The Eukaryotes: Fungi, Algae, Protozoa, and Helminths – Study Notes

Fungi: General Characteristics

Defining Features of Fungi

Fungi are a diverse group of eukaryotic organisms that play essential roles in ecosystems as decomposers, symbionts, and pathogens. The study of fungi is known as mycology.

• Cell Type: Eukaryotic

• Nutrition: Chemoheterotrophs (obtain energy and carbon from organic compounds)

• Cell Wall: Composed of glucans, mannans, and chitin (never peptidoglycan)

• Cell Membrane: Contains sterols

• Growth: Aerobic or facultative anaerobic

• Symbiosis: Many plants form mutualistic associations with fungi called mycorrhizae to enhance nutrient absorption

Comparison with Bacteria: Fungi are eukaryotic, have sterols in their membranes, and possess chitinous cell walls, while bacteria are prokaryotic, lack sterols (except Mycoplasma), and have peptidoglycan cell walls.

Fungal Structure and Growth

Hyphae and Mycelium

Fungi grow as multicellular filaments called hyphae, which collectively form a mycelium (the fungal body). Hyphae can be:

• Septate hyphae: Contain cross-walls (septa) dividing cells

• Coenocytic hyphae: Lack septa, forming continuous cytoplasmic masses with many nuclei

• Growth: Occurs at the tips of hyphae

Aerial and Vegetative Hyphae

Fungi have specialized hyphae:

• Vegetative hyphae: Involved in nutrient absorption

• Aerial (reproductive) hyphae: Involved in spore production and dispersal

Yeasts

Yeasts are unicellular, non-filamentous fungi. They reproduce by budding (uneven division) or fission (even division). Some yeasts can form chains of cells called pseudohyphae if buds fail to detach.

• Budding: Parent cell forms a bud, nucleus divides, one nucleus migrates into the bud, and a new cell wall forms.

• Facultative anaerobes: Can grow with or without oxygen (respiration or fermentation).

Dimorphic Fungi

Dimorphic fungi can exist as either yeast (unicellular) or mold (filamentous) forms, often depending on temperature. Many pathogenic fungi are dimorphic.

• Yeastlike growth: At 37°C (body temperature)

• Moldlike growth: At 25°C (environmental temperature)

Fungal Reproduction and Life Cycle

Asexual and Sexual Reproduction

Fungi reproduce by forming spores, either asexually (via mitosis) or sexually (via fusion of nuclei from different mating types).

• Asexual spores: Conidiospores, arthroconidia, blastoconidia, chlamydoconidia, sporangiospores

• Sexual reproduction: Involves three phases—plasmogamy (fusion of cytoplasm), karyogamy (fusion of nuclei), and meiosis (formation of haploid spores)

Classification of Fungi

Major Fungal Phyla

• Zygomycota (Conjugation Fungi): Coenocytic hyphae, asexual sporangiospores, sexual zygospores

• Ascomycota (Sac Fungi): Septate hyphae, asexual conidiospores, sexual ascospores in saclike ascus

• Basidiomycota (Club Fungi): Septate hyphae, asexual conidiospores, sexual basidiospores formed on basidium (mushrooms)

• Microsporidia: Obligate intracellular parasites, lack mitochondria, no observed sexual reproduction

Fungal Diseases (Mycoses)

Types of Mycoses

• Systemic mycoses: Deep within the body (e.g., soil fungi)

• Subcutaneous mycoses: Beneath the skin

• Cutaneous mycoses: Affect hair, skin, and nails (dermatophytes)

• Superficial mycoses: Localized, e.g., hair shafts

• Opportunistic mycoses: Normally harmless fungi become pathogenic in immunocompromised hosts

Lichens

Structure and Function

Lichens are mutualistic associations between a fungus and a green alga or cyanobacterium. They colonize harsh environments and contribute to soil formation.

• Thallus: Body composed of medulla (hyphae around algal cells), rhizines (holdfasts), and cortex (protective layer)

• Types: Crustose (encrusted), foliose (leaflike), fruticose (fingerlike)

• Roles: Alga produces carbohydrates; fungus provides structure and protection

Algae

General Characteristics

Algae are a diverse group of mostly aquatic, photosynthetic eukaryotes. They are not true plants and lack roots, stems, and leaves.

• Cell wall: Usually cellulose

• Photosynthetic pigments: Vary by group (chlorophylls, carotenoids, phycobiliproteins)

• Reproduction: Asexual and sexual (alternation of generations in some)

Major Groups of Algae

Group	Cell Wall	Pigments	Storage	Pathogenicity
Brown Algae	Cellulose, alginic acid	Chlorophyll a, c, xanthophylls	Carbohydrate	None
Diatoms	Pectin, silica	Chlorophyll a, c, carotene, xanthophylls	Oil	Toxins
Dinoflagellates	Cellulose	Chlorophyll a, c, carotene, xanthins	Starch	Toxins
Red Algae	Cellulose	Chlorophyll a, d, phycobiliproteins	Glucose polymer	Some toxins
Green Algae	Cellulose	Chlorophyll a, b	Starch	None

Protozoa

General Characteristics

Protozoa are unicellular eukaryotes with animal-like nutrition. They inhabit water and soil and often have complex life cycles.

• Movement: By pseudopods, flagella, or cilia

• Reproduction: Asexual (fission, budding, schizogony) and sexual (conjugation)

Helminths

General Characteristics

Helminths are multicellular parasitic worms, including flatworms (Platyhelminthes) and roundworms (Nematoda). They are specialized for parasitism with reduced digestive, nervous, and locomotor systems, but complex reproductive systems.

• Dioecious: Separate male and female individuals

• Monoecious (hermaphroditic): Both reproductive organs in one individual

Arthropods as Vectors

Role in Disease Transmission

Arthropods are animals with segmented bodies, exoskeletons, and jointed legs. Some act as vectors, transmitting pathogenic microorganisms to humans.

• Mechanical transmission: Pathogen is carried on the body surface

• Biological transmission: Pathogen multiplies within the vector and is transmitted via bite

• Definitive host: Where the pathogen undergoes sexual reproduction

Vector	Disease
Dermacentor (tick)	Rocky Mountain spotted fever
Ixodes (tick)	Lyme disease, babesiosis, ehrlichiosis
Aedes (mosquito)	Dengue, Zika virus disease, heartworm
Anopheles (mosquito)	Malaria
Triatoma (kissing bug)	Chagas disease