Cell structure and function for microbiologists Prokaryotes Eukaryotes Both have the same types of biological molecules metabolism, 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 (cell walls are more complex) Divide by binary fission

Prokaryotes include eubacteria and archaea How do you tell them apart? They’re all small! morphology chemical composition nutritional and energy requirements

Typical shapes of bacteria Most bacteria retain a particular shape; a few are 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” colonies fruiting bodies Etc.

Typical prokaryotic structures Working from the outside in…

Extracellular components Protection dehydration immune mechanisms Attachment Glycocalyx- polysaccharide, protein capsule if organized slime 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 or chemical stimuli Flagellin protein is unique to prokaryotes

Peritrichousmonotrichous (or amphi, or lophotrichous Cocci do not have flagella

Pili- attachment; motility; conjugation

Cell membrane structure is similar in structure and function to that of eukaryotes Phospholipid bilayer (everything moves through it, since there are no organelles) carrier proteins generally involve proton motive force (i.e, require energy and moving against the concentration gradient)

Cell wall- hallmark of prokaryotes Their reaction with Gram stain allows bacterai to be divided into two groups Positive-lots of peptidoglycan Negative- thin layer, with an outer membrane and “periplasmic space” in between Many 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 protective lipid A- strong inflammatory response (endotoxin) O-linked polysaccharide- antigenic

Significance of Gram-positive vs Gram-negative antibiotic sensitivity sensitivity to lysozyme reaction with Gram reagents crystal violet iodine alcohol safranin

Mycoplasma do not have a cell wall Lots of variety in archaea- but none have peptidoglycan

Internal components Nucleoid- with single, circular, supercoiled DNA molecule Many bacteria have plasmids small, extrachromosomal, circular piece of DNA genes present are usually not required but may be advantageous (antibiotic resistance, resistance to metals) Now used for genetic engineering

Ribosomes Involved in protein synthesis Prokaryotic ribosomes are smaller than eukaryotic (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) that enable 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