Lecture 2 Chapters 2 & 3

Prokaryotic Structure & Function

Gram StainingThe Gram stain depends on the ability of bacteria to retain a purple color (based on cellular envelope).

Gram Negative• More numerous• Does not retain

dye coloringpurple red

red dye

alcohol

purple dye

Gram negative has an outer membrane made up of an assortment of: • Lipopolysaccharides• Proteins

The Difference:

Cell Membrane = Inner Membrane

GRAM POSITIVE• Thick cell wall that protects the membrane

from high internal turgor pressure• Made of peptidoglycan called mureinMurein does a lot!– Protector, provides rigidity and shape– Not the only determinant of bacteria cell shape– Provides hydrophilic layerMurein is unique to bacteria making it a good target for antibiotics through preventing murein synthesis.

Osmotic Pressure

• Murein allows for the cell to survive under different osmotic pressures.

• When lysozymes hydrolyze part of the murein layer the cell acts more sensitively:– Low osmotic pressure outside the cell: lyses– High osmotic pressure outside the cell: the cell

changes shape (rod to sphere)In gram negative this is called protoplastsIn gram positive this is called spheroplasts

High osmotic pressure

Low osmotic pressure outside the cell lyse High osmotic pressure outside the cell shape change

Low osmotic pressure

Shape Change

The Different Shapes of Bacteria

Singular Spirillum Bacillus Coccus

Plural

Cell Membrane + Outer Membrane

GRAM NEGATIVE• Thin murein layer connected to the outer membrane by a

lipoptein• Periplasm - space between the two membranes• The outer membrane contains LPS• LPS is unique to prokaryotes!

– lipid A• Causes fever and shock

– Core– O antigen

• Immunogenic/toxic, highly variable

Hydrophilic polysaccharides on O antigen keep out hydrophobic compounds just like in gram positive

LPS Structure

Lipid A Core O Antigen

Portion that sticks out of cell

Hydrophilic

Gram Negative

Gram Positive

Transportation

GRAM NEGATIVE• The outer membrane contains special

channels called porins that allow hydrophilic compounds such as sugars, aa’s, and ions inside the cell (nonspecific).

• Large molecules – rely on active transport and energy usually coupled with inner membrane transport

Capsules and slime layers

Found in both +/-• Capsule: slime attached to

cells• Slime layer: looser layerRole:• Enables bacteria to adhere to

surfaces• Bacterial defense against

phagocytosis• Prevents

desiccation(dehydration)

Flagella

• Flagella – helical filaments that rotate and act as propellers

• Composed of three parts: filament connected via a hook to a basal body.

Different types of Flagella

1. Filament - composed of a single protein, flagellin2. Hook – connects filament to the cell3. Basal Body – composed of 15 proteins that

aggregate to form a rod

Flagella – (Regulated) Process1. Basal body assembled and inserted into cell

envelope2. Hook is added3. Filament is assembled progressively by the

addition of new flagellin subunits to its growing tip.

• Flagella grow from the tip outward with a hollow channel where the flagellin molecules are extruded

• A regulated process – inhibitor secreted once basal body is inserted

Pili• Common in gram-negative

bacteria• Functions include:– Gene transfer– Motility – Adhere to mucosal surfaces– Inhibit phagocytic ability of

white blood cells

Motility• Pili are straight rods – do not rotate!• “Twitching Motility” – move by pulling

themselves across a solid surfaceGrowth• Grow from the inside of the cell outward

unlike flagella

Pili vs. Flagella

The Cell Interior

Prokaryote

Eukaryote

The Nucleoid

• Contains DNA consisting of a large circular chromosome– With exceptions

• No membrane – With exceptions, an

example is: Plantctomycetes

DNA Packaging

• Long DNA is tightly folded into nucleoid

• Condensing DNA requires high ion concentration and DNA binding proteins

• Supercoiling of DNA also aids in packaging

DNA Supercoiling• Supercoiled circular DNA lowers the energy

barrier for strand separation• Balance of two enzymes:– DNA gyrase which does the supercoiling– Topoiseomerase I which does the uncoiling

Genome Comparison

Prokaryotic Range580,000 base pairs – 10 million base pairs

Eukaryotic Range2.9 million base pairs – 4 billion base pairs

DNA Copies & Plasmids

• Often multiple chromosome copies are seen in the cell because of rapid growth

• Decreasing growth rate single nucleoid

Plasmids• carry extrachromosomal material• “Disposable”

Cytoplasm

• Crowded inside the cell• Viscous (gel-like)• Chemical reactions within a bacterial cell

take place in an environment totally different from what is common in test tube studies

Gas Vesicles

• Structures filled with gas similar to that of the environment

• Vesicles surrounded by protein shell• Allow buoyancy for the cell so it can set level

in water column (and get the right amount of light)

• Ultimate purpose: control buoyancy for photosynthetic bacteria

Other Internal Structures

• Other structures found in bacteria and archaea play roles in:– Photosynthesis– Chemoautotrophy– Carbon fixation– Gorwth on certain substrates– Magnetosomes (bacteria contain tiny magnets

and they can orient themselves by responding to the magnetic fields on earth)