Introduction to Materials - Sample Preparation Topic for Transmission Electron Microscopy - page 30

more difficult. They may be considered to be poorly structured heterogeneous
structures.
Viruses are inevitably parasites and are slightly protected by envelopes. Therefore
they are very fragile. In particular, they are very sensitive to osmotic or ionic vari-
ations that can destroy them. Because of their small size, on the order of some
tens of microns maximum, they can all be considered as fine particles in terms of
preparation techniques.
4.4 Role of Structures on Functional Properties
The microstructures defined above all perform vital functions for the organism. The
development of certain organ functions results in an adapted and often very par-
ticular structure. For example, muscle is formed by very large cells that contain
myofibrils (actin and myosin) in addition to the organelles mentioned above. These
muscle cells are responsible for muscle contraction. Bone favors the construction
of an extracellular connective framework that, as it calcifies, gives it rigidity and
solidity. These cellular differentiations are defined by the genetic component of
the cell and through its relationships with its surroundings. Sometimes these struc-
tural modifications are provoked by external aggression (production of collagen in
response to a tear or a wound and development of the immune system in response
to a viral or bacterial attack). Thus, the absence or exaggerated development of cer-
tain organelles in a cell could indicate pathology. An important development in the
endoplasmic reticulum results in high synthetic activity, either natural or diverted
for the purpose of replicating RNA or DNA for a virus. The disorganization or loss
of myofibrils in muscle is the consequence of a myopathy.
Likewise, on the molecular scale, changing the conformation of a protein will
reflect different properties. The best known case, because it is the most dramatic, is
that of the prion. The presence of certain enzymes on the surface of nucleic acids
will enable them to carry out syntheses or replications.
Contrary to materials science, where the structure is considered to determine the
different characteristics of a material and therefore its function, in biology it appears
preferable to consider the functional role as determining the structure. Through
genetic engineering and the creation of the genetically modified organism (GMO),
for example, we are starting to see the possibility of intervening on the structure (of
DNA or RNA) to change functional properties.
Bibliography
Alberts, B., Bray, D., Lewis, J., Raff, M., Roberts, K., and Watson, J.-D. (1989). Biologie
Moléculaire de la Cellule , 2e édition. Médecine Sciences Flammarion, Paris.
Ayache, J. (2002). Growth induced microstructure in thin films and heterostructures. In Crystal
Growth in Thin Film Superconductors (eds. M. Guilloux, V. Perrin, and A. Perrin). Research
Signpost, Trivandrum.