Microbiology, 2021

96 3 • The Cell
Group translocation also transports substances into bacterial cells. In this case, as a molecule moves into a cell
against its concentration gradient, it is chemically modified so that it does not require transport against an
unfavorable concentration gradient. A common example of this is the bacterial phosphotransferase system, a
series of carriers that phosphorylates (i.e., adds phosphate ions to) glucose or other sugars upon entry into
cells. Since the phosphorylation of sugars is required during the early stages of sugar metabolism, the
phosphotransferase system is considered to be an energy neutral system.
Photosynthetic Membrane Structures
Some prokaryotic cells, namely cyanobacteria and photosynthetic bacteria, have membrane structures that
enable them to perform photosynthesis. These structures consist of an infolding of the plasma membrane that
encloses photosynthetic pigments such as green chlorophylls and bacteriochlorophylls. In cyanobacteria,
these membrane structures are called thylakoids; in photosynthetic bacteria, they are called chromatophores,
lamellae, or chlorosomes.
Cell Wall
The primary function of the cell wall is to protect the cell from harsh conditions in the outside environment.
When present, there are notable similarities and differences among the cell walls of archaea, bacteria, and
eukaryotes.
The major component of bacterial cell walls is called peptidoglycan (or murein); it is only found in bacteria.
Structurally, peptidoglycan resembles a layer of meshwork or fabric (Figure 3.25). Each layer is composed of
long chains of alternating molecules of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). The
structure of the long chains has significant two-dimensional tensile strength due to the formation of peptide
bridges that connect NAG and NAM within each peptidoglycan layer. In gram-negative bacteria, tetrapeptide
chains extending from each NAM unit are directly cross-linked, whereas in gram-positive bacteria, these
tetrapeptide chains are linked by pentaglycine cross-bridges. Peptidoglycan subunits are made inside of the
bacterial cell and then exported and assembled in layers, giving the cell its shape.
Since peptidoglycan is unique to bacteria, many antibiotic drugs are designed to interfere with peptidoglycan
synthesis, weakening the cell wall and making bacterial cells more susceptible to the effects of osmotic
pressure (see Mechanisms of Antibacterial Drugs). In addition, certain cells of the human immune system are
able “recognize” bacterial pathogens by detecting peptidoglycan on the surface of a bacterial cell; these cells
then engulf and destroy the bacterial cell, using enzymes such as lysozyme, which breaks down and digests the
peptidoglycan in their cell walls (see Pathogen Recognition and Phagocytosis).
Figure 3.25
Peptidoglycan is composed of polymers of alternating NAM and NAG subunits, which are cross-linked by peptide bridges
linking NAM subunits from various glycan chains. This provides the cell wall with tensile strength in two dimensions.
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