1. Cell Structure and Function
Chapter 3
Cell Structure and Function

2. Prokaryotic Cells: An Overview
Prokaryotes
Do not have membrane surrounding their DNA; lack a nucleus
Lack various internal structures bound with phospholipid membranes
Are small, ~1.0 µm in diameter
Have a simple structure
Composed of bacteria and archaea

3. Prokaryotic & Eukaryotic Cells: An Overview
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4. External Structures of Prokaryotic Cells
Glycocalyces
Gelatinous, sticky substance surrounding the outside of the cell
Composed of polysaccharides, polypeptides, or both

5. External Structures of Prokaryotic Cells
Two Types of Glycocalyces
Capsule
Composed of organized repeating units of organic chemicals
Firmly attached to cell surface
Protects cells from drying out
May prevent bacteria from being recognized and destroyed by host
Slime layer
Loosely attached to cell surface
Water soluble
Sticky layer that allows prokaryotes to attach to surfaces

6. External Structures of Prokaryotic Cells
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7. External Structures of Prokaryotic Cells
Flagella
Are responsible for movement
Have long structures that extend beyond cell surface
Are not present on all prokaryotes

8. External Structures of Prokaryotic Cells
Flagella
Structure
Composed of filament, hook, and basal body
Flagellin protein (filament) deposited in a helix at the lengthening tip
Base of filament inserts into hook
Basal body anchors filament and hook to cell wall by a rod and a series of either two or four rings of integral proteins
Filament capable of rotating 360º

9. External Structures of Prokaryotic Cells

10. External Structures of Prokaryotic Cells
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11. External Structures of Prokaryotic Cells
Flagella
Function
Rotation propels bacterium through environment
Rotation reversible, can be clockwise or counterclockwise
Bacteria move in response to stimuli (taxis)
Runs
Tumbles

12. External Structures of Prokaryotic Cells
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13. External Structures of Prokaryotic Cells
Fimbriae and Pili
Rod-like proteinaceous extensions

14. External Structures of Prokaryotic Cells
Fimbriae
Sticky, bristlelike projections
Used by bacteria to adhere to one another, to hosts, and to substances in environment
Shorter than flagella
May be hundreds per cell
Serve an important function in biofilms

15. External Structures of Prokaryotic Cells
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16. External Structures of Prokaryotic Cells
Pili
Tubules composed of pilin
Also known as conjugation pili
Longer than fimbriae but shorter than flagella
Bacteria typically only have one or two per cell
Mediate the transfer of DNA from one cell to another (conjugation)

17. External Structures of Prokaryotic Cells
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18. Prokaryotic Cell Walls
Provide structure and shape and protect cell from osmotic forces
Assist some cells in attaching to other cells or in eluding antimicrobial drugs
Not present in animal cells, so can target cell wall of bacteria with antibiotics
Bacteria and archaea have different cell wall chemistry

19. Prokaryotic Cell Wall
Bacterial Cell Walls
Most have cell wall composed of peptidoglycan
Peptidoglycan is composed of sugars, NAG, and NAM
Chains of NAG and NAM attached to other chains by tetrapeptide crossbridges
Bridges may be covalently bonded to one another
Bridges may be held together by short connecting chains of amino acids
Scientists describe two basic types of bacterial cell walls: Gram-positive and Gram-negative

20. External Structures of Prokaryotic Cells
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21. External Structures of Prokaryotic Cells
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22. Prokaryotic Cell Walls
Bacterial Cell Walls
Gram-positive cell walls
Relatively thick layer of peptidoglycan
Contain unique polyalcohols called teichoic acids
Some covalently linked to lipids, forming lipoteichoic acids that anchor peptidoglycan to cell membrane
Retain crystal violet dye in Gram staining procedure; so appear purple
Up to 60% mycolic acid in acid-fast bacteria helps cells survive desiccation

23. Prokaryotic Cell Walls
[INSERT FIGURE 3.14a]

24. Prokaryotic Cell Walls
Bacterial Cell Walls
Gram-negative cell walls
Have only a thin layer of peptidoglycan
Bilayer membrane outside the peptidoglycan contains phospholipids, proteins, and lipopolysaccharide (LPS)
May be impediment to the treatment of disease
Appear pink following Gram staining procedure

25. Prokaryotic Cell Walls
[INSERT FIGURE 3.14b]

26. Prokaryotic Cell Walls
Archaeal Cell Walls
Do not have peptidoglycan
Contains variety of specialized polysaccharides and proteins
Gram-positive archaea stain purple
Gram-negative archaea stain pink

27. Prokaryotic Cytoplasmic Membranes
Structure
Referred to as phospholipid bilayer; composed of lipids and associated proteins
Approximately half composed of proteins that act as recognition proteins, enzymes, receptors, carriers, or channels
Integral proteins
Peripheral proteins
Glycoproteins
Fluid mosaic model describes current understanding of membrane structure

28. Prokaryotic Cytoplasmic Membranes
[INSERT FIGURE 3.15]

29. Prokaryotic Cytoplasmic Membranes
Function
Energy storage
Harvest light energy in photosynthetic prokaryotes
Selectively permeable
Naturally impermeable to most substances
Proteins allow substances to cross membrane
Occurs by passive or active processes
Maintain concentration and electrical gradient
Chemicals concentrated on one side of the membrane or the other
Voltage exists across the membrane

30. Prokaryotic Cytoplasmic Membranes
[INSERT FIGURE 3.16]

31. Prokaryotic Cytoplasmic Membranes
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32. Prokaryotic Cytoplasmic Membranes
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33. Cytoplasm of Prokaryotes
Cytosol – liquid portion of cytoplasm
Inclusions – may include reserve deposits of chemicals
Endospores – unique structures produced by some bacteria that are a defensive strategy against unfavorable conditions

34. Cytoplasm of Prokaryotes

35. Cytoplasm of Prokaryotes
Nonmembranous Organelles
Ribosomes – sites of protein synthesis
Cytoskeleton – plays a role in forming the cell’s basic shape

36. Eukaryotic Cell Walls & Cytoplasmic Membranes
Fungi, algae, and some protozoa have cell walls but no glycocalyx
Composed of various polysaccharides
Fungal cell walls composed of cellulose, chitin, and/or glucomannan
Algal cell walls composed of cellulose, proteins, agar, carrageenan, silicates, algin, calcium carbonate, or a combination of these

37. Cytoplasm of Eukaryotes
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