Biology Reference
In-Depth Information
microfibrils are cross-linked by branched molecules of hemicellulose. The flat arrays of cellu-
lose microfibrils are laid on top of each other at different angles, and hemicellulose and
pectins are used to form cross-links between as well as within the layers. The angles between
different layers give the whole wall an ability to resist forces from all directions. The wall is
so good at resisting forces that it can contain cells that are under positive pressure due to
osmotic swelling of the cell; this pressure can reach up to 10 bar.
1
In plant cells, therefore,
the main tension-bearing elements are external.
The shapes of plant cells are determined by the shapes and mechanical properties of their
cell walls. During the period of plant cell morphogenesis, the cell is surrounded only by its
primary cell wall: later on, a thick secondary cell wall may be added on the inside for
strength, but the secondary cell wall is largely beyond the scope of this chapter. Therefore,
wherever this discussion mentions a cell wall, the primary cell wall should be assumed.
Plant cells synthesize cellulose directly at the surface of their plasma membranes, rather
than in the endoplasmic reticulum/golgi system where protein-based extracellular matrix
molecules are made. New cellulose is made from cytoplasmic UDP-glucose by rosette-like
complexes of cellulose synthase.
2
As soon as new cellulose polymers are synthesized and
leave the rosettes, they associate with one another to form microfibrils and take their place
in the microfibril array. Because the already synthesized section of a cellulose polymer
rapidly associates with the other molecules of the cell wall, parallel to them, the continuing
polymerization of the cellulose tends to push the cellulose synthase complex along the
membrane, parallel to the microfibrils in the layer of the wall nearest the plasma membrane
unless any other influences intervene.
DIFFUSE CELL ELONGATION IN PLANTS
Much of the elongation of plant tissues is driven by diffuse elongation of their constituent
cells, a process that takes place after mitotic proliferation has ended. By definition, diffuse
elongation takes place throughout the cell and it is achieved by allowing cell growth more
in one axis than in the others. This distributed pattern of growth contrasts with focused elon-
gation, described later in this chapter, in which elongation takes place only at one part of a cell.
The force that drives any expansion of plant cells, whether diffuse or focused, is mainly
osmotic turgor pressure. Because the inside of the cell is liquid, turgor pressure pushes
equally in all directions so, for plant cells to expand more in one axis than others, cell walls
have to yield to the internal pressure more easily in some directions than in others. A sheet
composed of parallel arrays of filaments, like the microfilaments of the cell wall, will usually
show most resistance to stretching along the axis of the filaments and least resistance orthog-
onal to that axis.
3
Therefore, if the direction in which cellulose microfilaments are laid down
is biased during the growth of a cell wall, the direction in which the internal pressure of the
cell can force the wall to expand will be set orthogonally to the microfilaments (
Figure 6.2
).
)
)
A quick classroom demonstration of this can be made by partially inflating a balloon, wrapping parallel
lengths of sticky tape around it like lines of latitude on a globe to represent filaments, then inflating the
balloon more. Its expansion will be anisotropic, at least until the extreme stresses at the tape edges cause it to
burst (this end-point will, at least, wake the sleepy souls at the back of the room).
