Chapter 3: bacterial cell structure
How a tiny microbe can cause much damage in human body tissues: is due to their diversity
of transporters.
Transporters are specific for 1 type of molecule; ions, amino acids, vitamins or sugars.
Cell shape
The 2 most common shapes of bacterial cells:
1. Cocci (s. coccus):
- Roughly spherical cells
- Exist singly or in characteristic arrangements
- Diplococci: when cocci divide and remain together to form
pairs.
2. Rods (s. bacillus):
- Differ in their length-to-width ratio
- Shape often varies between species:
o Flat
o Rounded
o Football-shaped
o Bifurcated
Less common cell shapes and arrangements:
- Bivrios: comma shaped
- Spirilla: rigid spiral-shaped cells
- Spirochetes: flexible, spiral shaped bacteria
Many actinobacteria form long filaments called; hyphae = unit of structure of most fungi and
some bacteria; a tubular filament. (In a network called a mycelium).
E. coli
- Rod-shaped bacterium
- Size range is far beyond average
What causes a bacterial species to have a particular size and
shape?
- Microbes are small to increase the surface area-to-volume
ratio (S/V ratio).
As the ratio increases, the uptake of nutrients and the
diffusion of these and other molecules within the cell
become more efficient; facilitates rapid growth
- However: bacteria can be large; they have other
characteristics that maximize their S/V ratio.
Cell organization
Common features:
- Cells are surrounded by several layers; collectively = cell envelope
- Plasma membrane
- Cell wand
- Capsule/sim layer
- Chemical complex wall; covers the plasma membrane
- Genetic material is localizes in the nucleoid
- Not separated from the surrounding cytoplasm by a membrane.
- Inclusions are scattered about the cytoplasm
- = ribosomes and larger masses
- Filamentous structures (pili) may protrude from the surface
- Facilitating gene transfer/attachment to surfaces.
1
, Plasma membrane
Cell envelope = the plasma membrane and all the surrounding layers external to it.
Plasma membrane = most important, because:
- It encompasses the cytoplasm and defines the cell
- When removed/damages; cell’s contents spill into the environment; cell dies.
- Responsible for much of the cell’s relationships with the outside world
- They are selectively permeable barriers
- They allow particular molecules to pass either into/out of the cell
- In bacteria they play additional roles: They are the location of
several crucial metabolic processes
- Respiration
- Photosynthesis
- Synthesis of lipids and cell wall constituents
Plasma membrane structure = dynamic
Composed of 2 sheets of lipid molecules arranged end-to-end
Chemical nature = amphipathic
- Structurally asymmetric; with polar and nonpolar ends
- Polar ends are hydrophilic
- Nonpolar are hydrophobic
2 types of membrane proteins are able to separated from the
membrane:
1. Peripheral membrane proteins
- Are loosely connected to the membrane; easily removed
2. Integral membrane proteins
- Their hydrophobic regions are buried in membrane lipids, whereas the hydrophilic
portions project form the membrane surface.
- Not easily extracted from membranes, they are
amphipathic
Integral membrane proteins functions
- Transport proteins
- Involved in energy-conserving processes (e.g. electron
transport chains)
Hopanoids = similar in structure to cholesterol found in
eukaryotic membranes, their rigid planar structure makes them
more hydrophobic than phospholipids.
- They insert easily into membranes (are hydrophobic),
but they distort the regular bilayer structure (they’re not
amphipathic)
- Determines where certain integral membrane proteins
can reside
these form functional membrane microdomains, that serve as
platforms for large proteins complexes.
Growth factors must be obtained from the environment (microbes are unable to synthesize
certain molecules).
Passive diffusion
- Molecules move from a region of higher concentration to
one of lower concentration.
- Rate of diffusion depends on the size of the
concentration gradient.
2
How a tiny microbe can cause much damage in human body tissues: is due to their diversity
of transporters.
Transporters are specific for 1 type of molecule; ions, amino acids, vitamins or sugars.
Cell shape
The 2 most common shapes of bacterial cells:
1. Cocci (s. coccus):
- Roughly spherical cells
- Exist singly or in characteristic arrangements
- Diplococci: when cocci divide and remain together to form
pairs.
2. Rods (s. bacillus):
- Differ in their length-to-width ratio
- Shape often varies between species:
o Flat
o Rounded
o Football-shaped
o Bifurcated
Less common cell shapes and arrangements:
- Bivrios: comma shaped
- Spirilla: rigid spiral-shaped cells
- Spirochetes: flexible, spiral shaped bacteria
Many actinobacteria form long filaments called; hyphae = unit of structure of most fungi and
some bacteria; a tubular filament. (In a network called a mycelium).
E. coli
- Rod-shaped bacterium
- Size range is far beyond average
What causes a bacterial species to have a particular size and
shape?
- Microbes are small to increase the surface area-to-volume
ratio (S/V ratio).
As the ratio increases, the uptake of nutrients and the
diffusion of these and other molecules within the cell
become more efficient; facilitates rapid growth
- However: bacteria can be large; they have other
characteristics that maximize their S/V ratio.
Cell organization
Common features:
- Cells are surrounded by several layers; collectively = cell envelope
- Plasma membrane
- Cell wand
- Capsule/sim layer
- Chemical complex wall; covers the plasma membrane
- Genetic material is localizes in the nucleoid
- Not separated from the surrounding cytoplasm by a membrane.
- Inclusions are scattered about the cytoplasm
- = ribosomes and larger masses
- Filamentous structures (pili) may protrude from the surface
- Facilitating gene transfer/attachment to surfaces.
1
, Plasma membrane
Cell envelope = the plasma membrane and all the surrounding layers external to it.
Plasma membrane = most important, because:
- It encompasses the cytoplasm and defines the cell
- When removed/damages; cell’s contents spill into the environment; cell dies.
- Responsible for much of the cell’s relationships with the outside world
- They are selectively permeable barriers
- They allow particular molecules to pass either into/out of the cell
- In bacteria they play additional roles: They are the location of
several crucial metabolic processes
- Respiration
- Photosynthesis
- Synthesis of lipids and cell wall constituents
Plasma membrane structure = dynamic
Composed of 2 sheets of lipid molecules arranged end-to-end
Chemical nature = amphipathic
- Structurally asymmetric; with polar and nonpolar ends
- Polar ends are hydrophilic
- Nonpolar are hydrophobic
2 types of membrane proteins are able to separated from the
membrane:
1. Peripheral membrane proteins
- Are loosely connected to the membrane; easily removed
2. Integral membrane proteins
- Their hydrophobic regions are buried in membrane lipids, whereas the hydrophilic
portions project form the membrane surface.
- Not easily extracted from membranes, they are
amphipathic
Integral membrane proteins functions
- Transport proteins
- Involved in energy-conserving processes (e.g. electron
transport chains)
Hopanoids = similar in structure to cholesterol found in
eukaryotic membranes, their rigid planar structure makes them
more hydrophobic than phospholipids.
- They insert easily into membranes (are hydrophobic),
but they distort the regular bilayer structure (they’re not
amphipathic)
- Determines where certain integral membrane proteins
can reside
these form functional membrane microdomains, that serve as
platforms for large proteins complexes.
Growth factors must be obtained from the environment (microbes are unable to synthesize
certain molecules).
Passive diffusion
- Molecules move from a region of higher concentration to
one of lower concentration.
- Rate of diffusion depends on the size of the
concentration gradient.
2
