Kingdom Protista & Fungi: Characteristics, Classification & Examples
Classification of Protists
Protists are divided into three groups: Protozoa, Algae, and Fungi-like protists.
A) Protozoa
Protozoa are unicellular, microscopic organisms with a standard cellular structure. Some species have a hard carapace surrounding them, which can be made of calcium carbonate, silica, etc. They are always heterotrophs. Most live freely, but there are also symbionts and parasites of both animals and plants. The principal edges are:
- Zoomastigophora: They have one or more flagella. Some are free-living, while others are parasites. Example: Trypanosoma.
- Rhizopoda: They have pseudopodia to move. Examples: Amoebas and Radiolarians (which have a silica shell surrounding them).
- Ciliophora: They move and capture their food using cilia. Most are free-living. Example: Paramecium.
- Sporozoa: They are all parasites. Example: Plasmodium.
B) Algae
Algae are unicellular and multicellular photosynthetic protists. Multicellular algae can have a filamentous or thallus type of organization. Algal cells are similar to plant cells. They have a cellulose wall and chloroplasts. However, these chloroplasts are more primitive than those of plant cells. They possess different types of photosynthetic pigments that capture light, providing them with color: chlorophyll, carotenoids, etc. Because of this, they absorb light of different wavelengths, allowing them to live at different depths. Most algae are aquatic, but they can also live in humid terrestrial environments. Unicellular algae float in the water (forming phytoplankton) or associate with fungi to form lichens. Multicellular algae are found in marine environments or fixed to floating objects.
C) Fungi-like Protists
These protists are distinguished by their cellular wall, which is made of chitin. There are two groups:
- Slime molds: Also called Myxomycetes. They are unicellular and motile. In fact, they resemble amoebae with multiple nuclei inside (plasmodia). When reproducing, they first form a structure that resembles a mushroom. Inside, the nuclei multiply and form cells that become independent afterward.
- Oomycetes: Formed by filaments of cells joined in a row (hyphae). Their structure is very similar to fungi. They differ only in that their cell walls are made of cellulose. Example: Mildew (a parasite of the vine).
The Fungi
Previously classified in the plant kingdom, fungi are immobile and fixed to the ground. However, the discovery that they do not photosynthesize and that their cellular walls are made of another polysaccharide, chitin, instead of cellulose, made it necessary to include them in a separate kingdom. It is assumed that fungi evolved from protists.
General Characteristics
Fungi are eukaryotic heterotrophs, with cells that store energy reserves as glycogen and have cellular walls made of chitin. They can be unicellular, like yeasts, but most are multicellular with a thallus type of organization, meaning they do not have true tissues. Their basic structure is called mycelium, and it is formed by filaments of aligned cells called hyphae. These hyphae can be simple or branched. If they have partitions separating the cells, they are called septate hyphae. If there are no partitions, they are called coenocytic hyphae. Fungi reproduce both asexually and sexually.
– Asexual Reproduction
Asexual reproduction is usually by budding (in yeasts), fragmentation, or sporulation. Asexual spores or conidia are formed by mitosis in structures that are not shaped like a globe and are supported by a peduncle. These structures are called conidiophores. When spores germinate, they form a new mycelium.
– Sexual Reproduction
Hyphae from two different fungi fuse. The cells of these hyphae are also united, forming a multicellular zygote that grows. This zygote forms structures called sporangia, in which a pair of fused cells undergoes meiosis, resulting in four haploid spores. In some fungi, sporangia are found in structures called carpophores or fruiting bodies, formed by very intertwined hyphae. For example, mushrooms are carpophores that carry sporangia loaded with spores in their gills.
Classification
The five fungal phyla are:
- A) Zygomycetes: They have hyphae without septa. They reproduce both asexually by conidia and sexually. They are very simple fungi. Molds belong to this group, for example, Mucor (bread mold).
- B) Deuteromycetes: They have septate hyphae. They only reproduce asexually by conidia. Example: Penicillium.
- C) Ascomycetes: They have septate hyphae and reproduce by conidia and also sexually by haploid spores, the ascospores, which are born inside an elongated sporangium called an ascus. This group includes both single-celled fungi, like yeast, and multicellular fungi, like truffles.
- D) Basidiomycetes: They have septate hyphae and produce sexual spores called basidiospores. The basidia are sporangia that are grouped in a structure, the mushroom. Example: Mushrooms.
- E) Lichens: They are Ascomycetes or Basidiomycetes that live in symbiosis with green algae or cyanobacteria. They reproduce asexually by fragmentation. Small pieces of the lichen body are formed by algae cells surrounded by hyphae of the fungus. Sexual reproduction occurs only in the fungus. When the fungal spore germinates, it forms a new mycelium. If the algae is properly associated, they form the lichen.