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Structure and Function of Bacterial Cells (page 1)
(This chapter has 10 pages)
© Kenneth Todar, PhD
Drawing of a typical
bacterial cell, by Vaike Haas, University of Wisconsin-Madison
Primary Structure of
Biological Macromolecules Determines Function
Procaryotic structural components consist
of
macromolecules
such as DNA, RNA, proteins, polysaccharides, phospholipids, or some
combination
thereof. The macromolecules are made up of primary subunits such as
nucleotides, amino acids and sugars (Table 1). It is the
sequence in which the
subunits are put together in the macromolecule, called the primary
structure,
that determines many of the properties that the macromolecule
will
have. Thus, the genetic code is determined by specific nuleotide base
sequences in chromosomal DNA; the amino acid sequence in a protein
determines the properties and function of the protein; and sequence of
sugars in bacterial lipopolysaccharides determines unique cell wall
properties for pathogens. The primary structure of a
macromolecule will drive its function, and differences within
the primary structure of biological macromolecules
accounts for the immense diversity of life.
Table 1.
Macromolecules
that make up cell material
|
Macromolecule
|
Primary
Subunits
|
Where
found
in cell
|
|
Proteins
|
amino acids
|
Flagella, pili, cell walls,
cytoplasmic
membranes, ribosomes, cytoplasm
|
|
Polysaccharides
|
sugars (carbohydrates)
|
capsules, inclusions
(storage), cell walls
|
|
Phospholipids
|
fatty acids
|
membranes
|
|
Nucleic Acids
(DNA/RNA)
|
nucleotides
|
DNA: nucleoid (chromosome),
plasmids
rRNA: ribosomes; mRNA, tRNA: cytoplasm
|
Procaryotic
Cell Architecture
At one time it was
thought that bacteria
and other procaryotes were essentially "bags of enzymes" with no
inherent cellular
architecture. The
development of the electron microscope in the 1950s revealed the
distinct anatomical features of bacteria and confirmed the suspicion
that they lacked
a nuclear membrane. Procaryotes are cells of
relatively
simple construction, especially if compared to eucaryotes. Whereas
eucaryotic
cells have a preponderance of organelles with separate cellular
functions,
procaryotes carry out all cellular functions as individual units.
A
procaryotic
cell has five essential structural components: a nucleoid (DNA),
ribosomes,
cell
membrane, cell wall, and some sort of surface layer,
which
may or may not be an inherent part of the wall.
Structurally,
there are
three architectural regions: appendages (attachments to the
cell surface)
in the form of flagella and pili (or fimbriae); a cell
envelope consisting of a capsule, cell wall and plasma
membrane; and a cytoplasmic region that contains the cell chromosome
(DNA) and ribosomes and various sorts of inclusions
(Figure 1).
Figure 1.
Cutaway drawing of a typical
bacterial cell illustrating structural components. See Table 2
below
for chemical composition and function of the labeled components.
Table
2. Summary of characteristics
of typical bacterial cell structures
Structure
Flagella |
Function(s)
Swimming movement
|
Predominant chemical
composition
Protein
|
| Pili |
| Sex pilus |
Stabilizes mating
bacteria during DNA transfer by conjugation |
Protein |
| Common pili or fimbriae |
Attachment to surfaces;
protection
against phagotrophic engulfment |
Protein |
| Capsules (includes
"slime
layers" and glycocalyx) |
Attachment to surfaces;
protection
against phagocytic engulfment, occasionally killing or digestion;
reserve
of nutrients or protection against desiccation |
Usually polysaccharide;
occasionally
polypeptide |
| Cell wall |
| Gram-positive bacteria |
Prevents osmotic lysis
of cell
protoplast and confers rigidity and shape on cells |
Peptidoglycan (murein)
complexed
with teichoic acids |
| Gram-negative bacteria |
Peptidoglycan prevents
osmotic
lysis and confers rigidity and shape; outer membrane is permeability
barrier;
associated LPS and proteins have various functions |
Peptidoglycan (murein)
surrounded
by phospholipid protein-lipopolysaccharide "outer membrane" |
| Plasma membrane |
Permeability barrier;
transport
of solutes; energy generation; location of numerous enzyme systems |
Phospholipid and protein |
| Ribosomes |
Sites of translation
(protein synthesis) |
RNA and protein |
| Inclusions |
Often reserves of
nutrients; additional
specialized functions |
Highly variable;
carbohydrate,
lipid, protein or inorganic |
| Chromosome |
Genetic material of
cell |
DNA |
| Plasmid |
Extrachromosomal
genetic material |
DNA |
Figure
2 . Electron micrograph
of an ultra-thin section of a dividing pair of group A streptococci
(20,000X).
The cell surface fimbriae (fibrils) are
evident.
The bacterial cell wall is
seen as the light staining region between the fibrils and the dark
staining
cell interior. Cell division in progress is indicated by the new septum
formed between the two cells and by the indentation of the cell wall
near
the cell equator. The streptococcal cell diameter is equal to
approximately
one micron. Electron micrograph of Streptococcus pyogenes by
Maria
Fazio and Vincent A. Fischetti, Ph.D. with permission. The
Laboratory of Bacterial Pathogenesis and Immunology, Rockefeller
University.
chapter continued
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