chapter 4

• Eukaryotic flagella are about ten times thicker than bacterial flagella and are structurally more complex
• Eukaryotic cilia are similar to flagella in structure, but smaller and more numerous, found only in a single group of protozoa and certain animal cells
• The glycocalyx is an outermost layer composed of polysaccharides that comes into direct contact with the environment
• The cell walls of fungi are rigid, provide structural support and shape, and have a thick inner layer of polysaccharide fibers composed of chitin or cellulose
• The cell membrane is a typical bilayer of phospholipids with embedded protein molecules and contains sterols for stability
• Mitochondria generate energy for the cell, have a smooth outer membrane with inner folded membrane containing cristae, and divide independently of the cell
• Chloroplasts are found in algae and plant cells, capable of converting sunlight energy into chemical energy through photosynthesis
• Ribosomes in eukaryotic cells are distributed throughout the cell, with large and small subunits forming an 80S ribosome
• The cytoskeleton functions include anchoring organelles, moving RNA and vesicles, permitting shape changes, and movement
• Nearly 300 species of fungi can cause human disease, with three types of fungal diseases in humans
• Fungi play an essential role in decomposing organic matter, returning minerals to the soil, and have been engineered to produce various substances
• Fungi have heterotrophic nutrition, can be saprobic or parasitic, and secrete enzymes to absorb nutrients from the substrate
• Cells of most microscopic fungi grow in loose associations or colonies, with yeasts having a soft, uniform texture and filamentous fungi having a cottony, hairy, or velvety texture
• Mycelium is the mass of hyphae that makes up the body of a mold, and reproductive hyphae produce spores
• Fungi can propagate by outward growth of hyphae or fragmentation, with spores being the primary reproductive mode dispersed through the environment
• Asexual spores like sporangiospores are formed within a sporangium, while conidiospores are free spores not enclosed by a sac
• Sexual spore formation involves mixing DNA from two parent fungi to create offspring with different gene combinations
• Protozoa are single-celled creatures, with harmless, free-living species and some pathogenic species responsible for diseases
• Protozoan Form and Function:

  • Single cells containing all major eukaryotic organelles
  • Cytoplasm divided into two parts:
  • Ectoplasm: clear outer layer involved in locomotion, feeding, and protection
  • Endoplasm: granular inner region housing the nucleus, mitochondria, and food and contractile vacuoles
• Some organelles act as a primitive nervous system to coordinate movement

  • Can move through fluids by means of pseudopods ("false feet")
  • Cell membrane regulates food, wastes, and secretions
  • Cell shape can remain constant (as in most ciliates), or change constantly (as in amoebas)
  • Size of most protozoans range from 3 to 300 μm
  • Giant amoebas and ciliates range from 3 to 4 mm
• Nutritional and Habitat Range:

  • Heterotrophic, require food in a complex organic form
  • Free-living species scavenge dead plant or animal debris or graze on bacteria and algae
  • Pathogenic species may live on the fluids of their host, such as plasma and digestive juices, or actively feed on tissues
  • Main limiting factor is availability of moisture
  • Predominant habitats are fresh and marine water, soil, plants, and animals
  • Can survive in extremes of temperature and pH
• Life Cycles:

  • Trophozoite: motile feeding stage requiring ample food and moisture to stay active
  • Cyst: dormant, resting stage when conditions in the environment become unfavorable
  • Resistant to heat, drying, and chemicals
  • Can be dispersed by air currents
  • Important factor in the spread of disease
• Life Cycles and Transmission:

  • Some protozoan groups exist only in the trophozoite phase
  • Many alternate between the trophozoite and cyst stage, depending on the habitat
  • Trichomonas vaginalis, a common STD, does not form cysts and must be transmitted by intimate contact
  • Entamoeba histolytica and Giardia lamblia form cysts and are readily transmitted in contaminated water and foods
• Reproduction:

  • All protozoa reproduce by relatively simple, asexual mitotic cell division or multiple fission
  • Sexual reproduction also occurs in most protozoa
  • Ciliates participate in conjugation in which two cells fuse and exchange micronuclei
  • This results in new and different genetic combinations that can be advantageous in evolution
• Major Pathogenic Protozoa:

  • Amoeboid Protozoa:
  • Entamoeba histolytica: Amoebiasis (intestinal and other symptoms)
  • Naegleria, Acanthamoeba: Brain infection
  • Ciliated Protozoa:
  • Balantidium coli: Balantidiosis (intestinal and other symptoms)
  • Flagellated Protozoa:
  • Giardia lamblia: Giardiasis (intestinal distress)
  • Trichomonas vaginalis: Trichomoniasis (vaginal symptoms)
  • Trypanosoma brucei, T. cruzi: Trypanosomiasis (intestinal distress and widespread organ damage)
• • Leishmania donovani, L. tropica, L. brasiliensis: Leishmaniasis (either skin lesions or widespread involvement of internal organs)
• General Worm Morphology:

  • Multicellular animals equipped with organs and organ systems
  • In pathogenic helminths, the most developed organ is the reproductive tract
  • Reduction in the digestive, excretory, nervous, and muscular systems
• Life Cycles and Reproduction:

  • Complete life cycle includes the fertilized egg, larval, and adult stages
  • Adults derive nutrients and reproduce sexually in a host’s body
  • Nematodes have separate sexes that are different in appearance
  • Trematodes can have separate sexes or be hermaphroditic
  • Cestodes are generally hermaphroditic
• Life Cycles and Reproduction:

  • Helminth life cycle:
  • Must transmit an infective form (egg or larva) to the body of another host
  • Intermediate host: where the larva develops
  • Definitive host: where adulthood and mating occur
  • Transport host: intermediate host that experiences no parasitic development
  • Sources for human infection are contaminated food, soil, water, or infected animals
• Examples of Helminths and How They Are Transmitted:

  • Roundworms—Nematodes:
  • Ascaris lumbricoides: Ascariasis
  • Enterobius vermicularis: Pinworm
  • Trichinella spiralis: Trichina worm
  • Onchocerca volvulus: River blindness
  • Dracunculus medinensis: Guinea worm
  • Flatworms—Trematodes:
  • Schistosoma japonicum: Blood fluke
  • Flatworms—Cestodes:
  • Taenia solium: Pork tapeworm
  • Diphyllobothrium latum: Fish tapeworm
• Egg Laying:

  • Fertilized eggs are released to the environment
  • Provided with a protective shell and extra food to aid their development into larvae
  • Vulnerable to heat, cold, drying, and predators
  • Certain helminths can lay from 200,000 to 25 million eggs a day to assure successful completion of their life cycle
• A Helminth Cycle: The Pinworm:

  • Enterobius vermicularis: Pinworm or seatworm
  • Common infestation of the large intestine
  • Range from 2 to 12 mm long with a tapered, curved cylindrical shape
• A Helminth Cycle: The Pinworm:

  • Life cycle:
  • Microscopic eggs are swallowed: picked up from another infected person or objects they have touched
  • Eggs hatch in the intestine
  • Larvae mature into adults within 1 month
  • Male and female worms mate
  • Female migrates to the anus to deposit eggs
  • Intense itching ensues
  • Scratching spreads the eggs
• Distribution and Importance of Parasitic Worms:

  • About 50 species of helminths cause disease in humans
  • Distributed worldwide with higher incidence in tropical areas
  • Billions of cases yearly, not confined to developing countries
  • Conservative estimate of 50 million helminth infections in North America alone