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7.3.2. Protoctists

Protist Kingdom

The Protoctista kingdom contains all those eukaryotic organisms that cannot be classified in any of the other three eukaryotic kingdoms : they are neither of the kingdom fungi (fungi), nor of the kingdom animalia or metazoans, nor of the kingdom plantae or metaphytes. Therefore, this group is very heterogeneous, they can be unicellular and multicellular, and can be grouped into:


Protozoa are microscopic, unicellular, eukaryoticheterotrophic, and predatory or detritivorous organisms. They are generally free-living and can move using flagella, cilia or pseudopodia, although there are also some parasites and causes diseases such as malaria or malaria. In adverse conditions, they tend to encyst to resist, and germinate again when conditions are favourable.

They reproduce by cell division. From one cell, two cells arise that share their content.

Protozoa are classified according to their structure and the way they move in four groups:

Flagellates (or mastigophora)

Flagellates are the most primitive protozoa, the original form from which all eukaryotes derive. They are characterized by the possession of one or more long filaments, called flagella, with which they move.

Although most are free-living, some are parasites, such as theTrypanosoma gambiense that through the African tsetse fly can transmit sleeping sickness to humans .

Trypanosoma con flagelo

By Photo Credit:Content Providers: CDC/Dr. Myron G. Schultz [Public domain], via Wikimedia Commons

Other flagellates live in symbiosis with other organisms. For example, associated with termites, along with bacteria and fungi, so that they can digest wood.


Ciliates are protozoa that live in fresh water and are surrounded by cilia. These cilia are short and numerous filaments and allow them to move. Cilia are similar in structure to flagella and are related to the cytoskeleton and centrioles.

An example of a ciliated protozoan is the paramecium.


Rhizopods or sarcodines are protozoans that move by pseudopodsPseudopods are deformations of the cytoplasm and the plasma membrane that are generated in the direction of displacement, dragging behind them the rest of the cell. The pseudopods also serve them to catch their prey which they then digest by phagocytosis.

Most rhizopods live in water, but some, such as Entamoeba histolytica, are parasitic, causing diseases such as amebiasis.


Sporozoa are parasitic protozoa  without the ability to move. An example of a sporozoan is Plasmodium, which transmits malaria or malaria through some mosquitoes, such as Anopheles.

The seaweed

Algae are  photosynthetic autotrophic eukaryotic organisms, which can be unicellular and multicellular. Like plants, they use the sun's energy to synthesize organic matter from water, CO2 and mineral salts, but they have no roots, no stems, no leaves, no vascular tissue, and they do not produce flowers or seeds.

Unicellular algae

Unicellular algae can live free, such as Euglena, or associated in colonies, such as the genus Volvox.

Diatoms are a type of unicellular algae that are covered by a layer of silica.

These algae are part of the phytoplankton, so they are important in aquatic ecosystems, since they serve as food for aquatic heterotrophic organisms and produce oxygen.

Multicellular algae

Multicellular algae are made up of many cells but they are not differentiated into tissues, instead forming a structure called a thallus.

They are located at different depths depending on the pigments they have for photosynthesis, and in addition to chlorophyll they can have other photosynthetic pigments. They are classified into three groups:

Algae are very important living beings, since they have:

  • Ecological importance:
  • Industry Significance.
    • Feeding. Although it is not common in our country, it is used as food in some countries and can be a source of food for a future in which overpopulation makes other foods scarce.
    • Biofuels. To manufacture bioethanol, biobutanol and biodiesel.
    • Cosmetics They are used for their  moisturizing, antioxidant and regenerating properties.
    • Farming. To make fertilizers.

Green algae

Green algae live in fresh and salty surface waters. Green algae are closely related to land plants as they have the same types of chlorophyll and also synthesize starch as an energy reserve substance. Plants are believed to have evolved from green algae.

Brown algae

Brown algae photosynthesize deeper than green algae, thanks to a yellowish-brown pigment.

Red algae

Red algae live at great depths, but they can carry out photosynthesis thanks to a red pigment that is very sensitive to light, even if the light is very scarce. Some of these algae accumulate calcium carbonate and contribute to the formation and maintenance of coral reefs.