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• All living cells have a cell (plasma) membrane,
genetic information is stored in DNA and all are
classified either as prokaryotic or eukaryotic.
– prokaryotes lack a nucleus and other membraneenclosed structures, all are bacteria (archeobacteria are
important ecologically).
– eukaryotes have various membrane-enclosed structures,
include all plants, animals, fungi, and protists. We will
study some fungi and various parasites, plus
interactions between microbes and eukaryotes.
– Differences between prokaryotes and eukaryotes are
important when looking for ways to control diseasecausing microbes. Treatments generally have to
damage prokaryotic cells (bacteria) while leaving our
eukaryotic cells unscathed.
– Viruses do not fit in either category, they are acellular
– See Table 4.1 (page 73)
Prokaryotes are very small – most range from 0.5 to 2.0 micrometers – but there
are always exceptions. Because of their small size, bacteria have a large
surface-to-volume ratio. No internal part of the cell is very far from the surface
and nutrients can reach. Example, bacteria with a diameter of 2μm have a
surface area of 12μm2 and a volume of 4μm3 which give a surface-to-volume
ratio of 3:1. In contrast, large eukaryotic cells need internal membrane since
their surface-to-volume ratio is much smaller.
Most bacteria come in three basic shapes – spherical, rodlike, and
spiral – with variations. Even bacteria of the same kind
sometimes vary in size and shape due to conditions like
availability of nutrients, or aging cultures with build up of waste
products. Some bacteria vary widely even within a single culture
– pleomorphism.
Many bacteria are found in
characteristic arrangements due
to the way they divide and
whether they separate.
Cocci that divide in one plane
form pairs or chains
Division in two planes forms
tetrads.
Division in three planes froms
sarcina
Prokaryotes divide by
binary fission (not
sexual), not mitosis or
meiosis. (see Table 4.1)
Checklist page 76
Random division planes form
grapelike clusters (staphylo)
Bacilli divide in only one plane
but can remain attached .
Bacteria cells consist of
1. A cell membrane, usually surrounded by a cell wall and
sometimes by an outer layer
Fig. 4-3
2. An internal cytoplasm with ribosomes, a nuclear region, and
sometimes granules and/or vesicles
3. A variety of external structures
capsules, flagell, pili
A semi-rigid cell wall is found
outside the cell membrane in
nearly all bacteria.
Two major functions include
1.
it maintains the shape of the
cell
2.
it prevents the cell from
bursting due to osmotic
pressure (described later).
The cell wall usually does not
regulate the entry and exit of
material from the cell. The
plasma membrane does that.
The cell wall is usually very
porous.
Peptidoglycan is the most important
component of the cell wall. It
is extremely large. Gram
positive bacteria can have up
to 40 layers.
Fig. 4-6
Peptidoglycan is
composed of sugar chains
(glycan) joined by
peptides (small sequences
of amino acids).
The sugar backbone
consists of two alternating
sugar molecules.
The peptide crosslinker is
a tetrapeptide.
Gram-positive organisms
also have teichoic acid, a
very long polymer that
extends beyond the rest of
the cell wall.
Fig. 4-6
The outer membrane is found mainly in gram-negative bacteria.
Proteins called porins form large channels. Little control over
transport in and out of the cell. Again, the cell membrane does
Fig. 4-5
that
Lipopolysaccharide (LPS), also called endotoxin, can be used to identify
gram-negative bacteria. It is released only when the cell wall is broken
down. LPS consists of sugars (polysaccharides) sticking out from the
cell wall and lipid A, which holds LPS in the outer membrane. Lipid A is
toxic, it causes fever and dilates blood vessels, leading to a drop in blood
pressure. LPS is released only when gram-negative bacteria are dying.
So antibiotics given late in an infection can even lead to the death of the
patient.
Fig. 4-6
Gram-positive bacteria have a thick
layer of peptidoglycan closely attached
to the cell membrane. If it is digested the
cell becomes a protoplast (no cell wall).
Gram-negative bacteria have a thinner
but more complex cell wall, including
an outer membrane and a periplasmic
space which contains enzymes that
protect the bacteria – make it less
susceptible to antibiotics. If cell wall is
digested the cell becomes a spheroplast
with both a cell membrane and most of
the outer membrane.
Acid-fast bacteria have a thick wall
made up of lots of lipid. They grow
slowly and stain gram-positive.
• Mycoplasma have no cell walls. Their cell
membranes are reinforced with sterols, like
eukaryotes (we have cholesterol in our cell
membranes).
• When treating with antibiotics that block cell wall
construction, some bacteria may survive in the
form of L-forms (bacteria that no longer form cell
walls). After treatment is stopped these bacteria
can revert back to walled form and go on to regrow an infecting population.
• Checklist on page 80
• The cell membrane is dynamic and constantly
changing, unlike the static cell wall.
• It is made up primarily of phospholipids and
proteins (a mosaic), both constantly moving
(fluid) – fluid-mosaic model.
• The main function of the cell membrane is to
controls what goes in and what comes out of the
cell.
• Bacterial cell membranes synthesize cell wall
components, assist with DNA replication, secrete
proteins, carry on respiration, capture energy as
ATP, etc.
Phospholipids form the
lipid bilayer
Fluid Mosaic Model
Fig. 4-7
Internal Structure: The cytoplasm of prokaryotic cells is semifluid,
about 4/5 water plus enzymes and other proteins, carbohydrates,
lipids, and ions. Chemical reactions take place in the cytoplasm,
including protein syntheisis on
ribosomes.
Ribosomes consist of RNA and protein. They are often seen in
the cytoplasm as polyribosomes. They include two subunits, the
large 70S subunit and the small subunit. Eukaryotic cells have a
larger ribosome (80S). Here is a good target for antibiotics since
it is different than our eukaryotic ribosomes.
Target the 70S ribosomal
subunit and disrupt
protein synthesis in
bacteria.
A defining feature of prokaryotic cells is the lack of a nucleus.
Bacteria have a nuclear region or nucleoid centrally located
containing one circular DNA chromosome. There are
exceptions. Figure 4.9 Some bacteria also contain smaller
circular DNA molecules called plasmids. These often contain
genes for antibiotic resistance.
Prokaryotes lack intracellular membrane-bound organelles.
However, there are exceptions. Photosynthetic bacteria and
cyanobacteria contain internal membrane systems derived from
the cell membrane. These contain pigments used to capture
energy from light for making sugars – carbohydrates. Figure
4.10
• Vegetative cells (cells that are actively metabolizing
nutrients) produce resting stages called endospores. One
bacteria makes one spore in its cytoplasm, often when
nutrients are scarce.
• They are highly resistant to heat, drying, acids, bases, etc.
and allow for the survival of bacteria when vegetative cells
cannot survive. Killing endospores in food is important.
Figure 4.11
• Endospores can survive adverse environmental conditions
for long periods of time. When the conditions are more
favorable, endospores germinate and begin to develop into
functional vegetative cells.
Exterior structures include flagella and pili,
capsules and slime layers.
About ½ of known bacteria are motile, and
move with speed and direction, usually by
means of flagella. Bacteria with a single
polar flagellum are monotrichous; with two,
one at each end are amphitrichous; with two or
more at one or both ends are lophotrichous; and
those with flagella all over are peritrichous.
Bacteria with no flagella are atrichous. Cocci
rarely have flagella. See Figure 4.12.
Prokaryotic flagellum have a diameter about
1/10th that of eukaryotes.
Fig. 4-13
Gram negative
Gram positive
Most flagella rotate. When they rotate counterclockwise, they bundle,
and the bacteria move in a straight line (“run”). When the flagella rotate
clockwise, the bundle comes apart, and the bacteria “tumble” randomly.
Tumbles allow random
changes in direction. Runs
allow bacteria to cover
more ground in the
Molecules in the cell
preferred direction
membrane detect
attractants and repellants.
(chemotaxis).
Chemotaxis is moving toward
(positive) or away from
(negative) a high concentration
of some substance or molecule.
Example: Bacteria will move
toward a nutrient. They move
up the concentration gradient
toward the area of high glucose
concentration
Fig. 4-14
• Spirochetes have axial filaments which lie between the
outer sheath and the cell wall. When these filaments
rotate, the body of the spirochete moves like a cork screw
(Figure 4.15).
• Pili are tiny, hollow projections used to attach bacteria to
surfaces, not for movement.
– Conjugation pili (sex), found in certain groups of bacteria, attach
two cells together and allow transfer of genetic DNA. This adds
genetic variety and allows transfer of genes important to bacteria,
including antibiotic resistant genes (Figure 4.16).
– Attachment pili help bacteria attach to surfaces. These pili
contribute to pathogenicity since they allow bacteria to adhere and
form colonies on cell surfaces within an organism.
• Example, Neisseria gonorrhoeae adheres to the epithelial cells of the
urogenital system. N gonorrhoeae without pili rarely cause disease.
• Glycocalyx = all polysaccharide-containing substances
found external to the cell wall, including the thick capsule
to the thin slime layers.
– A capsule is a protective structure outside the cell wall secreted by
the bacteria. Usually consist of complex polysaccharide molecules
in a loose gel. Each capsule is unique to the strain of bacteria.
Capsules also contribute to pathogenicity since they help the
bacteria evade the immune responses such as phagocytosis.
• Example, when a bacteria loses its ability to secrete a capsule, it often
loses its ability to cause disease.
– A slime layer is less tightly bound to the cell wall and is usually
thinner than a capsule. It protects the cell from drying and allows
bacteria to adhere to objects like your teeth (dental plaque).
– Checklist on page 89.
Eukaryotic Cells are larger and more complex with a diameter of more than 10 microns. They
contain a variety of highly specialized structures and compartments and are the basic unit of all
organisms in the kingdoms protista, plantae, fungi, and animalia.
The plasma membrane has the same
fluid-mosaic structure as prokaryotic
cells. It contains a greater variety of
lipids (sterols) and is functionally less
versatile – specific functions are
delegated to membrane-bound
organelles. Example: Mitochondrial
membranes contain the enzymes
necessary for making ATP, not the
plasma membrane of eukaryotic cells.
The cytoplasm is much like the
prokaryotic cytoplasm except that it
contains components of the
cytoskeleton.
The nucleus is surrounded by the
nuclear membrane (double) and
contains nucleoplasm, nucleoli, and
paired chromosomes.
Fig. 4-18
The nuclear envelope
consists of a double
membrane with nuclear
pores which allow specific
molecules to enter and
leave.
The gel within the nucleus
is called nucleoplasm
(instead of cytoplasm).
The nucleoli contain RNA
and proteins needed to
construct ribosomes
(protein factories).
Also contains
chromosomes (DNA plus
structural and regulatory
proteins) which are in the
form of chromatin unless
the cell is dividing.
The nuclei of eukaryotic cells divide by
The cytoskeleton forms a
spindle apparatus which
guides the movement of
chromosomes.
Fig. 4-20
Duplicated, condensed
chromosomes
During sexual reproduction, the nuclei of
sex cells divide by meiosis.
One replication, two divisions
Since the two chromosome pairs
are positioned together, they are
separated into two nuclei with
only one of each chromosomes
(haploid). These cells go on to
divide again, forming gametes
with one copy of one of each of
the chromosome pairs.
Mitochondria (power plant) have an inner and an outer membrane.
The fluid-filled interior is called the matrix. The inner membrane is folded
into cristae. The mitochondria contains some DNA, replicates on its own,
and contains the enzymes needed for oxidation of “food” into ATP, the
energy form used by the cell for most of their activities.
Fig. 4-21
Chloroplasts contain an inner and an outer membrane also. They also contain
DNA and replicate independently. Chloroplasts capture the energy from light
during photosynthesis.
Fig. 4-22 a&b
Ribosomes in eukaryotes are
larger than in prokaryotes and
are assembled in the
nucleolus. Ribosomes are
protein factories. Some
make proteins in the cytosol,
others make proteins on the
ER – for export or for
insertion into membrane.
Endoplasmic reticulum
(ER) is an extensive system
of membranes, smooth (lipid
synthesis) and rough (protein
synthesis). Vesicles transport
lipids and proteins to the
golgi.
Golgi Apparatus
Lysosomes
Cytoskeleton
Fig. 4-18
Golgi Apparatus receives
vesicles from the ER.
Modifies the proteins and the
lipids, “labels and packages”
them for export to the
appropriate location via
vesicles. If a protein is to be
secreted, it is transported from
the Golgi via secretory
vesicles to the plasma
membrane
Lysosomes contain digestive
enzymes. Lysosomes fuse
with vesicles containing
material to be digested.
Cytoskeleton is a network of
protein fibers including
microtubules and
microfilaments which gives
Shape and mechanical support and is involved in movement of organelles inside the
cell and movement of the cell itself.
Fig. 4-18
External structure of eukaryotic cells include flagella, cilia, and cell walls.
Eukaryotic flagella are larger and
more complex than prokaryotic
flagellum. They consist of 2 central
and 9 peripheral pairs of microtubules.
Each microtubule is about the size of
the prokaryotic flagellum. Dynein
proteins use ATP energy to change
shape, causing the microtubules to slide
across each other and creating a wave
like motion. Flagella are most common
among protozoa. Spermatozoa are the
only human flagellated.
Cilia are shorter and more numerous
but have the same chemical
composition and basic arrangement.
Cilia are found mainly among protozoa.
Cilia move in a coordinated manner,
like oars on a large boat. Cilia are
found on the cells of our bronchial tract.
Fig. 4-23
Fig. 4-25
Eukaryotic cell walls do not contain
petidoglycan like bacteria. Instead
Algal cell walls consist of cellulose,
fungal cell walls consist of cellulose
and/or chitin. Cell walls give the cell
rigidity and protect it from osmotic
pressures which can burst a cell.
Cells without cell walls can move by
forming pseudopodia. The plasma
membrane extends in the direction of
the movement. The cytoplasm
streams into the new area and the
trailing plasma membrane “pulls in”.
The proteins in the plasma membrane
must be able to “hold on to” a solid
surface.
• The endosymbiotic theory offers a possible explanation for
the evolution of prokaryotic cells to the more complex
eukaryotic cells.
– 1st a prokaryotic cell developed a nucleus when an invagination
surrounded the DNA
– 2nd This new eukaryotic cell engulfed a bacteria which could use
oxygen to make ATP. This became mitochondria and/or a
eukaryotic cell engulfed a photosynthetic bacteria, which
eventually became a chloroplast.
– Evidence for this scenario includes
•
•
•
•
They are the same size as bacteria
They have their own DNA in a circle, like bacteria
They have their own ribosomes which are like prokaryotic ribosomes.
They carry out protein synthesis like bacteria , independently of the
cell.
• They replicate by binary fission, independently of the cell.
• They have a double-membrane.
The movement of substances across membranes: a cell membrane
separates a cell from its environment, carefully regulating what comes in
and what goes out. Lipid membranes like the plasma membrane are
semipermeable (selectively permeable).
Very small polar substances, water, small ions, probably pass through
pores.
Some nonpolar substances, lipids dissolve in and pass through the
membrane lipids.
Most substances are moved through the membrane in a controlled way by
specific carrier proteins. This process can be active (require energy) or
passive.
Passive transport moves substances
from high concentration to low
concentration only – down its
concentration gradient. This
includes simple diffusion,
facilitated diffusion and osmosis.
Fig. 4-28
Simple Diffusion
Simple diffusion occurs because of random movements of
particles. As the molecules move randomly, they eventually reach
equal concentration everywhere if there is no barrier. The net effect
is that molecules diffuse from high concentration to low
concentration.
Materials diffuse through small prokaryotic cells quickly, allowing
nutrients in and wastes out. Eukaryotic cells are larger and have
many membranes. Although many substances do diffuse in and out
of the cell, much of the movement of molecules across the cell
membrane is controlled by transport systems in these cells.
Fig. 4-29
Facilitated diffusion is diffusion down a
concentration gradient and across a
membrane with the assistance of special
pores or carrier proteins. Saturation occurs
when all the carriers are moving the
diffusing molecules/ions as fast as they can.
The rate of diffusion reaches a maximum, it
can not increase further.
Many different kinds of pores and carrier
proteins exist. These are specific for one or
a few molecules or ions.
Remember passive transfer does not require
energy from ATP.
Osmosis is the diffusion of water
molecules across a semi-permeable
membrane.
a. Sugar molecules
can not pass through
the membrane.
b/c. The net movement of water is
into the sugar solution because the
concentration of water there is
lower.
Fig. 4-30
The important thing for
a microbiologist to
know is how particles
dissolved in fluid affect
microorganisms.
Cells burst in hypotonic
solutions and shrink in
hypertonic solutions.
Bacteria have cell walls
which protect them
from these reactions to
some degree.
Fig. 4-31
Active transport requires energy (usually ATP). Active transport uses
energy to move molecules or ion against their concentration gradient.
This is like moving something up a hill, it requires energy.
Active transport is important for
microorganisms to move nutrients
that are present in low
concentrations in their environment.
Membrane proteins act as carriers
and enzymes. They are specific for
a single or a few molecules or ions.
The end result is that a gradient is
set up and maintained. These
carriers can be saturated.
Group translocation reactions
move a substance from outside to
inside a cell while modifying it at
the same time. Ex. Glucose is
phosphorylated and can not leave
the cell.
Fig. 4-32
Eukaryotic cells move substances by forming
membrane-enclosed vesicles – endocytosis and
exocytosis.
Endocytosis is used for intake of liquid, small
molecules, etc. For us the most important type
of endocytosis is phagocytosis. This is the
process in which white blood cells of our
immune system surround and take in
bacterium. The phagosome is then moved to
and fused with a lysosome (digestive
enzymes). Here the bacteria is digested.
Useful molecules are moved into the
cytoplasm while undigested parts are sent to
the plasma membrane where the vesicle fuses
and dumps its contents outside of the cell
(exocytosis).
Fig. 4-33