Bacterial Cell Structure and Function

1. Bacterial Appearance • Size • 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 http://www.cellsalive.com/howbig.htm http://www.ionizers.org/Sizes-of-Bacteria.html http://smccd.net/accounts/case/biol230/ex3/bact.jpeg

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 http://biyoloji_genetik.sitemynet.com/genel_biyoloji/genel_biyoloji_logos/phospholipids.gif

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 http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LipidBilayer.gif

6. Cell Membranes • 50/50 lipids and proteins • 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 http://www.conceptdraw.com/sampletour/medical/GramNegativeEnvelope.gif http://www.conceptdraw.com/sampletour/medical/GramPositiveEnvelope.gif

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. http://www.sp.uconn.edu/~terry/images/other/peptidoglycan.gif; http://www.alps.com.tw/cht/img/anti-allergy_002.jpg

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. http://www.visionengineer.com/env/normal_osmosis.gif

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. 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)