lecture 2

3.1 Use of the Term Prokaryote is Controversial

• Bacterial and Archaea Structure and Function

  • Prokaryotes differ from eukaryotes in size and simplicity

  • most lack internal membrane systems

  • term prokaryotes is becoming blurred

  • Bacteria and Archaea terms will be used

3.2 Bacteria are Diverse but Share Some Common Features

• Size, Shape, and Arrangement

  • Shape cocci and rods most common

  • various others

  • Arrangement

  • determined by plane of division

  • determined by separation or not

  • Size - varies

Cocci Shape and Arrangement

• Cocci (s., coccus) – spheres

• diplococci (s., diplococcus) – pairs

• streptococci – chains

• staphylococci – grape-like clusters

• tetrads – 4 cocci in a square

• sarcinae – cubic configuration of 8 cocci

Other Shapes and Arrangements

• Bacilli (s., bacillus) – rods

• coccobacilli – very short rods

• Vibrios – resemble rods, comma-shaped

• Spirilla (s., spirillum) – rigid helices

• Spirochetes – flexible helices

• Mycelium – network of long, multinucleate filaments

• Pleomorphic – organisms that are variable in shape

Size Ranges

• Smallest – $0.3\,\mu\text{m}$

  • (Mycoplasma)

• Average rod – $1.1{--}1.5 \times 2{--}6\,\mu\text{m}$

  • (E. coli)

• Very large – $600 \times 80\,\mu\text{m}$

  • Epulopisciumfishelsoni

Concept Check

• List the characteristics originally used to describe prokaryotic cells

• What are the most common shapes of a bacterial cell?

3.3 Bacterial Plasma Membranes Control What Enters and Leaves the Cell

• Bacterial Cell Organization – Common Features

  • Cell envelope – 3 layers

  • Cytoplasm

  • External structures

Bacterial Cell Envelope

• Plasma membrane (Cytoplasmic membrane)

• Cell wall

• Layers outside the cell wall

• Capsule

• Slime layer

Plasma Membrane Structure is Dynamic

• Also named cell membrane or cytoplasmic membrane

• Encompasses the cytoplasm; absolute requirement for all living organisms.

• Composed of phospholipids and proteins

• Fluid mosaic model

• Effective barrier to large and hydrophilic molecules

• O2, CO2, H2O, lipid substances can pass through

• Salts, sugars, amino acids, polymers, cannot.

• Involved primarily with transport

• Degradation and biosynthesis

• Site of ATP synthesis

Structure of a Phospholipid

• Amphipathic lipids

  • Polar head groups (hydrophilic- interact with water)

  • Non-polar tails (hydrophobic associate with each other to avoid water)

• Membrane proteins

  • Proteins can be peripheral – loosely connected to membrane; easily removed

  • or transmembrane (integral, embedded within membrane)

• Membranes Are Lipid Bilayers With Floating Proteins

Methods for Uptake of Nutrients

• Microbes can only take in dissolved particles across a selectively permeable membrane

• Some nutrients enter by passive diffusion

• Microorganisms use transport mechanisms

Nutrients

• Macroelements (macronutrients) — required in relatively large amounts

  • C, O, H, N, S, and P — found in organic molecules (proteins, lipids, carbohydrates, and nucleic acids)

  • K, Ca, Mg, and Fe — cations; serve in variety of roles including assisting enzymes and biosynthesis

• Micronutrients (trace elements) — required in trace amounts

  • Mn, Zn, Co, Mo, Ni, and Cu

  • Often supplied in water/media components; ubiquitous

  • Work to assist enzymes (cofactors)

  • Some unique substances may be required

Growth Factors

• Organic compounds

• Essential cell components (or their precursors) that the cell cannot synthesize

• Must be supplied by environment if cell is to survive and reproduce

• Classes include:

  • Amino acids — needed for protein synthesis

  • Purines and pyrimidines — needed for nucleic acid synthesis

  • Vitamins — function as enzyme cofactors

Passive Diffusion and Facilitated Diffusion

• Passive diffusion – Molecules move from high concentration to low concentration between the cell’s interior and the exterior such as $ext{H}<em>2 ext{O}, ext{O}</em>2, ext{and} ext{CO}_2$

• Facilitated diffusion – Similar to passive diffusion

  • movement of molecules is not energy dependent

  • Uses membrane bound carrier molecules (permeases)

  • Rate increases with the concentration gradient

  • Effectively transports glycerol, sugars, and amino acids

Active Transport

• Energy-dependent process—$ATP$ or $PMF$ used

• Move molecules against the gradient

• Concentrates molecules inside cell

• Involves carrier proteins (permeases)

Group Translocation

• Energy dependent transport that chemically modifies molecule as it is brought into cell

• Sugar phosphotransferase system (PTS) best known system

Iron Uptake

• Microorganisms require iron

• Ferric iron is very insoluble so uptake is difficult

• Microorganisms secrete siderophores - small organic molecules to aid uptake of ferric iron $ext{Fe}^{3+}$

• Siderophore complexes with ferric ion

• Complex is then transported into cell

Concept Check

• List the functions of bacterial plasma membranes

• What is a macronutrient, trace element and a growth factor

• Explain facilitated diffusion, active transport and group translocation and siderophores

3.4 Cell Walls have Many Functions

• Bacterial Cell Wall

  • cell wall functions

  • maintains shape of the bacterium

  • almost all bacteria have one

  • helps protect cell from osmotic lysis and toxic materials

  • may contribute to pathogenicity

  • All bacterial cell wall has a layer of peptidoglycan (murein)

  • rigid structure lying just outside the cell plasma membrane

  • two types based on Gram stain

  • Gram-positive: stain purple; thick peptidoglycan

  • Gram-negative: stain pink or red; thin peptidoglycan and outer membrane

Peptidoglycan Structure

• Meshlike polymer of identical subunits forming long strands

  • Glycan chain

  • N-acetylglucosamine (NAG)

  • N- acetylmuramic acid (NAM)

  • $\beta{-}1{,}4\text{ glycosidic bond}$

  • alternating D- and L- amino acids

Strands Are Crosslinked

• Peptidoglycan strands have a helical shape

• Peptidoglycan chains are crosslinked by peptides for strength

• interbridges may form

• peptidoglycan sacs – interconnected networks

• various structures occur

Gram-Positive Cell Walls

• Composed primarily of peptidoglycan

• May also contain teichoic acids (negatively charged)

• help maintain cell envelope

• protect from environmental substances

• may bind to host cells

• Some Gram-positive bacteria have layer of proteins on surface of peptidoglycan

Periplasmic Space of Gram + Bacteria

• Between plasma membrane and cell wall

• Periplasm has relatively few proteins

• Exoenzymes secreted by Gram- positive bacteria

• aid in degradation of large nutrients

Gram-Negative Cell Wall Structure

• More complex than Gram-positive

• Outer membrane (OM) composed of lipids, lipoproteins, and lipopolysaccharide (LPS)

• OM is more permeable than plasma membrane due to presence of porin proteins and transporter proteins

• porin proteins form channels to let small molecules (600–700 daltons) pass

• Consist of a thin layer of peptidoglycan ~5{-}10 ext{%} of cell wall weight

• surrounded by an outer membrane

• Braun’s lipoproteins connect OM to peptidoglycan

• Periplasmic space differs from that in Gram-positive cells

• may constitute 20{-}40 ext{%} of cell volume

• No teichoic acids

LPS – Lipopolysaccharide and its importance

• Contributes to negative charge on cell surface and stabilize OM structure

• Attachment to surfaces and biofilm formation

• Consists of three parts

  • lipid A embedded in OM and acts as an endotoxin

  • core polysaccharide

  • O side chain (O antigen) - Antibody response directed towards this antigen-(sero- typing)

Mechanism of Gram Stain Reaction

• Gram stain reaction due to nature of cell wall

• Shrinkage of the pores of peptidoglycan layer of Gram-positive cells

• constriction prevents loss of crystal violet during decolorization step

• Thinner peptidoglycan layer and larger pores of Gram-negative bacteria do not prevent loss of crystal violet

• alcohol may also remove/extract some lipids from outer layer of Gram- negative cell wall, making crystal violet dye removal easier

Concept Check

• Describe peptidoglycan structure

• Compare and contrast the cell walls of typical Gram- positive and Gram-negative bacteria

1. Practice Questions (from Concept Checks)

• 1. Bacteria that do not have a fixed shape are said to be pleomorphic. A. True B. False

• 2. The cell membrane is a rigid structure that provides bacteria with their characteristic shapes A. True B. False

• 3. Gram-positive bacteria have a structurally and chemically more complex cell wall than Gram-negative bacteria. A. True B. False