INTRODUCTION - Nanoindentation with Biological Applications - page 9

2.1 . Molecular building blocks
Here, briefly, the building blocks of biological materials are discussed
including the cell-associated components, proteins, sugars, biominerals
and water.
2.1.1. Cell-associated components
Cells are made up of carbon, hydrogen, nitrogen and oxygen—these
four atoms make up 99% of the cell by weight, including 70% water.
The four atoms make up four classes of small molecules: simple
sugars (monosaccharides), fatty acids (saturated and unsaturated),
amino acids and nucleotides. Macromolecules are then formed from
these small molecules: monosaccharides into polysaccharides, amino
acids polymerize into proteins, and nucleotides polymerize into nucleic
acids. The lipids in cells are fatty acids that form phospholipids, the
major component of bilayer cell membranes. Fatty acids are found
in many places and it is therefore not easy to categorize one main
set of macromolecules into which they polymerize. Nucleic acids are
based on the sugars ribose and deoxyribose, forming RNA and DNA,
respectively.
2.1.2. Proteins
Proteins are classed as either primarily structural—a category dominated
by the fibrillar proteins including the ubiquitous macromolecule
collagen—or mainly associated with non-mechanical functions, such as
catalysis by enzymes.
Protein chains are all synthesized from different linear combinations
of 20 amino acids. An amino acid structure is centered on an alpha
carbon atom - C ; off this atom is an amino group - NH 2 , a carboxyl group
- CO 2 H , a hydrogen atom - H and a side chain - R i , as shown in Fig. 1-5 .
These side chains can range from a single hydrogen atom to a very
complex long chain aromatic group, 10 each side chain creates a different
amino acid, which has different physicochemical properties: amino acids
can be basic or acidic, uncharged polar or nonpolar. A polypeptide chain