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Transport in Plants
• Explain the need for transport systems inmulticellular plants in terms of size and surfacearea:volume ratio;
• Describe, with the aid of diarams andphotoraphs, the distribution of xylem andphloem tissue in roots, stems and leaves ofdicotyledonous plants;
• Describe, with the aid of diarams andphotoraphs, the structure and function of xylemvessels, sieve tube elements and companioncells;
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Transport in Plants
• Plants need a transport
system so that cells deep
within the plants tissues
can receive the nutrients
they need for cellprocesses
• The problem in plants is
that roots can obtain
water, but not suar, andleaves can produce suar,
but can!t et water from
the air
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"hat substances need to be
moved#
• The transport system
in plants is called
vascular tissue
• $ylem tissuetransports water and
soluble minerals
•Phloem tissuetransports suars
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The %ascular Tissues
• $ylem and phloem
are found toether in
vascular bundles, that
sometimes containother tissues that
support and
strenthen them
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&oot vs' stem vs' leaf The vascular bundle
differs dependin on ifit is a root or stem
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&oot• The vascular bundle is found
in the centre• There is a lare central core ofxylem( often in an x(shape
• This arranement providesstrenth to withstand thepullin forces to which roots
are exposed• )round the vascular bundleare cells called the endodermiswhich help to et water into thexylem vessels
• *ust inside the endodermis is
the periycle which containsmeristem cells that can divide+for rowth
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-tem• The vascular bundles are found near the outer ede of the stem
• The xylem is found towards the inside of each vascular bundle, the
phloem is found towards the outside• .n between the xylem and phloem is a layer of cambium
• /ambium is a layer of meristem cells that divide to ma0e new xylemand phloem
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1eaf
• The vascular bundles
+xylem and phloem form
the midrib and veins of the
leaf
• ) dicotyledon leaf has a
branchin networ0 of
veins that et smaller as
they branch away from the
midrib• "ithin each vein, the
xylem can be seen on top
of the phloem
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Phloem
$ylem
-tem
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) 2 $ylem
3 2 Phloem
/4D 2 5pper41ower epidermis
1eaf
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$ylem vessel wall
Phloem
&oot
Endodermis
$ylem vessel
lumen
-tarch
rains
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-tructure of $ylem
• 5sed to transport
water and minerals
from roots to leaves
• /onsists of tubes forwater, fibres for
support and livin
parenchyma cells
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$ylem vessels
• 6bvious in dicotyledonous plants
• 1on cells with thic0 walls containin linin
• 1inin waterproofs walls of cells and strenthens
them• /ells die and ends decay formin a lon tube
• 1inin forms spiral, annular rins or bro0en rins+reticulate
• -ome linification is not complete and pores areleft called pits or bordered pits, allowin water tomove between vessels or into livin parts
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)daptations of $ylem to 7unction
• $ylem can carry water and minerals from roots
to shoot tips because:
• 8ade of dead cells formin continuous column
• Tubes are narrow so capillary action is effective
• Pits allow water to move sideways
• 1inin is stron and allows for stretchin
• 7low of water is not impeded as: there are noend walls, no cell contents, no nucleus, linin
prevents tubes collapsin
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-tructure of Phloem
• 7unction to transport
suars from one part
to another
• 8ade of sieve tubeelements and
companion cells
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-ieve Tubes
• -ieve tube elements not true cells as they
have little cytoplasm
• 1ined up end to end to form a tube
• -ucrose is dissolved in water to form a
sap
• Tubes +0nown as sieve tubes have a fewwalls across the lumen of the tube with
pores +sieve plates
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/ompanion cells
• .n between sieve tubes
• 1are nucleus, dense
cytoplasm
• 8any mitochondria toload sucrose into sieve
tubes
• 8any plasmodesmata
+aps in cell walls
between companion cells
and sieve tubes for flow
of minerals
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"ater route between cells
• )poplast: between cell
walls of neihbourin
cells
• -ymplast: throuh
plasma membrane and
plasmodesmata to
cytoplasms from cell to
cell
• %acuolar: same assymplast, but also
throuh vacuoles
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"ater upta0e from the soil
• Epidermis of roots contain root hair cells
• 8inerals absorbed by active transport
usin )TP
• 8inerals reduce the water potential in the
cell cytoplasm +more neative so water is
ta0en up by osmosis
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8ovement across the root• )ctive process occurrin at the endodermis +layer of cells surroundin the
xylem, some containin waterproof strip called casparian strip
• /asparian strip bloc0s the apoplast pathway +between cells forcin waterinto the symplast pathway +throuh the cytoplasm• The endodermis cells move minerals by active transport from the cortex into
the xylem, decreasin the water potential +more neative, thus water movesfrom the cortex throuh the endodermal cells to the xylem by osmosis
• ) water potential radient exists across the whole cortex, so water is movedalon the symplast pathway +throuh cytoplasm from the root hair cells
across the cortex and into the xylem
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/asparian -trip
• 3loc0s the apoplast pathway +cell walls
• "ater and dissolved nitrate ions have to pass
into the cell cytoplasm throuh cell membranes
• There are transporter proteins in the cellmembranes that actively transport nitrate ions
into the xylem lowerin the water potential +more
neative
• "ater enters xylem down concentration radient
and cannot pass bac0
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"ater movement up stem
• &oot pressure: minerals move into xylem byactive transport, forcin water into xylem andpushes it up the stem
• Transpiration Pull: loss of water at leavesreplaced by water movin up xylem' /ohesion(tension theory( cohesion between watermolecules and tension in the column of water+which is why xylem is strenthened with linin
means the whole column of water is pulled up inone chain• /apillary action: adhesion of water to xylem
vessels as they are narrow
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9ow water leaves the leaf
• Throuh stomata
• Tiny amount throuh the waxy
cuticle
• "ater evaporates from the
cells linin the cavity betweenthe uard cells, lowerin water
potential and meanin that
water enters them by osmosis
from neihbourin cells, which
is replaced by furtherneihbourin cells and so on
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Transpiration
• 1oss of water vapour from upper parts of theplant
• "ater enters leaf from xylem and passes to
mesophyll cells by osmosis• "ater evaporates from surface of mesophyll
cells to form water vapour +air spaces allowwater vapour to diffuse throuh leaf tissue
• "ater vapour potential rises in air spaces, sowater molecules diffuse out of the leaf throuhopen stomata
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Transpiration: three processes
• 6smosis from xylem to mesophyll cells
• Evaporation from surface of mesophyll
cells into intercellular spaces
• Diffusion of water vapour from intercellular
spaces out throuh stomata
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"ater use in plant
• Photosynthesis
• /ell rowth and elonation
• Turidity• /arriae of minerals
• /ools the plant
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8easurin transpiration
• Potometer is used to
estimate water loss
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7actors affectin transpiration
• 1eaf number: more leaves, more transpiration• umber, size, position of stomata: more and lare, more
transpiration, under leaf, less transpiration• /uticle: waxy cuticle, less evaporation from leaf surface
• 1iht: more as exchane as stomata are open• Temperature: hih temperature, more evaporation, more
diffusion as more 0inetic enery, decrease humidity somore diffusion out of leaf
• 9umidity: hih humidity, less transpiration
• "ind: more wind, more transpiration• "ater availability: less water in soil, less transpiration
+e'' in winter, plants lose leaves
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Too much water loss
• 1ess turidity
• on(woody plants wilt and die
• 1eaves of woody plants die first then it willdie if water loss continues
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$erophytes
• -maller leaves reducin surface area e'' pine tree• Densely pac0ed spony mesophyll to reduce surface area, so less
water evaporatin into air spaces• Thic0 waxy cuticle e'' holly leaves to reduce evaporation• /losin stomata when water availability is low
• 9airs on surface of leaf to trap layer of air close to surface whichcan become saturated with water, reducin diffusion
• Pits containin stomata become saturated with water vapourreducin diffusion
• &ollin the leaves so lower epidermis not exposed to atmospherealso traps air which becomes saturated
• 8aintain hih salt concentration to 0eep water potential low andprevent water leavin
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8arram rass1eaf rolled up to trap
air inside
Thic0 waxy cuticle to
reduce water
evaporation from the
surface
Trapped air in the
centre with a hih
water potential +less
neative
9airs on lower
surface reduce
movement of air-tomata in pits to
trap air with moisture
close to the stomata
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8ovement of -uars
• Translocation: movement of assimilates +suars
and other chemicals throuh the plant
• -ource: a part of the plant that releases sucrose
to the phloem e'' leaf • -in0: a part of the plant
that removes sucrose from
the phloem e'' root
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-ucrose Enterin the Phloem
• )ctive process +re<uires enery• /ompanion cells use )TP to transport hydroen
ions out of their cytoplasm• )s hydroen ions are now at a hih
concentration outside the companion cells, theyare brouht bac0 in by diffusion throuh specialco(transporter proteins, which also brin thesucrose in at the same time
• )s the concentration of sucrose builds up insidethe companion cells, they diffuse into the sievetubes throuh the plasmodesmata +apsbetween sieve tubes and companion cell walls
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-ucrose movement throuh phloem
• -ucrose enterin sieve tube lowers the water
potential +more neative so water moves in by
osmosis, increasin the hydrostatic pressure
+fluid pushin aainst the walls at the source• -ucrose used by cells surroundin phloem and
are moved by active transport or diffusion from
the sieve tube to the cells' This increases water
potential in the sieve tube +ma0es it lessneative so water moves out by osmosis which
lowers the hydrostatic pressure at the sin0
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8ovement alon the phloem
• "ater enterin the phloem at the source,
movin down the hydrostatic pressure
radient and leavin at the sin0 produces
a flow of water alon the phloem thatcarries sucrose and other assimilates'
This is called mass flow' .t can occur
either up or down the plant at the sametime in different phloem tubes
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Evidence for translocation
• &adioactively labelled carbon from carbon dioxide can appear in thephloem
• &inin a tree +removin a rin of bar0 results in suars collectinabove the rin
• )n aphid feedin on the plant stem contains many suars when
dissected• /ompanion cells have many mitochondria• Translocation is stopped when a metabolic poison is added that
inhibits )TP• p9 of companion cells is hiher than
that of surroundin cells
• /oncentration of sucrose is hiher atthe source than the sin0
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Evidence aainst translocation
• ot all solutes move at the same rate
• -ucrose is moved to parts of the plant at
the same rate, rather than oin more
<uic0ly to places with low concentrations
• The role of sieve plates is unclear
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5seful &evision -ites
• http:44scienceaid'co'u04
• http:44www's(cool'co'u04
• http:44www'spar0notes'com4bioloy4