Chapter 5: Unicellular Eukaryotes

{{Unicellular Eukaryotes{{

$<strong>Characteristics</strong>$

• Diverse and abundant
  • Over 64,000 species have been named
  • Highly adaptable
  • Require moist environments
  • Marine or freshwater
  • Soil
  • Decaying organic matter
  • inside plants and animals
  • May be sessile or free-swimming
  • Form a large part of the floating plankton
• Many impact humans:
  • Feed on corneas or brain tissue
  • Cause diseases (e.g., malaria, dysentery)
  • Infect livestock
• The organisms formerly known as protists
  • Now recognize that they belong to many different taxonomic groups
• Correct classification is important
  • Otherwise, we can’t treat these disease agents
  • Susceptibility to particular drugs is often lineage-dependent

$<strong>Often Described By Body Type</strong>$

1. Ciliates

   1. Numerous cilia cover plasma membrane

2. Flagellates

   1. Have 1+ flagella used to propel cell

3. Amebas

   1. Have irregular shapes

      1. Caused by the flowing cytoplasm inside cell

   2. May have tests (shells) or be “naked”

{{Form and Function{{

$Plasma Membrane and Cytoskeleton$

• Plasma Membrane

  • Separates unicellular eukaryote from the surrounding environment
  • Composition: Selectively permeable phospholipid bilayer

• Cell Cortex with associated actin-binding proteins

  • Beneath PM and bound to it
  • Function: influences shape and other PM properties

• Cytoplasm

  • Composition:
  • Cytosol (fluid portion)
  • Organelles and membranes
  • * nucleus is not part of the cytoplasm

  

• Cytoskeleton

  • Fibers cross through cytoplasm

  • 3 types of cytoskeletal fibers:

  • Microtubules

    • Transport within cell
    • Internal structure to cilia and flagella
    • Stiffen PM and cell extensions

    

  • Intermediate Filaments

    • Intermediate in size between other fibers
    • Functions largely structural

  • Microfilaments

    • A.k.a. actin filaments
    • Form the cell cortex, which affects PM properties
    • Very important in cell movement and shape change, particularly in ameboid cells

{{Cell Extensions in Representative Unicellular Eukaryotes{{

Ameboid cells assume a wide variety of shapes and forms.

1. Flexible Plasma Membrane

   1. Responds to changes in dynamic cytoskeleton

      

2. Movement

   1. Pseudopodia (fake feet)

      1. General cell extensions

      https://www.youtube.com/watch?v=QTIYOC-vwl8

3. Feeding

   1. Phagocytosis: Engulfing food using plasma membrane extensions

      https://www.youtube.com/watch?v=mv6Ehv06mXY

$<strong>Foraminiferans</strong>$

• Reticulopodia

  • Pseudopodia that branch and rejoin extensively
  • Develop a branched net-like mesh
  • Reticulum
    • Uses:
    • Prey capture
    • Cytoskeleton formation
    • Cell attachment
    • Cell movement

  

{{Nutrition and Digestion{{

Two broad feeding modes in unicellular eukaryotes

• Autotrophs

• Heterotrophs

• Mixotrophs

  • Make some of their nutrients, but also eat other organisms

• Newly discovered feeding process: trogocytosis

  • Feeding mode for certain parasites
  • Parasite bites and consumes sections of the host cell membrane, cytoplasm, and organelles
  • Examples:
  • Entamoeba histolytica
  • Naegleria fowleri “brain-eating amoeba”
    • Causes sudden, severe, and usually fatal brain infection
    • Enters through nose
    • Found in warm and hot freshwater
    • Rivers, lakes, hot springs (very rare)

  

  

{{Reproduction and Life Cycles{{

$Asexual Reproduction$

• Fission

  • Cell-multiplication process that produces genetically identical individuals in unicellular eukaryotes
  • Binary Fission
  • Most common type

  

  • Budding
  • Progeny cell considerably smaller than the parent and then grows to adult size
  • Multiple Fission
  • Several nuclear divisions precede division of the cytoplasm
    • Numerous individuals produced almost simultaneously
  • Common among apicomplexans and certain amebas
  • When it leads to spore formation: sporogony

$Sexual Processes$

• All unicellular eukaryotes reproduce asexually
  • Some exclusively
• Sexual reproduction also widespread
  • Value of genetic diversity and recombination
  • Gametic Meiosis
  • Occurs during or just before gamete formation
  • Occurs in Ciliophora and some flagellated and ameboid groups
  • Intermediary Meiosis
  • Haploid and diploid generations alternate
  • Widespread among plants
  • Occurs in foraminiferans

$Encystment and Excystment$

• Unicellular eukaryotes are extremely tolerant to extreme environmental conditions
  • One reason: Cyst Formation (Encystment)
  • Dormant forms
  • Resistant external coverings
  • Undergo complete shutdown of metabolic machinery
  • Also important for many parasitic forms
    • Must survive harsh environment between hosts
  • Does not occur in Paramecium, rare or absent in marine forms
  • Excystment
  • Escape from cysts
  • Exact stimulus is usually unknown
  • In general: return to favorable conditions