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

Fungi

General Characteristics of Fungi

Fungi are a diverse group of eukaryotic organisms that play essential roles in ecosystems as decomposers, pathogens, and symbionts. The study of fungi is known as mycology. Fungi are chemoheterotrophs, meaning they obtain energy by decomposing organic matter. They can be aerobic or facultative anaerobes.

• Cell Type: Eukaryotic

• Cell Membrane: Contains sterols

• Cell Wall: Composed of glucans, mannans, and chitin (no peptidoglycan)

• Reproduction: Sexual and asexual spores

• Metabolism: Limited to heterotrophic; aerobic or facultatively anaerobic

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	Endospores (not for reproduction)
Metabolism	Heterotrophic	Heterotrophic, autotrophic

Nutritional Adaptations

• Grow best at pH 5

• Resistant to high sugar and salt concentrations (osmotic pressure)

• Can grow in low moisture environments

• Capable of metabolizing complex carbohydrates

Vegetative Structures

Fungi exist as molds (multicellular) or yeasts (unicellular). The main body of a mold is called a thallus, composed of filaments called hyphae. A mass of hyphae is termed a mycelium. Hyphae can be:

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

• Coenocytic (aseptate) hyphae: Lack septa, forming a continuous cell with many nuclei

Yeasts are nonfilamentous, unicellular fungi. They reproduce by:

• Budding: Uneven division (e.g., Saccharomyces)

• Fission: Even division

Dimorphic fungi can exist as either yeast-like or mold-like forms, often depending on temperature (yeast-like at 37°C, mold-like at 25°C).

Fungal Reproduction and Spores

Fungi reproduce by forming spores, which are important for classification. Spores can be:

• Asexual spores: Produced by mitosis (no genetic variation)

• Sexual spores: Produced by meiosis (genetic variation)

Asexual Spores

• Conidiospores: Not enclosed in a sac (e.g., Penicillium)

• Sporangiospores: Enclosed in a sac (sporangium; e.g., Absidia)

• Arthroconidia: Fragmentation of septate hyphae

• Blastoconidia: Buds of the parent cell

• Chlamydoconidium: Spore within a hyphal segment

Sexual Spores

• Zygospores: Fusion of similar cells (e.g., Rhizopus)

• Ascospores: Formed in a sac (ascus; e.g., cup fungi)

• Basidiospores: Formed externally on a pedestal (basidium; e.g., mushrooms)

The sexual reproductive cycle involves three phases:

1. Plasmogamy: Fusion of cytoplasm from two parent cells

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

3. Meiosis: Production of haploid sexual spores

Major Fungal Phyla

• Mucoromycota (Zygomycota): Conjugation fungi, coenocytic hyphae, produce sporangiospores (asexual) and zygospores (sexual). Example: Rhizopus stolonifer (black bread mold).

• Microsporidia: Obligate intracellular parasites, lack mitochondria, cause diseases like chronic diarrhea and keratoconjunctivitis.

• Ascomycota: Sac fungi, septate hyphae, produce conidiospores (asexual) and ascospores (sexual). Includes yeasts, truffles, and morels.

• Basidiomycota: Club fungi, septate hyphae, produce conidiospores (asexual) and basidiospores (sexual). Includes mushrooms.

Fungal Diseases (Mycoses)

Mycoses are fungal infections classified by the site of infection:

• Systemic mycoses: Deep within the body

• Subcutaneous mycoses: Beneath the skin

• Cutaneous mycoses (dermatomycoses): Affect hair, skin, and nails (e.g., ringworm)

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

• Opportunistic mycoses: Pathogenic in immunocompromised hosts

Some fungi produce mycotoxins (e.g., ergot toxin from Claviceps purpurea), which can cause severe health effects.

Economic Importance of Fungi

• Aspergillus niger: Production of citric acid

• Saccharomyces cerevisiae: Bread, wine, hepatitis B vaccine

• Trichoderma: Cellulase production

• Taxomyces: Taxol (anticancer drug)

• Tolypocladium inflatum: Cyclosporine (immunosuppressant)

Protists

General Characteristics

Protists are a diverse group of eukaryotic organisms that may be unicellular, multicellular, or multinucleated. They can be autotrophs or heterotrophs and reproduce sexually or asexually. Some have cell walls, while others do not.

Major groups include:

• Algae

• Slime molds

• Protozoans

Protozoa

Protozoa are unicellular eukaryotes that require a moist environment. They have animal-like nutrition and may be free-living or parasitic. Many have a protective pellicle and specialized feeding structures.

• Some are normal microbiota; others are pathogens (e.g., Plasmodium, Toxoplasma gondii, Giardia)

• Complex life cycles, often involving multiple hosts

Life Cycle

• Trophozoite: Active, feeding, and growing stage

• Cyst: Dormant, resistant stage for survival in harsh conditions

• Asexual reproduction: Fission, budding, or schizogony

• Sexual reproduction: Conjugation

Classification by Motility

Type	Motility	Example
Amoeboid	Pseudopods	Entamoeba histolytica
Flagellated	Flagella	Giardia lamblia
Ciliated	Cilia	Tetrahymena thermophila
Spore-forming	Non-motile	Plasmodium

Medically Important Protozoa

• Excavata: Flagellated, often lack mitochondria (e.g., Giardia intestinalis, Trichomonas vaginalis, Trypanosoma)

• Amoebozoa: Move by pseudopods (e.g., Entamoeba histolytica)

• Apicomplexa: Nonmotile, obligate intracellular parasites (e.g., Plasmodium, Toxoplasma gondii)

• Ciliates: Move by cilia (e.g., Balantidium coli)

Spore-Forming Protozoans (Apicomplexa)

Apicomplexans are the largest phylum of protozoans. They are obligate intracellular parasites with complex life cycles involving both sexual and asexual stages:

• Merogony: Asexual reproduction producing merozoites

• Gamogony: Sexual reproduction producing gametes

• Sporogony: Formation of sporozoites (infective stage)

Notable diseases include malaria (Plasmodium), toxoplasmosis, and cryptosporidiosis.

Helminths (Parasitic Worms)

General Features

Helminths are multicellular, eukaryotic animals, many of which are parasitic. They are classified into two main phyla:

• Platyhelminthes: Flatworms (includes trematodes and cestodes)

• Nematoda: Roundworms

Helminths have complex life cycles, often involving multiple hosts and larval stages. They may lack digestive systems and have reduced nervous and locomotor systems, but possess complex reproductive systems.

Phylum	Structure	Size Range	Reproduction	Examples
Roundworms (Nematodes)	Non-segmented, cylindrical	Microscopic–1 m	Sexual, two sexes	Hookworm, pinworm
Flatworms (Cestodes)	Segmented, ribbon-like	1 mm–10 m	Hermaphroditic	Tapeworms
Flatworms (Trematodes)	Non-segmented, leaf-shaped	1 mm–7 cm	Hermaphroditic (except blood flukes)	Liver fluke, blood fluke

Platyhelminths (Flatworms)

Trematodes (Flukes)

• Flat, leaf-shaped

• Ventral and oral suckers

• Absorb food through cuticle

• Examples: Paragonimus (lung fluke), Schistosoma (blood fluke)

Cestodes (Tapeworms)

• Segmented body with scolex (head) and proglottids (body segments)

• Absorb nutrients through cuticle

• Examples: Taenia solium (pork tapeworm), Echinococcus granulosus

Nematodes (Roundworms)

• Cylindrical, complete digestive system

• Dioecious (separate sexes)

• Examples: Ascaris lumbricoides, Enterobius vermicularis (pinworm), Necator americanus (hookworm)

Arthropods as Vectors

General Features

Arthropods are animals with segmented bodies, exoskeletons, and jointed legs. Many serve as vectors, transmitting pathogens to humans.

• Arachnida: Mites and ticks (8 legs)

• Crustacea: Crabs, crayfish (4 antennae)

• Insecta: Mosquitoes, flies, lice, fleas (6 legs)

Transmission can be mechanical (passive) or biological (pathogen multiplies in vector). The definitive host is where the microbe's sexual reproduction occurs.

Class	Order	Vector	Disease
Arachnida	Mites and ticks	Dermacentor, Ixodes	Rocky Mountain spotted fever, Lyme disease
Insecta	Flies, mosquitoes, lice, fleas	Aedes, Anopheles, Pediculus	Dengue, malaria, typhus