Study Notes: Microbial Life – Fungi (General Chemistry Context)

Microbial Life: Fungi

Introduction to Fungi

Fungi are a diverse group of eukaryotic organisms that play essential roles in ecosystems as decomposers, recyclers, and symbionts. They are more closely related to animals than to plants, primarily due to similarities in their cellular and genetic structures.

• Eukaryotes: Fungi possess membrane-bound organelles and linear chromosomes.

• Relation to Animals: Fungi absorb carbon as a food source and store carbohydrates as glycogen, similar to animals.

• Cell Walls: Fungal cell walls are composed of chitin, a polysaccharide also found in arthropods.

• Non-photosynthetic: Fungi do not contain chlorophyll and cannot perform photosynthesis.

• Reproduction: Most fungi reproduce both sexually and asexually, though some reproduce only asexually.

• Symbiosis: Fungi form mutualistic and parasitic relationships with other organisms.

• Applications: Fungi are used in food, beer, and wine production.

Fungal Cell Structure

Key Features of Fungal Cells

Fungal cells exhibit unique structural characteristics that distinguish them from plants and animals.

• Haploid Tissue: Most fungal tissue is haploid (n), containing only one set of chromosomes.

• Organelles: Fungi have mitochondria but lack chloroplasts.

• Cell Wall Composition: The cell wall is made of chitin and glucans, providing rigidity and strength.

• Ergosterol: Fungal cell membranes contain ergosterol instead of cholesterol.

• No Photosynthesis: Fungi do not have pigments for photosynthesis and do not require light for growth.

Fungal Growth

Growth Forms and Structures

Fungi can be unicellular (yeasts) or multicellular, with growth involving distinct morphological stages.

• Hyphae: Multicellular fungi produce hyphae, which are haploid filamentous structures.

• Mycelium: A network of hyphae forms the mycelium, the main body of the fungus.

• Cellular Organization: Hyphae can be divided by septa (walls) or be coenocytic (without septa, containing many nuclei).

Fungal Light & Oxygen Requirements

Environmental Needs

Fungi have specific requirements for growth, particularly regarding moisture, pH, and oxygen.

• Moisture: Fungi require moist environments for optimal growth.

• pH: Most fungi prefer slightly acidic conditions.

• Light: Fungi do not need light to grow.

• Oxygen: Most fungi are obligate aerobes, but some are obligate anaerobes or facultative anaerobes.

Fungal Nutrition

Modes of Nutrition

Fungi are heterotrophs, obtaining nutrients from organic and inorganic sources through external digestion.

• Heterotrophs: Fungi absorb nutrients after breaking down food externally using enzymes.

• Nitrogen Acquisition: Some fungi trap worms and insects to obtain nitrogen in poor soils.

• Storage: Fungi store carbohydrates as glycogen, not starch or cellulose.

• Saprophytes: Many fungi are saprophytes, decomposing organic matter and recycling nutrients in ecosystems.

• Decomposition: Fungi can break down lignin and cellulose, substances that are difficult for most organisms to digest.

Fungal Reproduction

Sexual and Asexual Reproduction

Fungi reproduce through both sexual and asexual means, with several mechanisms for propagation.

• Fragmentation: Hyphae break apart and grow into new mycelium.

• Budding: A new cell forms as a bulge from the parent cell, common in yeasts.

• Sporangia: Specialized structures produce haploid spores via mitosis, which develop into new vegetative cells.

Sexual Reproduction: Involves fusion of haploid hyphae to form a heterokaryotic stage (n + n), followed by formation of a diploid zygote and subsequent spore production.

Summary Table: Fungal Characteristics

Characteristic	Fungi	Plants	Animals
Cell Wall Composition	Chitin	Cellulose	None
Storage Carbohydrate	Glycogen	Starch	Glycogen
Photosynthesis	No	Yes	No
Mode of Nutrition	Absorptive (heterotrophic)	Autotrophic	Ingestive (heterotrophic)

Key Equations and Terms

• Haploid: $n$ (one set of chromosomes)

• Diploid: $2n$ (two sets of chromosomes)

• Heterokaryotic: $(n + n)$ (two genetically distinct nuclei in one cell)

Example: Fungal Role in Ecosystems

Fungi decompose organic matter, releasing nutrients such as nitrogen and phosphorus back into the soil, making them available to plants. This process is essential for nutrient cycling and ecosystem health.

Additional info:

• Some fungi form mutualistic relationships with plants (mycorrhizae), algae (lichens), and animals (e.g., leaf-cutter ants).

• Fungi can also be parasitic, causing diseases in plants, animals, and humans.

• Fungal pathogens such as Batrachochytrium dendrobatidis (Bd) have significant impacts on amphibian populations.