Prescott’s Microbiology, 9th Edition

5Eukaryotic Cell Structure

CHAPTER OVERVIEW

This chapter focuses on eukaryotic cell structure and function. Although prokaryotic organisms are immensely important in microbiology, eukaryotic microorganisms—such as fungi, algae, and protozoa—are also prominent members of many ecosystems, and some have medical significance as etiological agents of disease and as model organisms for research. The detailed discussion of eukaryotic microbial structure and function is followed by a comparison of eukaryotes and prokaryotes.

LEARNING OUTCOMES

After reading this chapter you should be able to:

•compare and contrast eukaryotic, bacterial, and archaeal cells in terms of their use of membranes, size, morphological diversity, and organelles

•identify the types of eukaryotic microbes that have cell walls and distinguish them from plant cell walls

•compare and contrast the cell envelopes of members of Bacteria, Archaea and Eukarya in terms of their component layers, molecular makeup, and function

•describe the functions of the cytoplasm

•identify the three filaments that make up the cytoskeleton of eukaryotic cells and describe their functions

•differentiate the two types of endoplasmic reticulum in terms of structure and function

•outline the pathway of molecules through the secretory pathway, noting the structures involved and their

role in the process

•list the endocytic pathways observed in mammalian cells, noting the structures involved and their role in the process, and noting those pathways that have been observed in eukaryotic microbes

•describe the structure of the nucleus, chromosomes, nucleolus, and eukaryotic ribosomes

•compare and contrast the chromosomes and ribosomes of bacterial, archaeal, and eukaryotic cells

•draw a mitochondrion and identify its components

•compare and contrast mitochondria and hydrogenosomes in terms of their structure and the chemical processes they carry out

•draw a chloroplast and identify its components parts

•describe the structure of eukaryotic flagella and cilia

•compare and contrast bacterial, archaeal, and eukaryotic flagella

•list the types of motility observed in eukaryotic microbes

•create a Venn diagram or concept map that clearly distinguishes bacterial, archaeal, and eukaryotic cells in terms of their genome organization, organelles, cell envelopes, ribosome size and component molecules, and cytoskeleton

•determine the type of microbe when given a description of a newly discovered microbe

CHAPTER OUTLINE

  1. A Typical Eukaryotic Cell
  2. Eukaryotic cells have membrane encased nuclei
  3. In addition to the nucleus, eukaryotic cells have membrane-bound organelles that perform specific functions within the cell; this allows simultaneous independent control of cellular processes
  4. Eukaryotic cells also contain a large intracytoplasmic membrane complex, which provides a large surface area allowing greater respiratory and photosynthetic activity; this complex of membranes also serves as a transport system to move materials to different cell locations
  5. Eukaryotic Cell Envelopes
  6. Eukaryotic membranes are phospholipid bilayers with sphingolipids and sterols, but the lipids in the outer layer have been shown to differ from those in the inner layer
  7. Certain microdomains in the membranes differ in lipid and protein content to suit specific functions
  8. The cell wall composition in eukaryotic microbes varies and can include the polymers cellulose, glucan, or chitin, while others have a rigid silica shell
  9. Cytoplasm of Eukaryotes
  10. The cytoplasmic matrix provides the complex structured environment required for many cellular activities
  11. The cytoskeleton is a vast network of interconnected filaments and tubules important for motion and act as a scaffold for maintaining cell organization
  12. Microfilaments (4 to 7 nm) composed of actin may be scattered throughout the matrix or organized into networks and parallel arrays; they play a major role in cell motion and changes in cell shape
  13. Intermediate filaments (8 to 10 nm), along with microfilaments and microtubules, are major components of the cytoskeleton, and are particularly prominent in nuclear lamina
  14. Microtubules are hollow cylinders (25 nm) composed of tubulin that help maintain cell shape, are involved (with microfilaments) in cellular movement, participate in intracellular transport of substances, and participate in organelle movements; they also form the mitotic spindle during cell division and are present in cilia and flagella
  15. Organelles of the Secretory and Endocytic Pathways
  16. Endoplasmic reticulum
  17. The endoplasmic reticulum (ER) is a complex interconnected array of internal membranous tubes and sacs that may have ribosomes attached (rough endoplasmic reticulum; RER) or may be devoid of ribosomes (smooth endoplasmic reticulum; SER)
  18. The ER has many important functions:
  19. It transports proteins, lipids, and other materials within the cell
  20. ER-associated enzymes and ribosomes synthesize lipids and many proteins
  21. It is a major site of membrane synthesis
  22. Proteins often begin their protein folding and transport sorting here.
  23. The Golgi apparatus
  24. The Golgi apparatus is a set of membranous sacs (cisternae) that is involved in the modification, packaging, and secretion of materials; the cisternae exist in stacks called dictyosomes
  25. The Golgi apparatus is present in many eukaryotic cells, but many fungi and ciliated protozoa lack it
  26. Lysosomes are membrane-bound vesicles that contain hydrolase enzymes needed for intracellular digestion of all types of macromolecules
  27. Secretory pathway
  28. Proteins destined for the cell membrane, lysosomes, or secretion are recognized by amino acid targeting sequences and are transported in vesicles that bud off the ER and join the cis face of the Golgi apparatus; the proteins are modified and transported in vesicles that bud off of the trans face of the Golgi apparatus
  29. Transport vesicles move the material to the cell membrane or lysosome; secretory vesicles hold their contents until signaled to release them through fusion with the plasma membrane
  30. Proteasomes are a nonlysosomal protein degradation system found in eukaryotic cells that degrades proteins marked with ubiquitin
  31. Endocytic pathway
  32. Endocytosis is the process in which the cell takes up solutes or particles by enclosing them in vesicles (endosomes) pinched off from the plasma membrane
  33. Phagocytosis—endocytosis of large particles by engulfing them into a phagocytic vacuole (phagosome)
  34. Pinocytosis—endocytosis of small amounts of liquid with its solute molecules; there are three types of pinocytosis: fluid-phase endocytosis; receptor-mediated endocytosis using clathrin-coated pits and vesicles; and a type of endocytosis that forms special vesicles (caveolae), whose contents are not degraded
  35. Most endosomes fuse with early lysosomes (newly formed lysosomes) to form late lysosomes; late lysosomes are important to cell functioning
  36. Some function as food vacuoles, digesting and releasing nutrients into the cytoplasm
  37. Some function to destroy invading bacteria
  38. Autophagosomes fuse lysosomes to selectively digest portions of the cell’s own cytoplasm as part of the normal turnover of cellular components (macrophagy)
  39. Undigested materials accumulate in lysosomes called residual bodies
  40. Organelles Involved in Genetic Control of the Cell
  41. Nucleus
  42. Nuclei are membrane-bound structures that house most of the genetic material of the cell
  43. Nuclear structure
  44. Chromatin is the dense fibrous material seen within the nucleoplasm of the nucleus; the nucleoplasm is the DNA-containing part of the nucleus; when the cell is dividing, chromatin condenses into visible chromosomes
  45. DNA is organized into chromatin by basic proteins known as histones which allow higher order packaging.
  46. The nucleus is bound by a double-membrane (two lipid bilayers) nuclear envelope structure which is penetrated by nuclear pores; the nuclear pores allow materials to be transported into or out of the nucleus
  47. The nucleolus is a very noticeable structure within the nucleus; it is involved in the synthesis of rRNA and the production of ribosomes
  48. Eukaryotic ribosomes
  49. Eukaryotic ribosomes are 80S, with 60S and 40S subunits
  50. Eukaryotic ribosomes are responsible for synthesis of cellular proteins; they can either be attached to the ER (rough ER) or are free in the cytoplasm
  51. ER-associated ribosomes synthesize integral membrane proteins or proteins that are secreted out of the cell
  52. Free ribosomes synthesize nonsecretory, nonmembrane proteins
  53. Organelles Involved in Energy Conservation
  54. The endosymbiotic hypothesis holds that the three energy-conserving organelles are thought to be derived from bacterial cells engulfed by host cells early in evolutionary history
  55. Mitochondria
  56. Mitochondria are the site of the energy-conserving tricarboxylic acid cycle and the generation of ATP by electron transport and oxidative phosphorylation
  57. Mitochondria have an outer membrane and an inner membrane enclosing a fluid matrix
  58. The inner and outer membranes have different lipids and enzymes
  59. The enzymes of the tricarboxylic acid cycle and the -oxidation pathway for fatty acid degradation are located within the matrix
  60. Electron transport and oxidative phosphorylation occur only in the inner mitochondrial membrane
  61. Mitochondria use their own DNA and their own ribosomes to synthesize some of their proteins; mitochondrial DNA and mitochondrial ribosomes are similar to bacterial DNA and ribosomes in terms of size and structure; mitochondria reproduce by binary fission
  62. Hydrogenosomes are small organelles in some anaerobic protists that conserve energy by fermentation and generate hydrogen; composed of a double membrane they share some features with mitochondria
  63. Chloroplasts
  64. Chloroplasts are the site of photosynthesis in protists and higher plants
  65. Chloroplasts are surrounded by two membranes; the inner membrane encloses a fluid matrix called the stroma; within the stroma is a system of flattened membrane sacs called thylakoids which often form stacks known as grana
  66. The formation of carbohydrates from carbon dioxide and water (dark reactions) occurs in the stroma
  67. The trapping of light energy to generate ATP, NADPH, and oxygen (light reaction) occurs in the thylakoid membranes of the grana
  68. External Structures
  69. Cilia and flagella are locomotor structures that differ in length and how they move the cell with cilia like oars and flagella like undulating propellers
  70. Cilia and flagella are structurally similar; both are membrane-bound cylinders composed of microtubules, in a 9+2 arrangement, embedded in a matrix; basal bodies have a 9+0 microtubule pattern
  71. Comparison Bacterial, Archaeal, and Eukaryotic Cells
  72. Eukaryotes have a membrane-delimited nucleus and many complex membrane-bound organelles, each of which performs a separate function for the cell
  73. Prokaryotes lack a membrane-delimited nucleus and internal membrane-bound organelles; they do not perform endocytosis
  74. Non-archaeal prokaryotes have a different ribosome structure that sediments at a slightly different rate.
  75. Despite the significant differences between prokaryotes and eukaryotes, they have remarkable biochemical similarities: the same basic chemical composition, the same genetic code, and some of the same basic metabolic processes

CRITICAL THINKING

1.Describe how the “division of labor” associated with internal, membrane-delimited organelles enables eukaryotic cells to be more efficient and thereby allows them to grow much larger than prokaryotes grow.

2.Both prokaryotes and eukaryotes are capable of using large particles as nutrient sources. Eukaryotes bring them into the cell by endocytosis. Prokaryotes cannot do this; they export digestive enzymes to digest the particles externally and then transport the resulting small nutrient molecules into the cell. Why is the prokaryotic process much less efficient? Use diagrams to support your answer.

3. The orderly packaging of eukaryotic DNA into chromatin using histones suggests that this packaging may

influence gene regulation. Comment on how loosening up a chromosome may influence the ability of its

genes to be read and expressed.

CONCEPT MAPPING CHALLENGE

Construct a concept map that allows you to describe the molecular makeup and function of eukaryotic cell structures. Use the concepts that follow, any other concepts or terms you need , and your own linking words between each pair of concepts in your map.

cell wall plasma membrane cytoplasm chloroplast mitochondrion endoplasmic reticulum

Golgi apparatus lysosome nucleus ribosomes cytoskeleton

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