Many people instinctively categorize fungi, such as mushrooms, alongside plants. This common perception stems from their stationary nature and growth in soil, much like vegetation. However, fungi possess fundamental biological characteristics that align them more closely with the animal kingdom. This surprising truth reveals a fascinating evolutionary story and unique adaptations.
Dispelling the Plant Myth
Fungi are not plants because they lack the ability to produce their own food through photosynthesis. Unlike plants, fungi do not contain chlorophyll or chloroplasts, the cellular structures where photosynthesis occurs.
Furthermore, the structural composition of fungal cells differs significantly from plants. Plant cell walls are primarily composed of cellulose, a complex carbohydrate. In contrast, fungal cell walls are made of chitin, a tough polysaccharide also found in the exoskeletons of insects and crustaceans.
A Shared Evolutionary Past
The surprising kinship between fungi and animals is rooted in their shared evolutionary history. Both fungi and animals belong to a large group of eukaryotes called Opisthokonta. This supergroup also includes various single-celled organisms, indicating a common ancestor that predates the divergence of multicellular animals and fungi. Molecular evidence, particularly through DNA sequencing, strongly supports this close relationship.
The name Opisthokonta itself provides a clue to this ancient connection; it refers to the single, posterior flagellum found in the motile cells of many organisms within this group. While most fungi do not have flagellated cells in their primary life stages, this characteristic is present in ancestral fungal groups, such as chytrids, and is also found in animal sperm cells.
Their Animal-Like Feeding Strategy
A significant characteristic linking fungi to animals is their method of obtaining nutrients. Fungi are heterotrophs, meaning they acquire organic compounds from external sources rather than producing their own. This nutritional strategy contrasts sharply with the autotrophic nature of plants, which produce their food internally through photosynthesis.
Fungi employ external digestion. They secrete digestive enzymes directly into their environment, breaking down complex organic molecules outside their bodies. Once these molecules are broken into simpler, soluble forms, the fungi then absorb the nutrients through their cell walls. This method of digesting food externally and then absorbing the resulting compounds is similar to how animals obtain their nutrition, albeit animals typically ingest food before internal digestion.
Structural and Metabolic Connections
Beyond their evolutionary lineage and feeding strategies, fungi and animals share specific structural and metabolic features. Fungal cell walls are made of chitin, the same robust polysaccharide that forms the exoskeletons of arthropods, a major group within the animal kingdom. Chitin provides structural support and protection.
Another shared metabolic trait is their primary energy storage molecule. Both fungi and animals store excess glucose as glycogen, a highly branched polysaccharide. In animals, glycogen is stored in the liver and muscles for quick energy release. This differs from plants, which typically store glucose as starch.