Download What Are Fungi?

Lesson Overview
21.4 Fungi
What Are Fungi?
– What are the basic characteristics of fungi?
– Fungi are heterotrophic eukaryotes with cell
walls that contain chitin.
What Are Fungi?
– Many fungi grow from the ground, but fungi
aren’t plants.
– Instead of carrying out photosynthesis, fungi
produce enzymes that digest food outside their
bodies. Then they absorb the small molecules
released by the enzymes.
– Many fungi feed by absorbing nutrients from
decaying matter in the soil. Others live as
parasites, absorbing nutrients from their hosts.
What Are Fungi?
– The cell walls of fungi are composed of chitin,
a polymer made of modified sugars that is also
found in the external skeletons of insects.
– The presence of chitin is one of several
features that show fungi are more closely
related to animals than to plants.
Structure and Function
– Yeasts are tiny fungi that live most of their
lives as single cells.
– Mushrooms and other fungi, on the other
hand, grow much larger. Their bodies are
made up of cells that form long, slender
branching filaments called hyphae.
–
Structure and Function
– In most fungi, cross walls divide the hyphae
into compartments resembling cells, each
containing one or two nuclei. In the cross
walls, there are openings through which
cytoplasm and organelles can move.
Structure and Function
– The body of a mushroom is actually the
fruiting body, the reproductive structure of
the fungus.
– The fruiting body grows from the mycelium,
the mass of branching hyphae below the soil.
Clusters of mushrooms are often part of the
same mycelium, which means they are part of
the same organism.
Structure and Function
– The mycelium of the soil fungus in a
fairy ring has grown so large that it has
used up all of the nutrients near its center.
– It grows and produces fruiting bodies—
the mushrooms—only at its edges, where
it comes in contact with fresh soil and
abundant nutrients.
Reproduction
– Fungi can reproduce asexually, primarily by
releasing spores that are adapted to travel
through air and water.
– Breaking off a hypha or budding off a cell
can also serve as asexual reproduction.
Reproduction
– Most fungi can also reproduce sexually. The
life cycle of the bread mold Rhizopus
stolonifer is shown.
Reproduction
– Sexual reproduction in fungi often involves
two different mating types. One mating type is
called “+” (plus) and the other “–” (minus).
– Hyphae of opposite mating types fuse
together, bringing + and – nuclei together in
the same cell.
Reproduction
– The + and – nuclei form pairs that divide as
the mycelium grows. Many of the paired
nuclei fuse to form diploid zygotes within a
zygospore.
Reproduction
– The zygospore germinates and a
sporangium emerges.
– The sporangium reproduces asexually,
releasing haploid spores produced by
meiosis.
– Each spore has a different combination of
parental genes, and each can make a new
mycelium.
Diversity of Fungi
– More than 100,000 species of fungi are
known. Biologists have placed fungi into
several distinct groups.
– The major groups of fungi differ from one
another in their reproductive structures.
Diversity of Fungi
The Ecology of Fungi
– How do fungi affect homeostasis in other
organisms and the environment?
– Fungi are champions of decomposition. Many
species help ecosystems maintain homeostasis
by breaking down dead organisms and
recycling essential elements and nutrients.
The Ecology of Fungi
– How do fungi affect homeostasis in other
organisms and the environment?
– Parasitic fungi can cause serious diseases in
plants and animals by disrupting homeostasis.
– Some fungi form mutualistic associations with
photosynthetic organisms in which both
partners benefit.
Decomposition
– 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.
– Many organisms remove important trace
elements and nutrients from the soil. Fungi
recycle these essential elements and
nutrients. If these materials were not
returned, the soil would quickly be
depleted.
Parasitism
– Parasitic fungi can cause serious diseases in
plants and animals by disrupting homeostasis.
Plant Diseases
– A number of parasitic fungi cause diseases
that threaten food crops. Corn smut, for
example, destroys corn kernels.
– Some mildews, which infect a wide variety of
plants, are also fungi.
Animal Diseases
– Fungal diseases also affect insects, frogs,
and mammals.
– For example, the Cordyceps fungus
infects grasshoppers in rain forests in
Costa Rica. Microscopic spores become
lodged in the grasshopper, where they
germinate and produce enzymes that
slowly penetrate the insect’s external
skeleton. The spores multiply in the
insect’s body, digesting all its cells and
tissues until the insect dies.
Animal Diseases
– Parasitic fungi can also infect humans.
– The fungus that causes athlete’s foot forms a
mycelium in the outer layers of the skin, which
produces a red, inflamed sore from which the
spores can easily spread from person to
person.
– The yeast Candida albicans is often
responsible for vaginal yeast infections and
for infections of the mouth called thrush.
Lichens
– A lichen is a symbiotic association between
a fungus and a photosynthetic organism. The
photosynthetic organism is either a green alga
or a cyanobacterium, or both.
– The protective upper surface of a lichen is
made up of densely packed fungal hyphae.
Below this are layers of green algae or
cyanobacteria and loosely woven hyphae.
The bottom layer contains small projections
that attach the lichen to a rock or tree.
Lichens
– Lichens are extremely resistant to drought
and cold. Therefore, they can grow in places
where few other organisms can survive—on
dry bare rock in deserts and on the tops of
mountains.
– Lichens are able to survive in these harsh
environments because the green algae or
cyanobacteria carry out photosynthesis,
providing the fungus with a source of energy,
while the fungus provides the green algae or
cyanobacteria with water and minerals.
Lichens
– Lichens are often the first organisms to enter
barren environments, gradually breaking
down the rocks on which they grow. In this
way, lichens help in the early stages of soil
formation.
– Lichens are also remarkably sensitive to air
pollution: They are among the first organisms
to be affected when air quality deteriorates.
Mycorrhizae
– Fungi also form mutualistic relationships with
plant roots. These symbiotic associations of
plant roots and fungi are called mycorrhizae.
– Researchers estimate that 80 to 90 percent of
all plant species form mycorrhizae with fungi.
– The hyphae collect water and minerals and
bring them to the roots, greatly increasing the
effective surface area of the root system. In
addition, the fungi release enzymes that free
Mycorrhizae
– The presence of mycorrhizae is essential for
the growth of many plants. The seeds of
orchids, for example, cannot germinate in the
absence of mycorrhizal fungi. Many trees are
unable to survive without fungal symbionts.
Mycorrhizae
– This graph illustrates the growth rates of
three species of trees with and without
mycorrhizae.
Mycorrhizae
– The roots of plants are plugged into
mycorrhizal networks that connect many
plants.
– In an experiment using isotopes to trace the
movement of carbon, ecologist Suzanne
Simard found that mycorrhizal fungi
transferred carbon from paper birch trees
growing in the sun to Douglas fir trees
growing in the shade nearby.