Overview of Prokaryotic Cell Structure and Function

1. 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 [INSERT FIGURE 3.2]

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 • Protects cells from drying out • Sticky layer that allows prokaryotes to attach to surfaces

6. External Structures of Prokaryotic Cells [INSERT FIGURE 3.5]

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 [INSERT FIGURE 3.8]

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 [INSERT FIGURE 3.9]

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 [INSERT FIGURE 3.10]

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 [INSERT FIGURE 3.11]

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 [INSERT FIGURE 3.12]

21. External Structures of Prokaryotic Cells [INSERT FIGURE 3.13]

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 [INSERT FIGURE 3.17]

32. Prokaryotic Cytoplasmic Membranes [INSERT FIGURE 3.19]

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 [INSERT TABLE 3.5]