Chapter 3: Cell Structure and Function in Bacteria and Archea

Question 1

Morphology is

Answer 1

Cell shape

Question 2

Major cell morphologies

Answer 2

• Coccus (pl. cocci): spherical or
ovoid
• Rod/bacillus: cylindrical shape
• Spirillum: loose spiral shape

Question 3

Cells with unusual shapes

Answer 3

Spirochetes, appendaged
bacteria, and filamentous
bacteria

Question 4

Cell Morphology

Answer 4

Morphology typically does not predict physiology, ecology,
phylogeny, etc. of a prokaryotic cell

Selective forces may be involved in setting the morphology
• Optimization for nutrient uptake (small cells and those with
high surface-to-volume ratio)
• Swimming motility in viscous environments or near surfaces
(helical or spiral-shaped cells)
• Gliding motility (filamentous bacteria)

Question 5

Size range for prokaryotes:

Answer 5

0.2 µm to > 700
µm in diameter

Question 6

Examples of very large prokaryotes

Answer 6

• Epulopiscium fishelsoni
• Thiomargarita namibiensis

Question 7

Size range for eukaryotic cells:

Answer 7

10 to >200 µm in diameter

Question 8

Most cultured rod-shaped bacteria are
between

Answer 8

0.5 and 4.0 µm wide and <15 µm long

Question 9

Advantages to being small

Answer 9

Small cells have more surface area relative to cell
volume than large cells (i.e., higher S/V)
– support greater nutrient exchange per
unit cell volume
– tend to grow faster than larger cells

Question 10

Lower Limits of Cell Size

Answer 10

• Cellular organisms <0.15 µm in diameter are unlikely
• Open oceans tend to contain small cells (0.2–0.4 µm in diameter)

Question 11

Cytoplasmic membrane:

Answer 11

• Thin structure that surrounds the cell
• 6–8 nm thick
• Vital barrier that separates cytoplasm from
environment
• Highly selective permeable barrier; enables
concentration of specific metabolites and excretion of
waste products

Question 12

Composition of Membranes

Answer 12

• General structure is phospholipid bilayer (Contain both hydrophobic and hydrophilic
components)
• Can exist in many different chemical
forms as a result of variation in the
groups attached to the glycerol backbone
• Fatty acids point inward to form
hydrophobic environment; hydrophilic
portions remain exposed to external
environment or the cytoplasm

Question 13

Cytoplasmic Membrane

Answer 13

• 6–8 nm wide
• Embedded proteins
• Stabilized by hydrogen bonds and
hydrophobic interactions
• Mg2+ and Ca2+ help stabilize membrane by
forming ionic bonds with negative
charges on the phospholipids
• Somewhat fluid

Question 14

Membrane Proteins

Answer 14

• Outer surface of cytoplasmic membrane can interact with a variety of
proteins that bind substrates or process large molecules for transport
• Inner surface of cytoplasmic membrane interacts with proteins
involved in energy-yielding reactions and other important cellular
functions

Integral membrane proteins
• Firmly embedded in the membrane

Peripheral membrane proteins
• One portion anchored in the membrane

Question 15

Membrane-Strengthening Agents

Answer 15

Sterols
• Rigid, planar lipids found in eukaryotic membranes Strengthen and stabilize
membranes
Hopanoids
• Structurally similar to sterols
• Present in membranes of many Bacteria

Question 16

Archaeal Membranes

Answer 16

• Ether linkages in phospholipids of
Archaea (Figure 3.6)
• Bacteria and Eukarya that have ester
linkages in phospholipids
• Archaeal lipids lack fatty acids, have
isoprenes instead
• Major lipids are glycerol diethers and
tetraethers (Figure 3.7a and b)
• Some archaeal lipids form monolayers
while others form bilayers, whereas
all bacterial lipids form bilayers.

Question 17

Functions of the Cytoplasmic Membrane

Answer 17

1. Permeability Barrier
   • Polar and charged molecules must be transported
   • Transport proteins accumulate solutes against the concentration gradient
2. Protein Anchor
   • Holds transport proteins in place
3. Energy Conservation

Question 18

Carrier-Mediated Transport Systems

Answer 18

• Show saturation effect
• Highly specific

Question 19

Three major classes of transport
systems in prokaryotes

Answer 19

• Simple transport
Driven by the energy in the proton motive force
• Group translocation
Chemical modification of the transported substance driven by phosphoenolpyruvate
• ABC system
Periplasmic binding proteins are involved and energy comes from ATP

Question 20

Three transport events are possible:

Answer 20

• Uniporters transport in one direction
across the membrane
• Symporters function as cotransporters
• Antiporters transport a molecule
across the membrane while
simultaneously transporting another
molecule in the opposite direction

Question 21

Simple Transport: Lac Permease of Escherichia coli

Answer 21

• Lactose is transported into E. coli by the simple transporter lac permease, a symporter
• Activity of lac permease is energy driven
• Other symporters, uniporters, and antiporters

Question 22

The Phosphotransferase System in E. coli

Answer 22

• Type of group translocation: substance transported is chemically modified during transport
across the membrane
• Best-studied system
• Moves glucose, fructose, and mannose
• Five proteins required
• Energy derived from phosphoenolpyruvate

Question 23

ABC (ATP-Binding Cassette) Systems

Answer 23

• >200 different systems identified in
prokaryotes
• Often involved in uptake of organic
compounds (e.g., sugars, amino acids),
inorganic nutrients (e.g., sulfate,
phosphate), and trace metals
• Typically display high substrate specificity
• Contain periplasmic binding proteins

Question 24

Protein Export: Translocases

Answer 24

responsible for exporting proteins through and inserting into prokaryotic membranes

Question 25

Sec translocase system

Answer 25

exports proteins and inserts integral
membrane proteins into the membrane

Question 26

Type III secretion system

Answer 26

common in pathogenic bacteria; secreted
protein translocated directly into host

Question 27

Peptidoglycan

Answer 27

Rigid layer that provides
strength to cell wall

Polysaccharide composed of
• N-acetylglucosamine and Nacetylmuramic acid
• Amino acids
• Lysine or diaminopimelic acid
(DAP)
• Cross-linked differently in
gram-negative bacteria and
gram-positive bacteria (Figure
3.17)

Question 28

Gram-Positive Cell Walls

Answer 28

Can contain up to 90% peptidoglycan

Common to have teichoic acids (acidic
substances) embedded in the cell wall

Lipoteichoic acids: teichoic acids covalently
bound to membrane lipids

Question 29

Prokaryotes That Lack Cell Walls

Answer 29

Mycoplasmas
• Group of pathogenic bacteria

Thermoplasma
• Species of Archaea

Question 30

Lipopolysaccharide (LPS) layer

Answer 30

• LPS consists of core polysaccharide and
O-polysaccharide
• LPS replaces most of phospholipids in outer half of outer membrane
• Endotoxin: the toxic component of LPS

Question 31

Porins

Answer 31

channels for movement of
hydrophilic low-molecular weight
substances

Question 32

Periplasm

Answer 32

space located between
cytoplasmic and outer membranes

~15 nm wide

Contents have gel-like consistency

Houses many proteins

Question 33

Cell Walls of Archaea

Answer 33

No peptidoglycan

Typically no outer membrane

Pseudomurein
• Polysaccharide similar to peptidoglycan
(Figure 3.21)
• Composed of N-acetylglucosamine and
N-acetyltalosaminuronic acid
• Found in cell walls of certain
methanogenic Archaea

Cell walls of some Archaea lack
pseudomurein

Question 34

S-Layers

Answer 34

• Most common cell wall type among
Archaea
• Consist of protein or glycoprotein
• Paracrystalline structure

Question 35

Ribosomes

Answer 35

Complex structures, sites of protein synthesis.
• Consisting of protein/RNA.

Entire ribosome.
• Bacterial/archaeal ribosome = 70S.
• Eukaryotic (80S) S = Svedburg unit.

Bacterial and archaeal ribosomal RNA.
• 16S small subunit.
• 23S and 5S in large subunit.
• At least one archaeon have additional 5.8S rRNA (also seen in eukaryotic large subunit).

Proteins in ribosomes vary.
• Archaea more similar to eukarya than to bacteria, but there are some that are unique to archaea.

Question 36

The Nucleoid

Answer 36

Irregularly shaped region in bacteria and archaea.

Usually not membrane bound (few exceptions).

Location of single circular chromosome and
associated proteins.

Some evidence for polyploidy in some archaeons.

Supercoiling and nucleoid-associated proteins (NAPs, including histones in some cases) aid in folding and chromosome condensation.

Question 37

Capsules and Slime Layers

Answer 37

Polysaccharide layers (Figure 3.23)
• May be thick or thin, rigid or flexible

Assist in attachment to surfaces

Protect against phagocytosis

Resist desiccation

Question 38

Fimbriae

Answer 38

• Filamentous protein structures
• Enable organisms to stick to surfaces
or form pellicles

Question 39

Pili

Answer 39

• Filamentous protein structures
• Typically longer than fimbriae
• Assist in surface attachment
• Facilitate genetic exchange between
cells (conjugation)
• Type IV pili involved in twitching
motility

Question 40

Cannulae

Answer 40

• Hollow, tubelike structures
on surface of thermophilic
archae in genus Pyrodictium.
• Function unknown.
• May be involved in
formation of networks of
multiple daughter cells.

Question 41

Hami

Answer 41

• Archaeal external structure still not well understood
• ‘Grappling hook’ appearance.
• Involvement in cell adhesion mechanisms

Question 42

Cell Inclusions

Answer 42

Carbon storage polymers
• Poly-b-hydroxybutyric acid
(PHB): lipid (Figure 3.26)
• Glycogen: glucose polymer

Polyphosphates:
accumulations of inorganic phosphate

Sulfur globules: composed of elemental sulfur

Magnetosomes: magnetic storage inclusions

Question 43

Gas Vesicles

Answer 43

Confer buoyancy in planktonic cells

Spindle-shaped, gas-filled structures
made of protein

Gas vesicle impermeable to water

Question 44

Molecular Structure of Gas Vesicles

Answer 44

• Gas vesicles are composed of two
proteins: GvpA and GvpC

• Function by decreasing cell density

Question 45

Endospores

Answer 45

Highly differentiated cells resistant to heat, harsh
chemicals, and radiation (Figure 3.32)

“Dormant” stage of bacterial life cycle (Figure 3.33)

Ideal for dispersal via wind, water, or animal gut

Only present in some gram-positive bacteria

Question 46

Endospore Structure

Answer 46

Structurally complex

Contains dipicolinic acid

Enriched in Ca2+

Core contains small acid-soluble
proteins (SASPs)

Question 47

The Sporulation Process

Answer 47

• Complex series of events
• Genetically directed

Question 48

Flagellum (pl. flagella):

Answer 48

structure that assists in swimming

different arrangements: peritrichous, polar, lophotrichous

helical in shape

Question 49

Flagellar Structure

Answer 49

Consists of several components

Filament composed of flagellin

Move by rotation

Question 50

Differences of Archaeal Flagella

Answer 50

• Flagella thinner
• More than one type of flagellin
protein
• Filament is not
hollow
• Hook and basal body
difficult to distinguish
• More related to type IV
bacterial pili
• Growth occurs at the base, not
the end

Question 51

Flagellar Synthesis

Answer 51

• Several genes are required for flagellar synthesis and motility
• MS ring made first
• Other proteins and hook made next
• Filament grows from tip

Question 52

Flagella increase or decrease
rotational speed in relation to strength
of the ______________

Answer 52

proton motive force

Question 53

Differences in swimming motions

Answer 53

• Peritrichously flagellated cells move
slowly in a straight line
• Polarly flagellated cells move more
rapidly and typically spin around

Question 54

Gliding Motility

Answer 54

• Flagella-independent motility
• Slower and smoother than
swimming
• Movement typically occurs along
long axis of cell
• Requires surface contact

Mechanisms
• Excretion of polysaccharide slime
• Type IV pili
• Gliding-specific proteins

Question 55

Taxis: directed movement in response to chemical or physical gradients

Answer 55

• Chemotaxis: response to chemicals
• Phototaxis: response to light
• Aerotaxis: response to oxygen
• Osmotaxis: response to ionic strength
• Hydrotaxis: response to water

Question 56

Chemotaxis

Answer 56

• Best studied in E. coli
• Bacteria respond to temporal, not spatial, difference in chemical concentration
• “Run and tumble” behavior (Figure 3.47)
• Attractants and receptors sensed by chemoreceptors

Question 57

Measuring Chemotaxis

Answer 57

• Measured by inserting a
capillary tube containing
an attractant or a repellent
in a medium of motile
bacteria
• Can also be seen under a
microscope