Prokaryotic Cells

Chapter 4 Prokaryotic Cell

Prokaryotic Cells - Bacteria

•         Genetic material not enclosed with a membrane

•         Absence of other membrane-enclosed organelles

•         Cell division by binary fission

•         Small; <1.0-3.0 μm in diameter

External Structures

•         Glycocalyx – Capsule or Slime

•         Flagella

•         Fimbriae and Pili

Internal Structures

•         Cytoplasm

•         Nucleoid

•         Ribosomes

•         Inclusion granules

 Flagella

•         Long, whip-like structures that extend beyond surface of cell

•         Responsible for movement

•         Present in some bacteria

 Arrangements of Flagella

•         Monotrichous - Single polar flagellum

•         Lophotrichous - Two or more flagella at one end of the cell

•         Amphitrichous  - a single flagellum or a tuft of flagella on each end of the cell

•         Peritrichous -   flagella distributed over the entire cell

Structure of Flagella:

•         basal body - anchors the flagellum to the cell wall and plasma membrane

•         hook anchors the flagellum to the basal body

•         filament - outermost region of the flagellum, composed of the protein – flagellin

Attachment of flagella

In Gram positive bacteria

•         1 pair of rings

•         Only inner pair is present

In Gram negative bacteria

•         2 pairs of rings

•         Outer pair of rings is attached to the cell wall

•         Inner pair anchored to the plasma membrane

Bacterial Movement

Rotation of flagella

-          Counter-clockwise – swims forward

-          Clockwise– tumble

Chemotaxis in bacteria

Motility of bacteria enables them to move towards or away from the environmental stimuli - taxis

-          chemotaxis - chemical stimulus

-          phototaxis - light as a stimulus

Bacteria contain receptors in the cell wall to pick up chemical stimuli

Attractant – towards the stimulus – many runs

Repellent – many tumbles

Appendages for attachment – Fimbriae

•         Sticky, proteinaceous, projections

•         Used by bacteria to adhere to each other a host, surfaces in the environment

•         May be hundreds per cell and; shorter than flagella

•         Serve an important function in biofilms

Appendages for mating –sex pili

Long hollow tubules composed of pilin

•         Bacteria typically only have one or two per cell

•         Join two bacterial cells and mediate the transfer of DNA from one cell to another (conjugation)

•         Also known as conjugation pili or sex pili

Bacterial Surface Coating – Glycocalix - Protects cells from drying

o   Slime layer – loosely attached to the cell surface; Water soluble

o   Capsule – Tightly bound to the cell surface

§  Higher pathogenicity - May prevent bacteria from being recognized and destroyed by host phagocytes

§  Enables attachment to surfaces (plant roots, water pipes, surface of teeth etc.)

The Cell Envelopes

•         Cell wall

•         Cell membrane

•         The outer membrane (some bacteria)

Cell wall

•         Surrounds the whole cell

•         Protects the cell from adverse effects of the outside environment

•         Withstands the pressure of the cell

•         Maintains the shape of bacterium

•         Main component: peptidoglycan

Peptidoglycan is composed of:

§  Sugars:

·         N-acetylglucosamin NAG

·         N-acetylmuramic acid NAM

§  Amino acids

Chains of NAG and NAM attached to other chains by tetrapeptide crossbridges

Gram + and Gram – bacteria

1884 Hans Christian Gram introduced Gram staining

Cell wall of Gr+ bacteria

Staining depends on the structure of the cell wall

•         Thick layer of peptidoglycan - (prevents the rinse out of a blue dye  complex during the Gram-staining - cell appear violet)

•         Contains also teichoic acid

•         Lipoteichoic acids anchor peptidoglycan to cell membrane

Cell wall of Gr- bacteria

o   Multilayered structure composed of:

§  Thin layer of peptidoglycan 

§  Outer membrane:

·         bilayer - phospholipids

·         channel proteins (porins)

·         lipopolysaccharide (LPS)

During Gram-staining procedure, the blue dye complex is washed out (by acetone), the second dye (Safranin – red) stains the cell - Cell appears red      

Outer membrane

Function:

•         Evading phagocytosis

•         Barrier to some antibiotics, lyzozyme, heavy metals…

Porins - Proteins - Act as a pore that permits the passage of certain molecules

LPS – Lipopolysaccharide

•         LPS also known as endotoxin

•         Released from dead cells when cell wall disintegrates

•         May trigger fever, vasodilation, inflammation, shock, and blood clotting – it is TOXIC

•         Can be released when antimicrobial drugs kill bacteria

Damage to the Cell Wall

Can be caused by:

•         Penicillin - interferes with the formation of the cell wall

•         Lysozyme - enzyme that ruptures the cell wall

Spheroplast -spherical cell without cell wall

Nontypical Cell Wall

Mycobacterium – most of their cell wall composed of waxy lipid - mycolic acid

Can be detected by acid-fast staining

Mycoplasmas - lack of cell wall – pleomorphic morphology

Structures Internal To the Cell Wall

Plasma (cytoplasmic) membrane – A thin structure surrounding the cell composed of:

·         Phospholipid bilayer

·         Proteins

Phospholipid bilayer: One layer consists of

                        - hydrophylic (waterloving)  heads

                        - hydrophobic (waterfearing) tails

Proteins

•         Integral proteins (channels)

•         Peripheral proteins (enzymes)

•         Glycoproteins (receptors)

Functions of cytoplasmic membrane:

•      Controls passage of substances into and out of the cell; selectively permeable

•      Site of energy production (ATP synthesis)

•      Harvests light energy in photosynthetic prokaryotes

•      Site of Flagella Insertion

•      Site of Pili, Fimbriae Insertion

•      DNA attached to one point on cytoplasmic membrane

Cytoplasm

A substance of the cell inside the plasma membrane

Consists of

80% of water, Proteins, Carbohydrates, Lipids, Minerals

Nuclear Area

•         Bacterial chromosome – a single circular molecule of DNA

•         Not surrounded by nuclear membrane

•         Plasmids – extrachromosomal genetic elements – small circular DNA molecules, carry genes for antibiotic resistance

Ribosomes

•         Small globular structures in cytoplasm

•         Composed of 2 subunits  - 70S (30S+50S)

•         Sites of protein synthesis

•         Consists of rRNA and proteins

•         Targeted by some antibiotics

Inclusions

•         Metachromatic granules – reserve of phosphate

•         Polysaccharide granules

•         Lipid inclusions

•         Sulfur granules

•         Gas vacuoles

•         Polyhidroxybutirate

Endospores

•         Resting cells – dehydrated cells with a thick cell wall

•         Formed in unfavorable conditions

•         Layers of spore coats provide resistance to dehydration, high temperatures, toxic chemicals, radiation

•         25-million-year-old spore germinated

•         Spores germinate into vegetative cells

Basic morphological shapes of bacteria

Ø  Cocci

o   Diplococci –n pairs, Streptococci – in chains; Tetrad - four, Sarcinae - eigth, Staphylococci – irregular clusters

Ø  Bacilli

o   Bacillus - single rods; Diplobacilli - in pairs; Streptobacilli in chains; Coccobacilli – short rods

Ø  Spiral Bacteria

o   Vibrio - curved rods; Spirilla - corkscrew – rigid; Spirochetes - helical – flexible

Dimensions of Bacteria

Average: 1.0-3.0 μm in diameter

Very small: Nanobacteria  -- 0.05 – 0.2 μm

Very big: 300 μm