Biomedical Engineering Reference
In-Depth Information
Plywood
Cell wall
layers
Microfibril
layers
Figure 19.10 If the microfibril directions between the separate cell wall layers are diverse enough, the wall will
provide multidirectional support and the cell has no directional preference for expansion. This principle is also the
biomimetic basis for the design of plywood. (From Brett C, Waldron K (1996) Physiology and Biochemistry of Plant
Cell Walls. Chapman & Hall, London. With permission of CRC Press.)
19.3.3
Guard Cells or How to Make a Pore
Although directional deposition of microfibrils provides different wall resistance to diametric and
axial cell expansion of stem cells, the wall resistance does not differ on both sides of the elongating
cell axis and therefore such cells have a directed but linear expansion. Guard cells, however, are an
example where asymmetrical wall resistance on the flanks of the cell axis lead to an unequal
expansion of the flanks and a curved shape of the cell. Since these cells are used to create openings
in a surface layer, guard cells always occur in pairs. Their asymmetric expansion leads to the
opening of a stomatal pore within the inexpandable flanks of two cells (Figure 19.11). A stomatal
pore is a reversible, nastic structure that operates as a valve in the leaf surface to balance the
photosynthetic intake of carbon dioxide (to be assimilated into sugars) with the loss of water exiting
the leaf in a process of forced evaporation that generates negative pressure.
Plants use the opening of these pores to (i) cool the leaf surface, (ii) access the carbon dioxide in
the air, and (iii) as the jets of a capillary vacuum pump that through evaporation generates
(absolute) negative pressures far beyond the capacity of human-made designs and lifts water
and ions in the capillary tubes of the xylem. Different models have been developed to explain
the asymmetric expansion of guard cells (Figure 19.12). All models agree that the basis for this
behavior is an asymmetry of cell wall characteristics but they differ in the specifics of this structural
requirement. The oldest model claims that this kind of expansion is due to the conspicuousthicken-
ing of the inner walls adjacent to the stomatal pore. Later models deny the role of wall thickening
and explain the asymmetric expansion of the guard cells and the resulting pore formation by either
the symmetry of guard cells (Cooke et al., 1977) or more convincingly with the arrangement of the
radial microfibril bundles that are tightly wound near the ends and looser near the center of the
guard cells (Aylor et al., 1973). Still others (including the authors of this chapter) think that it is not
so much the thickening of the inner walls but the enigmatic structure that is behind the thickening
— a ring made of cutin, a waterproofing resin polymer found as a common coating on the walls of
most epidermal cells of leaves. Even after the guard cell walls have been digested by
enzymatic mixtures, the cutin rings remain intact and float in the solution in the form of a gaping
mouth. Aside from separating the inner walls and water-proofing the pore walls, the ring itself is
the only direct physical connection between the two guard cells. Attached only to the inner
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