Download Powerpoint Presentation: Transport in Plants

TRANSPORT IN PLANTS
Flowering plants
(Angiosperms) have
2 transport systems
© 2016 Paul Billiet ODWS
http://www.hort.purdue.edu/hort/courses/HORT301/MikesLectures/TranslPhloem.html
Xylem
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Made of dead cells
(xylem vessels)
Uses physical
mechanisms to transport
the fluid (the transpiration
flow)
Transports water and
mineral salts only
From the root to the
leaves
© 2016 Paul Billiet ODWS
www.skidmore.edu/academics/biology/plant_bio/...
Phloem
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Made of living cells (sieve
tubes and their companion
cells)
Uses active transport to
load the phloem and unload
the phloem
Transports water, minerals
and organic molecules
From shoot to roots and
roots to shoot
© 2016 Paul Billiet ODWS
www.skidmore.edu/academics/biology/plant_bio/...
Transport in the xylem
www.richmond.edu/.../KMO_stem_2nd_xylem_20Xs.JPG
© 2016 Paul Billiet ODWS
universe-review.ca/I10-22a-xylem.jpg
Movement of minerals to the root
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Diffusion
Along fungal hyphae (mutualism)
In mass flow of soil water
© 2016 Paul Billiet ODWS
Root uptake
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Roots provide a large
surface area for
absorption
They are long, thin and
highly branched
Near the tip the is a
zone of root hairs
The soil solution can
penetrate into the root
up to the endodermis
via the free space =
the apoplasm
© 2016 Paul Billiet ODWS
www.epa.state.il.us/.../images/root-system.gif
Root uptake
© 2016 Paul Billiet ODWS
sps.k12.ar.us/massengale/images/modroothairs.jpg
Root uptake
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/X/Xylem.html
© 2016 Paul Billiet ODWS
Root uptake
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At any point across the root the minerals
can be taken up by a cell across a plasma
membrane
This uses active transport
Therefore movement into the root cell
cytoplasm is selective
Once a mineral is in the cytoplasm of a cell
it can move from cell to cell via the
plasmodesmata = the symplasm
© 2016 Paul Billiet ODWS
Root uptake
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Minerals cannot travel
in the apoplasm past
the endodermis
The cell walls of the
endodermis are water
proofed by a
Casparian strip
At the endodermis the
plant can have control
over what it absorbs
© 2016 Paul Billiet ODWS
universe-review.ca/I10-22a-strip.jpg
Xylem vessels
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Xylem is found in a vascular bundle in the middle of
the root
As they mature their wall becomes impregnated with
lignin
The tissue becomes wood
This provides support to the plant
When they are mature they lose the top and
bottom ends
The xylem forms continuous columns of water up
the plant to the leaves
© 2016 Paul Billiet ODWS
The cohesion-tension theory
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The upward flow of sap is created by the
evaporation of water from the surface of the leaves
(evapo-transpiration)
Most of the water evaporates through pores called
stomata (sing. stoma) = transpiration
The water is pulled up as the water evaporates from
the leaves = tension
BUT usually a column of water cannot be pumped
up more than 10m without breaking
Some trees can grow to 100m (e.g. sequoia and
eucalyptus)
© 2016 Paul Billiet ODWS
The cohesion-tension theory
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However when water travels
in thin tubes (like xylem
vessels) there is a strong
cohesion between the water
molecules
A column of water in a xylem
vessel has the same tensile
strength as a thread of steel
of the same diameter
The limit of this is reached at
about 100m
© 2016 Paul Billiet ODWS
lettres-histoire.ac-rouen.fr/histgeo/sequoia_...
Stomata
www.isv.cnrs-gif.fr/jg/images/stomata.jpg
© 2016 Paul Billiet ODWS
ABA = Abscissic acid
Stomata
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Stomata can open and close
Controlled by abscisic acid (ABA) hormone
Guard cells are found each side of the pore
When the guard cells are turgid the pore opens
When the guard cells are flaccid the pore closes
When the plant looses more water than it can
absorb its cells become flaccid
So a dehydrated plant closes its stomata
When the stomata close the plant economises on
water
© 2016 Paul Billiet ODWS
Factors affecting the transpiration flow
Anything that affects evaporation will affect
transpiration
 Humidity
 Temperature
 Wind speed
 But also light
When there is light the plant photosynthesizes
When it is photosynthesizing it needs CO2
So when it is light it opens the stomata to absorb CO2
and it lets out more water
© 2016 Paul Billiet ODWS
Factors affecting the transpiration flow
biology.unlv.edu/.../Leaves/FicusStomata2.jpg
© 2016 Paul Billiet ODWS
Stomata of fig (Ficus)
Translocation in the phloem
http://www.hort.purdue.edu/hort/courses/HORT301/MikesLectures/TranslPhloem.html
© 2016 Paul Billiet ODWS
Sieve tubes
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Sieve tubes = long thin
cells joined end to end
Each end has
perforated sieve plate
They do not grow thick
lignified cell walls
They keep their
cytoplasm but they lose
their nucleus
They are living cells
Difficult to experiment
with.
© 2016 Paul Billiet ODWS
www.uic.edu/.../bios100/lectf03am/phloem.jpg
Using aphids to trace phloem flow
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Aphids are bugs
(hemiptera)
Their mouth parts
are specialised for
piercing plants and
sucking sap from the
phloem (stylet).
© 2016 Paul Billiet ODWS
© P Billiet
Aphid stylets
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The phloem sap is
under pressure
So the sap flows out
even when the aphid
is removed
The stylet pierces a
single phloem sieve
tube
Transport in a single
sieve can be traced.
© 2016 Paul Billiet ODWS
Stylet (stained red) of the aphid
Sitobion yakini terminating in a
single sieve tube
Using radioisotopes
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Leaves 4, 6 and 8 labelled with a 5 min pulse of
14CO in three plants, then left for 1 hour
2
The radioisotope is traced to other leaves by
autoradiography.
© 2016 Paul Billiet ODWS
Tracing to other parts of the plant
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Leaf 8 labelled using
a 5 min pulse of 14CO2
Left for 1 hour
Autoradiograph taken
for 13 days.
© 2016 Paul Billiet ODWS
Pressure flow hypothesis
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Movement in the phloem is still not completely
understood
Movement seems to work on the principle of source
to sink
Organic molecules (e.g. sugars) are loaded into the
sieve tubes where they are produced (e.g.
photosynthesising leaves) = the source
This requires active transport
This is followed by osmosis, water follows the sugar
into the cell
The pressure rises and the liquid flows away
© 2016 Paul Billiet ODWS
Pressure flow hypothesis
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The sugars are removed from the sieve tubes where
they are used (e.g. respiring root cells) = the sink
The water follows the sugars out of the sieve tube by
osmosis
The pressure is lower in the sink than in the source
so the sap flows from source to sink
In summer the photosynthesising leaves are the source
and root storage organs are sinks
BUT after the winter in spring, the roots are sources
providing nutrients for the growing shoots
So phloem transport is in two directions and it may
depend on the season
© 2016 Paul Billiet ODWS
Pressure flow hypothesis
http://www.hort.purdue.edu/hort/courses/HORT301/MikesLectures/TranslPhloem.html
© 2016 Paul Billiet ODWS
THE END!
© 2016 Paul Billiet ODWS