Fungi are eukaryotic organisms. This means that their DNA-containing chromosomes are enclosed within a nucleus inside their cells. (The chromosomes of bacteria and archaea are not walled off inside nuclei, making them prokaryotic organisms.)
Many decades ago, scientists thought that fungi were primitive kinds of plants. New studies looking at the DNA of fungi have confirmed that these organisms are not plants.
Unlike plants, fungi do not make their own food energy via photosynthesis, but dine on organic matter like rotting leaves, wood, and other debris, or upon the tissues of living plants and animals.
Fungi, along with bacteria, are the planet’s major composters and recyclers.
Although fungi may seem like a nuisance when they dine in your fruit bowl or refrigerator, their ability to degrade some of the toughest organic materials, including tree wood and insect exoskeletons, means that our planet is not cluttered with a mass of debris.
Fungi dine at home, eating whatever they’re growing on. Fungi secrete digestive enzymes in order to break down complex food sources, such as animal corpses and tree stumps, into smaller components they can absorb.
Fungi include single-celled creatures that exist individually—the yeasts—and multicellular bunches, such as molds or mushrooms. Yeast cells look like little round or oval blobs under a microscope. They're too tiny to see as individuals, but you can see large clusters of them as a white powdery coating on fruits and leaves.
Molds are described as filament-like, or filamentous, because they form long filament-like, or thread-like, strands of cells called hyphae (high-fee). These hyphae are what give mold colonies their fuzzy appearance. They also form the fleshy body, or mushroom, that some species grow. It may seem odd to think of something as big as a mushroom as a microbe. But the cells of the hyphae making up that mushroom are connected in a closer way than the cells of other multicellular creatures, like you and me, are. The cell walls separating the cells in hyphae usually have openings that allow the protoplasm <pro-toe-plazm>, or fluid that fills cells, to flow between them. Essentially, a fungal hypha is like a tube. Your cells, on the other hand, are completely walled off from each other and the cell fluid, or cytoplasm <sigh-toe-plazm>, inside doesn't mingle between cells.