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Overview of Prokaryotic Cell Structure and Function

This chapter provides an overview of prokaryotic cells, including their external structures, cell walls, and cytoplasmic membranes. It discusses the composition, function, and importance of glycocalyces, flagella, fimbriae, pili, and cell walls in prokaryotes. The differences between Gram-positive and Gram-negative cell walls are also explained.

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Overview of Prokaryotic Cell Structure and Function

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  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]

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