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.co
m/howbig.htm
http://www.ionizers.org/S
izes-of-Bacteria.html
http://smccd.net/accounts/case/biol230/ex3/bact.jpeg
Overview of
prokaryotic cell.
2
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.
Structure of phospholipids
http://biyoloji_genetik.sitemynet.com/genel_biyoloji/genel_biyoloji_logos/phospholipids.gif
4
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/BiologyPa
ges/L/LipidBilayer.gif
Cell Membranes
6
• 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.
7
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.
8
Division of the Eubacteria:
Gram Negative and Gram Positive
9
• 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/s
ampletour/medical/GramNegat
iveEnvelope.gif
http://www.conceptdraw.com/s
ampletour/medical/GramPositi
veEnvelope.gif
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”.
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12
General Structure of peptidoglycan
NAM and NAG
are 2 complex
sugars that
alternate.
Everything else
are the amino
acids that
crosslink.
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Ways to think about peptidoglycan
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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
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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.
Osmosis
16
• 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).
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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
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”.
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Effect of osmotic pressure on cells
• Hypotonic:
water rushes in;
PG prevents cell
rupture.
• Hypertonic:
water leaves
cell, membrane
pulls away from
cell wall.
19
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
20
Cell Wall Exceptions
21
• 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)