1. Lecture 3 Structure and Function of Prokaryotes
(Text Chapter: )

2. Bacterial cell

3. Cell Membrane

4. Cell Membrane
Lipid bilayer : hydrophilic exteriors and a hydrophobic interior.
Highly selective permeability : The attraction of the nonpolar fatty acid portions of one phospholipid layer for the other layer helps to account for the selective permeability of the cell membrane.
Hopanoids in bacteria (similar to sterols in eukaryotes), may strengthen the membrane as a result of their rigid planar structure.
Integral proteins involved in transport and other functions traverse the membrane.

5. Prokaryotic vs Eukaryotic Membranes
Different composition of phospholipid
Absence of sterols in prokaryotic membranes
In many bacteria hopanoids instead

6. Bacteria vs Archaea Membranes
Archaea contain ether-linked lipids
No fatty acids but isoprene (5 carbon hydrocarbon) in Archaea
Some Archaea have lipid monolayers

7. Monolayer Membrane of Archaea
Monolayer results from covalent linkage of glycerol side chains

8. Function of Cell Membrane
Permeability barrier
Prevents leakage of cytoplasmic metabolites into the environment
Selective permeability also prevents diffusion of most solutes

9. Anchor for membrane proteins involved in
Specific transport mechanisms to accumulate nutrients against the concentration gradient
Bioenergetics
Chemotaxis
Site for energy conservation in the cell

10. Membrane Transport Systems

11. Cell Walls : Peptidoglycan
This material consists of strands of alternating repeats of N-acetylglucosamine and N-acetylmuramic acid, with the latter cross-linked between strands by short peptides. Many sheets of peptidoglycan can be present, with the number of sheets dependent on the organism.
Each peptidoglycan repeating subunit is composed of four amino acids (L-alanine, D-alanine, D-glutamic acid, and either L-lysine or meso-diaminopimelic acid) and two N-acetyl-glucose-like sugars (Figure 4.29).

12. Peptidoglycan
Tetrapeptide cross-links formed by the amino acids from one chain of peptidoglycan to another provide the cell wall of prokaryotes with extreme strength and rigidity (Figure 4.30).

13. Cell wall of Gram negative vs positive
Gram-negative Bacteria have only a few layers of peptidoglycan (Figure 4.27b), but gram-positive Bacteria have several layers (Figure 4.27a),
as well as a negatively charged teichoic acid polyalcohol group (Figure 4.31).

14. Surface Structure of Gram Positive Bacteria

15. The Outer Membrane of Gram-Negative Bacteria
Outer membrane consisting of lipopolysaccharide (LPS), protein, and lipoprotein
Porins allow for permeability across the outer membrane by creating channels that traverse the membrane.
Periplasm, which contains various proteins involved in important cellular functions.

16. Lipopolysaccharide (LPS)
Lipopolysaccharide (LPS) is composed of lipid A, a core polysaccharide, and an O-specific polysaccharide (Figure 4.34).
Lipid A of LPS has endotoxin properties, which may cause violent symptoms in humans (fever and, if the concentration is high enough, shock).

17. Cell Wall and Gram Stain
The structural differences between the cell walls of gram-positive and gram-negative Bacteria are thought to be responsible for differences in the Gram stain reaction.
Alcohol can readily penetrate the lipid-rich outer membrane of gram-negative Bacteria and extract the insoluble crystal violet-iodine complex from the thin peptidoglycan layer.

18. Exceptions: Bacteria without Cell Walls
Some prokaryotes are free-living protoplasts that survive without cell walls because they have unusually tough membranes or live in osmotically protected habitats, such as the animal body.
Species of mycoplasmas have no cell walls because they live in animal cells and they possess stronger membrane due to higher content of sterol, which they assumed from the host.

19. Cell Walls of Archaea Pseudopeptidoglycan
: Consist of N-acetyl talosaminuronic acid (NAT)
b-1,3 bonds instead of the b-1,4 bonds of peptidoglycan (Lysozyme insensitive)

20. Bacterial Shape: Common Shapes
Coccus
Round, spherical
E.g. Staphylococcus epidermidis
Rod (bacillus)
E.g. E. coli, B. anthracis
Spiral
Fixed: spirilla
Flexible: spirochetes (Treponema pallidum)
Filamentous

21. Bacterial Shape: Additional Shapes
Unusual
Star-shaped
Square
Triangular
Pleomorphic
Within a population various shapes (Corynebacteria)
No fixed shape
Cellwall-less: Mycoplasma/Ureaplasma
Shape is influenced by environmental conditions, age of culture, antibiotic pretreatment!

22. Surface Structures
Prokaryotic cells contain various surface structures
Fimbriae (F) and pili (P)
S-layers (S)
Capsules (C)
Slime layers (SL)
Key functions
Attaching cells to a solid surface or other cells (F, P,SL)
Providing protection (S, C)

23. Surface Structures: Fimbria and Pili
Protein filaments
Shorter and thinner: fimbriae
Longer and fewer: pili
filaments that are best known for their function in conjugation (bacterial sex)

24. Surface Structures: S-Layer
Two-dimensional array of protein called an S-layer
Often hexagonal symmetry
Selective sieve allowing the passage of low-molecular-weight substances while excluding large molecules and structures
Protection
Mainly in Archaea, but also found in some Bacteria

25. Surface Structures: Capsules and Slime Layer
Carbohydrates
Protection against desiccation
Capsule
more rigid and thicker
excludes more particles
sometimes made of protein
more protective, antiphagocytic
Slime layer
Attachment

26. Inclusion Bodies
Prokaryotic cells often contain internal granules that function as energy reserve, reservoir for building blocks, or in magnetotaxis
Lipids, sugars
Poly--hydroxyalkanoates (PHAs) such as poly--hydroxybutyrate (PHB)
Glycogen
Inorganic substances
Polyphosphate
Elemental sulfur (in periplasm)
Magnetite

27. Inclusion Bodies: Magnetosomes
Intracellular particles of the iron mineral magnetite (Fe3O4) that allow organisms to respond to a magnetic field
Orientation and alignment along Earth’s magnetic field, probably to deeper sediments

28. Gas Vesicles
Small gas-filled structures made of protein that confer buoyancy on cells by decreasing their density
Contain two different proteins arranged to form a gas-permeable structure impermeable for water and solutes (Figure 4.46)
Means of motility, which allows organisms in water to position themselves for optimum light harvesting
Common in many species of cyanobacteria

29. Gas Vesicles in Cyanobacteria
longitudinal
cross section

30. Endospores Highly resistant differentiated bacterial cell
Produced by certain gram-positive (Bacillus sp, Clostridium sp)
Enable the organism to endure extreme environmental conditions
Endospore formation leads to a highly dehydrated structure
Contain essential macromolecules and a variety of substances absent from vegetative cells

31. Endospores (II)
Significantly different from the vegetative, or normal functioning cells
Calcium–diplicolinic acid complexes
Reduce water availability within the endospore thus helping to dehydrate it
Intercalate in DNA and stabilizing it to prevent heat denaturation
Small acid-soluble proteins
Protect DNA from UV radiation, desiccation, and dry heat
Serve as a carbon and energy source during germination
Can remain dormant indefinitely but germinate quickly when the appropriate trigger is applied

33. Endospores (III)
Emergence of the vegetative cell is the result of endospore activation, germination, and subsequent outgrowth

34. Flagella
Motility in most microorganisms is accomplished by flagella. In prokaryotes, the flagellum is a complex structure made of several proteins, most of which are anchored in the cell wall and cytoplasmic membrane.
The flagellum filament, which is made of a single kind of protein (flagellin), rotates at the expense of the proton motive force, which drives the flagellar motor.
Flagella move the cell by rotation, much like the propeller in a motor boat (Figure 4.56). An appreciable speed of about 60 cell lengths/second can be achieved.

35. Hook and basal body differ in gram-positive and gram-negative bacteria!