Understanding Protozoa: Characteristics and Classification

The Protozoa (Kingdom Protista)

Protozoa are unicellular organisms classified within the Kingdom Protista. There are more than 30 million species living in diverse habitats. In each case, the relationship with the host can range from a strictly coincidental union to a parasitic one. Many serve as a staple food source for other animals and even other protozoa.

They are microscopic organisms ranging from 0.4 microns to 1 mm, though macroscopic species can measure up to several millimeters. Some fossils from the Secondary and Tertiary periods, such as Orbitolina, reached up to 12 cm. Some flagellate protozoa contain chlorophyll and resemble green algae, leading botanists to classify them as such. This kingdom was first proposed by Ernst Heinrich Haeckel due to the difficulty of distinguishing unicellular organisms as strictly animals or plants.

Classification of Protozoa

• Mastigophora: Move by flagella; examples include Euglena, trypanosomes, and Ceratium.
• Sarcodina or Rhizopoda: Use pseudopodia for locomotion; examples include amoebas, Globigerina, and Actinophrys.
• Ciliates: Possess cilia as organs of locomotion throughout their lifetime.
• Sporozoans: Produce spores during reproduction and lack specialized organelles for locomotion.
• Suctoria: Possess cilia only during the juvenile stage.

The Amoeba

Some species can be found under decaying leaves, in wetlands, and in salt water. They feed on algae, bacteria, protozoa, and organic detritus. The amoeba genus is cosmopolitan and includes many species.

Amoeba Structure

An amoeba consists of a single independent cell containing cytoplasm and a nucleus, without permanent organelles. Its shape is irregular, and it constantly changes by extending projections. The cytoplasm flows around the nucleus, consisting of an outer ectoplasm (plasmogel) and an inner, more granular endoplasm (plasmosol).

Parasitic Amoebas

In humans, there are harmless amoeba species found in the gut (Entamoeba coli) and mouth (Entamoeba gingivalis). However, Entamoeba histolytica causes amoebic dysentery. This species lives in the large intestine of humans and frequently causes infections known as amebiasis. E. histolytica presents in various stages: trophozoite, precyst, cyst, and metacyst.