Introduction to Materials - Sample Preparation Topic for Transmission Electron Microscopy - page 26

with a composition that is regulated for an entire organism. The study of this whole
ensemble constitutes a third level of organization that always must be taken into
account.
4.2 Singularity of Biological Materials: Importance
of the Liquid Phase
Biological material is a multiphase material in liquid solution. Biological structures
are composed of carbon compounds such as liquid polymers that are in a liquid
solution called the milieu intérieur . These compounds are bound to one another by
covalent bonds to form methyl, hydroxyl, carboxyl, and amine groups. The cell's
small organic molecules have a molecular weight ranging from 100 to 1,000 Da and
contain up to 30 carbon atoms. They are usually linear, branched, or crosslinked.
They each have a particular spatial conformation that determines their properties.
This spatial conformation depends not only on temperature, as with polymers, but
also on the water content of their surroundings and its physical characteristics
(e.g., pH and ionic composition). Based on their main property, each molecule is
grouped together into one of four families of molecules: carbohydrates, fatty acids,
amino acids, and nucleotides. Carbohydrates are most often in solution and are
not conserved by the preparation methods, with the exception of certain complex
polysaccharides such as glycogen, starch, and cellulose. Some amino acids and
nucleotides group together to form large polymers called macromolecules, which
can be viewed under a microscope. Nonpolar fatty acids such as phospholipids,
cholesterol, and glycolipids are amphiphilic. In the aqueous medium of organic tis-
sue, they combine with each other in double layers to constitute membranes or in
lipid pools.
Organic matter can be considered as a colloidal solution with an amorphous
structure, but the presence of amphiphilic molecules “partition” this medium, caus-
ing phase separations and creating compartments. In the living organism, these
partitions are in constant motion; the molecules frequently combine and sepa-
rate very quickly following changes in pH, ionic composition, and water content.
Furthermore, the double-lipid layer is crossed by many membrane proteins whose
specific functions vary widely. These partitions, which are not at all impermeable,
are semi-permeable partitions. They delimit the cells and intracellular compartments
that will be described later.
The preparation processes result in the blocking of these molecules, either
through freezing while immobilizing the water or through the creation of bridges
with a chemical fixing agent, so as to manufacture long insoluble chains that will
enable subsequent dehydration. What follows is a denaturing of the proteins and
a loss of all of the small diffusible elements. Chemical fixation results in a rigid
image of a continuous barrier that has long been attributed to the membrane sur-
rounding the cell, but which is in fact a preparation artifact. In biology, observations
are made on an organic material that has been profoundly reworked in comparison