Skip to main content
Biology LibreTexts

48: Protozoa


  • Identify common species of protozoa
  • Differentiate among the major classes of protozoa
  • Identify different types of motility


The protozoa are contained within the kingdom Protista along with the unicellular algae.  The classes of protozoa are categorized by a variety of factors: cell architecture, motility structure, even hosts. They do not photosynthesize, rather being chemoheterotrophic like animals. This means that they use chemicals for energy production and they get their carbon from the same compounds, e.g. sugar.  However, there are some pigmented protozoa and there are even a few that seem to be crossover organisms, being claimed by the botanists because of their photosynthetic ability.  for example, the flagellate Euglena will photosynthesize in light (it contains chlorophyll) or will switch over to regular aerobic respiration (chemoheterotrophism) without light.

Many of the protozoa form a resistant, dormant structure called a cyst. Parasitic protozoa are identified by the active feeding stage, called a trophozoite, in addition to the cyst stage, both of which may be found in the feces.

For our purposes, there are only 4 groups of protozoa that will be covered here:  these groups are separated by motility and cell structure.

  • Amebas (representative: Ameba proteus)
  • Flagellates (representative: Trypanosoma, Euglena)
  • Ciliates (representative: Paramecium)
  • Apicomplexa (representative: Plasmodium)

Many protozoa are found in the gut of warm-blood animals and cold-blooded animals, as well as in insects  such as termites and cockroaches.  In addition, there are quite a few protozoans that live in blood. You will see some of these examples in lab.

Ameobas move by cytoplasmic streaming, having no motility structure.  You will likely see some freshwater amebas in the pond water, some of which may have tests or 'shells' that surround their cytoplasm.  The flagellates have flagella or an undulating membrane for motility.  The ciliates have cilia.  The Apicomplexa have a unique arrangement of microtubules, called the apical complex (used in the takeover of the host cell), in the cell. This last class has most of the human and animal pathogens in it.


  • prepared slides: Trypanosoma and Plasmodium
  • fresh specimens: Ameba and Paramecium
  • pond water


  1. Make wet mounts of the pond water.
    • Go down to the bottom of the container or in the gunk to get your specimen.
    • Do NOT stir the specimen: you will get fewer that way.
    • Start with the 10X and go to 40X. Oil-immersion will magnify too much for most pond water protozoa.
    • Once you have found your objects on brightfield, change over to darkfield and phase contrast for even better viewing.
  2. Look at the READY VIEW (also called DEMOSLIDE) slide/ tubes containing Paramecium and Ameoa. a use culture tube/ slide for studying microscopic organisms. It is a tapered tube with a compressed viewing chamber at its tip, so you can just place the tube into a holder slide that fits right on the microscope stage. The protozoa will be at the bottom of the tube, making it very easy to focus on them. The optimal magnification for these organisms is 10X, maybe up to 40X.  You can use both brightfield microscopy as well as phase-contrast microscopy (be sure to use the appropriate phase condenser setting)
  3. Look at the prepared blood slides of Trypanosoma and Plasmodium on 100X, using brightfield microscopy. Trypanosoma will be easy to see: it is far larger than the red blood cells. However, Plasmodium will be difficult since the parasite will be inside of the RBCs.








Plasmodium                                                                                 Trypanosoma


Molecular Expressions


  1. Where is the malarial parasite located---in the RBC or in the plasma outside of the RBC?
  2. By what major criterion are the protozoa subcategorized into classes?