Functional Anatomy of Prokaryotic and Eukaryotic Cells

Lecture 2

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Comparing Prokaryotic and Eukaryotic Cells

Common features:DNA and chromosomesCell membraneCytosol and Ribosomes

Distinctive features: ?

Eukaryotic CellEukaryotic Cell(protist, animal)(protist, animal)

EukaryoEukaryo

Eukaryotic CellEukaryotic Cell(plant)(plant)

Differences Between Cell Types

Prokaryotic Cell Eukaryotic Cell

Single circular chromosome

Multiple linear chromosomes

Chromosome found in a cytoplasmic region called the nucleoid.

Chromosomes found in a membrane-bound nucleus.

No internal membranes

Some infolded plasma membrane

Extensive network of internal membranes Specialized areas called organelles

Prokaryotes

One circular chromosome, not membrane bound

No histones No organelles Peptidoglycan cell walls Binary fission

Size and Shape of Prokaryotic Cells

Average size: 0.2 -1.0 µm 2 - 8 µm

Three basic shapes

1. Bacillus, 2. Coccus, 3. Spirals (Vibrio,

Spirillum, Spirochete)

Most monomorphic, some pleomorphic

Variations in cell arrangements (esp. for cocci)

Rod-Shaped Bacteria

Spherical Bacteria

Spiral-Shaped Bacteria

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Cell Arrangement Pairs: Diplococci,

diplobacilli

Clusters: Staphylococci

Chains: Streptococci, streptobacilli

Cytoplasm and Internal Structures

Location of most biochemical activities Nucleoid: nuclear region containing DNA

(up to 3500 genes). Most bacterial chromosomes are circular

Plasmids: small, nonessential, circular DNA (5-100 genes); replicate independently

Ribosomes (70S vs. 80S)

Inclusion bodies: granules containing nutrients, monomers, Fe compounds (magnetosomes)

Prokaryotic CellProkaryotic Cell

Closed Circular Chromosome

Also Plasmids, which are smaller, circular

pieces of DNA.

Plasmids usually encode

expendable functions, e.g.,

antibiotic resistance.

Ribosomes: Sites of Translation

On order of 10,000 per cell!

Internal Structures: Cell MembraneAnalogous to eukaryotic cell membrane: Phospholipid bilayer with proteins (Fluid mosaic

model) Permeability barrier (selectively permeable) Diffusion, osmosis and transport systems

Different from eukaryotic cell membrane: Role in Energy transformation (electron transport

chain for ATP production)

Damage to the membrane by alcohols, quaternary ammonium (detergents), and polymyxin antibiotics causes leakage of cell contents.

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Cell Wall

Rigid for shape & protection prevents osmotic lysis

Consists of Peptidoglycan (murein) polymer of two disaccharide subunits N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

Linked by polypeptides with tetrapeptide side chain attached to NAM

Fully permeable to ions, aa, and sugars (Gram positive cell wall may regulate movement of cations)

Gram + Cell Wall

Thick layer of peptidoglycan

Negatively charged teichoic acid on surface

Thin peptidoglycan No teichoic acids Outer membrane

Gram – Cell Wall

Gram-positive Cell Wall

Fig 4.13

Gram-negative Cell WallLipid A of LPS acts as endotoxin; O antigens for typing, e.g., E. coli O157:H7

Gram neg. bacteria are less sensitive to medications because outer membrane acts as additional barrier.

LPS layer = outer layer of outer membrane

(protein rich gel-like fluid)

Gram Stain Mechanism Crystal violet-iodine crystals form in cell.

Gram-positive

Alcohol dehydrates peptidoglycan

CV-I crystals do not leave

Gram-negative

Alcohol dissolves outer membrane and leaves holes in peptidoglycan.

CV-I washes out

Atypical Bacteria: Mycoplasmas

Lack cell walls

Instead, have cell membrane which incorporates cholesterol compounds (sterols), similar to eukaryotic cells

Cannot be detected by typical light microscopy

This EM shows some typically pleomorph mycoplasmas, in this case M. hyorhinis

Acid-fast Cell Walls

Genus Mycobacterium and Nocardia

mycolic acid (waxy lipid) covers thin peptidoglycan layer

Do not stain well with Gram stain use acid-fast stain

Damage to Cell Wall Lysozyme digests disaccharide in peptidoglycan.

Penicillin inhibits peptide bridges in peptidoglycan.

Protoplast is a wall-less cell.

Spheroplast is a wall-less Gram-positive cell.

L forms are wall-less cells that swell into irregular shapes.

Protoplasts and spheroplasts are susceptible to osmotic lysis.

External Structures located outside of cell wall

Flagella Axial filaments Fimbriae Pili Glycocalyx

Glycocalyx Many bacteria secrete external surface layer

composed of sticky polysaccharide (EPS), polypeptide, or both

A capsule is neatly organized A slime layer is unorganized and loose Allows cells to attach key to biofilms

Prevents phagocytosis virulence factor

E.g.: B. anthracis, S. pneumoniae, S. mutans

Flagellum – Flagella Anchored to wall and membrane Number and placement determines if

atrichous, monotrichous, lophotrichous, amphitrichous, or peritrichous

Allows for move toward or away from stimuli: Chemotaxis (phototaxis and magnetotaxis)

Flagella proteins are H antigens (e.g., E. coli O157:H7)

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also “atrichous”

e.g., E. coli

Polar Flagellum

Axial Filaments Endoflagella In spirochetes Anchored at one end

of a cell Rotation causes cell

to move

Fimbriae allow attachment

Pili are used to transfer DNA from one cell to another

Fimbriae and Pili

Fimbriae

Endospores

Dormant, tough, non-reproductive structure; germination vegetative cells

Spore forming genera: __________

Resistance to UV and radiation, desiccation, lysozyme, temperature, starvation, and chemical disinfectants

Relationship to disease

Sporulation: Endospore formation

Germination: Return to vegetative state