1. Bacterial Appearance Size Shape Arrangement 0.2 µm – 0.1 mm
Most 0.5 – 5.0 µm
Shape
Coccus (cocci); rod (bacillus, bacilli); spiral shapes (spirochetes; spirillum, spirilla); filamentous; various odd shapes.
Arrangement
Clusters, tetrads, sarcina, pairs, chains
See Chapter 11

2. Overview of
prokaryotic cell.

3. From Membrane Out: Examination of layers of bacterial cell
Cell membrane
Cell wall
Then next week after the test:
Gram negative cell wall
Layers and structures outside the cell wall
Examination of inside of bacterial cell
A look at how things get into cells
Brief review of eukaryotic cell structure.

4. Structure of phospholipids

5. How phospholipids work
Polar head groups associate with water
but hydrophobic tails associate with
each other to avoid water.
When placed in water, phospholipids
associate spontaneously side by side
and tail to tail to form membranes.
Lipid Bilayer

6. Cell Membranes 50/50 lipids and proteins Fluid mosaic model
Proteins can be on inner or outer surface or extend through the membrane.
Fluid mosaic model
Membrane like a soap bubble, proteins float around on/in the membrane
Effective barrier to large and hydrophilic molecules
O2, CO2, H2O, lipid substances can pass through
Salts, sugars, amino acids, polymers, cannot.
Special proteins needed (transport proteins) to allow molecules to pass through the membrane.

8. Outside the cell membrane: the Cell Wall
Animal cells do not have a cell wall outside the cell membrane.
Plant cells and fungal cells do.
So do most prokaryotic cells, providing structural support and influencing the shape of the cell.
The polymer found in the cell walls of nearly all Eubacteria is a complex polysaccharide called peptidoglycan.

9. Division of the Eubacteria: Gram Negative and Gram Positive
Gram stain invented by Hans Christian Gram
The color that the cells stain usually matches with a particular cell wall architecture.
Architecture:
Gram positives have a thick peptidoglycan layer in the cell wall;
Gram negatives have a thin peptidoglycan layer and an outer membrane.
Stain is valuable in identification.
Gram positives stain purple; Gram negatives stain pink.

10. Gram Negative Gram Positive

11. Function and Structure of peptidoglycan
Provides shape and structural support to cell
Resists damage due to osmotic pressure
Provides some degree of resistance to diffusion of molecules
Single bag-like, seamless molecule
Composed of polysaccharide chains cross linked with short chains of amino acids: “peptido” and “glycan”.

12. General Structure of peptidoglycan
NAM and NAG are 2 complex sugars that alternate. Everything else are the amino acids that crosslink.

13. Ways to think about peptidoglycan

14. Peptidoglycan is a 3D molecule
Cross links are both horizontal and vertical between glycan chains stacked atop one another.

15. 2nd Law of Thermodynamics
All things tend toward entropy (randomness).
Molecules move (diffuse) from an area of high concentration to areas of low concentration.
Eventually, molecules become randomly distributed unless acted on by something else.

16. Osmosis Osmosis: a special case of diffusion
Water flows from where it is more concentrated (a dilute solution) to where it is less concentrated (a solution with many solute molecules)
Osmosis requires a “semi-permeable” membrane
One which water, but not dissolved substances, can pass through.
Cells typically have lots of dissolved substances; the net flow of water is into the cell (unless resisted).

17. Osmosis
Yellow spots cannot move through membrane in middle. Water moves into compartment where spots are most concentrated, trying to dilute them, make concentration on both sides of the membrane the same.
In this example, gravity limits how much water can flow. In a bacterium, the peptidoglycan provides the limit.

18. Osmosis definitions
Movement of water across a semi permeable membrane.
If the environment is:
Isotonic: No NET flow.
Hypertonic: Water flows OUT of cell.
Hypotonic: Water flows IN.
Water can flow both ways; we are considering NET flow.
Terms are comparative terms, like the word “more”.

19. Effect of osmotic pressure on cells
Hypotonic: water rushes in; PG prevents cell rupture.
Hypertonic:
water leaves cell, membrane pulls away from cell wall.

20. Bacteria and Osmotic pressure
Bacteria typically face hypotonic environments
Insides of bacteria filled with proteins, salts, etc.
Water wants to rush in, explode cell.
Protection from hypertonic environments is different, discussed later.
Peptidoglycan provides support
Limits expansion of cell membrane
Chemicals that damage peptidoglycan can kill cells
Penicillin, cephalosporin prevent PG synthesis
Lysozyme in body secretions cut bonds of PG

21. Mycobacterium and relatives
Cell Wall Exceptions
Mycobacterium and relatives
Wall contains lots of waxy mycolic acids
Attached covalently to PG
Mycoplasma: no cell wall
Parasites of animals, little osmotic stress
Archaea, the 3rd domain
Pseudomurein and other chemically different wall materials (murein another name for PG)