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Kingdom of Protists


Protists are living beings unicellular and eukaryotes; therefore they have individualized nucleus, surrounded by membrane.

They also have membranous organelles several. This group includes the protozoa and the unicellular algae.

The protozoans

Protozoan is a word of Greek origin meaning "primitive animal". The protozoans were named because, in the past, some of them, when studied, were mistaken for animals.

Protozoa are beings heterotrophs. They can live in isolation or form colonies, live free or associate with other organisms, and inhabit a variety of environments. Some species are parasites of various beings, even humans.

Types of protozoan locomotion

There are several species of protozoa, and they can be classified into several groups. The criterion most used by scientists for this classification is the type of locomotion:

  • Sarcodynia or rhizopods- are protozoa that move around extending pseudopods, expansions in their cells that act as "false feet". Amoebas are an example of a sarcodyne.


Image of an amoeba in an electron microscope.

  • Flagellates - are those who "swim" with the aid of flagella (long strands that vibrate and allow locomotion). An example of flagellate is giardia.


Flagellated protozoans of the genus Leishmania cause leishmaniasis, a disease that affects 12 million people worldwide

  • Ciliates - are beings that use cilia (small filaments throughout the body) in locomotion, such as the paramecium.


Illustration (left) and electron microscopy (right) of a paramecium.

  • Sporozoa- are protozoa that do not have locomotion structures. They are all parasites and cause disease. Among them is the plasmodium, which causes malaria.


Red blood cells infected with the Plasmodium falciparum, causer of malaria (on arrows)

For an organism that has no locomotion structures to capture food, parasitism is an important adaptation because it allows it to survive by removing the nutrients it needs from the parasite.

Reproduction of protozoa

Most protozoa reproduce asexual, mainly by cissiparity. But some species can reproduce sexually.

Notice, in the scheme below, the asexual reproduction of a paramecium:


A paramecium dividing in two, asexual reproduction by cissiparity.

See also: Human diseases caused by protozoa.