Chapter 10 – Dependable systems Chapter 10 Dependable Systems1.
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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
Particular requirements of plant cellsParticular requirements of plant cells
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
Particular requirements of plant cellsParticular requirements of plant cells
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
H2O reaches the steleH2O reaches the stele
H2O reaches the steleH2O reaches the stele
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
H2O reaches the steleH2O reaches the stele
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.
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
How translocation occursHow translocation occurs
- 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
How translocation occursHow translocation occurs
- 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
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 for the mechanism of phloem transport
Evidence for the mechanism of phloem 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
- 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
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
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
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
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