Chapter 2

Prokaryotic Structure

Introduction to the Cell

§     All living things are composed of living cells

§     The smallest unit of life maintaining all the properties of life.

§     Properties of life

–    Metabolism

–    Reproduction

–    Differentiation

–    Evolution

•     Two types of cells:

–    Prokaryotic

–   Archaea and bacteria

–    Eukaryotic: plants, algae, fungi, protists, and animals (variety)

Prokaryote vs Eukaryote

§    Comparing prokaryotic and eukaryotic cells

–  Prokaryote comes from the Greek words for prenucleus.

–  Eukaryote comes from the Greek words for true nucleus.

Prokaryote

§     Absence of nucleus

§     One circular chromosome, not in a membrane

§     No histones

§     No organelles

§     Peptidoglycan cell walls

§     Binary fission for cell division

§     Smaller

Eukaryote

§     Contain nucleus

§     Paired chromosomes,
in nuclear membrane

§     Histones

§     Membrane enclosed organelles

§     Simple (polysaccharide) cell walls

§     Cell division by mitosis or meiosis

§     Larger

Prokaryotic Cells: Shapes

�  Average size: 0.2 –1.0 µm ´ 2 – 8 µm

§    Most bacteria are monomorphic

§    A few are pleomorphic

MORPHOLOGY

§    Coccus

§    Bacillus

§    Spiral

§    Unusual shapes

–  Star-shaped Stella

–  Square Haloarcula

Arrangements

§    Pairs: Diplococci, diplobacilli

§    Clusters: Staphylococci

§    Chains: Streptococci, streptobacilli

External Structure - Glycocalyx

§     Outside cell wall

§     Gelatinous, sticky substance

§     A capsule is neatly organized

§     A slime layer is unorganized and loose

§     Made inside and exported

§     General functions

–    Adherence - Extracellular polysaccharide allows cell to attach

§   Streptococcus mutans and tooth decay

–    Capsules prevent phagocytosis (virulence factor)

§   Protection of pathogen from host immune system

–   Bacillus anthracis and Klebsiella pneumoniae

–    Protection from dehydration

External Structures - Flagella

§    Outside cell wall

§    Long filaments that propel some prokaryotes

§    Structure

–   Filament: Made of chains of flagellin; responsible for the actual movement

–   Attached to a protein hook

–   Anchored to the wall and membrane by the basal body

Flagella Arrangement

Motile Cells

§     Rotate flagella (clockwise or counterclockwise) to run or tumble

§     Move toward or away from stimuli (taxis)

§     Taxis

–    Movement toward or away from a favorable or unfavorable environment

§     Stimuli:  signals that stimulate movement (attractants or repellants)

§     Type of stimuli

–    Chemical à chemotaxis

–    Light à phototaxis

Understand Run and Tumble

–   Runs and tumbles are random without stimuli

–   Ratio of runs to tumbles increase when stimuli present

–   Receptors for stimuli in plasma membrane of cells

External Structure – Axial Filaments

§     Also called endoflagella

§     Move spirochetes

–   Treponema pallidum

–   Borrelia burgdorferi

§     Fibril bundles beneath a sheath anchored at one end of a cell

§     Spiral movement

–   Rotation causes cell to move

Other External Structures

§      Pilin (protein)

–    Arranged helically around cell

–    Mostly Gram negative

–    Two types:

§   Fimbriae

–   Found at ends or distributed evenly

–   Sticky bristle-like projections

–    Allow attachment or adherence to surfaces

§   Neisseria gonorrhoeae attachment to mucous lining

§   Pili

–   Longer than fimbriae

–   Straight hairlike appendages

–   Facilitates transfer of DNA from one cell to another

§   Ex. Sex pili during conjugation

PILI

Cell Wall

§    Purpose/significance

–   Prevents osmotic lysis

–   Cell shape

–   Differentiation

–   Anchor flagella

–   Virulence

§    More rigid and complex than eukaryotes

§    Made of peptidoglycan (in bacteria)

–   Sugar backbone (NAM-NAG), tetrapeptide side chains, peptide crossbridge

Peptidoglycan

§    Polymer of disaccharide
N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

§    Linked by polypeptides

Peptidoglycan Structure

§     NAM-NAG backbone units attached via amino acid cross-bridges (cross-bridges attached to side chains).

§     Side chain amino acids connect layers of NAM-NAG backbones.

Gram-Positive
Cell Walls

§    Thick peptidoglycan

§    Teichoic acids

§    In acid-fast cells, contains mycolic acid

Gram-Negative

Cell Walls

§    Thin peptidoglycan

§    No teichoic acids

§    Outer membrane

§    Periplasmic space

Gram-Positive Cell Walls

§     Retains crystal violet used in Gram staining procedure

§     Teichoic acids

–    Two classes

§   Lipoteichoic acid links to plasma membrane

§   Wall teichoic acid links to peptidoglycan

–    May regulate movement of cations.

–    Role in cell growth

–    Polysaccharides provide antigenic variation.

Gram-Negative Cell Walls

§     Thinner peptidoglycan layer attached to outer membrane (phospholipid and lipopolysaccharide) via lipoproteins

§     No teichoic acids

§     Outer Membrane

–   Lipopolysaccharides, lipoproteins, phospholipids

–   Forms the periplasm between the outer membrane and the plasma membrane.

–   Protection from phagocytes, complement, and antibiotics

–   O polysaccharide antigen, e.g., E. coli O157:H7

–   Lipid A is an endotoxin

–   Porins (proteins) form channels through membrane.

Gram-Negative Cell Wall

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 Cell Walls

§    Mycobacterium

–   Cell walls contain mycolic acids

–   Acid Fast stain

§    Mycoplasmas

–   Lack cell walls

–   Sterols in plasma membrane

§    Archaea

–   Wall-less or

–   Walls of pseudomurein (lack NAM and D amino acids)

Damage to Cell Walls

§    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.

§    Protoplasts and spheroplasts are susceptible to osmotic lysis.

Plasma Membrane

§    Prokaryotes vs. Eukaryotes

§    Basic Structure

–  Phospholipid bilayer

–  Integral membrane proteins

§  Transmembrane proteins

–  Peripheral proteins

Fluid Mosaic Model

§    Membrane is as viscous as olive oil.

§    Proteins move to function.

§    Phospholipids rotate and move laterally.

Functions of Plasma Membrane

§     Semi-permeable membrane

–    Selective permeability

§   Allows passage of needed molecules (nutrients, water, etc.) and barrier to harmful chemicals

§   Passage through transport proteins

§     Breakdown of nutrients

§     Enzymes for ATP production

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

 Passive Transport

§     High concentration to low

§     No energy

§     Types

–   Simple diffusion

§   Movement of solute across bilayer from an area of high concentration to an area of low concentration

–  Ex. CO2 and O2

 Passive Transport

§      Facilitated diffusion

§   Solute combines with a transporter protein in the membrane.

OSMOSIS

–   The movement of water across selectively permeable membrane from an area of high water concentration to an area of lower water until concentration of solute is equal on both sides (equilibrium)

§  Isotonic solution:  same concentration of solute as inside of cell

§  Hypotonic solution:  concentration of solute is higher inside of cell

§  Hypertonic solution:  concentration of solute is lower inside of cell

§  Osmotic pressure

–  The pressure needed to stop the movement of water across the membrane

OSMOSIS
Fig 4.18

§    Cell in hypotonic solution:  expands

§    Cell in hypertonic solution:  shrinks

Active Transport

§    Movement of solute from low concentration to high

§    Requires energy (ATP) and transport proteins to move solute across membrane

§    Used to get nutrients when nutrient concentration is low in environment

§    Group translocation

–   Requires a transporter protein and PEP

§   Ex. Glucose to glucose-6-phosphate

Inside of Prokaryotic Cell - Cytoplasm

§    Cytoplasm is the substance inside the plasma membrane.

§    80% water

§    Contains all internal structures

Inside of Prokaryotic Cell

§    Nucleoid

–  Contains DNA

–  Not membrane bound and no histones

–  Circular single bacterial chromosome (4,300 genes)

–  Weakly bound to proteins in plasma membrane (help to replicate and separate during cell division

–  20% of cell

Inside of Prokaryotic Cell

§    Plasmids

–  Can contain 1 or more

–  Extrachromosomal DNA (5-100 genes)

–  Often transferred to other prokaryotes (conjugation)

–  Contains non-essential genes

§  Antibiotic resistance

Inside of Prokaryotic Cell

§    Prokaryotic ribosomes

–   Site of protein synthesis

–   Protein and RNA

–   70S ribosome:  30S and 50S subunits

• Eukaryotic = 80S

Inside of Prokaryotic Cell

§    Inclusion bodies

–  Storage granules

–  Dependent on species

Inclusions

§     Metachromatic granules (volutin)   Phosphate reserves

§     Polysaccharide granules                Energy reserves

§     Lipid inclusions                               Energy reserves

§     Sulfur granules                               Energy reserves

§     Carboxysomes                                Ribulose 1,5-diphosphate carboxylase for CO₂

fixation

§     Gas vacuoles                                  Protein covered cylinders

§     Magnetosomes                               Iron oxide
(destroys H₂O₂)

ENDOSPORES

§     Resting cells

§     Highly durable

–    Resistant to desiccation, heat, chemicals

§     Survival mechanism for some species

–    Bacillus and Clostridium spp.

–    Sporulation

§   Endospore formation

–    Germination

§   Vegetative cells

§   Return to vegetative state