This document describes the key structures and characteristics of prokaryotic cells. It discusses the differences between eukaryotic and prokaryotic cells, as well as the external structures of bacterial cells such as flagella, fimbriae, pili, and glycocalyx. It also summarizes the internal structures of bacterial cells, including the cell membrane, cytoplasm, chromosome, plasmids, ribosomes, inclusions, and endospores. Finally, it covers the typical shapes of bacterial cells.

1. 1
Characteristics of Cells
Eukaryotic cells: animals, plants, fungi, and
protists
– Contain membrane-bound organelles that
compartmentalize the cytoplasm and perform specific
functions
– Contain double-membrane bound nucleus with DNA
chromosomes
Prokaryotic cells: bacteria and archaea
– No nucleus or other membrane-bound organelles

2. 4.3 Prokaryotic Profiles
2

3. Figure 4.1 Structure of a bacterial cell
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4. 4
4.3 External Structures
• Appendages
– Two major groups of appendages:
• Motility – flagella and axial filaments (periplasmic
flagella)
• Attachment or channels – fimbriae and pili
• Glycocalyx – surface coating

5. 5
Flagella
• 3 parts:
– Filament – long, thin, helical structure composed of protein
Flagellin
– Hook – curved sheath
– Basal body – stack of rings firmly anchored in cell wall
• Rotates 360o
• Number and arrangement of flagella varies:
– Monotrichous, lophotrichous, amphitrichous, peritrichous
• Functions in motility of cell through environment

6. Figure 4.2 Flagella
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7. 7
Flagellar Arrangements
1. Monotrichous – single flagellum at one end
2. Lophotrichous – small bunches emerging
from the same site
3. Amphitrichous – flagella at both ends of cell
4. Peritrichous – flagella dispersed over surface
of cell; slowest

8. Figure 4.3 Electron micrographs of
flagellar arrangements
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9. 9
Flagellar Responses
Guide bacteria in a direction in response to external
stimulus:
Chemical stimuli – chemotaxis; positive and negative
Light stimuli – phototaxis
Signal sets flagella into rotary motion clockwise or
counterclockwise:
Counterclockwise – results in smooth linear direction –
run
Clockwise – tumbles

10. 10
Fimbriae
• Fine, proteinaceous, hairlike bristles
emerging from the cell surface
• Function in adhesion to other cells and
surfaces

11. 11
Pili
• Rigid tubular structure made of pilin protein
• Found only in gram-negative cells
• Function to join bacterial cells for partial DNA
transfer called conjugation

12. 12
Glycocalyx
• Coating of molecules external to the cell wall,
made of sugars and/or proteins
• Two types:
1. Slime layer - loosely organized and attached
2. Capsule - highly organized, tightly attached
• Functions:
– Protect cells from dehydration and nutrient loss
– Inhibit killing by white blood cells by phagocytosis,
contributing to pathogenicity
– Attachment - formation of biofilms

13. 13

14. 14

15. Figure 4.11 Biofilm on a catheter
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16. Table 4.1 Comparison of Gram-Positive and Gram-Negative
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17. 17
Nontypical Cell Walls
• Some bacterial groups lack typical cell wall
structure, i.e., Mycobacterium and Nocardia
– Gram-positive cell wall structure with lipid
mycolic acid (cord factor)
• Pathogenicity and high degree of resistance to certain
chemicals and dyes
• Basis for acid-fast stain used for diagnosis of infections
caused by these microorganisms
• Some have no cell wall, i.e., Mycoplasma
– Cell wall is stabilized by sterols
– Pleomorphic

18. Figure 4.15 Extreme variation in shape of
Mycoplasma pneumoniae
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19. 19
Cell Membrane Structure
• Phospholipid bilayer with embedded proteins –
fluid mosaic model
• Functions in:
– Providing site for energy reactions, nutrient processing, and
synthesis
– Passage of nutrients into the cell and the discharge of wastes
• Cell membrane is selectively permeable

20. 20
4.5 Bacterial Internal Structures
• Cell cytoplasm:
– Dense gelatinous solution of sugars, amino acids,
and salts
– 70-80% water
• Serves as solvent for materials used in all cell functions

21. 21
• Chromosome
– Single, circular, double-stranded DNA
molecule that contains all the genetic
information required by a cell
– Aggregated in a dense area called the nucleoid
• DNA is tightly coiled
Bacterial Internal Structures

22. Figure 4.17 Chromosome structure
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23. 23
Bacterial Internal Structures
• Plasmids
– Small circular, double-stranded DNA
– Free or integrated into the chromosome
– Duplicated and passed on to offspring
– Not essential to bacterial growth and metabolism
– May encode antibiotic resistance, tolerance to toxic
metals, enzymes, and toxins
– Used in genetic engineering - readily manipulated
and transferred from cell to cell

24. 24
Bacterial Internal Structures
• Ribosomes
– Made of 60% ribosomal RNA and 40% protein
– Consist of two subunits: large and small
– Prokaryotic differ from eukaryotic ribosomes in
size and number of proteins
– Site of protein synthesis
– Present in all cells

25. 25
Bacterial Internal Structures
• Inclusions and granules
– Intracellular storage bodies
– Vary in size, number, and content
– Bacterial cell can use them when environmental
sources are depleted
– Examples: glycogen, poly b-hydroxybutyrate, gas
vesicles for floating, sulfur and phosphate
granules (metachromatic granules), particles of
iron oxide

26. Figure 4.19 Bacterial inclusion bodies
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27. 27
Bacterial Internal Structures
• Endospores
– Inert, resting, cells produced by some G+ genera:
Clostridium, Bacillus, and Sporosarcina
• Have a 2-phase life cycle:
– Vegetative cell – metabolically active and growing
– Endospore – when exposed to adverse environmental conditions;
capable of high resistance and very long-term survival
– Sporulation - formation of endospores
• Hardiest of all life forms
• Withstands extremes in heat, drying, freezing, radiation, and
chemicals
• Not a means of reproduction
– Germination - return to vegetative growth

28. Figure 4.22 Sporulation cycle
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29. 29
Endospores
• Resistance linked to high levels of calcium and
dipicolinic acid
• Dehydrated, metabolically inactive
• Thick coat
• Longevity verges on immortality, 250 million
years
• Resistant to ordinary cleaning methods and boiling
• Pressurized steam at 120oC for 20-30 minutes will
destroy

30. 30
4.6 Bacterial Shapes,
Arrangements, and Sizes
• Vary in shape, size, and arrangement but
typically described by one of three basic
shapes:
– Coccus – spherical
– Bacillus – rod
• Coccobacillus – very short and plump
• Vibrio – gently curved
– Spirillum – helical, comma, twisted rod,
• Spirochete – spring-like

31. Figure 4.23 Common bacterial shapes
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