This document provides an overview of the bacterial cell, including its key parts and structures. It discusses the plasma membrane and transport mechanisms, the cell wall and other outer layers, the nucleoid which contains the bacterial chromosome, and how bacterial cells divide. Modern research techniques like cell fractionation allow scientists to isolate and study the various subcellular components that work together in a coordinated, ordered structure to carry out vital bacterial functions.

1. Chapter Overview
● A synopsis of the bacterial cell
● How cell parts are studied
● The plasma membrane and transport
● The cell wall and other outer layers
● The nucleoid: structure and expression
● How bacterial cells divide
● Specialized structures, including pili & stalks
● Bacterial flagella and chemotaxis
1

2. The Bacterial Cell: An Overview
Most prokaryotes share fundamental traits.
- Thick, complex outer envelope
- Compact genome
- Tightly coordinated cell functions
Modern research shows that the cell’s parts
fit together in a structure that is ordered,
though flexible.
2

3. Figure 3.1
3

4. The Bacterial Cell: An Overview
Cytoplasm = Consists of a gel-like network
Cell membrane = Encloses the cytoplasm
Cell wall = Covers the cell membrane
Nucleoid = Non-membrane-bound area of
the cytoplasm that contains the
chromosome in the form of looped coils
Flagellum = External helical filament whose
rotary motor propels the cell
4

5. Biochemical Composition of
Bacteria
All cells share common chemical
components.
- Water
- Essential ions
- Small organic molecules
- Macromolecules
Cell composition varies with species, growth
phase, and environmental conditions.
5

6. How We Study Cell Parts
Cell study requires isolation of cell parts.
- Cell disruption
- Subcellular fractionation
- Structural analysis
- Genetic analysis
6

7. Isolating Parts of Cells
Cells must be broken up by techniques that
allow subcellular parts to remain intact.
Examples of such techniques include:
- Mild detergent analysis
- Sonication
- Enzymes
- Mechanical disruption
7

8. Subcellular Fractionation
A key tool of
subcellular
fractionation is the
ultracentrifuge.
-The high rotation rate
produces centrifugal
forces strong enough
to separate particles
by size.
Figure 3.4 8

9. The Cell Membrane
The structure that defines the existence of a
cell is the cell membrane.
Figure 3.7
9

10. Membrane Constituents
Membranes have
approximately equal parts Figure 3.8
of phospholipids and
proteins.
A phospholipid consists of
glycerol with ester links to
two fatty acids and a
phosphoryl head group.
- May have side chain
10

11. Membrane Constituents
Membrane proteins serve numerous functions,
including:
- Structural support
- Detection of environmental signals
- Secretion of virulence factors and
communication signals
- Ion transport and energy storage
Have hydrophilic and hydrophobic regions that
lock the protein in the membrane
11

12. Transport across the Cell Membrane
The cell membrane acts as a semipermeable
barrier.
Selective transport is essential for survival.
- Small uncharged molecules, such as O 2
and CO2, easily permeate the membrane by
diffusion.
- Water tends to diffuse across the
membrane in a process called osmosis.
12

13. Weak acids and weak bases exist partly in an
uncharged form that can diffuse across the
membrane and change the pH of the cell.
Figure 3.9
13

14. Polar molecules and charged molecules
require transport through specific protein
transporters.
Figure 3.10
- Passive transport =
Molecules move along
their concentration
gradient
- Active transport =
Molecules move against
their concentration
gradient
- Requires energy
14

15. Membrane Lipids
Phospholipids vary with respect to their
phosphoryl head groups
& their fatty acid side chains.
Figure 3.11
Figure 3.12
15

16. Membranes also include planar molecules that fill
gaps between hydrocarbon chains.
In eukaryotic membranes, the reinforcing agents
are sterols, such as cholesterol.
In bacteria, the same function is filled by
hopanoids, or hopanes.
Figure 3.13
16

17. Archaea have the most extreme variations in
phospholipid side-chain structures.
- Ether links between glycerol and fatty acids
- Hydrocarbon chains are branched terpenoids.
Figure 3.14
17

18. The Cell Wall
The cell wall confers shape and rigidity to the
cell, and helps it withstand turgor pressure.
The bacterial cell wall, or the sacculus,
consists of a single interlinked molecule.
Figure 3.16 18

19. Peptidoglycan Structure
Most bacterial cell walls are made up of
peptidoglycan (or murein).
The molecule consists of:
- Long polymers of two disaccharides called
N-acetylglucosamine and N-acetylmuramic
acid, bound to a peptide of 4-6 amino acids
- The peptides can form cross-bridges
connecting the parallel glycan strands.
19

20. Figure
3.17
20

21. Gram-Positive and
Gram-Negative Bacteria
Most bacteria have additional envelope layers that
provide structural support and protection.
Envelope composition defines:
- Gram-positive bacteria (thick cell wall)
- Example: The phylum Firmicutes
- Gram-negative bacteria (thin cell wall)
- Example: The phylum Proteobacteria
Figure 1.1
21

22. Gram-Positive Cell Envelope
Capsule (not all species)
- Made of polysaccharides
S-Layer (not all species)
- Made of protein
Thick cell wall
- Amino acid cross-links in
peptidoglycan
- Teichoic acids for strength
Plasma membrane
Figure 3.18a 22

23. Mycobacterial Cell Envelopes
Mycobacterium Figure 3.21
tuberculosis and
M. leprae have very
complex cell
envelopes.
- Include unusual
membrane lipids
(mycolic acids) and
unusual sugars
(arabinogalactans)
23

24. Gram-Negative Outer Membrane
The thin peptidoglycan
layer consists of one
or two sheets.
-Covered by an outer
membrane, which
confers defensive
abilities and toxigenic
properties on many
pathogens
Figure 3.18b
24

25. Figure
3.22
25

26. Table 3-2 Subcellular location of proteins in Gram-negative bacteria.
26

27. Figure 3-23 Outer membrane analysis by centrifugation.
27

28. Figure 3-24 Sucrose porin.
28

29. The Nucleoid
An important function of the cell envelope is
to contain and protect the cell’s genome.
Eukaryotes have a
membrane-bound
nucleus.
Prokaryotes have a
nucleoid region that
extends throughout
the cytoplasm.
Figure
29
3.26

30. The E. coli nucleoid The nucleoid forms about
appears as clear 50 loops or domains.
regions that exclude Within each domain, the
the ribosome and
contain the DNA DNA is supercoiled by
strands. DNA-binding proteins.
Figure Figure
3.27 3.28 30

31. Cell Division
Cell division, or cell fission, requires highly
coordinated growth and expansion of all the
cell’s parts.
Unlike eukaryotes, prokaryotes synthesize RNA
and proteins continually while the cell’s DNA
undergoes replication.
Bacterial DNA replication is coordinated with the
cell wall expansion and ultimately the separation
of the two daughter cells.
31

32. DNA Replication
In prokaryotes, a circular chromosome begins to
replicate at its origin, or ori site.
Two replications forks are generated, which
proceed outward in both directions.
- At each fork, DNA is synthesized by DNA
polymerase with the help of accessory proteins
(the replisome).
As the termination site is replicated, the two forks
separate from the DNA.
32

33. Figure
3.32
33

34. DNA Replication
Animation: Replisome Movement in a
Dividing Cell
Click box to launch animation
34

35. Septation Completes Cell Division
Replication of the termination site triggers growth of
the dividing partition, or septum.
The septum grows inward, at last constricting and
sealing off the two daughter cells.
Figure
3.33
35

36. Septation Completes Cell Division
The spatial
orientation of
septation has
a key role in
determining
the shape and
arrangement
of cocci.
Figure
3.34 36

37. The Bacterial Cytoskeleton
Shape-determining proteins Special Topic 3.2
Figure 2
- FtsZ = Forms a “Z ring”
in spherical cells
- MreB = Forms a coil
inside rod-shaped cells
- CreS “Crescentin” =
Forms a polymer along
the inner side of crescent-
shaped bacteria
37

38. Cell Attachment
Pili or fimbriae are straight
filaments of protein
monomers called pilin.
Sex pili are used in
conjugation.
Figure
Stalks are membrane- 3.40
embedded extensions
of the cytoplasm.
- Tips secrete adhesion
factors called holdfasts.
Figure 3.41
38

39. Rotary Flagella
Prokaryotes that are motile generally swim by
means of rotary flagella.
Peritrichous cells have
flagella randomly
distributed around the cell.
Figure 3.42a
Lophotrichous cells have flagella at the
end(s).
Monotrichous cells have a single flagellum.
39

40. Each flagellum is a spiral filament of protein
monomers called flagellin.
The filament is rotated by a motor driven by
the proton motive force.
Note: Flagella
rotate either
clockwise
(CW) or
counterclock
wise (CCW)
relative to
the cell. Figure
40
3.43

41. Chemotaxis
Chemotaxis is the movement of a bacterium in
response to chemical gradients.
Attractants cause CCW rotation.
- Flagella bundle together.
- Push cell forward
- “Run”
Repellents cause CW rotation.
- Flagellar bundle falls apart.
- “Tumble” = Bacterium briefly
stops, then changes direction
Figure
41
3.42

42. Chemotaxis
The alternating runs and tumbles cause a
“random walk.”
- Receptors detect attractant concentrations.
- Sugars, amino acids
- Attractant concentration increases and
prolongs run.
- This is termed a “biased random walk.”
- Causes a net movement of bacteria
toward attractants (or away from repellents)
42

43. Figure
3.44
43

44. Chemotaxis
Animation: Chemotaxis: Molecular Events
Click box to launch animation
44

45. Chapter Summary
● While prokaryotes are diverse, they share certain
fundamental traits and biochemistry.
● The study of cells employs various methods
including subcellular fractionation, structural
analysis, and genetic analysis.
● The cell membrane consists of a phospholipid
bilayer containing proteins.
- Bacterial phospholipids are ester-linked, while
those of Archaea contain ether linkages.
● The Gram-negative cell envelope is much more
complex than that of Gram-positive cells.
45

46. Chapter Summary
● The DNA of prokaryotes is organized into loops in
the nucleoid.
- Transcription and translation are coupled.
● Most bacteria divide by binary fission.
- Cell growth and DNA replication are coordinated.
● Bacteria may have specialized structures, including
thylakoids, storage granules, and magnetosomes.
● Pili and stalks are used for attachment.
● Flagella are rotary appendages used for movement
and chemotaxis.
46

47. Concept Quiz
Which one of these membranes is not
found in Gram-negative bacteria?
a) Plasma membrane
b) Inner membrane
c) Nuclear membrane
d) Outer membrane
47

48. Concept Quiz
Peptidoglycan is composed primarily of
a) sugars and amino acids.
b) sugars and nucleic acids.
c) nucleic acids and lipids.
d) amino acids and lipids.
48

49. Concept Quiz
An extension of the cytoplasm that attaches
bacteria to a surface is called a
a) pilus.
b) flagellum.
c) fimbrium.
d) stalk.
49

50. Concept Quiz
The structure in prokaryotes that performs
the same function as mitochondria in
eukaryotes is the
a) cell membrane.
b) chloroplast.
c) outer membrane.
d) cell wall.
50

51. Concept Quiz
In archaeal membranes, the glycerol is
linked to the hydrocarbon chains by _____
bonds.
a) ether
b) ester
c) glycan
d) peptide
51

52. Concept Quiz
All of the following statements about prokaryotic
flagella are correct except
a)they are driven by the proton motive force.
b)they are found in both Gram-positive and
Gram-negative bacteria.
c) they move with a whiplike motion.
d)they are used for chemotaxis.
52