12 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 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 or facultative anaerobic	Heterotrophic, autotrophic; aerobic, facultative anaerobic, anaerobic

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) dividing cells

• Coenocytic hyphae: Lack septa, forming a continuous cytoplasmic mass 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 yeast: Divide unevenly, forming a bud that may break off or remain as a pseudohypha

• Fission yeast: Divide evenly to produce two new cells

• Facultative anaerobes: Can use aerobic respiration or fermentation

Dimorphic Fungi

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

• Yeast form: Typically at 37°C (body temperature)

• Mold form: Typically at 25°C (environmental temperature)

Fungal Reproduction and Life Cycle

Asexual Reproduction

Asexual spores are produced by mitosis and are genetically identical to the parent. Main 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, resulting in genetic recombination. The process includes:

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; asexual reproduction observed

Fungal Diseases (Mycoses)

Types of Mycoses

• Systemic mycoses: Deep within the body, often from soil fungi

• Subcutaneous mycoses: Beneath the skin, usually from soil fungi

• 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

Economic Importance of Fungi

• Saccharomyces cerevisiae: Used in bread, wine, and vaccine production

• Trichoderma: Produces cellulase for fruit juice clarification

• Taxomyces: Source of taxol, a cancer drug

• Entomophaga: Biocontrol agent against insect pests

Lichens

Characteristics and Structure

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

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

• Three morphologies: Crustose (encrusted), foliose (leaflike), fruticose (fingerlike)

• Nutrition: Alga provides carbohydrates via photosynthesis; fungus provides structure and protection

Algae

General Features

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

Major Groups of Algae

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

Roles of Algae in Nature

• Primary producers: Fix CO2 into organic molecules

• Oxygen production: Responsible for ~80% of Earth's oxygen

• Algal blooms: Can produce toxins or deplete oxygen, affecting aquatic life

• Symbiosis: Some algae live in mutualistic relationships with animals

Protozoa

General Characteristics

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

• Movement: By pseudopods, flagella, or cilia

• Reproduction: Asexual (fission, budding); sexual (conjugation in ciliates)

Examples of Protozoa

• Amebae: Move by pseudopods (e.g., Entamoeba histolytica causes amebic dysentery)

• Plasmodium: Causes malaria, transmitted by mosquitoes

Helminths

General Features

Helminths are multicellular parasitic worms, classified into two main phyla:

• Platyhelminthes: Flatworms (e.g., flukes, tapeworms)

• Nematoda: Roundworms

They are specialized for parasitism, often with reduced digestive and nervous systems and complex reproductive cycles.

Life Cycles

• Dioecious: Separate male and female individuals

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

Arthropods as Vectors

Definition and Importance

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

Representative classes include Arachnida (ticks, mites), Crustacea (crabs, crayfish), and Insecta (mosquitoes, flies, lice).

Examples of Arthropod Vectors and Diseases

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