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 (no peptidoglycan)

• Reproduction: Both sexual and asexual spores

• Metabolism: Aerobic or facultative anaerobic

• Symbiosis: Many plants have symbiotic fungi (mycorrhizae) to aid in mineral and water absorption

Comparison: Fungi vs. Bacteria

Feature	Fungi	Bacteria
Cell Type	Eukaryotic	Prokaryotic
Cell Membrane	Sterols present	Sterols absent (except Mycoplasma)
Cell Wall	Glucans, mannans, chitin	Peptidoglycan
Spores	Sexual and asexual reproductive spores	Endospores (not for reproduction); some asexual spores
Metabolism	Heterotrophic; aerobic/facultative anaerobic	Heterotrophic, autotrophic; aerobic, anaerobic, facultative

Fungal Structure and Growth

Hyphae and Mycelium

The main body of multicellular fungi is composed of filaments called hyphae. A mass of hyphae forms a mycelium. Hyphae can be:

• Septate hyphae: Contain cross-walls (septa)

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

• Growth: Occurs at the tips of hyphae

Aerial vs. Vegetative Hyphae

• Vegetative hyphae: Involved in nutrient absorption

• Aerial (reproductive) hyphae: Involved in reproduction, often bear spores

Yeasts

Yeasts are unicellular, non-filamentous fungi. They are typically spherical or oval and reproduce by budding or fission.

• Budding yeasts: Divide unevenly; a bud forms, the nucleus divides, and one nucleus migrates into the bud

• Fission yeasts: Divide evenly to produce two new cells

• Pseudohyphae: Chains of yeast cells formed when buds fail to detach

Dimorphic Fungi

Dimorphic fungi can exist as either yeast or mold, depending on environmental conditions (often temperature).

• Yeastlike growth: At 37°C, reproduce by budding

• Moldlike growth: At 25°C, produce hyphae

Fungal Reproduction and Life Cycles

Asexual Reproduction

Asexual spores are produced by mitosis and are genetically identical to the parent. Types include:

• Conidiospore: Not enclosed in a sac

• Arthroconidia: Formed by fragmentation of septate hyphae

• Blastoconidia: Buds of the parent cell

Chlamydoconidium: Thick-walled spore within a hyphal segment

• Sporangiospore: Enclosed in a sac (sporangium)

Sexual Reproduction

Sexual reproduction involves the fusion of nuclei from two opposite mating types, followed by meiosis. The three main phases are:

1. Plasmogamy: Fusion of cytoplasm from two parent mycelia

2. Karyogamy: Fusion of nuclei to form a diploid zygote

3. Meiosis: Production of haploid sexual spores

Classification of Fungi

Major Fungal Phyla

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

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

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

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

Fungal Diseases (Mycoses)

Types of Mycoses

• Systemic mycoses: Deep within the body

• Subcutaneous mycoses: Beneath the skin

• Cutaneous mycoses: Affect hair, skin, and nails

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

• Opportunistic mycoses: Pathogenic in immunocompromised hosts

Lichens

Structure and Function

Lichens are mutualistic associations between a fungus and a green alga or cyanobacterium. They are important pioneers in ecological succession and serve as environmental indicators.

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

• Alga: Produces carbohydrates via photosynthesis

• Fungus: Provides structure and protection

• Morphologies: Crustose (encrusted), foliose (leaflike), fruticose (shrubby/fingerlike)

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 Arrangement: Unicellular, colonial, or multicellular

• Cell Wall: Usually cellulose

• Photosynthetic Pigments: Chlorophylls and accessory pigments

• Reproduction: Asexual (fragmentation, mitosis), sexual (alternation of generations in some)

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 Features

Protozoa are unicellular eukaryotes found in water and soil. They exhibit animal-like nutrition and complex life cycles, often involving multiple hosts.

• Movement: Pseudopods, flagella, or cilia

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

• Examples: Entamoeba histolytica (amebic dysentery), Plasmodium (malaria)

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 and animals.

• Mechanical transmission: Pathogen is carried on the body surface

• Biological transmission: Pathogen multiplies within the vector

• Examples: Mosquitoes (malaria), ticks (Lyme disease)

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