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PROKARYOTIC CELL STRUCTURE AND FUNCTION
I.
The cell theory:
A. Developed by:
Schleiden and Schwann 1838-39
B. Basic structural unit of all organisms (with the exception of the viruses) is the cell.
C. Cells always come from preexisting cells.
Parent cell grows and divides  new cells.
D. Cells never arise de novo (spontaneously):
Produced only by a parent cell.
II. Electron Microscope (1960- 65):
A. Showed two structurally different types of cells (see pages 89 - 91)
1. Prokaryotic cells:
Structural-unit of bacteria and blue-green bacteria
2. Eukaryotic cells:
Structural unit of algae, fungi, protozoa, higher plants and animals
III. SHAPE AND ARRANGEMENT
A. Most common bacterial shapes are:
1.
Spherical (coccus, cocci)
2.
Cylindrical (bacillus, bacilli)
3.
Cocci arranged:
a.
Groups of two (diplococcus)
b.
Chains:
Cells remain together after repeated divisions
in one plane (Streptococcus, Enterococcus, and Lactococcus)
4.
5.
Irregular grape-like clumps:
a.
Cells divide in random planes: Staphylococcus
Symmetrical clusters:
Cells divide in two or three planes:
Square groups of four cells (tetrads): Result from division in two planes:
Micrococcus
Cubical packets of eight cells:
Result from division in three planes:
Sarcina
6. Bacilli (rod-shaped or cylindrical bacteria)
May be:
a. Short and fat:
Resemble cocci (coccobacilli)
b. Long and skinny:
Single cells
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Chains
Curved rods: Vibrios
7. Other bacterial shapes:
a.
Long filaments or hyphae:
Produce a network or mycelium:
Actinomycetes
b.
Spirals
Flexible spirals:
Spirochetes
c.
Rigid spirals:
Spirilla
8. Pleomorphic bacteria:
Variable in shape:
Lack a single characteristic form:
May have a generally rod-like form:
Corynebacterium
B. SIZE
1. Units of Measure
meter
centimeter
millimeter
micrometer
nanometer
Ångstrom
m
cm
mm
m
nm
Å
39.37 inches
1/100m
1/1000m
1/1,000,000m
1/1,000,000,000m
1/10,000,000,000m
1 m
10-2m
10-3m
10-6m
10-9m
10-10m
2. Bacteria have a wide range of sizes:
100 - 200 nm in diameter:
Mycoplasma
7m in diameter:
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Oscillatoria
1 m to 500 m in length
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IV. PROKARYOTIC CELL ORGANIZATION
A. Cell wall (chemically complex):
Outer boundary of all prokaryotic cells:
Except Mycoplasma
B. Periplasmic space:
Separates cell wall from cell membrane:
Contains hydrolytic enzymes
Protein binding sites
C. Cell membrane:
Composed of:
Phospholipids
Proteins
D. Chemical composition and structure similar in prokaryotic and eukaryotic cells
1. Phospholipids:
Amphipathic:
o Have polar and non-polar ends:
o Polar ends interact with H2O:
Hydrophilic
o Non-polar ends interact with each other:
o Insoluble in H2O: Hydrophobic
2. Membrane proteins:
Primarily hydrophobic:
Associate primarily with fatty acids
Have a few hydrophilic groups:
Allow them to associate with H2O
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3. Carbohydrates:
May be attached to outer surface of membrane proteins
4. Membrane proteins:
Can diffuse laterally to new locations
Do not flip-flop or rotate through the lipid bilayer
E. FLUID MOSAIC THEORY OF MEMBRANE STRUCTURE:
o Proposed by Jonathan Singer and Garth Nicholson (1972)
o Sidedness:
Different areas of the membrane have different functions
F. HOPANOIDS
o Pentacyclic steroid-like molecules:
o Found in most bacterial membranes
o Stabilize the bacterial membrane
G. MEMBRANE FUNCTIONS:
o Encloses cell:
Retains cytoplasm (must be intact)
o Selectively permeable barrier:
o Regulates passage of substances into and out of cell
o Keeps various molecules in the cell
o Keeps various molecules out of cell
H. Location of a number of metabolic processes:
o Respiration
o Photosynthesis
o Synthesis of lipids
o Synthesis of cell wall components
Contains receptor molecules:
Help bacteria detect and respond to molecules in their environment.
I.
INTERNAL MEMBRANE SYSTEMS
1. CELL INTERIOR
Lacks compartmentalization:
Areas with specialized functions:
Not enclosed in phospholipid membranes:
Not segregated from the rest of the cell
2. CYTOPLASMIC MATRIX:
70% water
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3.
Substance between the plasma membrane and the nucleoid:
Contains:
Ribosomes
Plasmids
Inclusion bodies
Mesomes:
Invaginations of the cell membrane:
Form:
Tubules
Vesicles
Lamellae
Function unknown:
May be involved in:
Cell wall formation
4. RIBOSOMES:
Found in all cells
o Prokaryotic
o Eukaryotic.
o Essential to life
o Site of protein synthesis
o 10,000 in prokaryotic cell, more in eukaryotic cell.
o Prokaryotic ribosome smaller (70s) than eukaryotic ribosome (80s).
o Composed of:
Ribonucleic acid (RNA):
rRNA (ribosomal)
Protein
5. PLASMIDS:
Small circular molecules of DNA
Contain limited amount of genetic information:
Antibiotic resistance
Mating type (sex)
Resistance to toxic materials
Not essential for life.
Bacteria may contain none, one or more.
6. INCLUSION BODIES
Reserve materials
Gas vacuoles
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RESERVE MATERIALS:
Cells accumulate various biochemicals:
Act as nutrient reserves in times of need:
Metachromatic granules = Volutin
Polyphosphate:
Phosphorus reserves:
Used in synthesis of:
ATP
RNA
NADP
Polysaccharides - Energy reserves:
Glycogen
DNA
NAD
Starch
Lipid-like material - Energy reserve:
Poly-beta-hydroxybutyric acid
7. GAS VACUOLES:
Present in bacteria that float on surface of ponds or lakes:
Many blue-green bacteria
Allow:
Adjustment of cell's buoyancy
Photosynthetic cells to move to areas where light conditions are best
for photosynthesis.
Surrounded by a gas-permeable protein membrane:
Impermeable to dissolved solids and to liquids
8. NUCLEOID
Located in the cytoplasm
One to several per cell
Each contains single long molecule of DNA:
10. BACTERIAL CHROMOSOME:
Single helical molecule of DNA
Arranged in a circle:
No beginning, no end.
Not associated with protein (naked)
NUCLEOID NOT SURROUNDED BY A MEMBRANE
MAJOR DIFFERENCE BETWEEN PROKARYOTIC AND EUKARYOTIC CELLS
11. THE PROKARYOTIC CELL WALL
Surrounds the cell membrane of all prokaryotic cells except the Mycoplasma.
Functions:
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Protects against osmotic shock.
Gives cell rigidity and shape:
Without cell wall most prokaryotic cells would LYSE (burst)
Relatively porous:
Allows small molecules to move to and from the cell membrane:
Salt
Sugars
Amino acids
H2O
Does not allow large molecules through:
DNA
RNA
Proteins
Polysaccharides
Chemical composition:
Peptidoglycan or murein
Macromolecule found only in prokaryotic cell wall
Composed of:
N-acetylglucosamine
Amino sugars
N-acetylmuramic acid
Amino sugars and amino acids linked together to form a rigid layer
Cell walls organized differently in the two major groups of bacteria:
Gram positive
Gram negative
Peptidoglycan found in nearly all prokaryotic cell walls:
In different amounts
Associated with different biochemicals
Differences in structure used to classify the two major groups of bacteria:
Gram positive cell wall structure:
Crystal violet-iodine complex retained when cell is washed with
organic solvent
Composition:
Thick peptidoglycan layer:
90% of cell wall material
Large amounts of Teichoic acids
Polymers of glycerol or ribitol joined by PO4
V. GRAM REACTION
A. Gram reaction is due to differences in physical structure of cell wall.
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B. Pores in Gram positive cell wall swell shut when cell is treated with organic
solvent (ethyl alcohol)
Crystal violet-iodine complex trapped in the periplasmic space - cell remains
blue/purple
C. Gram Negative Cells
Very thin peptidoglycan layer
Larger pores than Gram positive cell wall
Solvent gets into the periplasmic space and dissolves the crystal violet-iodine
complex.
VI. COMPONENTS EXTERNAL TO THE CELL WALL
A. Bacterial capsules, slime layers, and S layers
Cell wall may be surrounded by layer of polysaccharide &/or proteins.
1. Capsule - well organized, not easily washed off
Major factor in determining pathogenicity, because:
It protects the cell from phagocytosis
Allows disease-causing bacteria to attach to host tissue
Allows saprophytic bacteria to attach to areas where food is available
2. Slime layer:
Similar to capsule, but more water soluble
Zone of diffuse unorganized material:
Easily removed
Protects against:
Dehydration
Loss of nutrients
Restricts:
Movement of substances away from cell
Attaches cell to solid surfaces:
Rocks
Plants
Detritus
Teeth (Streptococcus mutans)
3. S layer:
Occurs in many Gram-negative and Gram-positive cells
Composed of:
Protein or glycoprotein
Arranged in pattern like floor tiles
Protects cell against:
Ion and pH fluctuations
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Osmotic stress
Enzymes
Predacious bacteria (Bdellovibrio)
B. Pili and Fimbriae:
Found only on Gram negative bacteria
1. Fimbriae:
Short hair-like projections from the cell surface
Up to 10,000/cell
Slender tubes
3 -10 nm in diameter
Several m long
Composed of pilin:
Small helical protein molecule
Originate from the cell membrane
Several types with different functions:
Attach bacteria to surfaces:
Rocks in streams
Host tissues: Neisseria gonorrhoeae:
Attaches to cells in the urogenital tract via fimbriae
Formation of surface films
2. Sex Pili:
Similar to fimbriae:
1 - 10/cell
Larger than fimbriae
9 -10 nm in diameter
Presence genetically determined by a plasmid
Required for conjugation:
Transfer DNA during mating
Serve as attachment site for viruses
C. Flagella
Size:
20 nm in diameter
15-20 m long:
10x the length of the cell
Composed of flagellen:
Small helical protein molecule
Originate from the cell membrane
Function:
Propel motile cells through liquid media
Arrangement varies:
Often used in classification of bacteria:
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Polar flagella
Lophotrichous flagella
Amphitrichous flagella
Peritrichous flagella
D. Bacterial Endospores:
Formed by several species of bacteria in response to decreased food supply:
Bacillus
Clostridium
Sporosarcina
Resistant to:
Heat
Drying
Radiation
Chemicals
Cold
Endospores may remain dormant for years:
Viable spores:
Found in Egyptian mummy wrappings:
7500 years old
Germinate in a few hours when conditions become favorable
Endospore position in sporangium (mother cell) used in identification:
Central spore
Sub-terminal spore
Terminal spore
Terminal spore with
swollen sporangium
Composed of:
Dipicolonic acid:
May stabilize spore's nucleic acid
Calcium:
Calcium dipicolinate is found only in prokaryotic endospores
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