All Fungi Are Heterotrophs
Fungi are eukaryotic heterotrophs that have cell walls. The cell walls of fungi are made up of chitin, a
complex carbohydrate that is also found in the external skeletons of insects. Recall that heterotrophs depend on
other organisms for food. Unlike animals, fungi do not ingest their food. Instead, they digest food outside of
their bodies and then absorb it. Many fungi feed by absorbing nutrients from decaying matter in the soil. Others
live as parasites, absorbing nutrients from the bodies of their hosts.
Many fungi are saprobes, organisms that obtain food from decaying organic matter. Others are parasites, which
harm other organisms while living directly on or within them. Still other fungi are symbionts that live in close
and mutually beneficial association with other species. Although most fungi feed on decaying matter, a few feed
by capturing live animals.
Structure and Function of Fungi
All fungi are multicellular. Multicellular fungi are composed of tiny filaments called hyphae (HY-fee; singular:
hypha). Each hypha is only one cell thick. In some fungi, cross walls divide the hyphae into cells containing one
or two nuclei, as shown below. In the cross walls, there are tiny openings through which the cytoplasm and
nuclei can move. Other hyphae lack cross walls and contain many nuclei.
The bodies of multicellular fungi are composed of many hyphae tangled together into a thick mass called
a mycelium. The mycelium (my-SEE-lee-um; plural: mycelia) is well suited to absorb food because it permits a
large surface area to come in contact with the food source through which it grows.
What you recognize as a mushroom is actually the fruiting body of a fungus. A fruiting body is a reproductive
structure that develops from a mycelium that grows below the surface of the ground. Clusters of mushrooms are
often part of the same mycelium, which means that they are part of a single organism.
Fungi as Parasites
Parasitic fungi cause serious plant and animal diseases. A few fungi cause diseases in humans.
Plant Diseases Fungi cause diseases. Mildews, which infect a wide variety of fruits, are also fungi. Fungal diseases are
responsible for the loss of approximately 15 percent of the crops grown in temperate regions of the world. In
tropical areas, where high humidity favors fungal growth, the loss of crops is sometimes as high as 50 percent.
Fungi are in direct competition with humans for food. Unfortunately for us, sometimes fungi win that
competition.
Human Disease Fungal parasites can also infect humans. One deuteromycete can infect the areas between the toes,
causing the infection known as athlete’s foot. The fungus forms a mycelium directly within the outer layers of
the skin. This produces a red, inflamed sore from which the spores can easily spread from person to person.
When the same fungus infects other areas, such as the skin of the scalp, it produces a red scaling sore known as
ringworm, which is not a worm at all.
A different type of fungal disease is caused by the yeast Candida albicans, which grows in moist regions
of the body. Usually its growth is kept in check by competition from bacteria that grow in the body and by the
body’s immune system. This normal balance can be upset by many factors, including the use of antibiotics,
which kill bacteria, or by damage to the immune system. When this happens, Candida may produce thrush, a
painful mouth infection.
Reproduction in Fungi
Most fungi reproduce both asexually and sexually. Asexual reproduction takes place when cells or
hyphae break off from a fungus and begin to grow on their own. Some fungi also produce spores, which can
scatter and grow into new organisms.
In some fungi, spores are produced in structures called sporangia (spoh-RAN-jee-uh; singular:
sporangium). Sporangia are found at the tips of specialized hyphae called sporangiophores (spoh-RAN-jee-ohfawrz).
Sexual reproduction in fungi usually involves two different mating types. Because gametes of both
mating types are about the same size, they are not called male and female. Rather, one mating type is referred to
as “+” (plus), and the other is referred to as “−” (minus). When the hyphae of opposite mating types meet, each
hypha forms a gametangium (gam-uh-TAN-jee-um; plural: gametangia), or structure that forms gametes. Then,
the two gametangia join, and the haploid nuclei contained in them fuse to form a diploid nucleus, or zygote.
After the zygote forms, meiosis takes place, producing haploid nuclei that dominate the remainder of the life
cycle. In most fungi, the zygote is the only cell in the entire life cycle that is diploid.
Fungi as Decomposers
Fungi are found in every ecosystem, where they recycle nutrients by breaking down the bodies and
wastes of other organisms. Many fungi feed by releasing digestive enzymes that break down leaves, fruit, and
other organic material into simple molecules. These molecules then diffuse into the fungus. The mycelia of
fungi produce digestive enzymes that speed the breakdown of dead organisms, thereby helping to recycle
nutrients and essential chemicals.
Imagine a world without fungi and other decomposers. Without decay, the energy-rich compounds that
organisms accumulate during their lifetimes would be lost forever. Many organisms, especially plants, remove
important trace elements and nutrients from the soil. If these materials were not eventually returned, the soil
would quickly be depleted, and Earth would become lifeless and barren.
How Fungi Spread
Fungal spores are found in almost every environment. This is why molds seem to spring up in any
location that has the right combination of moisture and food. Many fungi produce dry, almost weightless spores.
These spores scatter easily in the wind. On a clear day, a few liters of fresh air may contain hundreds of spores
from many species of fungi.
If these spores are to germinate, they must land in a favorable environment. There must be the proper
combination of temperature, moisture, and food so that the spores can grow. Even under the best of
circumstances, the probability that a spore will produce a mature organism can be less than one in a billion.