THE PROKARYOTIC CELL
The
members of the prokaryotic world make up a vast het erogenous group of very
small unicellular organism. Prokaryotes include bacteria and archaea. The
majority of prokaryotes including the photosynthesizing cyanobacteria are
included in the bacteria.
SIZE, SHAPES OF BACTERIAL CELLS
SIZE
- 0.2 to 2.0 diameter and from 2 to
8 in length
SHAPES
COCCUS ---- SPHERICAL
BACILLUS ----- ROAD
COCCI ---- ROUND
STREPTOBACILLI ---- CHAIN
(a)
Division in one plane produces diplococci and streptococci.
(b)
Division in two planes produces tetrads.
(c)
Division in three planes produces sarcine.
(d)
Division in multiple plances produces staphylococci.
"Bacillus"
has 2 meanings in microbiology.as we have just used it bacillus refers to a
bacterial shape. Bacteriathat look like curved road are called vibrios.
 |
(a)
Vibrios (b) Spirillum (c) Spirochele
The spirilla, which use whiplike external appendages called flagella to move,
spirochetes move by means ofaxial fila-ments, which resemble flagella but are
contained within a flexible external
sheath. In addition to the three basic shapes, there are star shaped
cells (genus Stella); rectangular, flat cells (halophilic archaea) of the genus Haloarcula and triangular cells The shape of a bacterium is determined by
heredity. Genetically, most bacteria are monomorphic; that is, they.
STRUCTURES EXTERNAL CELL WALL
Structure
external to the prokaryotic cell wall are the glycocalyx, flagella, axial
filaments, fimbriae, pili.
GLYCOCALYX
Many prokaryotes secrete on their surface a substance called glycocalyx.
Glycocalyx (meaning sugar coat) is the general term used for substances that surround cells.The bacterial glycocalyx is a viscous (sticky), gelatinous
polymer that is external to the cell wall and composed of poly saccharide, polypeptide, or both. Its chemical
composition varies widely with the
species. For the most part, it is made inside the cell and secreted to the cell surface. If the stance is organized and is firmly attached to the cell
wall the glycocalyx is described as
a capsule. The presence of a capsule
can be determined by using negative stainin.stance is unorganized and only
loosely attached to the cell. wall, the glycocalyx is described as a slime
layer.In certain species, capsules are important in contributing to bacterial
virulence (the degree to which a pathogencauses disease). Capsules often
protect pathogenic bacteria from
phagocytosis by the cells of the host. (As you will see later, phagocytosis is
the ingestion and digestio microorganisms
and other solid particles.)
For example,
Bacillus anthracis produces a capsule of D-glutamic acid.Because only encapsulated B.
anthracis causes anthrax, it is
speculated that the capsule may prevent its being destroyed by phagocytosis.
Another example involves Streptococcus pneumoniae (strep-tô-kok'kus
nü-mo'ne-T), which causes pneumonia only
when the cells are protected by a polysaccharide cap-sule. Unencapsulated S.
pneumoniae cells cannot cause pneumonia
and are readily phagocytized. The polysaccha-ride capsule of Klebsiella
(kleb-se-el'lä) also prevents phagocytosis
and allows the bacterium to adhere to and colonize the respiratory tract. A glycocalyx made of sugars is called an extracellular polysaccharide (EPS). The EPS enables a bacterium to survive by attaching to
various surfaces in its natural environment in order to survive. Through
attachment, bacteria can grow on diverse surfaces such as rocks in fast-moving streams , plant roots,human teeth,
medical implants, water pipes, and even other bacteria. Streptococcus mutans (mū'tans), an important cause of
dental caries, attaches itself to the surface of teeth by a glycocalyx. S. mutans may use its capsule
as a source of nutrition by breaking
it down and utilizing the sugars
when energy stores are low. A glycocalyx also can protect a cell against dehydration, and its viscosity
may inhibit the movement of
nutrients out of the cell.
FLAGELLA
Some prokaryotic cells have flagella (singular: flagellum meaning whip), which
are long filamentous appendages that
propel bacteria. Bacteria that lack flagella are referred to as atrichous. Those that have flagella may have one
of four arrangements of flagella
monotrichous (a single polar
flagellum), amphitrichous (a tuft of flagella at each end of the cell), lophotrichous (two or more
flagella at one or both ends of the cell), and peritrichous (flagella distributed over the entire cell)
A flagellum has three basic parts.The long outermost region, the
filament, is constant in diameter and contains
the globular (roughly spherical) protein flagellin arranged in several chains that intertwine and form a
helix around a hollow core. In most
bacteria, filaments are not covered by membrane or sheath as in eukaryotic
cells.
AXIAL FILAMENTS
Spirochetes are a group of bacteria that have unique
structure and
motility. One of the best-known spirochetes is Treponema pallidum, the causative agent of syphilis. Another
spirochete is Borrelia burgdorferi,
the causative agent of Lyme disease.
Spirochetes move by means of axial filaments, or
endoflaella, bundles of fibrils that arise at the ends of the
cell beneath an outer sheath and spiral around the cell.Axial filaments, which
are anchored at one end of the spirochete, have a structure similar to that of
flagella. The rotation of the
filaments produces a movement of the outer sheath that propels the spirochetes in a spiral motion. This type of movement is similar to the way a corkscrew
moves through a cork. This corkscrew
motion probably enablesa bacterium
such as T. pallidum to move effectively through body fluids
(a)
A photomicrograph of the spirochete leptospira showing an
axial filament.
(b)
A diagram of axial filaments wrapping around part of a
spirochete.
(c)
A cross sectional diagram of the spirochete showing the
the position of axial filaments.
FIMBRIAE AND PILI
Many gram-negative bacteria contain hairlike appendages that are shorter,
straighter, and thinner than flagella and are used for attachment and transfer
of DNA rather than for motility.
These structures, which consist of a protein called pilin arranged helically around a central core,
are divided into two types, fimbriae and pili, having very different functions.
(Some microbiologists use the two terms interchangeably to refer to all such structures, but we distinguish
between them.)
Fimbriae (singular: fimbria) can occur at the poles of
the bacterial cell, or they can be evenly distributed over the entire surface of
the cell. They can number anywhere from
a few to several hundred per cell Like the glycocalyx, fimbriae enable a cell to adhere to
surfaces, including the surfaces of
other cells. For example, fimbriae attached
to the bacterium Neisseria gonomhoeae(n-sere-ago-nor-rẽ'ї), the causative agent
of gonorrhea, help the microbe
colonize mucous membranes. Once colonization occurs, the bacteria can
cause disease. When fimbriae are absent
(because of genetic mutation), colonization cannot happen, and no disease ensues.Pili (singular: pilus) are usually longer than
fimbriae jal and number only
one or two per cell. Pili join bacterial.
THE CELL WALL
The cell wall of the bacterial cell is a complex, semirigid structure responsible
for the shape of the cell.The cell wall surrounds the underlying, fragile plasma (cytoplasmic) membrane and protects it and the interior of the cell
from adverse changes in the outside
environment .Almost all prokarvotes
have cell walls.The major function of the cell wall is to prevent bacterial
cells from rupturing when the water pressure inside the cell is greater than that outside the cell. It also
helps maintain the shape of a bacterium and serves as a point of anchorage for
flagella. As the volume of a bacterial cell increases, its plasma membrane and cell wall extend as needed.Clinically, the cellwa is important because it
contributes to the ability of some species to cause disease and is the site of
action of some antibiotics. In addition the chemical composition of the cell wall is used to differentiate major
types of bacteria.
COMPOSITION AND CHARACTERISTICS
The bacterial cell wall is composed of a macromolecular
net-work called peptidoglycan (also known as murein), which is present either alone or
in combination with other substances.Peptidoglycan consists of a repeating disaccharide attached by polypeptides to form a lattice that
surrounds and protects the entire cell. The disaccharide portion is made up of monosaccharides called N acetylglucosamine (NAG)
and N-acetylmuramic acid (NAM) (from
murus, meaning wall),which are related to glucose. The structural formulas for NAG and NAM are shown.The various components of
peptidoglycan are assem bled in the
cell wall Alternating NAM and NAG
molecules are linked in rows of 10 to 65 sugars to form a carbohydrate "backbone" (the glycan
portion of peptidoglycan). Adjacent
rows are linked by polypeptides (the
peptide portion of peptidoglycan). Although the structure of the polypeptide link varies, it always
includes tetrapeptide side chains,
which consist of four amino acids at tached
to NAMs in the backbone. The amino acids occur in an alternating pattern of D and L forms.This is
unique because the amino acids found in other proteins are L forms. Parallel tetrapeptide side chains may be directly bonded to each other or linked by a
peptide cross-bridge, consisting of
a short chain of amino acids Penicillin interferes with the final linking of
the peptidoglycan rows by peptide cross-bridges.As a result, the cell wall is
greatly weakened and the cell
undergoes lysis, destruction caused by rupture of the plasma membrane and the
loss of cytoplasm.
(a)
The structure of peptidoglycan in gram positive bacteria.
(b)
A Gram positive cell wall
(c)
A Gram negative cell wall
GRAM-POSITIVE CELL WALLS
In most gram-positive bacteria, the cell wall consists of ny layers of
peptidoglycan, forming a thick,rigid structure.By contrast, gram-negative
cell walls contain only a thin layer of peptidoglycan In addition, the cell walls of gram-positive bacteria
contain teichoic acids, which consist primarily of an alcohol (such as glycerol or ribitol) and phosphate.
There are two classes of
Teichoic acids: lipoteichoic acid, which spans the
peptidoglycan layer and is linked to the plasma membrane, and wall teichoic acid, which is
linked to the peptidoglycan layer Because
of their negative charge (from the phosphate groups), teichoic acids may bind and regulate the
movemen of cations (positive ions)
into and out of the cell.They may
also assume a role in cell growth,
preventing extensive wall breakdown
and possible cell lysis. Finally, teichoic acids provide much of the wall's
antigenic specificity and thus make it possible to identify bacteria by certain laboratory tests .Similarly,
the cell walls of gram-positive streptococci are covered with various
polysaccharides that allow them to
be grouped into medically significant types.
GRAM-NEGATIVE CELL WALLS
The cell walls of gram-negative bacteria consist of one or a very few layers of
peptidoglycan and an outer membrane. The
peptidoglycan is bonded to lipoproteins (lipids covalently linked to
proteins) in the outer membrane and
is in the periplasm, a gel-like fluid between the outer membrane and the plasma
membrane.The periplasm contains a high
concentration of degradative enzymes and transport proteins.Gram-negative cell walls do not contain teichoic acids.Because the cell
walls of gram-negative bacteria
contain only a small amount of peptidoglycan,
they are more susceptible to mechar breakage.The outer membrane of the gram-negative cell consists of lipopolysaccharides (LPS), lipoproteins, and
phospholipidsThe outer membrane has several specialized functions. Its strong negative charge is an important factor in evading phagocytosis and the
actions of complement (lyses cells
and promotes phagocytosis), two components
of the defenses of the host.The outer membrane also provides a barrie to certain antibiotics (for example, penicillin),
digestive enzymes such as lysozyme,
detergents, heavy metals, bile salts and
certain dyes.
CELL WALLS AND THE GRAM STAIN MECHANISM
Now that you have studied the Gram stain and the chemistry of
the bacterial cell wall (in the previous section), it is easier to understand the
mechanism of the Gram stain.The mechanism is based on differences in the
structure of the cell walls of gram-positive and gram-negative bacteria and how each reacts to the various reagents (substances used for producing a
chemicalreaction). Crystal violet, the primary stain, stains both gram-positive and gram-negative cells purple because
the dye enters the cytoplasm of both
types of cells. When iodine (the mordant) is applied, it forms large crystals
with he dye that are too large to
escape through the cell wall he application of alcohol dehydrates the peptidoglycan
of gram-positive cells to make it more
impermeable to the crystal
violet-iodine. The effect on gram-negative cells is the different; alcohol dissolves the outer membrane
of gram-negative cells and even
leaves small holes in the thin peptidoglycan layer through which crystal
violet-iodine diffuse. Because
gram-negative bacteria are colorless after the alcohol wash, the addition of safranin (the counter-stain) turns the
cells pink. Safranin provides a contrasting color to the primary stain (crystal violet). Although gram positive and gram-negative cells both absorb safranin,
the pink color of safranin is masked
by the darker purple dye previously
absorbed by gram-positive cells. In
any population of cells, some gram-positive cells will give a
gram-negative response. These cells are usually quit.
ACID-FAST CELL WALLS
The acid-fast stain is used to are,identify all bacteria of
the genus Mycobacterium and pathogenic species of Nocardia. These bacteria
contain high concentrations (60%) of a hydrophobic waxy
lipid (mycolic acid) in their cell wall that prevents the uptake of dyes, including
those used in the Gram stain. The my colic
acid forms a layer outside of a thin layer of peptidoglycan.The mycolic acid
and peptidoglycan are held together
by a polysaccharide.The hydrophobic waxy cell wall causes both cultures of
Mycobacterium to clump and to stick
to the walls of the flask. Acid-fast bacteria can bestained with carbolfuchsin;
heating enhances penetration of the
stain.The carbolfuchsin penetrates the cell wall,binds to cytoplasm, and
resists removal by washing with acid-alcohol.
Acid-fast bacteria retain the red color of car bolfuchsin because it is more soluble in the cell wall
my colic acid than in the
acid-alcohol. If the mycolic acid layer is removed from the cell wall of
acid-fast bacteria,they will stain gram-positive with the Gram stain.Chemicals that
damage bacterial cell walls, or interfere with their synthesis, often do not harm the cells of an
animal host because the bacterial
cell wall is made of chemicals unlike those
in eukaryotic cells. Thus, cell wall synthesis is the target for some antimicrobial
drugs.One way the cell wall can be
damaged is by exposure to the digestive enzyme lysozyme.This enzyme occurs naturally in some eukaryotic cells
and is a constituent of tears,
mucus, and saliva. Lysozyme is particularly active on the major cell wall
components of most gram-positive
bacteria, making them vulnerable to lysis. Lysozyme catalyzes hydrolysis
of the bonds between the sugars in the repeating disaccharide
"backbone" of peptidoglycan. This act is analogous to cutting the
steel supports of a bridge with a
cutting torch: the gram-positive cell wall is almost completely destroyed by
lysozyme.The cellular contents that remain surrounded by the plasma membrane
may remain intact if lysis does not
occur; this wall-less cell is termed
a protoplast. Typically, a protoplast is spherical and is still capable of carrying on metabolism.Some members of the genus Proteus, as well as other genera, can lose their cell walls and swel into
irregularly.
STRUCTURES INTERNAL
Thus far, we have discussed the prokaryotic cell wall and structures external to
it. We will nowlook inside the prokarvotic
cell and discuss the structures and functions of the plasma membrane and components within the
cytoplasm of the cell.Describe the structure, chemistry, and functions of the prokaryotic plasma membrane Define simple diffusion,
facilitated diffusion, osmosis, active transport, and group translocation.The
plasma (cytoplasmic) membrane (or inner membrane) is a thin structure lying
inside the cell wall andenclosing the cytoplasm of the cell.The plasma membrane of prokaryotes consists primarily of
phospholipids which are the most
abundant chemicals in the membrane, and proteins. Eukaryotic plasma membranes
also contain carbohydrates and sterols, such as cholesterol. Because the lack sterols, prokaryotic plasma membranes are less
rigid than eukaryotic membranes. One exception is the wall less prokaryote
Mycoplasma, which contains membrane sterols.
STRUCTURE
In electron micrographs, prokaryotic and eukaryotic plasma membranes (and the
outer membranes of gram-negative bacteria)
look like two-layered structures;There are two dark lines with a light space
between the lines The phospholipid
molecules are arranged in two parallel
rows, called a lipid bilayer. As each phospholipid molecule contains:a polar head, composed of a phosphate group and glycerol that is hydrophilic (water-loving) and soluble in
water, and nonpolar tails, composed
of fatty acids that are hydrophobic (water-fearing) and insoluble in water
polar heads are on the two surfaces of the lipid bilayer,and the nonpolar tails
are in the interior of the bilayer.The
protein molecul arranged in a variety of ways. Some, called peripheral
proteins, are easily removed from the membrane by mild treatments and lie at
the inner or outer surface of the membrane.They
may function as enzymes that catalyze chemical reactions, as a "scaffold"
for support, and as mediators of changes
in membrane shape during movement.Other proteins,called
integral proteins, can be removed from the membrane only after disrupting
the lipid bilayer (by using detergents,for example).Most integral proteins penetrate
the membrane completely and are called transmembrane proteins.Some integral
proteins are channels that have a pore, or hole, through which substances enter and exit the cell.Many of the proteins and some of the lipids on the
outer surface of the plasma membrane
have carbohydrates attached to them. Proteins attached to carbohydrates are called glycoproteins; lipids attached to carbohydrates
are called glycolipids. Both
glycoproteins and glycolipids help protect and lubricate the cell and are
involved in cell-to-celles in the membrane can be interactions. For example, glycoproteins play a role
in certain infectious diseases.The influenza virus and the toxins that cause cholera and botulism enter their target
cells by first binding to
glycoproteins on their plasma membranes.Studies
have demonstrated that the phospholipid and protein molecules in membranes are not static but move quite freely within the membrane surface. This
movemen is most probably associated with the many functions performed by
the plasma membrane. Because the fatty acidils cling together, phospholipids in
the presence of water a self-sealing
bilayer, with the result that breaks and tears in the membrane will heal themselves.The membrane about as viscous as olive oil which allows proteins to move freely enough to perform their ions without destroying the structure of the
membrane.This dynamic arrangement of phospholipids and proteins is referred to as the fluid mosaic model.
FUNCTIONS
The most important function of the plasma membrane is to serve as a selective
barrier through which materials enter and
exit the cell In this function plasma membranes have selective permeability (sometimes called
semipermeability).This term indicates that certain molecules and ions pass through the membrane, but others are prevented from
passing through it. The permeability of the membrane depends on several factors. Large molecules (such as proteins)
cannot pass through the plasma membrane, possibly because these molecules are larger than the pores in integral
proteins that function as channels. But smaller molecules (suchas water,
oxygen, carbon dioxide, and some simple sugars).What is the function of
chromatophores!? usually pass
through easily. Ions penetrate the membrane.slowly. Substances that dissolve easily in lipids (such as very dioxide,
and nonpolar organic molecules) oxygen,
carbon
enter and exit more easily than other substances because the membrane consists
mostly of phospholipids.The movement of materials across plasma membranes also
depends on transporter molecules,
which will be described shortly Plasma
membranes are also important to the breakdown of nutrients and the production
of energy. The plasma membranes of
bacteria contain enzymes capable of catalyzing the chemical reactions that
break down nutrients and produce ATP. In some bacteria, pigments and en-zymes
involved in photosynthesis are found in infoldings of the plasma membrane that extend into the cytoplasm.These membranous structures are called
chromatophores or thylakoids.When
viewed with an electron microscope, bacterial plasma membranes often appear to contain one or more large, irregular folds called mesosomes. Many
functions have been proposed for mesosomes. However, it is now known that
they are artifacts, not true cell structures.Mesosomes are believed to be folds
in the plasma membrane that develop by the process used for preparing specimens
for electron microscope.
DESTRUCTION OF THE PLASMA MEMBRANE
BY ANTIMICROBIAL AGENTS
Because the plasma membrane is vital to the bacterial cell,it
is not surprising that several antimicrobial agents exert their effects at this
site. In addition to the chemicals that damage the cell wall and thereby indirectly expose the membrane to injury, many compounds specifically
damag plasma membranes.These compounds include certain alcohols and
quaternary ammonium compounds, which are used as disinfectants. By disrupting the membrane's phospholipids, a
group of antibiotics known as the polymyxins cause leakage of intracellular contents and subsequent
cell death.
THE MOVEMENT OF MATERIALS ACROSS MEMBRANES
Materials move across plasma membranes of both prokaryotic
and eukaryotic cells by two kinds of processes: passive and active. In passive
processes, substances cross the membrane from an area of high concentration to
an area of low concentration (move with the concentration gradient, or difference), without any expenditure of energy (ATP)
by the cell. In active processes,
the cell must use energy (to move substances from areas of low concentration to
areas high concentration (against
the concentration gradient) Passive Processes Passive processes include simple
diffision, facilitated diffusion, and osmosis. Simple diffusion is the net
(overall) movement of molecules or ions from an area of high concentration
to an area of low concentration.The
movement continues until the
molecules or ions are evenly distributed. The point of even distribution is
called equilibrium Cells rely on
simple diffusion to transport certain small molecules, such as oxygen and carbon dioxide, across their cell membranes.In facilitated diffusion, the substance
(glucose, for example) to be trarısported combines with a plasma membrane
protein called a transporter (sometimes called a permease). In one proposed
mechanism for facilitated diffusion,transporters bind a substance on one side
of the membrane and, by changing
shape, move it to the other side of the membrane, where it is released.Facilitate diffusion is similar to simple diffusion in that the
cell does not need to expend energy
because the substance move from a
high to a low concentration. The process differs from simple diffusion in its use of transporters.
What is osmosis?
In some cases, molecules that bacteria need are too large to be transported
into the cells by these methods.Most bacteria, however, produce enzymes that
can breakdown large molecules into simpler ones (such as proteins into amino acids, or polysaccharides into simple
sugars) Such enzymes, which are
released by the bacteria into the surrounding
medium, are appropriately called extracellular enzymes. Once the enzymes degrade the large
molecules,the subunits move into the cell with the help of transporters. For
example, specific carriers retrieve DNA bases, such as the purine guanine, from
extracellular media and bring them
into the cell's cytoplasm Osmosis is
the net movement of solvent molecules across
a selectivelv permeable membrane from an area with a high concentration of solvent molecules (low
concentration of solute molecules) to an area of low concentration of solvent
molecules (high concentration of solute molecules). In living systems, the chief solvent is water Osmosis may be demonstrated with the apparatus.A sack
constructed from cellophane, such as the purine guanine, from extracellular
media and bring them into the cell's
cytoplasm.Osmosis is the net movement of
solvent molecules across a
selectivelv permeable membrane from an area with a high concentration of solvent molecules.(low concentration of
solute molecules) to an area of low concentration of solvent molecules (high
concentration of solutemolecules). In living systems, the chief solvent is
water.Osmosis may be demonstrated with
the apparatus. A sack constructed
from cellophane,which is a selectively permeable membrane, is filled with a solution of 20% sucrose (table sugar). The cellophane
sack is placed into a beaker containing distilled water. Initially the concentrations of water on either side of the
membrane are different. Because of the sucrose molecules, the concentration of water is lower inside the cellophane
saclk Therefore, water moves from
the beaker (where its concentration is higher) into the cellophane sack (where
its concentration is lower).There is no movement of sugar out of the cellophane sack into the beaker, however, because the cellophane
is in permeable to molecules of sugar-the sugar molecules are too large to go through the pores of the membrane. As water moves into the cellophane sack, the sugar
solution becomes increasingly dilute, and, because the cellophane sack has expanded to its limit as a result of an
increased volume of water, water
begins to move up the glass tube. In time,
the water that has accumulated in the cellophane sack and the glass tube exerts a downward pressure
that forces water molecules out of
the cellophane sack and back into the beaker. This movement of water
through a selectively permeable membrane produces a pressure called osmotic
pressure. Osmotic pressure is the pressure required to prevent the movement of pure water (water with no solutes) into a solution containing some solutes. In
other words, osmotic pressure is the
pressure needed to stop the flow of
water across the selectively perımeable membrane.(cellophane). When water
molecules leave and enter the cellophane
sack at the same rate, equilibrium is reached.A bacterial cell may be subjected to any of three kinds of osmotic solutions: isotonic, hypotonic, or
hypertonic An isotonic solution is a
medium in which the overall concentration of solutes equals that found
inside a cell (isomeans equal). Water leaves and enters the cell at the same rate (no net change); the cell's contents are in
equilibrium with the solution
outside the cell wall. Earlier we mentioned that lysozyme and certain
antibiotics damage bacterial cell walls, causing the cells to rupture, or lyse.
Such rupturing occurs because bacterial cytoplasm usually contains such a high concentration of solutes that, when the wall is weakened or removed,
additional water enters the cell by osmosis. The damaged (orremoved) cell wall
cannot constrain the swelling of the cytoplasmic
membrane, and the membrane bursts. This is an example of osmotic lysis
caused by immersion in a hypotonic cell
is a medium whose concentration of solutes is lower than that inside the cell (hypbacteria live in
hypotonic solutions, and swelling is contained by the cell wall. Cells with
weak cell walls, such as gram-negative
bacteria, may burst or undergo osmotic lysis as a result of excessive water intake.
CYTOPLASM
For a prokaryotic cell, the term cytoplasm refers to the substance of the cell
inside the plasma membrane . cytoplasm is about 80% water and contains primarily proteins (enzymes), carbohydrates , lipids,
inor ganic ions, and many
low-molecular-weight compounds norganic
ions are present in much higher concentrations in cytoplasm than in most media. Cytoplasm is thick aqueous, semitransparent, and elastic. The major
structures in the cytoplasm of prokaryotes are a nuclear area(containing DNA),
particles called ribosomes, and reserve posits called inclusions. Protein
filaments in the cytoplasm are most likely responsible for the rod and helical cell shapes of bacteria. Prokaryotic cytoplasm lacks certain features of
eukaryotic cytoplasm, such as a cytoskeleton and cytoplasmic streaming.
These features will be described later
THE NUCLEAR AREA LEARNING OBJECTIVE
Identify the functions of the nuclear area,ribosomes, and inclusions.The nuclear
area, or nucleoid, of a bacterial cell usually contains a single long,
continuous, and frequently
circularly arranged thread of double-stranded DNA called the bacterial chromosome. This is the
cell's genetic information, which
carries all the information required for the cell's structures and functions.
Unlike the chromosomes of eukaryotic cells, bacterial chromosomes are not surrounded by a nuclear envelope (membrane) and do not include histones. The nuclear area can be
spherical,elongated, or dumbbell-shaped. In actively growing bacteria,as much
as 20% of the cell volume is occupied by DNA because such cells presynthesize
nuclear material for future cells.
The chromosome is attached to the plasma membrane.Proteins in the plasma
membrane are believed to be responsible for replication of the DNA and
segregation of the new chromosomes
to daughter cells during cell division In addition to the bacterial chromosome, bacteria often contain small usually circular, double-stranded
DNA molecules called plasmids (see
the F factor in FigureThese molecules are Extra chromosom algenetic
elements; that is, they are not connected to the main bacterial chromosome, and they replicate
independently of chromosomal DNA.Research indicates that plasmids are
associated with plasma membrane proteins.lasmids usually contain from 5 to 100
genes that are generally not crucial for the survival of the bacterium under normal environmental conditions; plasmids may be
gained or lost without harming the
cell. Under certain conditions,however,
plasmids are an advantage to cells. Plasmids may carry genes for such
activities as antibiotic resistance,tolerance to toxic metals, the production
of toxins, and the synthesis of enzymes. Plasmids can be transferred
from one bacterium to another. In
fact, plasmid DNA is used for gene
manipulation in biotechnology.
RIBOSOMES
All eukaryotic and prokaryotic cells contain ribosomes,which
function as the sites of protein synthesis. Cells that have high rates of
protein synthesis, such as those that are vely growing, have a large number of ribosomes.The cytoplasm of a prokaryotic cell contains tens of
thousands of these very small
structures, which give the cytoplasm a granular appearance.Ribosomes are
composed of two subunits, each of which
consists of protein and a type of RNA called ribosomal RNA (rRNA). Prokaryotic ribosomes differ
from eukaryotic ribosomes in the
number of proteins and rRNA molecules they contain; they are also somewhat
smaller and less dense than
ribosomes of eukaryotic cells. Accordingly, prokaryotic ribosomes are called
70S ribosomes and those of
eukaryotic cells are known as 80S
ribosomes. The letter S refers to Svedberg units,which indicate the relative
rate of sedimentation during ultra-high-speed
centrifugation. Sedimentation rate is a function of the size, weight, and shape of a particle. The subunits
of a 70S ribosome are a small 30S subunit containing one molecule of rRNA and a
larger 50S subunit containing two
molecules of rRNA Several
antibiotics work by inhibiting protein synthesis on prokaryotic ribosomes.
Antibiotics such as streptomycin and gentamicin attach to the 30S subunit
and interfere with protein synthesis. Other antibiotics, such as erythromycin and chloramphenicol, interfere with
protein synthesis by attaching to
the 50S subunit. Because of differences in prokaryotic and eukaryotic ribosomes,
the microbial cell can be killed by the antibiotic while the eukaryotic host cell remains unaffected.
INCLUSIONS
Within the cytoplasm of prokaryotic cells are several kinds of reserve deposits,
known as inclusions. Cells may accumulate certain nutrients when they are
plentiful and use them when the
environment is deficient. Evidence suggests that macromolecules concentrated in
inclusions avoid the increase in
osmotic pressure that would result if the
molecules were dispersed in the cytoplasm. Some in clusions are common to a wide variety of bacteria,
whereas others are limited to a small number of species and there fore
serve as a basis for identification.
METACHROMATIC GRANULES
Metachromatie granules are large inclusions that take their namefrom the fact
that they sometimes stain red with certain
blue dyes such as methylene blue. Collectively they are known as volutin. Volutin represents a reserve of
inor- Tganic phosphate (polyphosphate) that can be used in the synthesis of ATP. It is generally formed by cells that
grow in phosphate-rich environments. Metachromatic granules are found in algae, fungi, and protozoa, as well as in
bacteria.These granules are characteristic of Corynebacterium diphrtheriae
(kô-rĩ-ne-bak-ti're-um dif-thi'rē-ī), the causative agent of diphtheria; thus, they have diagnostic
significance.
POLYSACCHARIDE GRANULES
Inclusions known as polysaccharide granules typically consist of glycogen and
starch, and their presence can be demonstrated
when iodine is applied to the cells. In the presence of iodine, glycogen granules appear reddish brown and starch granules appear blue.
LIPID INCLUSIONS
Lipid inclusions appear in various species of Mycobacterium Bacillus, Azotobacter
(а-25-to-bak, tér), Spirillum (spril, lum) and other genera. A common
lipid-storage material, one unique
to bacteria, is the polymer poly-B-hydroxybutyric acid. Lipid inclusions
are revealed by staining cells with fat-soluble
dyes, such as Sudan dyes
SULFUR GRANULES
Certain bacteria-for example, the "sulfur bacteria"
that belong
to the genus Thiobacillus--derive energy by oxidizing sulfur and
sulfur-containing compounds. These bacteria may deposit sulfur granules in the cell, where they serve as an energy reserve
CARBOXYSOMES
Carboxysomes are inclusions that contain the enzymeribulose
1,5-diphosphate carboxylase. Photosynthetic bacteria use carbon dioxide as
their sole source of carbon andrequire this enzyme for carbon dioxide fixation.
Among the bacteria containing carboxysomes are nitrifying bactria,
cyanobacteria, and thiobacilli.
GAS VACUOLES
Hollow cavities found in many aquatic
prokaryotes cluding cyanobacteria, anoxygenic photosynthetic bacteria, and
halobacteria are called gas vacuoles. Each vacuole consists of rows of several individual gas
vesicle which are hollow cylinders
covered by protein. Gas vacuoles maintain buoyancy so that the cells can remain
at the depth in the water
appropriate for them to receficient amounts of oxygen, light, and nutrients.chromosome
and a small portion of cytoplasm are
isolated by an ingrowth of the
plasma membrane called a spore septum. The spore septum becomes a
double-lavered membrane that surrounds the chromosome and cytoplasm. This structure, entirely enclosed
within the original cell, is called a forespore. Thick layers of peptidoglycan are laid down between the two membrane layers. Then a thick spore coat of protein
forms around the outside membrane;
this coat is responsible for the
resistance of endospores to many harsh chemicals. The original cell is degraded, and the endospore is
released The diameter of the
endospore may be the same as,smaller than, or larger than the diameter of the
vegetative cell. Depending on the
species, the endospore might be located terminally (at one end), subterminally
(near one end; or centrally inside the vegetative cell. When the endospore matures, the vegetative cell wall
ruptures (lyses), killing the cell,
and the endospore is freed Most of
the water present in the forespore cytoplasm is eliminated by the time sporulation is complete, and
endospores do not carry out metabolic reactions. The highly dehydrated
endospore core contains only unts of
RNA, ribosomes, enzymes, and a few important small molecules. The latter
include a strikingly large amount of an organic acid called dipicolinic
acid (found in the cytoplasm), which
is accompanied by a large number of
calcium ions. These cellular components are essential for resuming metabolism later and is known as DNA, small Endospores can
remain dormant for thousands of years,An endospore returns to its vegetative
state by a process called
germination. Germination is triggered by physical or chemical damage to the endospore's coat. The
endospore's enzymes then break down
the extra layers surrounding the endospore,
water enters, and metabolism resumes.Because one vegetative cell forms a
single endospore, which, after germination,
remains one cell, sporulation in bacteria is not a means of reproduction.This process does not
increase the number of cells.Bacterial endospores differ from spores
formed by (prokaryotic) actinomycetes and the eukaryotic fungi and algae, which detach from the
parent and develop into another
organism and, therefore, represent reproduction Endospores are important from a clinical viewpoint and in the food industry because they are resistant to processes that normally kill vegetative cells. Such processes include heating, freezing, desiccation, use
of chemicals, and radiation. Whereas most vegetative cells.are killed by temperatures above 70°C, endospores can
survive in boiling water for several hours or more. Endospores of thermophilic (heat-loving) bacteria can survive in
boiing water for 19 hours. Endospore-forming bacteria are a problem in the food industry because they are likely
to survive underprocessing, and, if conditions for growth occur some
species produce toxins and disease. Special methods for controlling organisms that produce endospores.
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