Fungi are a diverse and intriguing group of organisms that often capture attention through their visible forms, such as mushrooms and molds. Despite their stationary appearance and growth in soil, which might lead some to associate them with plants, fungi represent a unique branch of life. This article clarifies their position among living organisms.
Challenging Assumptions
However, this perception overlooks fundamental biological distinctions. Fungi do not belong to the Kingdom Plantae; instead, they occupy their own distinct biological kingdom, Fungi. Their unique characteristics set them apart from both plants and animals.
Shared Traits with Animals
Fungi share several significant biological characteristics with animals, indicating a closer evolutionary relationship than with plants. Both fungi and animals are heterotrophs, meaning they cannot produce their own food. Fungi obtain nutrients by releasing digestive enzymes into their environment and then absorbing the broken-down organic compounds, a process known as absorptive nutrition. Animals, in contrast, typically ingest their food and then digest it internally.
Another shared trait is the composition of their cell walls. Fungi possess cell walls made of chitin, a complex carbohydrate also found in the exoskeletons of insects and crustaceans, which are animals. This chitin provides structural support to fungal cells. Plants, by comparison, have cell walls primarily composed of cellulose.
Furthermore, both fungi and animals store excess glucose as glycogen, a branched polysaccharide. Plants, however, store their energy as starch. Like animals, fungi do not perform photosynthesis and therefore lack chloroplasts, the organelles responsible for converting light energy into chemical energy in plants.
Key Differences from Plants
Despite superficial resemblances, fungi exhibit fundamental differences from plants. Unlike plants, which are autotrophs that produce food through photosynthesis using chlorophyll, fungi are heterotrophs, lacking chlorophyll and relying on external nutrition. Structurally, fungi do not possess true roots, stems, or leaves like plants. Instead, many fungi grow as filamentous structures called hyphae, which collectively form a mycelium. While both kingdoms reproduce using spores, fungal spores are distinct from plant spores or seeds and are dispersed by various means, including wind or animals.
Evolutionary Journey
Molecular and genetic evidence provides a clear picture of the evolutionary relationships between these kingdoms. This evidence indicates that fungi and animals share a more recent common ancestor with each other than either does with plants. This shared lineage is recognized by their inclusion in the supergroup Opisthokonta, which encompasses animals, fungi, and their closest unicellular relatives, such as choanoflagellates.
The term “Opisthokonta” refers to the presence of a single posterior flagellum in the motile cells of their ancestral forms, a feature retained in animal sperm and some fungal spores. Plants diverged from this broader lineage much earlier in evolutionary history. This placement within the tree of life solidifies the understanding that while fungi may appear plant-like, their deep biological connections lie with the animal kingdom.