1. Flowering plants (Angiosperms) have 2 transport systems
TRANSPORT IN PLANTS
Flowering plants (Angiosperms) have 2 transport systems
© 2016 Paul Billiet ODWS

2. Xylem 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

3. Phloem 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

4. Transport in the xylem © 2016 Paul Billiet ODWS
© 2016 Paul Billiet ODWS
universe-review.ca/I10-22a-xylem.jpg

5. Movement of minerals to the root
Diffusion
Along fungal hyphae (mutualism)
In mass flow of soil water
© 2016 Paul Billiet ODWS

6. Root uptake 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

7. Root uptake © 2016 Paul Billiet ODWS
sps.k12.ar.us/massengale/images/modroothairs.jpg

8. Root uptake © 2016 Paul Billiet ODWS
© 2016 Paul Billiet ODWS

9. Root uptake
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

10. Root uptake 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
universe-review.ca/I10-22a-strip.jpg
© 2016 Paul Billiet ODWS

11. Xylem vessels
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

12. The cohesion-tension theory
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

13. The cohesion-tension theory
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_...

14. Stomata ABA = Abscissic acid © 2016 Paul Billiet ODWS
ABA = Abscissic acid
© 2016 Paul Billiet ODWS

15. Stomata 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

16. 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

17. Factors affecting the transpiration flow
biology.unlv.edu/.../Leaves/FicusStomata2.jpg
Stomata of fig (Ficus)
© 2016 Paul Billiet ODWS

18. Translocation in the phloem
© 2016 Paul Billiet ODWS

19. Sieve tubes 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

20. Using aphids to trace phloem flow
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

21. Aphid stylets 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.
Stylet (stained red) of the aphid
Sitobion yakini terminating in a
single sieve tube
© 2016 Paul Billiet ODWS

22. Using radioisotopes
Leaves 4, 6 and 8 labelled with a 5 min pulse of 14CO2 in three plants, then left for 1 hour
The radioisotope is traced to other leaves by autoradiography.
© 2016 Paul Billiet ODWS

23. Tracing to other parts of the plant
Leaf 8 labelled using a 5 min pulse of 14CO2
Left for 1 hour
Autoradiograph taken for 13 days.
© 2016 Paul Billiet ODWS

24. Pressure flow hypothesis
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

25. Pressure flow hypothesis
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

26. Pressure flow hypothesis
© 2016 Paul Billiet ODWS

27. THE END!
© 2016 Paul Billiet ODWS