Biomedical Engineering Reference
In-Depth Information
cellulose is the strongest and highest energy storing material known (4). After expansion
ceases, some plant cells deposit thick (or secondary) walls, which often contain higher
percentages of cellulose. For example, the secondary walls within the water-conducting
and supportive xylem tissue contain 40-50% cellulose. These thick walls confer bulk
material strength to plant cells and tissues, as well as supporting other functions such
as water conduction. The accumulation of xylem becomes most pronounced in large
woody trees, which, together with the abundance of aquatic algae, results in cellulose
being the most abundant renewable biomaterial.
In other resonances with nanoscience, cellulose nanofibrils are produced in associa-
tion with one of nature's most remarkable biological nanomachines, a cellulose synthesis
complex (CSC), which is the focus of this chapter. (Historically, other names for such
complexes have been microfibril terminal complex (TC) or 'rosette' for the particular
CSC of land plants and their close relatives (see below). Recently, the acronym CSC has
sometimes been used to denote 'cellulose synthase complex', but we prefer 'cellulose
synthesis complex' to allow the possibility that other proteins besides cellulose synthase
(CS, or CesA in plants) will be identified as part of this complex.) It has also been noted
that the plant CSC functions within a
100 nm planar area bridging the cortical cyto-
plasm, plasma membrane, and exoplasmic space (or the area where cellulose nanofibrils
crystallize before they are integrated into the cell wall) (5).
Given the importance of cellulose in nature as well as the energy and material needs
of human civilization, it is astonishing that we understand so few details about the
mechanistic operation of the CSC. The purpose of this chapter is to summarize cur-
rent knowledge and remaining questions about how the CSC of land plants acts as a
nanomachine to produce cellulose I fibrils.
∼
2.2
Background
Before 1999, there were several major advances in our understanding of plant cellulose
biogenesis. The reviews of Delmer (1999) and Tsekos (1999) can be consulted for fur-
ther details and primary citations on the background findings that are summarized below.
Freeze fracture transmission electron microscopy (FF-TEM) generates micrographs of
metal replicas of cryo-fractured, shadowed, membranes including their embedded pro-
teins. Beginning in the 1970s, FF-TEM was used to see high resolution views of large,
distinctive, protein aggregates (CSCs) in the plasma membranes of cells that synthe-
size cellulose. Often the CSCs were at the termini of cellulose fibril impressions in
the plasma membrane. However, the geometry of individual CSCs and the arrange-
ment within the membrane of multiple CSCs varied between organisms, and especially
between major lineages. As observations of CSCs in various organisms accumulated,
it became apparent that cellulose nanofibril lateral dimensions, extent of crystallinity,
modes of crystallization (to form cellulose I
α
or Iß, see below), and manner of possible
macrofibril assembly varied in parallel with CSC geometry and, occasionally, higher
order aggregation of CSCs.
The ability to interfere with cellulose crystallization
in vivo
in both prokaryotic and
eukaryotic cells by addition of cellulose-binding molecules demonstrated that there was a
temporal lag between polymerization and crystallization and led to the unifying principle
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