Protist Kingdom

The complexity of a protozoan's eukaryotic cell is so great that it alone performs all the functions that tissues, organs, and systems perform in a complex multicellular being.

Locomotion, breathing, excretion, water control, reproduction and relationship with the environment are all performed by a single cell, which has some structures capable of performing some of these specific roles, as in a multicellular organism.

According to the classification of living beings into five kingdoms (Whittaker - 1969), one of them, the Protists, groups organisms eukaryotes, unicellular, autotrophs and heterotrophs. In this kingdom are placed the lower algae: euglenophytes, pyrrophytes (dinoflagellates) and chrysophytes (diatoms), which are autotrophists (photosynthesizers). The protozoa are heterotropic protists.

The cell

The cell of a protist is similar to the cells of animals and plants, but there are peculiarities. Algae plastids differ from those of plants in their internal organization of photosynthetic membranes.

Occur eyelashes and flagella for locomotion. The protozoan cell has a simple membrane or is reinforced by protein outer shells or by mineral carapaces, such as certain amoebae (weavers).

Radiolaria and heliozoans have a intracellular skeleton composed of silica.

The foraminifera are endowed withexternal hoods made of calcium carbonate. Diatom algae have silicaous carapaces.

Protists may also have shape and structure adaptations according to their way of life: parasite, or free-living.

The cytoplasm is differentiated into two zones, one external, hyaline, the ectoplasm, and the other, granular, the endoplasma. In this there are digestive vacuoles and inclusions.


Protozoa are a group of eukaryotes with about 20,000 species. It is a diverse, heterogeneous group that evolved from unicellular algae. In some cases this origin becomes very clear, as for example in the group of flagellates. There are fossil records of protozoa with carapaces (foraminifera) that lived more than 1.5 billion years ago in the Proterozoic Era. Large expanses of the seafloor have thick layers of shell deposits of certain species of radiolaria and foraminifera. These are the so-called vasas.

Electron microscopy of the shell present outside the cell of a species of radiolaria.


Protozoa are, for the most part, aquatic, living in seas, rivers, tanks, aquariums, puddles, mud and damp land. There are species mutualistic and many are parasites of invertebrates and vertebrates. They are microscopic organisms, but there are species of 2 to 3 mm. Some form colonies free or sessile.

They are part of plankton (set of beings that live suspended in the water of rivers, lakes and oceans, passively carried by waves and currents). In plankton two groups of organisms are distinguished:

  • phytoplankton: Producing organisms (photosynthesizers), mainly represented by dinoflagellates and diatoms, form the basis for sustaining the food chain in the seas and lakes. They are responsible for over 90% of photosynthesis on the planet.
  • zooplankton: consuming organisms, ie heterotrophs, represented mainly by protozoa, small crustaceans and larvae of many invertebrates and fish.


In free-living species there is formation of digestive vacuoles. Food particles are either enclosed by pseudopods or penetrate through a pre-existing membrane opening, the cytostome.

Digestion occurs inside the cell, and undigested solid residues are expelled at any point on the periphery by extrusion of the vacuole or at a certain point on the membrane. cytopigy or cytoprotto.

The exchange of respiratory gases takes place throughout the cell surface.

Soluble excretion products can be eliminated throughout the cell surface. In freshwater protozoa there is a contractile vacuole, which collects the excess water absorbed by the cell, expelling it from time to time by a sudden contraction. The vacuole is therefore osmoregulator.