BIOL 1406

PreLab 4.4

Which organisms are composed of prokaryotic cells and which are composed of eukaryotic cells?

Although generally not visible to the naked eye, organisms composed of prokaryotic cells still dominate the earth. Their collective mass is at least 10 times greater than the mass of all eukaryotes combined. Organisms composed of prokaryotic cells are classified into 2 main groups: archaea and bacteria. Archaea most closely resemble the first primitive cells on our planet. They are often adapted to the extreme environments that were common on the primitive earth (such as high temperature, high salt concentration, and no O2). Bacteria, on the other hand, are a much more diverse and widespread group.  They are, by far, the most common prokaryotes on our planet today.

Prokaryotic organisms are usually unicellular, but a few form simple colonies. Unicellular means an organism is composed of a single cell. Colonial means an organism is composed of attached cells, but the cells are mostly similar in structure and function. If separated from the colony, each cell can usually survive on its own and grow into a new colony. Almost all prokaryotic cells have a rigid cell wall external to the plasma membrane, and about half can actively move themselves through their environment, usually with the use of flagella.

In spite of their relatively simple structure, prokaryotic cells have much greater metabolic diversity than eukaryotic cells. Although most prokaryotes are heterotrophs, some are autotrophs. Autotrophs can manufacture their own food (organic molecules) from inorganic substances, while heterotrophs cannot. Most autotrophic bacteria are photosynthetic (they use light energy to manufacture organic molecules) but some are chemosynthetic (they use energy from inorganic chemicals to manufacture organic molecules.)

The three domains of life


Organisms composed of eukaryotic cells are divided into 4 main groups:

  1. Protista are mostly unicellular or colonial eukaryotes. They may be either photosynthetic autotrophs or heterotrophs.  This is a very diverse group that is actually composed of multiple kingdoms.  In terms of the diagram above, it includes the following groups: Entamoebae, Myxomycota, Ciliates, Flagellates, Trichomonads, Microsporidia, and Diplomonads.
  2. Plantae are mostly multicellular, photosynthetic autotrophs. Like colonial organisms, multicellular organisms are composed of many cells. However, in a multicellular organism there is considerable specialization and division of labor among the cells.
  3. Fungi are mostly multicellular heterotrophs that secrete digestive enzymes and digest their food outside the body before absorbing it through the body surface.
  4. Animalia are mostly multicellular heterotrophs that digest their food inside an internal digestive tract.
Your Turn
Prokaryotes include a more primitive group called and a more recent group called Check your answer.
Organisms composed of cells that are mostly similar in structure and function are called Check your answer.
Organisms that can manufacture their own food (organic molecules) from inorganic substances are called Check your answer.
Organisms that use energy from inorganic chemicals to manufacture organic molecules are called Check your answer.
Which group of eukaryotes usually has an internal digestive tract Check your answer.
Which group contains mostly unicellular or colonial eukaryotes Check your answer.
Which group of eukaryotes includes mostly multicellular, photosynthetic autotrophs Check your answer.
Which eukaryotes generally digest their food outside the body before absorbing it through the body surface Check your answer.


 


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