Movement of water through the plant cells by diffusion, osmosis and imbibition,. The importance of diffusion to plant cells

1. LECTURE 2
PLANT WATER RELATIONS
By
Dr. Luma H. Abdul –Qadir
Department of Biology

6. Diffusion

24. Absorption of Water and Ions by Roots
There Appear to Be Limits to Tree Height
As we have seen, the tensile strength of water is great enough to
prevent the pulling apart of adjacent water molecules under the
tension required to move water up the xylem of tall trees, the
tallest of which is a giant redwood at 115.6 meters.
A study on the limits to tree height in red wood indicates,
however, that the maximum tension exerted on the water columns
in eight of the tallest redwoods (including the 115.6-meter-tall tree)
is close to the point of embolism.

25. If so, this value would be a major factor in the control of
tree height. In the same study, it was noted that as trees
grow taller, increasing water stress in the leaves due to
gravity and increasing path-length resistance may ultimately
limit leaf expansion and lead to a decline in photosynthesis
in the leaves.
Such a decline would impose another constraint on tree
height. Barring mechanical damage, a maximum possible
tree height of 122 to 130 meters was predicted.

26. Water Absorption by Roots Is Facilitated by Root Hairs
The root system serves to anchor the plant in the soil and, above all, to
meet the tremendous water requirements of the leaves resulting from
transpiration.
Most of the water that a plant takes from the soil enters through the
younger parts of the root.
Absorption takes place directly through the epidermis of younger roots.
The root hairs, located several millimeters above the root tip, provide an
enormous surface area for absorption. From the root hairs, the water
moves through the cortex, the outer layer or layers of which may be
differentiated as an exodermis (a subepidermal layer of cells with
Casparian strips).
From there, the water progresses through the endodermis (the innermost
layer of cortical cells) and into the vascular cylinder.

27. Water May Follow One or More of Three Possible Pathways
across the Root
The pathway followed by water across the root depends largely on
the degree of differentiation of the various tissues that make up the
root.
In each of the tissues, water may follow one or more of three
possible pathways :
(1) apoplastic (around the protoplasts via the cell walls),
(2) symplastic (from protoplast to protoplast via plasmodesmata),
(3) transcellular (from cell to cell, across the plasma membranes
and tonoplasts).

28. For example, in a root without an exodermis, water can move
apoplastically as far as the endodermis.
At the endodermis, however, because of the water-impermeable
Casparian strips in the radial and transverse walls of the endodermal
cells, water is forced to traverse the plasma membranes and
protoplasts of these tightly packed cells.
In roots having an exodermis, by contrast, the Casparian strips in the
radial and transverse walls of the compactly arranged exodermal cells
prevent apoplastic movement of water across that cell layer.
Water could follow either a symplastic or transcellular pathway across
such cells.

29. (The permeability of the transcellular pathway is
determined in large part by aquaporins in the plasma
membranes.)
If, however, the outer tangential walls of the exodermal
cells possess suberin lamellae, movement across that
surface may be limited to the symplast.
Having traversed the exodermis, subsequent movement
of the water across the cortex could be by one or more
of the three possible pathways listed above.