Protozoa: Characteristics and Classification

• Protozoa are single-celled eukaryotic organisms.
• They range from 3 to 1,000 microns in size, they are microscopic.
• They lack germ layers, tissues, and organs.
• Protozoa possess specialized intracellular organelles.
• They exhibit various symmetries: bilateral, radial, spherical, and asymmetrical.
• Protozoa have diverse life habits: free-living, commensal, parasitic, or mutualistic.
• Most protozoa species are naked, but some have protective exoskeletons called tests.
• Protozoa nutrition can be autotrophic (holophytic), saprozoic, saprophytic, or holozoic.
• Protozoa move using pseudopodia, flagella, or cilia.
• They reproduce both asexually and sexually.
• Asexual reproduction includes binary fission, multiple fission (sporogony & schizogony), and budding.
• Sexual reproduction methods include syngamy, autogamy, and conjugation.

Classification of Protozoa

• Protozoa are classified into four sub-phyla: Sarcodina, Mastigophora, Apicomplexa, and Ciliophora.

Sub-Phylum Sarcodina

• Sarcodina are amoeba-like organisms that move using pseudopodia.
• They have a single nucleus or monomorphic nuclei.
• Examples include Amoeba (naked and free-living), Arcella (possesses a calcareous test and is free-living), and Entamoeba (lacks a test).
• Pseudopodia are extensions of the cytoplasm, consisting of a rigid ectoplasm (gel state) and a less rigid endoplasm (sol state).
• Alternation between these states allows for pseudopodial extension.
• Types of pseudopodia are: Lobopodia (large and blunt), Filopodia (thin extensions), Reticulopodia (thin extensions forming a mesh), and Axopodia (long and thin, supported by an axial filament).

Sub-Phylum Mastigophora

• Mastigophora use one or two flagella for movement.
• They have a single nucleus or monomorphic nuclei.
• Some are photosynthetic (with chloroplasts), while others are mutualistic or parasitic.
• Most are bilaterally symmetrical.
• Examples include Euglena (contains chloroplasts), Trypanosoma (heterotrophic and parasitic, causing sleeping sickness), Leishmania (causes leishmaniasis), Giardia (causes giardiasis/stomach infection), and Volvox (a colonial species).

Sub-Phylum Apicomplexa

• Apicomplexa lack locomotory structures and are all parasitic.
• Asexual reproduction involves multiple fission (schizogony).
• Plasmodium, the causative agent of malaria, is a key example, with types like Plasmodium vivax, ovale, malariae, and falciparum.
• Plasmodium falciparum is the most virulent, causing over 50% of the global malaria burden.
• Toxoplasma gondii causes toxoplasmosis, an opportunistic infection affecting immunocompromised individuals, transmitted through contact with cats and dogs.

Sub-Phylum Ciliophora (Ciliates)

• Ciliophora move using cilia and usually possess two sizes of nuclei: macronucleus and micronucleus.
• Reproduction occurs asexually through transverse binary fission and sexually through conjugation.
• Examples include Paramecium, Vorticella, and Stentor.
• Flagella and cilia share a similar internal structure called an axoneme, consisting of a 9+2 arrangement of microtubules, providing strength.
• Flagella are generally longer and fewer, while cilia are shorter and more numerous.