass into these cells by symplast or cytoplasm movment

Controlling the route of water in root

Controlling the route of water in root

Endodermis cell layer surrounding vascular cylinder of root lined with impervious Casparian strip forces fluid through

selective cell membrane & into symplast

filtered & forced into xylem vessels

Endodermis cell layer surrounding vascular cylinder of root lined with impervious Casparian strip forces fluid through

selective cell membrane & into symplast

filtered & forced into xylem vessels

Aaaaah…Structure-Function

yet again!

in a young root the suberin deposits form bands in the cell walls called Caspian strips

The symplast path remains open

in a young root the suberin deposits form bands in the cell walls called Caspian strips

The symplast path remains open



in older root, entire cells become suberised, closing the symplast path too

Only passage cells are then permeable to water

in older root, entire cells become suberised, closing the symplast path too

Only passage cells are then permeable to water

Suberin depositsSuberin deposits

- as cell ages, some cells become completely blocked and can only

use passage cells - allows the plant to control

incoming nutrients - builds root pressure - H2O crosses the pericycle into

the xylem

- as cell ages, some cells become completely blocked and can only

use passage cells - allows the plant to control

incoming nutrients - builds root pressure - H2O crosses the pericycle into

the xylem


XylemXylem

dual function (support & transport) Made up of four different types of cells

parenchyma cells – standard plant cells, no chloroplast

fibers – elongated cells with dead lignifies walls

vessel elements tracheids

dual function (support & transport) Made up of four different types of cells

parenchyma cells – standard plant cells, no chloroplast

fibers – elongated cells with dead lignifies walls

vessel elements tracheids

XylemXylem

Plant tissuesPlant tissues

Dermal “skin” of plant single layer of tightly

packed cells that covers & protects plant

Vascular transport materials

between roots & shoots xylem & phloem

Ground everything else:

storage, photosynthetic bulk of plant tissue

Dermal “skin” of plant single layer of tightly

packed cells that covers & protects plant

Vascular transport materials

between roots & shoots xylem & phloem

Ground everything else:

storage, photosynthetic bulk of plant tissue

Plant cell types in tissuesPlant cell types in tissues

Plant cell types in tissues

Plant cell types in tissues

Parenchyma “typical” plant cells = least specialized photosynthetic cells, storage cells tissue of leaves, stem, fruit, storage roots

Collenchyma unevenly thickened primary walls = support

Sclerenchyma very thick, “woody” secondary walls =

support rigid cells that can’t elongate dead at functional maturity

Parenchyma “typical” plant cells = least specialized photosynthetic cells, storage cells tissue of leaves, stem, fruit, storage roots

Collenchyma unevenly thickened primary walls = support

Sclerenchyma very thick, “woody” secondary walls =

support rigid cells that can’t elongate dead at functional maturity

Those would’vebeen great names

for my kids!

Plant cell types in tissuesPlant cell types in tissues

ParenchymaParenchymaParenchyma cells are relatively unspecialized, thin, flexible & carry out many metabolic functions

all types of cells develop from parenchyma

Xylem - vessel elementsXylem - vessel elements

make up the xylem vessels began as normal cell, but lignin in

cells walls impermeable to water - as it builds up, cells die,

empty lumen left - no lignin at plasmodesmata –

kept open = pits

make up the xylem vessels began as normal cell, but lignin in

cells walls impermeable to water - as it builds up, cells die,

empty lumen left - no lignin at plasmodesmata –

kept open = pits

Xylem - tracheidsXylem - tracheids

- non living lignified walls - not open ended, not tubes - H2O moves through pits - primitive plants, ferns &

conifers

- non living lignified walls - not open ended, not tubes - H2O moves through pits - primitive plants, ferns &

conifers

XylemXylemdead cells water-conducting cells of xylem

XylemXylem

XylemXylem

H2O movement in the xylem

H2O movement in the xylem

H2O crossed cortex, epidermis, pericycle into xylem vessels

- moves up vessels to the leaves - in root - xylem in center - in stem – near the outer region

H2O crossed cortex, epidermis, pericycle into xylem vessels

- moves up vessels to the leaves - in root - xylem in center - in stem – near the outer region

How does H20 get from the xylem to the leaf?

How does H20 get from the xylem to the leaf?

- H20 evaporates from cell walls of mesophyll

- H20 replaced from xylem

- H20 evaporates from cell walls of mesophyll

- H20 replaced from xylem

Ascent of xylem “sap”Ascent of xylem “sap”Transpiration pull generated by leaf

Rise of water in a tree by bulk flow

Rise of water in a tree by bulk flow

Transpiration pull adhesion & cohesion

H bonding brings water &

minerals to shoot Water potential

high in soil low in leaves

Root pressure push due to flow of H2O

from soil to root cells upward push of

xylem sap

Transpiration pull adhesion & cohesion

H bonding brings water &

minerals to shoot Water potential

high in soil low in leaves

Root pressure push due to flow of H2O

from soil to root cells upward push of

xylem sap

From leaf to atmosphere – transpiration

From leaf to atmosphere – transpiration

- inside leaf mesophyll – air spaces that are saturated with H20 vapor

- H20 evaporates fro mesophyll cell walls

- internal leafs spaces are in direct contact with the atmosphere through the stomata

- diffuse down the gradient to the atmosphere

- inside leaf mesophyll – air spaces that are saturated with H20 vapor

- H20 evaporates fro mesophyll cell walls

- internal leafs spaces are in direct contact with the atmosphere through the stomata

- diffuse down the gradient to the atmosphere

transpirationtranspiration

loss of H20 - low humidity – transpiration

HIGH, large gradient - high humidity – transpiration

LOW - high temperature/wind –

transpiration rate HIGH

loss of H20 - low humidity – transpiration

HIGH, large gradient - high humidity – transpiration

LOW - high temperature/wind –

transpiration rate HIGH

Control of transpirationControl of transpirationStomata function

always a compromise between photosynthesis & transpirationleaf may transpire more than its weight in

water in a day…this loss must be balanced with plant’s need for CO2 for photosynthesis

a corn plant transpires 125 L of water in a growing season

Stomata functionalways a compromise between

photosynthesis & transpirationleaf may transpire more than its weight in

water in a day…this loss must be balanced with plant’s need for CO2 for photosynthesis

a corn plant transpires 125 L of water in a growing season

Regulation of stomataRegulation of stomata Microfibril mechanism

guard cells attached at tips microfibrils in cell walls

elongate causing cells to arch open = open stomata

shorten = close when water is lost

Ion mechanism uptake of K+ ions by guard

cells proton pumps water enters by osmosis guard cells become turgid

loss of K+ ions by guard cells water leaves by osmosis guard cells become flaccid

Microfibril mechanism guard cells attached at tips microfibrils in cell walls

elongate causing cells to arch open = open stomata

shorten = close when water is lost

Ion mechanism uptake of K+ ions by guard

cells proton pumps water enters by osmosis guard cells become turgid

loss of K+ ions by guard cells water leaves by osmosis guard cells become flaccid

Other cues light trigger

blue-light receptor in plasma membrane of guard cells triggers ATP-powered proton pumps causing K+ uptake

stomates open

depletion of CO2

CO2 is depleted during photosynthesis (Calvin cycle)

circadian rhythm = internal “clock” automatic 24-hour cycle

Other cues light trigger

blue-light receptor in plasma membrane of guard cells triggers ATP-powered proton pumps causing K+ uptake

stomates open

depletion of CO2

CO2 is depleted during photosynthesis (Calvin cycle)

circadian rhythm = internal “clock” automatic 24-hour cycle

stomatastomata

stomata open/close – control rate of transpiration

- CO2 must come in, so H20 will leaves (HIGH transpiration)

- bright day, CO2 more needed for photosynthesis

- but then more water loss through transpiration

- plant will have to compromise

stomata open/close – control rate of transpiration

- CO2 must come in, so H20 will leaves (HIGH transpiration)

- bright day, CO2 more needed for photosynthesis

- but then more water loss through transpiration

- plant will have to compromise

Hydrostatic pressureHydrostatic pressure

pressure exerted by liquid - H20 removed from the xylem

reduces pressure - creates a high to low gradient - (e.g. suck on a straw, reduces

pressure at the top, liquid rises) - H20 flows up the xylem

pressure exerted by liquid - H20 removed from the xylem

reduces pressure - creates a high to low gradient - (e.g. suck on a straw, reduces

pressure at the top, liquid rises) - H20 flows up the xylem

Hydrostatic pressureHydrostatic pressure - H20 moves by mass flow (moves as

one body of liquid) -H20 molecules attracted to each

other (H bonds) – cohesion -H20 molecules attracted to lignin (H

bonds) – adhesion - air bubbles stop movement (break

water column) but pits allow H20 to move around

- H20 moves by mass flow (moves as one body of liquid)

-H20 molecules attracted to each other (H bonds) – cohesion

-H20 molecules attracted to lignin (H bonds) – adhesion

- air bubbles stop movement (break water column) but pits allow H20 to move around

Root pressureRoot pressure

transpiration reduces hydrostatic pressure at top of xylem

- plants also increases root pressure to increase pressure differences

- active secretion of solutes, mineral ions, into the water of the xylem vessels in the root

transpiration reduces hydrostatic pressure at top of xylem

- plants also increases root pressure to increase pressure differences

- active secretion of solutes, mineral ions, into the water of the xylem vessels in the root

Root pressureRoot pressure

cells surrounding the xylem use active transport to pump solutes across their membranes and into the xylem

- the presence of solutes lowers the water potential drawing water from the surrounding root cells

- increases pressure at the base of the xylem vessels

- root pressure is not essential

cells surrounding the xylem use active transport to pump solutes across their membranes and into the xylem

- the presence of solutes lowers the water potential drawing water from the surrounding root cells

- increases pressure at the base of the xylem vessels

- root pressure is not essential

Comparing rates of transpiration

Comparing rates of transpiration

- hard to measure H2O vapor leaving the plant

- since most H2O lost by a plant was taken in by the roots

- a measurement of transpiration could be approximated by measuring the amount of water uptake

- hard to measure H2O vapor leaving the plant

- since most H2O lost by a plant was taken in by the roots

- a measurement of transpiration could be approximated by measuring the amount of water uptake

potometerpotometer apparatus used to measure uptake - must be airtight - plant stem in water with rest of

plant above the airtight seal - cut the stem with a slanting cut - as water evaporates it is drawn

into the xylem from the capillary tubing and can be measured

- an experiment can expose the plant to varying conditions to compare the rate of transpiration

apparatus used to measure uptake - must be airtight - plant stem in water with rest of

plant above the airtight seal - cut the stem with a slanting cut - as water evaporates it is drawn

into the xylem from the capillary tubing and can be measured

- an experiment can expose the plant to varying conditions to compare the rate of transpiration

XerohytesXerohytes

plants that live in places where water is scarce

- adaptations include -dune grass - eaves can roll up

exposing a tough waterproof cuticle to the outside air while the stomata remain open in the enclosed middle of the roll

plants that live in places where water is scarce

- adaptations include -dune grass - eaves can roll up

exposing a tough waterproof cuticle to the outside air while the stomata remain open in the enclosed middle of the roll

XerohytesXerohytes

hairs help trap a layer of moist air close to the leaf surface

- cactus – flattened photosynthetic stem, stores water, leaves are reduced to spines which reduces surface area for transpiration

- waxy coating cuts down on water loss

hairs help trap a layer of moist air close to the leaf surface

- cactus – flattened photosynthetic stem, stores water, leaves are reduced to spines which reduces surface area for transpiration

- waxy coating cuts down on water loss

All Cacti are xerophytes

Transverse Section Through Leaf of Xerophytic Plant

Left and right Epidermis of the cactus Rhipsalis dissimilis. Left: View of the epidermis surface. The crater-shaped depressions with a guard cell each at their base can be seen.Right: X-section through the epidermis & underlying tissues. The guard cells are countersunk, the cuticle is thickened. These are classic xerophyte adaptations.

Marram grass possesses: rolled leaves, leaf hairs and sunken stomata. These adaptations make it resistant to dry conditions and of course sand-dunes which drain very quickly retain very little water.

http://www.blackpoolsixth.net.uk/Biology/resource/micro/source/38.html

BYB3 June 2001 Question 8 part c

BYB3 June 2001 Question 8 part cANSWERS

TranslocationTranslocation

- term used to describe the transport of soluble organic substances (assimilates) – such as sugars

- assimilates are transported in sieve tube elements in the phloem tissue

- term used to describe the transport of soluble organic substances (assimilates) – such as sugars

- assimilates are transported in sieve tube elements in the phloem tissue

Phloem tissue Phloem tissue

types of cellssieve (tube) elementscompanion cellsparenchymafibers

types of cellssieve (tube) elementscompanion cellsparenchymafibers

Sieve elementsSieve elements

sieve tube – made up of many sieve elements joined end to end vertically to form a continuous column

a living cell with a cellulose cell wall, a plasma membrane, cytoplasm containing ER and mitochondria

sieve tube – made up of many sieve elements joined end to end vertically to form a continuous column

a living cell with a cellulose cell wall, a plasma membrane, cytoplasm containing ER and mitochondria

Sieve elementsSieve elements

amount of cytoplasm is very small, only providing a thin layer inside the cell wall,

no nucleus or ribosomessieve plate – end walls made of

perforated sieve plate pores allow free movement of

liquids

amount of cytoplasm is very small, only providing a thin layer inside the cell wall,

no nucleus or ribosomessieve plate – end walls made of

perforated sieve plate pores allow free movement of

liquids

Companion cellsCompanion cells

each sieve elements has at least on companion cell lying close beside it

same organelles of other plant cells, including small vacuole and nucleus

each sieve elements has at least on companion cell lying close beside it

same organelles of other plant cells, including small vacuole and nucleus

Companion cellsCompanion cells

contain more mitochondria and ribosomes, are metabolically very active

numerous plasmodesmata pass through their cell walls making direct contact between the cytoplasm of the companion cell and sieve element

contain more mitochondria and ribosomes, are metabolically very active

numerous plasmodesmata pass through their cell walls making direct contact between the cytoplasm of the companion cell and sieve element

The contents of phloem sieve tubes

The contents of phloem sieve tubes

liquid called phloem sap or sap not easy to collect phloem sapwhen phloem tissue is cut, the

sieve elements respond by rapidly blocking the sieve pores in a process called clotting

liquid called phloem sap or sap not easy to collect phloem sapwhen phloem tissue is cut, the

sieve elements respond by rapidly blocking the sieve pores in a process called clotting

phloem sieve tubesphloem sieve tubes

pores become blocked first by plugs of phloem protein strands and then the carbohydrate callose

callose – molecular structure similar to cellulose, long chains of glucose units linked by 1-3 glycosidic bonds

pores become blocked first by plugs of phloem protein strands and then the carbohydrate callose

callose – molecular structure similar to cellulose, long chains of glucose units linked by 1-3 glycosidic bonds

Phloem: food-conducting cells

Phloem: food-conducting cells

sieve tube elements & companion cells

PhloemPhloemLiving cells at functional maturity

lack nucleus, ribosomes & vacuolemore room: specialized for

liquid food (sucrose) transport

Cells sieve tubes

end walls, sieve plates, have pores to facilitate flow of fluid between cells

companion cellsnucleated cells connected to the sieve-

tube help sieve tubes

Living cells at functional maturity lack nucleus, ribosomes & vacuole

more room: specialized for liquid food (sucrose) transport

Cells sieve tubes

end walls, sieve plates, have pores to facilitate flow of fluid between cells

companion cellsnucleated cells connected to the sieve-

tube help sieve tubes

Aaaaah…Structure-Function

again!

PhloemPhloemsieve plate

sieve tubes

Phloem: food-conducting cellsPhloem: food-conducting cells sieve tube elements & companion cells

Vascular tissue in herbaceous stemsVascular tissue in herbaceous stemsdicot

trees & shrubsmonocot

grasses & lilies

Root structure: monocotRoot structure: monocot

Root structure: dicotRoot structure: dicot

xylemphloem

How are aphids used to sample sap?

How are aphids used to sample sap?

- aphids have a tubular mouthpart – stylets

- stylet is inserted through the surface of the plant’s stem/leaf into the phloem

- phoem sap flows through the stylet

- aphids have a tubular mouthpart – stylets

- stylet is inserted through the surface of the plant’s stem/leaf into the phloem

- phoem sap flows through the stylet

Why doesn’t the stylet clot?Why doesn’t the stylet clot?

- the diameter of the stylet is so small it does not allow the sap to flow out rapidly enough to switch on the plant’s phloem clotting mechanism

stimulus-response

- the diameter of the stylet is so small it does not allow the sap to flow out rapidly enough to switch on the plant’s phloem clotting mechanism

stimulus-response

How translocation occursHow translocation occurs

- like water movement in the xylem, sap moves by mass flow

- mass flow in xylem resulted from pressure difference created by the water potential gradient between the soil and air, this required no energy from the plant

- to create the pressure differences needed for mass flow in the phloem, the plant has to use energy, an active process

- like water movement in the xylem, sap moves by mass flow

- mass flow in xylem resulted from pressure difference created by the water potential gradient between the soil and air, this required no energy from the plant

- to create the pressure differences needed for mass flow in the phloem, the plant has to use energy, an active process


- active loading – sucrose is actively loaded into the sieve elements , usually where photosynthesis takes place

- as sucrose is loaded, this decreases the water potential in the sap

- through osmosis water enter the sap, moving down the water potential gradient

- in the root, (or other area along the way), sucrose may be removed, again water follows by osmosis

- active loading – sucrose is actively loaded into the sieve elements , usually where photosynthesis takes place

- as sucrose is loaded, this decreases the water potential in the sap

- through osmosis water enter the sap, moving down the water potential gradient

- in the root, (or other area along the way), sucrose may be removed, again water follows by osmosis


- this creates a pressure difference: hydrostatic pressure is high in the part of the sieve tube in the leaf and lower in the part of the root

- water flows from high to low taking dissolved solutes with it

source – area of the plant where sucrose is loaded

sink - area of the plant where sucrose is taken out

- this creates a pressure difference: hydrostatic pressure is high in the part of the sieve tube in the leaf and lower in the part of the root

- water flows from high to low taking dissolved solutes with it

source – area of the plant where sucrose is loaded

sink - area of the plant where sucrose is taken out

Loading sucrose into phloemLoading sucrose into phloem

photosynthesis produces trioses which are converted into sucrose

sucrose, in solution, moves from the mesophyll cells to the phloem

- moves by the symplast pathway (cell-to-cell via the plamodesmata) or the apoplast pathway traveling along the cell walls

photosynthesis produces trioses which are converted into sucrose

sucrose, in solution, moves from the mesophyll cells to the phloem

- moves by the symplast pathway (cell-to-cell via the plamodesmata) or the apoplast pathway traveling along the cell walls

Loading sucrose into phloemLoading sucrose into phloem

- sucrose is loaded into companion cells by active transport

- ATP is used to pump H+ ions outside the companion cell

- H+ reenter the cell traveling down their concentration gradient traveling with sucrose (against its through carrier proteins (co-transporter proteins) in the plasma membrane

- sucrose is loaded into companion cells by active transport

- ATP is used to pump H+ ions outside the companion cell

- H+ reenter the cell traveling down their concentration gradient traveling with sucrose (against its through carrier proteins (co-transporter proteins) in the plasma membrane

Unloading sucrose from phloem

Unloading sucrose from phloem

little is known about this process - believed sucrose moves from the

phloem though diffusion down a concentration gradient

- this gradient is maintained through cells converting sucrose to something else to maintain the gradient

- invertase – enzyme that hydrolyzes sucrose to glucose and fructose

little is known about this process - believed sucrose moves from the

phloem though diffusion down a concentration gradient

- this gradient is maintained through cells converting sucrose to something else to maintain the gradient

- invertase – enzyme that hydrolyzes sucrose to glucose and fructose

Evidence for the mechanism of phloem transport


- the role of sieve pores and phloem proteins were topics of considerable arguments in the 70’s and 80’s

- now known phloem proteins are not present in living active phloem tissue

- evidence that sap moves by mass flow is considerable

- rate of phloem transport is 10,000 X faster than diffusion

- actual rates of transport measured match closely with those calculated from measured pressure differences at source and sink

- the role of sieve pores and phloem proteins were topics of considerable arguments in the 70’s and 80’s

- now known phloem proteins are not present in living active phloem tissue

- evidence that sap moves by mass flow is considerable

- rate of phloem transport is 10,000 X faster than diffusion

- actual rates of transport measured match closely with those calculated from measured pressure differences at source and sink



- evidence of ‘active’ loading of sucrose into sieve elements in sources such as leaves

- - phloem sap always has a relatively high pH about 8, expected if H+ ions are being transported out of the cells

- - electrical potential of -150mV, consistent with an excess of H+ ions outside the cell compared to inside

- - ATP is present in phloem sieve elements in large amounts, expected it is required for active transport

- evidence of ‘active’ loading of sucrose into sieve elements in sources such as leaves

- - phloem sap always has a relatively high pH about 8, expected if H+ ions are being transported out of the cells

- - electrical potential of -150mV, consistent with an excess of H+ ions outside the cell compared to inside

- - ATP is present in phloem sieve elements in large amounts, expected it is required for active transport

Comparison of sieve elements and xylem vessels


Similarities - liquid moves in mass flow along

a pressure gradient - liquid moves in tubes formed

from cells stacked end to end

Similarities - liquid moves in mass flow along

a pressure gradient - liquid moves in tubes formed

from cells stacked end to end


Comparison of sieve elements and xylem vessels Differences - water transported through dead xylem

vessels - translocation through live phloem sieve tubes

requires active loading of sucrose at sources thus requiring living cells

- xylem has lignified walls, phloem does not - lignified, dead xylem vessels are entirely

empty providing a tube through which water can flow unimpeded

Differences - water transported through dead xylem

vessels - translocation through live phloem sieve tubes

requires active loading of sucrose at sources thus requiring living cells

- xylem has lignified walls, phloem does not - lignified, dead xylem vessels are entirely

empty providing a tube through which water can flow unimpeded


Comparison of sieve elements and xylem vessels - lignified, dead xylem vessels have

strong walls for plant support - end walls of elements disappear where

phloem sieve elements form sieve plates which prevent the phloem sieve elements from collapsing

- sieve plates allow the phloem to seal itself if damaged, i.e. part of a blade of grass is eaten by a herbivore

- lignified, dead xylem vessels have strong walls for plant support

- end walls of elements disappear where phloem sieve elements form sieve plates which prevent the phloem sieve elements from collapsing

- sieve plates allow the phloem to seal itself if damaged, i.e. part of a blade of grass is eaten by a herbivore


Comparison of sieve elements and xylem vessels - phloem sap has a high turgor

pressure because of its high solute content, it would leak out quickly if the sieve elements did not seal

- phloem sap contains valuable substances such as sucrose, which clotting protects from loss

- clotting may also prevent the entry of microorganisms which might feed on the nutritious sap or cause disease

- phloem sap has a high turgor pressure because of its high solute content, it would leak out quickly if the sieve elements did not seal

- phloem sap contains valuable substances such as sucrose, which clotting protects from loss

- clotting may also prevent the entry of microorganisms which might feed on the nutritious sap or cause disease

Education

Chapter 10