Fungi are a major group of eukaryotic organisms characterised by distinctive cellular, nutritional and reproductive features. They include unicellular forms such as many yeasts, filamentous forms commonly called moulds, and larger reproductive structures commonly called mushrooms. Fungal cells possess nuclei and membrane-bound organelles and generally have cell walls containing chitin and other structural polymers rather than the cellulose characteristic of plants.
Most fungi obtain nutrients by secreting extracellular enzymes that break down organic material and then absorbing soluble products, making them fundamentally absorptive rather than photosynthetic organisms. The term fungi is the plural form of fungus and refers to a biological kingdom or broad evolutionary group whose members share common ancestry. Fungi are neither plants nor animals, despite historical classifications that sometimes placed them near plants because of their stationary lifestyle. Molecular and cellular evidence demonstrates that fungi are evolutionarily closer to animals than to plants, although fungi possess a distinctive evolutionary history and biology of their own.
Fungal bodies commonly consist of hyphae, elongated microscopic filaments that can form extensive networks called mycelia. Hyphae may be divided by septa or may contain continuous cytoplasm with multiple nuclei depending on the lineage. Growth occurs primarily at hyphal tips, allowing fungi to explore substrates and penetrate complex environments. The surface area of a fungal mycelium can be large relative to its biomass, supporting efficient acquisition of nutrients from decomposing material or living hosts.
Some fungi are dimorphic and can switch between yeast-like and filamentous growth forms in response to environmental conditions, including temperature and nutrient availability. Fungal growth is influenced by moisture, temperature, pH, oxygen, nutrient composition and interactions with other organisms. Their ability to grow through solid substrates distinguishes many fungi ecologically from microorganisms that remain primarily suspended in liquids. Mycelial networks can also connect spatially separated nutrient sources and transport materials within the fungal body, creating complex growth patterns that contribute to their ecological success.
Fungi are among the most important decomposers in terrestrial ecosystems. Their extracellular enzymes allow them to break down complex organic compounds such as cellulose, hemicellulose, lignin, proteins and lipids. This ability makes fungi particularly important in the decomposition of wood and plant residues, where lignin can limit access to other organic components. Saprotrophic fungi release enzymes outside their cells, digest substrates externally and absorb the resulting compounds.
Other fungi are parasites or pathogens that obtain nutrients from living hosts, while mutualistic fungi exchange resources with plants or other organisms. Mycorrhizal associations are especially widespread and involve nutrient and carbon exchange between fungi and plant roots. Fungi therefore occupy multiple ecological roles and cannot be characterised simply as decomposers. Their interactions range from beneficial symbioses to destructive disease, and the same fungal lineage may interact differently with different hosts or under different environmental conditions.
Fungal reproduction can be sexual, asexual or involve combinations of both strategies depending on the lineage and environmental conditions. Asexual reproduction may occur through spores, budding or fragmentation and can allow rapid multiplication when conditions are favourable. Sexual reproduction involves fusion of compatible genetic material and can generate genetic variation through recombination and meiosis. Fungal life cycles vary greatly, with haploid, dikaryotic and diploid stages occurring in different groups and sometimes within the same life cycle.
Spores are important dispersal structures and can travel through air, water, soil, animals or human activities. Their resistance to environmental stress varies among species, but many spores allow fungi to survive periods when active growth is impossible. Reproductive structures can therefore have both ecological and evolutionary significance. The enormous diversity of fungal life cycles is one reason fungi cannot be adequately described using a single reproductive model derived from familiar mushrooms or laboratory yeasts.
Fungi have major economic and medical importance. They are used to produce foods, beverages, enzymes, organic acids, pharmaceuticals and industrial chemicals. Some fungi produce medically valuable compounds, including antimicrobial substances, immunologically active molecules and cholesterol-lowering compounds. Other fungi cause diseases in humans, animals and plants, with clinical manifestations ranging from superficial infections to severe systemic disease in susceptible individuals. Plant-pathogenic fungi contribute substantially to agricultural losses by infecting crops and damaging roots, leaves, stems, fruits or seeds.
Fungi can also spoil food and materials by growing under favourable moisture conditions. Their ability to degrade organic compounds is useful in biotechnology but can also damage timber, textiles, buildings and stored products. Consequently, fungal biology has both beneficial and harmful applications, and the distinction depends heavily on species, environment and interaction with a host or substrate.
Fungal taxonomy has changed considerably as molecular phylogenetics has revealed relationships that are not obvious from morphology alone. Historically, fungi were classified largely according to visible reproductive structures and spore characteristics, but DNA sequencing has transformed the understanding of fungal evolution. Major fungal lineages differ in cell structure, life cycles, reproductive biology and ecological roles. The term mould is primarily descriptive rather than a formal taxonomic category, because filamentous growth has evolved in multiple fungal groups.
Similarly, the term mushroom generally refers to a macroscopic reproductive structure rather than a complete organism or taxonomic group. This distinction is important because a mushroom is comparable to a fruiting structure, while the main fungal body may remain largely underground or within another substrate. Modern fungal classification therefore integrates morphology, developmental biology, genetics, physiology and molecular phylogenetics rather than relying solely on appearance.
Fungi interact extensively with other organisms through competition, predation, parasitism and mutualism. Some fungi form lichens through symbioses involving photosynthetic partners, while others associate with insects and contribute to complex ecological relationships. Fungal pathogens can manipulate host physiology, evade immune responses and produce enzymes or toxins that facilitate colonisation. Mycorrhizal fungi can alter plant nutrient acquisition and influence plant community structure, while decomposer fungi affect nutrient availability for plants and microorganisms.
Fungi also compete with bacteria and other fungi for space and resources and may produce secondary metabolites that inhibit competitors. These interactions make fungi important ecological engineers. Their activities influence carbon storage, nutrient cycling, soil formation and ecosystem succession. Understanding fungi therefore requires considering networks of organisms rather than treating each species as an isolated biological unit.
Modern fungal research combines microscopy, culture, physiology, genomics, transcriptomics, metabolomics and ecological field studies. Genome sequencing has revealed extensive biosynthetic potential, including genes for secondary metabolites that may not be produced under standard laboratory conditions. Environmental DNA and metabarcoding can detect fungal communities in soils, water, plant tissues and other habitats without requiring cultivation. However, DNA detection does not necessarily demonstrate that a fungus is active, abundant or causing a particular effect.
Fungal identification can also be complicated by cryptic species that are morphologically similar but genetically distinct. A common misconception is that all fungi are harmful or that visible mould indicates a single type of organism. In reality, fungi range from essential ecosystem components and food organisms to pathogens and destructive contaminants. Their diversity, metabolic versatility and ecological importance make Fungi a major biological group with significance across environmental science, medicine, agriculture, biotechnology and evolutionary biology.
