Circulation: Moving Water in Plants study guide that includes all the notes and relevant pictures from lecture as well as additional notes from the assigned readings

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16 Oct 2011
Department
Organismal Phys Lecture 13
Circulation 2: Moving Fluids in Plants
Xylem and Phloem (types of vessels)
Plants need a circulatory system in order to survive;
however they don’t have a pump
Have therefore evolved mechanisms and structures for
transport
Xylem (General)
Translocate water and other inorganic nutrients from the
roots to the leaves
Phloem (General)
Translocate sugars, proteins, and signalling molecules
from source tissues (ex. leaves) to sink tissues (ex. roots)
Xylem
Made up of tracheids and vessel elements
o These are dead cells
o Interconnect with one another (3 ways)
Laterally (both of them do this)
Edge to edge (tracheids)
End to end (vessel elements)
Need to be supported by fibres and other lignified cells
(in trees) because are not very strong
Have pits to allow transport of things between tracheids
Vessel Elements
Network of tubes, horizontally and vertically (most of the movement is vertical)
Primary vessel type in angiosperms (and others)
End-end stacking plus perforation plates = continuous tubes
Pits connect vessel elements laterally
Some lateral movement (but slow, compared to vertical)
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Tracheids
Act as long connected vessels
Secondary in angiosperms
Primary vessel type in conifers (trees)
o Have very permeable pit membranes
Allows lateral movement to provide long connected vessels
The pits allow for the lateral transfer and keep it very vertical
These are not end to end, they overlap
The Physics of Pulling Fluid through Tubes
Remember: plants PULL things; animals PUSH
How do plants move fluids if they don’t have a pump?
o Sometimes root pressure (due to evaporation from leaves) can
push H₂O into Xylem; but usually H₂O is pulled
o Atmospheric pressure will support a rise of water by about
10.3m
Capillary Action (Capillarity)
Sucks fluid up
Related to the diameter of the tube (needs to be very narrow)
2 Forces: wettable walls, and surface tension
o Wettable walls
Strong adhesion between the liquid and the walls of the vessels
o Surface tension
The interactions between water molecules
However, this only accounts for a few extra metres (from the rise of water due to atmosphere)
Cohesion-tension Theory
Evaporation at the leaves cause negative pressure
The cohesive properties of the H₂O molecules transfer this tension through the length of the
water column
This requires a continuous water column
Due to the fact that there is a lot of tension, dense wood acts to support the xylem; always the
danger of cavitation
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Cavitation
Results from the fact that Xylem are under a lot of tension (negative pressure)
o Have this tension because the higher the pressure, the more energy we can get into the
system
At these lower pressures, the lowest energy state for water should be as a vapour
o Water in the Xylem is meta-stable (small change will result in the formation of ice)
It is very common at a nucleus for a bubble to spontaneously form
o Can detect this occurring by the noise of the bubbles forming
Consequences of Cavitation
Formation of a bubble will block the Xylem
o This is known as an ‘Embolism’
o One nucleated, the bubble will continue to grow (like ice does!)
When a bubble occurs, there will no longer be a continuous column of water
o Water will be unable to get from the roots to the leaves
Coping with Cavitation 1: Block it off and Re-Route
If a bubble forms there is a sudden change in pressure in the trachea where it was
formed
o This pressure difference causes the torus (pit connecting 2 tracheids) to
collapse on itself, cutting off a lateral connection
Tori (plural for torus) in conifer trees (example in diagram)
o Block off otherwise permeable pit-pairs between tracheids (below pic)
o Will need to re-route the fluid elsewhere
Vessel Elements
o Perforation plates provide some barrier (stops bubble from travelling
elsewhere)
o Air bubbles often stop at perforation plates
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