Fungi are a diverse group of eukaryotic organisms, distinct from both plants and animals, playing many roles in Earth’s ecosystems. They are found across the globe in various environments, from rich soil to aquatic settings, and even in extreme Arctic and Antarctic regions. Fungi are primarily decomposers, breaking down organic matter, but also engage in parasitic or symbiotic relationships with other organisms. These organisms are often hidden, with their most visible parts, like mushrooms, being only a small portion of their overall structure.
Fundamental Fungal Components
Fungi grow as filamentous structures called hyphae, which are the primary units of fungal growth. These thread-like filaments extend by growing at their tips, a process known as apical growth. Each hypha consists of one or more cells surrounded by a protective cell wall.
A collective network or mass of hyphae forms the mycelium, which can be macroscopic or microscopic. This intricate web is the main body of the fungus, responsible for absorbing nutrients from the environment. Mycelium also anchors the fungus to its substrate and provides stability.
The fungal cell wall provides structural support and protection, primarily composed of chitin. Chitin is a complex polysaccharide which contributes to the wall’s rigidity and shields the cell from desiccation and predators. Fungal cell walls also contain glucans and glycoproteins, contributing to their protective functions.
Within hyphae, internal cross-walls called septa may be present, dividing the hypha into individual cells. Septate hyphae have these partitions, which often contain pores allowing the movement of cytoplasm, organelles, and sometimes even nuclei between cells. Conversely, aseptate hyphae lack these cross-walls, forming a continuous tube with multiple nuclei, a condition known as coenocytic. Zygomycetes, for example, typically exhibit aseptate hyphae, while Ascomycetes and Basidiomycetes generally have septate hyphae.
Structures for Nutrient Acquisition and Attachment
Fungi employ specialized structures to acquire nutrients from their surroundings. Haustoria are modified hyphae that parasitic fungi use to penetrate the host’s cells and absorb nutrients. Haustoria do not typically penetrate the host cell’s plasma membrane, but rather draw nutrients from the space between the cell wall and the membrane.
Some fungi, like certain Zygomycetes, develop rhizoids, which are root-like structures. These thin filaments anchor the fungus to its substrate and assist in the absorption of digested organic material.
Appressoria are flattened, adhesive structures formed by pathogenic fungi, enabling them to attach firmly to host surfaces. After attachment, these structures generate immense turgor pressure, which, along with secreted extracellular enzymes, helps them penetrate the host’s cuticle and epidermal cells.
Fungi obtain nutrients through extracellular digestion, secreting enzymes outside their bodies to break down complex organic matter. These enzymes, such as cellulases, proteases, and lipases, convert large molecules like cellulose, proteins, and lipids into simpler compounds. The simpler molecules, such as sugars, amino acids, and fatty acids, are then absorbed by the fungal hyphae.
Reproductive Structures and Spores
Spores are the primary reproductive units of fungi, serving for dispersal, survival, and initiating new fungal growth. Fungi produce a vast quantity of these single-celled units, which can be either asexual or sexual. Spores are typically dispersed by environmental forces such as wind or water, including raindrops.
Many fungi produce macroscopically visible reproductive structures called fruiting bodies, such as mushrooms and puffballs.
Sporangia are sac-like enclosures where spores are formed, particularly asexual spores called sporangiospores, in some fungal groups like bread molds. These structures are typically produced at the end of specialized hyphae called sporangiophores. During asexual reproduction, sporangiospores are released and can germinate to form new haploid hyphae.
Conidiophores are specialized hyphae that bear asexual spores called conidia externally. Conidia are non-motile spores, resembling dust, and are often dispersed by wind, water, or insects. The morphology of conidiophores can vary greatly between species and was historically used for fungal identification.
Microscopic structures called asci and basidia are where sexual spores are formed in specific fungal phyla. Asci are sac-shaped cells found in Ascomycetes. Basidia are club-shaped structures characteristic of Basidiomycetes. These structures highlight the diverse reproductive strategies within the kingdom Fungi.