Fungi are eukaryotes. Some fungi are unicellular, while others are multicellular. Single-celled fungi are called yeast. Some fungi alternate between single-celled yeast and multicellular forms depending on what stage of the life cycle they are in. Fungi cells have a nucleus and organelles like plant and animal cells do.
Protists are eukaryotes. Most of the protists are unicellular and some of the protists are multicellular. They could be viewed as those eukaryotes that cannot be classified as one of the other cell types.
Fungi are heterotrophic, which means they rely solely on carbon obtained from other organisms for their metabolism and nutrition. Fungi obtain nutrients in three different ways. First, saprotrophic fungi decompose dead organic matter. Second, as parasites, fungi live in or on other organisms and get their nutrients from their host. Third, they live mutualistically with other organisms.
Protists exhibit many forms of nutrition and may be aerobic or anaerobic. Photosynthetic protists (photoautotrophs) are characterized by the presence of chloroplasts. Other protists are heterotrophs and consume organic materials (such as other organisms) to obtain nutrition.
The major role of fungi is as decomposers. Most of the fungi help ecosystems maintain homeostasis by breaking down dead organisms and recycling essential nutrients. They also have symbiotic relationships with other plants.
Protists are primary producers. They are the important foundation in the food chain. The position of photosynthetic protists at the base of the food chain makes much of the diversity of aquatic life possible. They also produce a lot of oxygen.
Fungi are nonmotile. However, the fungal mycelium can grow quickly in any direction if it is in a suitable environment.
Some animal-like protists live symbiotically within other organisms such as termites. Others recycle nutrients by breaking down dead organic matter. Many animal-like protists live in seas and lakes, where they are eaten by tiny animals, which in turn serve as food for larger animals and contribute to the food chain.
Fungi reproduce asexually by fragmentation, budding, or producing spores. Fragments of hyphae can grow new colonies. Mycelial fragmentation occurs when a fungal mycelium separates into pieces with each component growing into a separate mycelium.
Most protists have the ability to move. Protists have three types of appendages for movement. They may have flagella, cilia, or pseudopods (“false feet”). There may be one or more whip-like flagella. Cilia are similar to flagella, except they are shorter and there are more of them. They may completely cover the surface of the protist cell. Pseudopods are temporary, foot-like extensions of the cytoplasm.
Reproduction of fungi can be sexual or asexual. In the life cycle of a sexually reproducing fungus, a haploid phase alternates with a diploid phase. The haploid phase ends with nuclear fusion, and the diploid phase begins with the formation of the zygote (the diploid cell resulting from the fusion of two haploid sex cells).
Protists have complex life cycles. Many have both asexual and sexual reproduction. An example is a protist called Spirogyra, which is shown in the figure. It usually exists as haploid cells that reproduce by binary fission.
This is a picture of Ringworm. Ringworm is a skin infection produces a ring-shaped rash, but it isn’t caused by a worm. It’s caused by the same fungus that causes athlete’s foot. The rash may occur on the arms, legs, head, neck, or trunk. The same fungi cause athlete’s foot when they infect the skin between the toes. Athlete’s foot is the second most common skin disease in the U.S. There are millions of different fungal species on Earth, but only about 300 of those are known to make people sick.
Most protist diseases in humans are caused by animal-like protists or protozoa. Protozoa make us sick when they become human parasites. Trypanosoma protozoa cause Chagas disease and sleeping sickness. Giardia protozoa cause giardiasis, and Plasmodium protozoa cause malaria.
There are over 100,000 described living species of protists. Nearly all protists exist in some type of aquatic environment, including freshwater and marine environments, damp soil, and even snow.