7.3 General Characteristics of Fungi

Fungi are a diverse kingdom of eukaryotic organisms that are not differentiated into roots, stems, and leaves. They are essential to ecosystems as decomposers, symbionts, and pathogens. The study of fungi is known as mycology.

Eukaryotic Nature

Fungi are eukaryotic, meaning their cells contain a true nucleus and other membrane-bound organelles.

Eukaryotic Cells→

Heterotrophic Nutrition

They lack chlorophyll and cannot perform photosynthesis. Instead, they obtain nutrients through external digestion (extracellular digestion), secreting enzymes to break down organic matter and then absorbing the simpler molecules. Because of this, they are often referred to as absorptive heterotrophs.

Unique Cell Structure

Fungi possess a rigid cell wall made primarily of chitin, a nitrogen-containing polysaccharide, which distinguishes them from plants (cellulose) and animals (no cell wall). They store carbohydrates as glycogen, similar to animals.

Cell Wall→

Body Plan (Thallus)

The main body of most fungi is a multicellular network of filaments called a mycelium. This network is composed of individual threads called hyphae, which maximize the surface area for nutrient absorption.

Hyphae Types

• Septate: Divided by cross-walls called septa. Example: Penicillium.
• Nonseptate (Aseptate): Lack cross-walls, resulting in a continuous cytoplasmic mass with many nuclei. This form is also called coenocytic hyphae. Example: Rhizopus.

Habitat and Mode of Life

Fungi are found in a wide variety of habitats, including terrestrial (forests, grasslands) and aquatic (lakes, rivers) environments. Their modes of life are diverse:

• Saprophytic: Act as decomposers, breaking down dead organic matter.
• Mutualistic: Form symbiotic relationships where both organisms benefit (e.g., Mycorrhizae, a partnership between fungi and plant roots).
• Parasitic: Live on or in other organisms, causing harm (e.g., fungal diseases in humans and crops).
• Predatory: Actively trap and consume other microorganisms.

Size and Locomotion

• Size: Varies from microscopic single-celled organisms like yeasts (3–10 micrometers) to massive organisms like Armillaria ostoyae (honey mushroom), which is considered the largest living organism on Earth.
• Locomotion: Fungi are non-motile and lack flagella at any stage of their life cycle. They grow towards food sources.

Reproduction

Fungi exhibit both asexual and sexual reproduction to alternate between stages.

Asexual Reproduction

• Budding: A small outgrowth or "bud" forms on the parent cell and detaches. Common in yeasts.
• Fragmentation: The mycelium breaks into fragments, each capable of growing into a new individual.
• Mitotic Spores: Spores are produced through mitosis and can develop into new, genetically identical individuals.

Sexual Reproduction

Involves the fusion of specialized sexual cells from different mating types, leading to genetic recombination and diversity.

Why Fungi are a Separate Kingdom

Fungi were once classified as plants but are now in their own kingdom because they share characteristics with both plants and animals, while also having unique features.

Nuclear Mitosis

A unique form of cell division in fungi where the mitotic spindle forms inside the nucleus, and the nuclear envelope does not break down until late in the process.

Cell Division→

Biological Significance

Fungi are vital to ecosystems as primary decomposers, recycling nutrients back into the soil. They form critical symbiotic relationships (mycorrhizae) with plants, aid in food production (yeast in bread, cheese molds), and are sources of medicines (e.g., penicillin), but can also be significant pathogens.