Fungi are a biological kingdom distinct from both plants and animals. Their cellular organization is complex, ranging across a broad spectrum of cellularity. Fungi are eukaryotes, meaning their cells possess a membrane-bound nucleus and other complex internal structures like mitochondria. The kingdom Fungi includes organisms that are strictly single-celled and those that form massive, interconnected multicellular structures.
The Single-Celled Fungi
Some fungi exist primarily as single, independent cells, representing the simplest organization within the kingdom. These unicellular organisms are commonly referred to as yeasts, and they are typically spherical or oval in shape. A typical yeast cell measures between 3 and 4 micrometers in diameter, visible only under a microscope.
Their primary method of multiplication is budding. During budding, a small outgrowth forms on the parent cell, and the nucleus divides, with one copy migrating into the newly formed bud. The bud grows until it separates, leaving a scar on the parent cell. Certain yeast species, such as Saccharomyces cerevisiae, are important for fermentation, converting sugars into carbon dioxide and alcohol.
The Filamentous Fungi (Hyphae and Mycelium)
The vast majority of fungi, including molds and mushrooms, develop into larger, multicellular forms. This occurs through the growth of microscopic filaments called hyphae, which are long, branching, thread-like structures. Hyphae constitute the fundamental structural unit of a filamentous fungus. A dense, tangled network of hyphae forms the vegetative body of the fungus, known as the mycelium.
The mycelium can be extensive, often growing beneath the soil or within decomposing material, maximizing its surface area for nutrient absorption. The internal structure of the hyphae determines the organism’s cellularity and falls into two categories. Many fungi are classified as septate, possessing internal cross-walls called septa that divide the hypha into distinct compartments.
These septa are not solid barriers; they contain pores large enough to allow cytoplasm, organelles, and nuclei to pass freely between the compartments. This arrangement allows for regulated communication and sharing of resources, making septate fungi multicellular in a traditional sense. In contrast, some fungal groups, such as the Zygomycetes, are coenocytic, meaning their hyphae lack internal septa entirely.
The coenocytic structure results in a continuous, elongated tube of cytoplasm densely packed with multiple nuclei. This arrangement is effectively a large, multinucleated “supercell” rather than a chain of individual cells. The lack of cellular division facilitates rapid transport of nutrients and cytoplasm throughout the structure.
Unique Features of the Fungal Cell Wall
Regardless of whether a fungus is single-celled or filamentous, the cellular units are defined by their unique, rigid outer layer. The fungal cell wall is a distinguishing feature that differentiates it from both plant and animal cells. Unlike plant cell walls, which are composed primarily of cellulose, the fungal cell wall’s main component is chitin.
Chitin is a tough polysaccharide polymer, the same material that provides structural strength to the exoskeletons of insects and crustaceans. This durable material gives the fungal cell structural integrity and protection against environmental stress, including osmotic pressure and predation. The wall also contains other complex polysaccharides, such as glucans, which form a scaffold-like core along with the chitin.
The unique chemical composition of the cell wall is paired with other structural differences, such as the absence of chloroplasts. This confirms their classification as non-photosynthetic organisms. This distinct architecture reflects the fungus’s lifestyle as a heterotroph, relying on external sources of food, and is a defining characteristic of the kingdom.