5.3 Protists and Fungi
Key Takeaways
- Protista is a paraphyletic catch-all of eukaryotes that are not plants, animals, or fungi; it includes amoebas (pseudopodia), paramecia (cilia), euglenas (flagella), *Plasmodium* (malaria parasite), and algae.
- Fungi are eukaryotic heterotrophs with **chitin cell walls** that digest food externally by secreting exoenzymes and absorbing the breakdown products through a network of filaments called **hyphae** that form a **mycelium**.
- The four traditional fungal phyla are Chytridiomycota (chytrids - flagellated spores), Zygomycota (*Rhizopus* bread mold - zygospores), Ascomycota (sac fungi - yeasts, morels, *Penicillium*), and Basidiomycota (club fungi - mushrooms, rusts, smuts).
- **Mycorrhizae** are mutualistic fungus-root associations in which plants supply sugars and fungi increase mineral and water absorption; about 90% of land plants form them.
- **Lichens** are mutualistic composites of a fungus (usually an ascomycete) plus a photosynthetic partner (green alga or cyanobacterium); they are pioneer species and sensitive bioindicators of air quality.
Protists: A Paraphyletic Grab-Bag
Protists are eukaryotes that are not plants, animals, or fungi. The kingdom Protista is paraphyletic - it contains organisms more closely related to plants, animals, or fungi than to each other. Modern systematists divide them across several supergroups (Excavata, SAR, Archaeplastida, Amoebozoa, Opisthokonta), but the Praxis still tests the broad functional categories below.
Functional Categories
Protists are traditionally grouped by how they obtain food and how they move:
| Group | Locomotion / Feature | Representative Example |
|---|---|---|
| Amoeboid protists | Pseudopodia ("false feet") - extensions of cytoplasm | Amoeba proteus, Entamoeba histolytica (amoebic dysentery) |
| Ciliates | Many short cilia covering the cell | Paramecium (with macronucleus + micronucleus) |
| Flagellates | One or a few long flagella | Euglena (also photosynthetic), Trypanosoma (sleeping sickness) |
| Apicomplexans | Non-motile spore-formers; obligate parasites | Plasmodium (malaria), Toxoplasma |
| Algae (plant-like) | Photosynthetic; uni- or multicellular | Diatoms, dinoflagellates, Chlamydomonas, kelp |
| Slime molds | Aggregate to form motile masses | Dictyostelium, Physarum |
Disease-Causing Protists Worth Memorizing
- Plasmodium - causes malaria; vectored by female Anopheles mosquitoes; infects liver cells, then red blood cells.
- Trypanosoma brucei - African sleeping sickness, vectored by the tsetse fly.
- Giardia lamblia - intestinal protozoan causing giardiasis from contaminated water.
- Entamoeba histolytica - amoebic dysentery.
Algae and the Origin of Plastids
Endosymbiotic theory explains the origin of chloroplasts: an ancestral eukaryote engulfed a photosynthetic cyanobacterium that became a primary plastid. Red algae, green algae, and land plants inherit those primary plastids. Secondary endosymbiosis - a eukaryote engulfing a eukaryotic alga - produced the more complex plastids of diatoms, dinoflagellates, and brown algae. Green algae (Chlorophyta) share enough features with land plants that they are now grouped with them in the kingdom Plantae sensu lato (or clade Viridiplantae).
Fungi: Eukaryotic Decomposers
Fungi are monophyletic eukaryotic heterotrophs that have shaped terrestrial ecosystems for hundreds of millions of years. They are closer to animals (within the supergroup Opisthokonta) than to plants - both fungi and animals descend from a flagellated single-celled ancestor.
Defining Fungal Features
- Cell wall of chitin (the same polysaccharide in arthropod exoskeletons) - distinguishes fungi from plants (cellulose walls) and animals (no walls).
- Heterotrophic by absorption - fungi secrete exoenzymes that digest organic matter outside the cell, then absorb the small breakdown products.
- Body of hyphae - long, thin filaments that branch and intertwine into a mycelium (the main fungal body). Hyphae may have septa (cross-walls) or be coenocytic (continuous multinucleate cytoplasm).
- Reproduction by spores, dispersed through air, water, or animal vectors.
- Haploid-dominant life cycle in most fungi; diploid stage is brief.
The Four Major Phyla
| Phylum | Common Name | Key Features | Examples |
|---|---|---|---|
| Chytridiomycota | Chytrids | Flagellated spores (only fungi with flagella); aquatic and damp habitats | Batrachochytrium (frog skin pathogen) |
| Zygomycota | Zygote fungi | Form a tough zygospore during sexual reproduction; coenocytic hyphae | Rhizopus stolonifer (black bread mold) |
| Ascomycota | Sac fungi | Sexual spores produced in a sac-like ascus | Yeasts (Saccharomyces cerevisiae), morels, truffles, Penicillium, most lichens |
| Basidiomycota | Club fungi | Sexual spores on club-shaped basidia on a fruiting body | Mushrooms, puffballs, rusts, smuts |
A newer recognized phylum, Glomeromycota, contains the arbuscular mycorrhizal fungi that associate with the roots of about 80% of land-plant species.
Key Symbioses
Fungi are central players in two ecologically critical mutualisms.
Mycorrhizae
Mycorrhizae (literally "fungus roots") are mutualistic associations between fungal hyphae and plant roots. The plant supplies carbohydrates (sucrose) from photosynthesis, and the fungus dramatically expands the absorptive surface area, helping the plant take up water, phosphorus, and other minerals.
- Ectomycorrhizae sheath the outside of root cells; common in trees (pines, oaks, birches).
- Endomycorrhizae (arbuscular mycorrhizae) penetrate root cortex cells, forming highly branched arbuscules; common in most herbaceous plants and crops.
Mycorrhizae are so important that some plants (orchids, many forest seedlings) cannot germinate or survive without them.
Lichens
A lichen is a stable mutualistic composite of a fungus (the mycobiont, usually an ascomycete) and a photosynthetic partner (the photobiont - a green alga, a cyanobacterium, or both). The fungus provides structure, water retention, and protection; the photobiont provides sugars and (if cyanobacterial) fixed nitrogen.
Lichens grow on bare rock, tree bark, and soil; they are pioneer species that initiate ecological succession by weathering rock and fixing nutrients. They are also extremely sensitive to air pollution (especially SO₂), so their diversity is a classic bioindicator of air quality.
A student observes a single-celled eukaryote under a microscope. The cell is covered with thousands of short, hair-like extensions that beat rhythmically to propel it through pond water. It also has two visibly distinct nuclei. To which group of protists does this organism MOST likely belong?
What feature uniquely distinguishes fungi from both plants and animals?