Biomedical Engineering Reference
In-Depth Information
2.3. CELL CHEMISTRY
Living cells are composed of high molecular-weight polymeric compounds such as
proteins, nucleic acids, polysaccharides, lipids, and other storage materials (fats, polyhydrox-
ybutyrate, and glycogen). These biopolymers constitute the major structural elements of
living cells. For example, a typical bacterial cell wall contains polysaccharides, proteins,
and lipids; cell cytoplasm contains proteins mostly in the form of enzymes; in eukaryotes,
the cell nucleus contains nucleic acids mostly in the form of DNA. In addition to these
biopolymers, cells contain other metabolites in the form of inorganic salts (e.g. NH 3
,PO 3 4
,
K þ ,Ca 2 þ ,Na þ ,SO 2 4
), metabolic intermediates (e.g. pyruvate, acetate), and vitamins. The
elemental composition of a typical bacterial cell is 50% carbon, 20% oxygen, 14% nitrogen,
8% hydrogen, 3% phosphorus, and 1% sulfur, with small amounts of K þ ,Na þ ,Ca 2 þ ,Mg 2 þ ,
Cl , and vitamins.
Cellular macromolecules are functional only when in the proper three-dimensional config-
uration. The interactions among them are very complicated. Each macromolecule is part of
an intracellular organelle and functions in its unique microenvironment. Information transfer
from one organelle to another (e.g. from the nucleus to ribosomes) is mediated by special
molecules (e.g. messenger RNA). Most of the enzymes and metabolic intermediates are
present in the cytoplasm. However, other organelles, such as mitochondria, contain enzymes
and other metabolites. A living cell can be visualized as a very complex reactor in which more
than 2000 reactions take place. These reactions (metabolic pathways) are interrelated and are
controlled in a complicated fashion.
Despite all their complexity, an understanding of biological systems can be simplified by
analyzing the system at several different levels: the molecular level (molecular biology,
biochemistry), the cellular level (cell biology, microbiology), the population (microbiology,
ecology), and production (bioprocess engineering). This section is devoted mainly to the
structure and function of biological molecules.
2.3.1. Amino Acids and Proteins
Proteins are the most abundant organic molecules in living cells, constituting 40 e 70% of
their dry weight. Proteins are polymers built from amino acid monomers. Proteins typically
have molecular weights (MWs) of 6000 to several hundred thousand. The a -amino acids are
the building blocks of proteins and contain at least one carboxyl group and one
-amino
group, but they differ from each other in the structure of their R groups or side chains.
a
H
H 2 N
C
COOH
R
L - α -amino acid
H
HOOC
C
NH 2
R
D -
α
-amino acid
 
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