Cell structure and function for microbiologists

Prokaryotes

Eukaryotes

Both have the same types of biological moleculesmetabolism, protein synthesis, ATP

Eukaryotes have organelles

• Much larger; more complex than prokaryotes• Processes compartmentalized into organelles

– Nucleus– Protein synthesis (ribosomes, RER, Golgi)– Mitochondria; chloroplasts– Lysosomes– Plasma membranes have different modifications– Cytoskeleton

Eukaryotes may be multicellular

• Cells may be variable within the organism– Tissues– Organs

– See table 3.7 on p. 72

Prokaryotes:Have no nucleus; genome is circular

No histones

No membrane-bound organelles

Cell wall usually contains peptidoglycan (cellwalls are more complex)

Divide by binary fission

Prokaryotes include eubacteria and archaea

How do you tell them apart? They’re all small!morphologychemical compositionnutritional and energy requirements

Typical shapes of bacteria

Most bacteria retain a particular shape; a feware pleiomorphic

Characteristic grouping (or not grouping)

Even in groups, bacteria tend to be single-celled in structure and behavior

Some have “colonial” traits

Well-studied example: myxobacteria“hunting” coloniesfruiting bodies

Etc.

Typical prokaryotic structures

Working from the outside in…

Extracellular components

Protectiondehydrationimmune mechanisms

Attachment

Glycocalyx- polysaccharide, proteincapsule if organizedslime layer if not

May contribute to virulence

Some bacteria are motile (due to flagella)

Bacteria vary in the way flagella are attached

How they move: running, tumbling, swarming

Can move toward or away from light orchemical stimuli

Flagellin protein is unique to prokaryotes

Peritrichous monotrichous(or amphi, or lophotrichous

Cocci do not have flagella

Pili- attachment; motility; conjugation

Cell membrane structure is similar in structureand function to that of eukaryotes

Phospholipid bilayer(everything moves through it, since thereare no organelles)

carrier proteins

generally involve proton motive force (i.e,require energy and moving against theconcentration gradient)

Cell wall- hallmark of prokaryotes

Their reaction with Gram stain allows bacteraito be divided into two groups

Positive-lots of peptidoglycan

Negative- thin layer, with an outer membraneand “periplasmic space” in betweenMany secreted proteins are found here

Structure of peptidoglycan

Gram-positive cell wall

Outer membrane is made of lipopolysaccharide(LPS)

Porins allow molecules to pass through outer membrane

LPS is protectivelipid A- strong inflammatory response(endotoxin)O-linked polysaccharide- antigenic

Significance of Gram-positive vs Gram-negativeantibiotic sensitivity

sensitivity to lysozyme

reaction with Gram reagentscrystal violetiodinealcoholsafranin

Mycoplasma do not have a cell wall

Lots of variety in archaea- but none havepeptidoglycan

Internal components

Nucleoid- with single, circular, supercoiledDNA molecule

Many bacteria have plasmidssmall, extrachromosomal, circularpiece of DNA

genes present are usually not requiredbut may be advantageous

(antibiotic resistance, resistance to metals)Now used for genetic engineering

Ribosomes

Involved in protein synthesis

Prokaryotic ribosomes are smaller thaneukaryotic (70S vs 80S)

Some antibiotics bind to the 70S ribosome

How does that affect bacteria?

No membrane-bound organelles

Some have storage granules

Some aquatic bacteria have gas vesicles

Some have endospores (soil bacteria) thatenable them to lie dormant under“unfavorable” conditions

NOT a reproductive structure

Summary• Eukaryotes have membrane-bound

organelles• Eukaryotes may be multicellular with

highly specialized cells• Prokaryotes have simple shapes and are

classified according to their morphology• Certain structures are unique to

prokaryotes