Chapter 3A:

Prokaryotic Cells

1. Overview of Prokaryotic Cells

2. The Plasma Membrane & Membrane Transport

3. External Prokaryotic Cell Structures

4. Internal Prokaryotic Cell Structures

3.1 – Overview of Prokaryotic Cells

Prokaryotic

Cell Structure

Prokaryotic Morphologies

Morphology refers to the shape

of individual cells:

coccus = round or spherical

bacillus = rod-shaped

vibrio = curved rod

coccobacillus = short rod, oval

spirillum = spiral shape

spirochete = long, coiled/helical

Arrangements

Arrangement refers to the

characteristic grouping of

multiple prokaryotic cells

following cell division:

diplo- = pairs

tetrad = 4 cells in a square

strepto- = linear chain

staphylo- = irregular

clusters

3.2 – The Plasma Membrane &

Membrane Transport

The Plasma (Cell) Membrane

barrier between inside & outside of cells

Diffusion & Osmosis

OSMOSIS = diffusion of water across semi-permeable membrane

overall movement from high to low concentration

passive, no energy required

Tonicity in Cells with Cell Walls

Tonicity in Cells without Cell Walls

Facilitated Diffusion

Diffusion (high to low conc.)

across a membrane with

the help of a membrane

protein:

• Protein channels for small

molecules or ions (e.g.,

sodium channels)

• Carrier proteins for larger

molecules (e.g., glucose

carrier)

Active Transport

ACTIVE TRANSPORT = movement from low to high concentration via

protein pumps – requires energy (e.g., ATP)

Sodium-PotassiumPump

3.3 – External Structures

Bacterial Cell Walls

2 general cell wall structures – Gram-positive and Gram-negative

PEPTIDOGLYCAN LPS

Peptidoglycan & Lipopolysaccharide (LPS)

These cell wall materials are unique to bacteria.

Bacterial Glycocalyx (“sugar coat”)

Outermost layer that surrounds the bacterium

• called a slime layer if loosely attached, water soluble

• called a capsule if compact, tightly attached to cell wall

usually, but not always, made of polysaccharides

Bacterial Flagellum

*

*

*

rotates, used for motility

Flagellar Arrangements

Peritrichous bacteria

can “run” in a specific

direction by rotating

flagella in one direction,

and “tumble” by

reversing the rotation

Motility with Peritrichous Flagella

Motile bacteria can

undergo taxis or

movement in response

to something it senses.

• e.g., chemotaxis

(movement in

response to a

chemical

substance)

Somewhat indirect,

based on longer runs,

less tumbles in direction

of “good stuff”

Axial Filaments

Bundle of endoflagella

surrounded by an outer sheath

found in spirochetes

• anchored at one end of cell

and rotate in unison

• rotates cell like a “corkscrew”

to propel it forward

Fimbriae/Pili Non-motile appendages that are chemically and

functionally different than flagella.

• typically small, multiple structures

involved primarily in adhesion,

invasion of tissue

• one or more relatively large pili

can be involved in gene transfer

between bacteria during conjugation

(sex pilus or conjugation pilus)

3.4 – Internal Structures

Prokaryotic Ribosomes

Ribosomes consist of 1 large and 1 small subunit.

Carry out protein synthesis (i.e., translation of mRNA).

• both subunits are made of rRNA & ribosomal proteins

• smaller, somewhat structurally different from eukaryotic ribosomes

• are specifically targeted by some antibiotics

Endospores When conditions are bad, a few types of Gram+ bacteria can form

endospores:

• inactive, dormant cells

enclosed in a highly

resistant spore coat

• remain dormant until

conditions are good

(even for many years!)

• very resistant to heating,

freezing, desiccation,

radiation, etcendospore position is characteristic of the species

Endospore Formation

The Genetic MaterialA region called the nucleoid contains the circular, single bacterial

chromosome (DNA + non-histone proteins):

• usually several million base pairs (bp) in

size

• the E. coli genome, for example, is

~4 mega-bp’s (4 Mbp)

• contains all bacterial genes plus an

origin of replication (Ori)

• Ori is where DNA replication starts,

is essential to copy the chromosome

Plasmids

Some bacteria have extrachromosomal,

non-essential circular DNA molecules

called plasmids:

• much smaller than bacterial

chromosome

• have own Ori so they are copied when

cell divides

• several kilo-base pairs (usually 3-6 kb)

plasmid

map

What’s the Role of Plasmids?

Plasmids generally contain genes that confer some sort of

survival advantage:

1) genes providing protection from toxic substances

2) genes enabling the metabolism of additional sources of energy

3) genes for toxins to kill microbial competitors, enhance pathogenicity

4) genes involved in gene transfer by conjugation

• e.g., antibiotic resistance

Inclusions are deposits of various

materials (metals, gas, nutrients, etc)

found in certain types of bacteria

(e.g., magnetosomes).

Chromatophores are pigment-containing

infoldings of the plasma membrane in some

photosynthetic bacteria.

Inclusions & Chromatophores

Key Terms for Chapter 3A

• bacillus, coccus, vibrio, spirillum, spirochete

• glycocalyx, capsule, fimbriae/pili

• peptidoglycan, teichoic acid, LPS, endotoxin

• endospores, vegetative

• plasmids, nucleoid

• inclusions, chromatophores

• diplo-, strepto-, staphylo-, tetrad

• flagellum, motility; mono-, amphi-, lopho-, peritrichous