Biomedical Engineering Reference
In-Depth Information
attributes that distinguish a living organism from an inanimate object [9]. One
is the ability to reproduce - the power of having progeny belonging to the same
species, being sufficiently similar to the parental organism. Another attribute is
the ability to ingest certain materials and subject them to own metabolism. Also a
capacity to respond to environment, possibility of healing, as well as the memory,
and the capacity to learn are typical for living organisms. The complicacy of
chemical processes and the constancy of biological systems that we recognize so
easily in the heredity phenomenon appear to contradict our intuitions.
The cell theory states that all living beings are composed of cells, which can
be regarded as the basic units of life, and that cells come from other cells [10].
Botanist Matthias Jakob Schleiden (1804-1881) was a cofounder of the cell theory,
along with Theodor Schwann and Rudolf Virchow. The growth of an organism
is effected by consecutive cell divisions (such a cell division is called
mitosis
).
In biology, another division process, namely, meiosis is dealt with. Meiosis is a
process of reductional division in which the number of chromosomes per cell is
halved. In animals, meiosis always results in the formation of gametes, while in
other organisms it can give rise to spores.
Many important substances in cells and tissues occur as thin, of the order of 20
˚
A,
and highly elongate particles. (1 angstrom
1
˚
A
10
−
10
m.) Proteins
(such as myosin, collagen, and nerve-axon protein), nucleic acids (DNA and RNA),
and polysaccharides (cellulose and hyaluronic acid) are examples. Many of these
substances are themselves polymers (as the protein macromolecules are polymers
of many amino-acid residues) but, as monomers, these elongated macromolecules
polymerize end-to-end and laterally to form fibrous structures. Schmitt
et al
. [12]
suggested that tropocollagen (TC), the macromolecule of collagen, has dimensions
of about 14
≡
=
0
.
1nm
=
2800
˚
A [11-15].
The nucleic acids can be longer. DNA polymers contain millions of repeating
units called
nucleotides
. One nucleotide unit is 3.3
˚
A (0.33 nm) long. Two nucleotides
on opposite complementary DNA (or RNA) strands that are connected
via
hydrogen
bonds are called a
base pair
(bp). In DNA, adenine forms a base pair with thymine,
as does guanine with cytosine, and the DNA chain is 22-26
˚
Awide.Thelargest
human chromosome, chromosome number 1, is approximately 220 million bp
long. This gives a length of 0
.
33
×
10
−
9
×
×
220
×
10
6
=
72
.
6 mm [16-18].
In Figures 1.1 and 1.2, one sees results of interactive computer an-
imation - simulation of the famous Bragg-Nye bubble raft experiment.
Two-dimensional crystallization is realized by equalizing distribution of atoms
on torus, not only globally as enforced by Descartes-Euler Law, but also locally.
Consecutive iterations repel atoms and shift them to centroids of Voronoi
polygons. In consequence, the fraction of pentagon-heptagon pairs of defects
(disclinations, curvatures, vortices,
) among prevailing crystalline hexagons is
gradually diminishing. A progressive coalescence and coarsening of crystal grains
are occurring during rotations and growth of circular inclusions while five to seven
edge dislocations align into migrating and rearranging grain boundaries [19]. In
such systems, a local order cannot propagate throughout space. Contradiction
...
