Fungi: Characteristics and Structure

Defining Characteristics of Fungi

Fungi are a diverse group of eukaryotic organisms that play essential roles in decomposition, symbiosis, and disease.

• Mycology: The study of fungi.

• Chemoheterotrophs: Obtain energy and carbon from organic compounds.

• Decomposers: Break down organic matter, recycling nutrients.

• Symbiosis: Many plants rely on symbiotic fungi (mycorrhizae) for mineral and water absorption.

• Cell Structure: Eukaryotic cells with sterols in membranes; cell walls contain glucans, mannans, and chitin (not peptidoglycan).

• Metabolism: Limited to heterotrophic; aerobic or facultative anaerobic.

Comparison: Fungi vs. Bacteria

• Cell Type: Fungi are eukaryotic; bacteria are prokaryotic.

• Cell Wall: Fungi have chitin, glucans, and mannans; bacteria have peptidoglycan.

• Spores: Fungi produce sexual and asexual spores; bacteria produce endospores (not for reproduction).

Structure of Fungal Hyphae

Fungi grow as multicellular filaments called hyphae, which form a network called mycelium.

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

• Coenocytic hyphae: Lack septa, forming a continuous cytoplasm.

• Growth: Occurs at hyphal tips.

Aerial and Vegetative Hyphae

• Vegetative hyphae: Absorb nutrients.

• Aerial hyphae: Involved in reproduction, producing spores.

Yeasts: Structure and Reproduction

Characteristics of Yeasts

Yeasts are unicellular fungi, typically spherical or oval, and can grow aerobically or anaerobically.

• Budding yeasts: Divide unevenly, forming a bud that may break off or remain attached (pseudo-hypha).

• Fission yeasts: Divide evenly.

Pathogenic Yeasts: Candida albicans

• Candida albicans: Opportunistic pathogen, common in human gut flora.

• Can cause oral and vaginal yeast infections when overgrown.

Dimorphic Fungi

• Can grow as yeast (at 37°C) or mold (at 25°C).

• Pathogenic species often exhibit dimorphism.

Fungal Reproduction and Life Cycle

Asexual and Sexual Reproduction

Fungi reproduce by forming spores, either asexually (mitosis) or sexually (fusion of nuclei).

• Asexual spores: Conidiospore, arthroconidia, blastoconidia, chlamydoconidium, sporangiospore.

• Sexual spores: Formed by plasmogamy, karyogamy, and meiosis.

Major Fungal Phyla

Zygomycota (Conjugation Fungi)

• Saprophytic molds with coenocytic hyphae.

• Asexual spores: Sporangiospores.

• Sexual spores: Zygospores (fusion of nuclei).

Microsporidia

• Obligate intracellular parasites.

• No mitochondria; sexual reproduction likely occurs in host.

Ascomycota (Sac Fungi)

• Septate hyphae; produce conidiospores (asexual) and ascospores (sexual).

• Teleomorphic (sexual and asexual) and anamorphic (asexual only) forms.

Basidiomycota (Club Fungi)

• Septate hyphae; produce mushrooms.

• Conidiospores (asexual) and basidiospores (sexual, formed on basidium).

Fungal Diseases (Mycoses)

Types of Mycoses

• Systemic: Deep within the body.

• Subcutaneous: Beneath the skin.

• Cutaneous: Affect hair, skin, nails.

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

• Opportunistic: Pathogenic in compromised hosts.

Pathogenic Fungi Table

Economic Effects of Fungi

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

• Trichoderma: Produces cellulase for fruit juice clarification.

• Taxomyces: Source of taxol, a cancer drug.

• Entomophaga: Biocontrol agent against gypsy moths.

• Coniothyrium minitans: Kills crop-destroying fungi.

• Paecilomyces: Used to control termites.

Lichens: Structure and Function

Characteristics and Nutritional Needs

Lichens are mutualistic associations between a fungus and a green alga or cyanobacterium.

• Neither partner can exist alone.

• Secrete organic acids to break down substrates for nutrients.

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

Structure of Lichen Thallus

• Medulla: Hyphae grown around algal cells.

• Rhizines: Holdfasts anchoring the lichen.

• Cortex: Protective coating over algal layer.

• Alga: Produces carbohydrates via photosynthesis.

• Fungus: Provides structure and protection.

Economic Importance of Lichens

• Source of dyes, antimicrobial compounds, litmus (pH indicator), and food for herbivores.

• Environmental indicators due to sensitivity to air pollution.

Algae: Characteristics and Roles

Defining Characteristics

Algae are unicellular or multicellular photoautotrophs, mostly aquatic, lacking roots, stems, and leaves.

• Not considered plants.

• Require water for growth and reproduction.

Table: Characteristics of Selected Algae

Roles of Algae in Nature

• Fix CO2 into organic molecules.

• Produce 80% of Earth's O2.

• Algal blooms can release toxins or deplete oxygen.

• Oil production and symbiosis with animals.

Protozoa: Characteristics and Diseases

Defining Characteristics

Protozoa are unicellular eukaryotes with animal-like nutrition, inhabiting water and soil, and exhibiting complex life cycles.

• Move by pseudopods, flagella, or cilia.

• Some cause human diseases (e.g., malaria, amebic dysentery).

Helminths: Parasitic Worms

Characteristics of Helminths

Helminths are multicellular eukaryotic animals specialized for parasitism.

• May lack digestive system; reduced nervous system and locomotion.

• Complex reproductive system.

• Two phyla: Platyhelminthes (flatworms) and Nematoda (roundworms).

Life Cycle

• Dioecious: Separate male and female.

• Monoecious: Both reproductive systems in one animal.

Platyhelminths

• Trematodes (flukes): Flat, leaf-shaped, absorb food through cuticle.

• Cestodes (tapeworms): Scolex for attachment, proglottids with reproductive organs.

Nematodes

• Roundworms, cylindrical, complete digestive system.

• Dioecious; males have spicules.

• Free-living and parasitic forms.

Arthropods as Vectors

Defining Arthropod Vector

Arthropods are animals with segmented bodies, hard exoskeletons, and jointed legs. Vectors are arthropods that transmit pathogens.

• Mechanical transmission: Pathogen transferred without development in vector.

• Biological transmission: Pathogen multiplies in vector and is transmitted by biting.

• Definitive host: Microbe's sexual reproduction occurs in vector.

Table: Important Arthropod Vectors of Human Diseases

Summary

This chapter covers the defining characteristics, structure, reproduction, and classification of fungi, algae, protozoa, and helminths, as well as their roles in nature, disease, and economic importance. It also discusses arthropod vectors and their significance in transmitting human diseases.