Biomedical Engineering Reference
In-Depth Information
opposite the pits on the adjacent cell walls; the intercellular layer between the two
pits and two layers of primary walls make up the pit membrane, and two opposite
pits and pit membrane make up the pit pair. Pits on the secondary wall have two
types: the simple pit and the bordered pit. The basic difference between them is
that the secondary thickened wall uplifts toward the central part, hangs over the
pit cavity, and forms a dome-shaped edge so that the pit aperture is significantly
smaller, but this kind of dome-shaped edge does not exist in the simple pit.
The growth of the cell wall includes an increase in surface area and thickness; the
growth process is strictly controlled by biochemical reactions in the protoplast. The
growth of the cell wall should be in a relaxed state and have a high respiration
rate, protein synthesis rate, and water absorption rate. Most newly synthesized
microfibrils are superimposed on the original cell wall, but a few insert into the
original cell wall. In those cells that grow evenly, such as marrow cells, storage cells,
and culture cells, cell wall microfibrils randomly arrange in various directions and
form an irregular network. In contrast, in the extended-growth cells, the deposition
direction of microfibrils on the side walls makes an acute angle with the extended
direction of the cells. When the surface area of the cell is increased, the external
microfibrils arranged direction gradually is parallel with the long axis of the
cells. Substrates (such as pectin and hemicellulose) and glycoprotein are mainly
transported to the cell wall by the Golgi vesicles. The type of substrate is dependent
on the development stage of the cell. For example, at the expanding stage of cells,
the pectin is predominant in the matrix; otherwise, hemicelluloses predominate at
the shrinking stage [
5
].
2.3
Cellulose
Cellulose is the most abundant renewable organic resource on Earth and is
widespread in higher plants, bacteria, marine algae, and other biomass. The total
annual amount of cellulose is several billion tons, revealing the huge economic
value of it. Cellulose is the main component of the plant cell. Although some
animals (such as tunicates) and some bacteria contain cellulose, the content of
cellulose in these species is negligible when compared with plants. Cellulose was
first separated by Anselme Payen (1839) from timber that was alternately treated
with nitric acid and sodium hydroxide solution. It is a
-1,4-linked linear polymer
of glucose units and is insoluble in water, dilute acidic solutions, and dilute alkaline
solutions at normal temperatures. Although the structure and composition of the
cell walls of plants vary widely, the cellulose content usually accounts for 35-50 %
of dry weight and, peculiarly, almost 100 % for cotton. Study of the supramolecular
structure of natural cellulose showed that the crystalline and noncrystalline phases
intertwine to form the cellulose. The noncrystalline phase assumes an amorphous
state when tested by X-ray diffraction because most hydroxyl groups on glucose
are amorphous. However, large amounts of hydroxyl groups in the crystalline phase
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