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What makes fungi, well, fungi? Professor Emeritus of Mycology Dennis E. Desjardin, PhD, highlights five key facts.
Many different organisms, from mushrooms to mold to yeast, can be categorized as fungi. Approximately 150,000 species of fungi have been discovered and formally described; however, mycologists estimate that there are conservatively 3.8 million species of fungi on earth. We know only about 3.9% of all fungi that exist in nature.
Fungi were once thought to be plants, because animals move and plants don’t. However, fungi have such distinct features that they are now considered to be in their own separate taxonomic kingdom. Here are a few things that make them unique:
Cell membranes: The cell membranes of animals, plants, and fungi are similarly composed of fatty lipid molecules, proteins, and other associated molecules. Fungal cells, like plants and animals, must maintain a specific fluidity of their cell membrane which is accomplished by a structurally rigid group of compounds collectively known as sterols. Whereas animal cells utilize cholesterol for this purpose, fungi uniquely utilize a compound called ergosterol. Interestingly, under certain conditions, the ergosterol in mycelium and mushrooms can be converted into vitamin D, which is biologically active in humans.
Cell walls: Fungi and plants need to maintain a high internal hydrostatic pressure, known as “turgor,” to maintain the diffusion of important biomolecules inside of their cells. Like the fiber mesh you can see embedded into the rubber of your gardening hose, cell walls help provide much needed structural integrity for cells to maintain their turgor. Whereas plant cell walls are mostly composed of cellulose, fungi produce a cell wall rich in beta-glucan and chitin (which also composes the hard shells of insects, crabs, and other arthropods). This incredible turgor is a major driving force for expansion which enables a mycelium to bore through dense obstacles in nature like rock and wood in their search for resources.
Bodies: Most people equate fungi in nature (or in supermarket produce sections) to mushrooms. The truth is that the bodies of most multicellular or “filamentous” fungi go almost entirely unnoticed in the wild until they grow visible mushrooms for the purpose of reproduction. The structure of a filamentous fungus, known as a mycelium, is a highly branched network of ever-reaching strands of tube-likecells called hyphae (pl., hypha sing.), which three-dimensionally explores its environment in search of food (i.e., dead plants, animals, and their excrement). If you were to align all the hyphae contained in just 1 gram of soil end to end, the strand could reach as far as 120 meters in length (Hanssen et al, 1974). This is 10 meters longer than an American football field.
Nutrients: Fungi, like animals, are heterotrophic; they consume the products of other organisms to derive energy and synthesize their biomolecules. Plants are autotrophic in that they produce their own energy via photosynthesis in specialized organelles called chloroplasts, which animals and fungi do not have. Interestingly, at least 400 million years ago, some fungi, now known as lichens, swallowed up ancient cyanobacteria and green algae and established a symbiotic relationship in which they derive energy from their symbionts’ ability to photosynthesize sugars in exchange for key nutrients and security.
External Digestion: Whereas animals evolved an internal digestive system including tubes and chambers from mouth to you-know-what, fungi perform most of their digestion externally through an intricate process of chemical sensing and secretion of an enzymatic soup called exudate. The digestive enzymes break down large food molecules into small building blocks, which the hyphae absorb to support their metabolism for growth and development. In fact, hyphae are literally surfing through the environment in their own flood of exudate, in a constant state of cellular remodeling as they expand.
About the Author
Dennis E. Desjardin, PhD, is the chief mycologist for mushroom-ingredients supplier Sempera Organics (Morgan Hill, CA) and Professor Emeritus of Mycology at San Francisco State University.