Prokaryotic Cell
When cells were first visualized with microscopes, two basic cellular architectures were recognized: eukaryotic and prokaryotic,
the terms indicating the presence or absence, respectively, of a
membrane-bound nucleus that contains genetic material.
Prokaryotes are the simplest organisms. They consist of cytoplasm
surrounded by a plasma membrane, encased within a rigid
cell wall. Importantly, they have no distinct interior compartments
(figure below). A prokaryotic cell is like a one-room cabin in which
eating, sleeping, and watching TV all occur.
cell organization of a prokaryote. The nucleoid is visible as a
dense central region segregated from the cytoplasm. Some
prokaryotes have hairlike growths, called pili (singular, pilus), on
the outside of the cell.
Prokaryotic Cell Contains No Membrane-bound Organelles
Prokaryotes play a very important role in the ecology of living organisms. Some harvest light by photosynthesis; others break down dead organisms and recycle their components. Still others
cause disease or have uses in many important industrial processes.
There are two main domains of prokaryotes: archaea and bacteria.
Although prokaryotic cells do contain organelles such as
ribosomes, which carry out protein synthesis, most lack the
membrane-bounded organelles characteristic of eukaryotic cells.
It was long thought that prokaryotes also lack the elaborate cytoskeleton found in eukaryotes, but we have now found they
have molecules related to both actin and tubulin, which form two
of the cytoskeletal elements.The strength
and shape of the cell are determined by the cell wall, not by these
cytoskeletal elements (see figure above). However, cell-wall structure
is influenced by the cytoskeleton. For instance, the presence of
actin-like MreB fibers running the length of the cell can lead to a
rod-shaped cell when cell-wall fibers are arranged parallel to
MreB. When MreB protein is removed, these cells become
spherical. During cell division, cell-wall deposition is influenced
by the tubulin-like FtsZ protein.
The plasma membrane of a prokaryotic cell carries out
some of the functions organelles perform in eukaryotic cells. For
example, some photosynthetic bacteria, such as the cyanobacterium prochloron seen in figure below, have an extensively folded plasma membrane, with the folds extending into the cell’s interior. These membrane folds contain the bacterial pigments connected with photosynthesis. In eukaryotic plant cells,
photosynthetic pigments are found in the inner membrane of
the chloroplast.
bacterial cell. Extensive folded photosynthetic membranes
are shown in green in this false color electron micrograph of a
Prochloron cell.
Bacteria
Most bacterial cells are encased by a strong cell wall. This cell
wall is composed of peptidoglycon, which consists of a
carbohydrate matrix (polymers of sugars) that is crosslinked by short polypeptide units. Cell walls
protect the cell, maintain its shape, and prevent excessive
uptake or loss of water. The exception is the class Mollicutes,
which includes the common genus Mycoplasma, which lacks
a cell wall. Plants, fungi, and most protists also have cell
walls, but with a chemical structure quite different from
peptidoglycan.
The susceptibility of bacteria to antibiotics often
depends on the structure of their cell walls. The drugs penicillin and vancomycin, for example, interfere with the ability
of bacteria to cross-link the peptides in their peptidoglycan
cell wall. Like removing all the nails from a wooden house,
this destroys the integrity of the structural matrix, which
can no longer prevent water from rushing in and swelling the
cell to bursting.
Some bacteria also secrete a jelly-like, protective capsule of
polysaccharide around the cell. Many disease-causing bacteria
have such a capsule, which enables them to adhere to teeth, skin,
food, and practically any other surface that will support their
growth.
Archaea
We are still learning about the physiology and structure of archaea,
which do not have peptidoglycan cell walls. Many of these organisms are difficult to culture in the laboratory, and so this group has
not yet been studied in detail. More is known about their genome
than about any other feature.
The cell walls of archaea are composed of various chemical compounds, including polysaccharides polysaccharides and and proteins, and
possibly even inorganic components. A common feature distinguishing archaea from bacteria is the nature of their membrane lipids. The chemical structure of archaeal lipids is distinctly different from that of lipids in bacteria and can include saturated
hydrocarbons that are covalently attached to glycerol at both
ends, such that their membrane is a monolayer. These features
seem to confer greater thermal stability to archaeal membranes,
although the trade-off seems to be an inability to alter the degree
of saturation of the hydrocarbons—meaning that archaea with
this characteristic cannot adapt to changing environmental
temperatures.
The cellular machinery that replicates DNA and synthesized proteins in archaea is more closely related to eukaryotic
systems than to bacterial systems. Even though they share a
similar overall cellular architecture with prokaryotes, on a
molecular basis archaea appear to be more closely related to
eukaryotes.
Prokaryotic Cell Movement
"Many prokaryotic cells about by means of rotating flagella"
Flagella (singular, flagellum) are long, threadlike structures
protruding from the surface of a cell that are used in locomotion.
Prokaryotic flagella are protein fibers that extend far out from the
cell. There may be one or more per cell, or none, depending on the
species. Bacteria can swim at speeds of up to 70 cell lengths per
second by rotating their flagella like screws (figure below). The rotary
motor uses the energy stored in a gradient that transfers protons
across the plasma membrane to power the movement of the
flagellum. (Interestingly, the same principle, in which a proton
gradient powers the rotation of a molecule, is used in eukaryotic
mitochondria and chloroplasts by an enzyme that synthesizes ATP.)
flagella. The bacterial flagellum is a complex structure. The
motor proteins, powered by a proton gradient, are anchored in
the plasma membrane. Two rings are found in the cell wall. The
motor proteins cause the entire structure to rotate. As the
flagellum rotates, it creates a spiral wave down the structure.
This powers the cell forward.
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