BIO 112 Notes: Eukaryotes

Eukaryotes

Eukaryotic cells have a nucleus and other membrane-bound organelles, such as mitochondria and a Golgi apparatus. They are structurally more complex than prokaryotic cells.

  • A well-developed cytoskeleton enables eukaryotic cells to have asymmetrical forms and to change shape.
  • Eukaryotic cells arose through endosymbiotic events that gave rise to the energy-producing organelles within the eukaryotic cells such as mitochondria and chloroplasts.

Endosymbiosis

Eukaryotes have genes and cell characteristics derived from both archaea and bacteria, likely as a consequence of endosymbiosis. Endosymbiosis is a symbiotic relationship in which one organism lives inside the body or cell of another organism.

  • Key evidence supporting endosymbiotic theory:
    • Inner membranes of mitochondria and plastids are similar to plasma membranes of living bacteria
    • Replication of mitochondria and plastids is similar to cell division in bacteria – Mitochondria and plastids have circular DNA, like bacteria
    • Mitochondria and plastids transcribe and translate their own DNA into proteins
    • Ribosomes of mitochondria and plastids are more similar to bacterial than eukaryotic ribosomes

Inner membranes, cell division, circular DNA, and ribosomes of mitochondria and plastids are similar to bacteria. Mitochondria and plastids also transcribe/translate their own DNA into proteins.

Choanoflagellates and Animals

Morphologically, choanoflagellate cells and the collar cells of sponges, a basal group of animals, are almost indistinguishable.

  • Similar collar cells have been identified in other animals, including cnidarians, flatworms, and echinoderms- but they have never been observed in non-choanoflagellate protists or in plants or fungi.
  • DNA sequence data indicate that choanoflagellates and animals are sister groups. In addition, genes for signaling and adhesion proteins previously known only from animals have been discovered in choanoflagellates.

Protists and Locomotion

Flagella, cilia, and pseudopodia are used for locomotion by protists.

  • Flagella are long protein filaments of uniform length that are responsible for cell motility.
  • Cilia are short eyelash-like filaments that allow locomotion for protists.
  • Pseudopodia are a temporary protrusion of the surface of an amoeboid cell for movement and feeding.

Nutritional Strategies

Protists show a wide range of nutritional diversity. Photoautotrophy, heterotrophy, and mixotrophy have arisen independently in protists many times. Producers, or phototrophs, are organisms that use energy from light (or inorganic chemicals) to convert CO2 to organic compounds.

  • All other organisms in the community are consumers, or heterotrophs, that depend on producers for food.
  • Mixotrophs can be both photoautotrophs and heterotrophs depending on the circumstance.

Protist Reproduction

Protists can either reproduce asexually or sexually. Usually, they reproduce asexually.

  • Protists can reproduce asexually through binary fission, where one nucleus divides; multiple fission, where many nuclei divide; and budding, where a new organism sprouts off a parent organism.
    • During both types of fission, the organism replicates its nucleus and divides to form new organisms. Some unicellular protists even reproduce sexually and are able to create gametes, or sex cells, that can fuse together to form a new organism in a process known as syngamy.
  • Conjugation is another type of sexual reproduction that mainly only occurs in ciliates. In this process, nuclei from gametes come together and fuse to create a zygotic nucleus.

Charophytes and Land Plants

Charophytes share more traits with land plants than do other algae, according to structural features and DNA analysis.

  • Their chloroplasts have a similar structure and pigment composition to those of land plants.
  • Both charophytes and land plants show apical growth—growth from the tips of the plant rather than throughout the plant body. Consequently, land plants and the charophytes are now part of a new monophyletic group called Streptophyta.