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Chapter 31
Fungi
Heterotrophic Metabolism
• Fungi are heterotrophs, but they
do not ingest their food.
• They release exoenzymes that
digest their food while it’s still in
the environment.
• Then they absorb the nutrients.
Characteristics
of Fungi:
• Some are multicellular
• Others are small and single celled
• The morphology of fungi is primed to
take up nutrients
• The bodies form a network of hyphae
– Hyphae are composed of tubular cell walls
surrounding the plasma membrane and
cytoplasm of cells.
– The cell walls contain chitin.
Hyphae
• The fungal hyphae form a network
of interwoven fibers called
mycelium.
• The mycelium surrounds and
infiltrates the material on which
the fungus feeds.
• This acts to increase the surface
area to volume ratio of the fungus
– Makes feeding more efficient.
Fungi
• Fungi are immobile, thus it is important
for them to be able to increase the
length of the hyphae quickly and
efficiently to obtain food.
– In some fungi, individual cells are separated
by septa which usually contain large pores
that allow many things through.
– In other fungi, the bodies are huge,
multinucleated masses with no cytoplasmic
divisions.
Mycorrhizae
• These are beneficial
relationships
between fungi and
plant roots.
– The benefit comes from the ability of
the fungus to provide the plant with
nutrients that it couldn’t otherwise
obtain.
– The plants provide the fungus with
organic nutrients.
Haustoria
• Specialized
hyphae that
penetrate the
roots of their
hosts (plants).
• Site of nutrient
exchange.
Mycorrhizae
• 2 Different Types of mycorrhizae:
– Ectomycorrhizal fungi form sheaths of
hyphae over the surface of the roots
and also grow into the extracellular
spaces of the root cortex
– Endomycorrhizal fungi extend their
hyphae through the root cell wall and
into the tubes by pushing the root cell
membrane inward.
Mycorrhizae
• Almost all vascular
plants have
mycorrhizae. The
fungi provide
essential nutrients
to the plant; the
plant provides
nutrients to the
fungus.
Fungal Reproduction
• Fungi reproduce both sexually and
asexually.
• Either way, they produce large
numbers of spores.
Fungi:
Sexual Reproduction
• Sexual reproduction in fungi begins
when hyphae from two distinct mycelia
release pheromones.
• The fungi perceive the chemical signal
and extend hyphae toward the source of
the pheromone.
• This acts to attract different mycelia
towards one another.
• During the sexual life cycle of a fungi,
the haploid nuclei of the hyphae and
their spores become diploid.
Fungi:
Sexual Reproduction
• When the mycelia meet they
perform a compatibility test to
ensure mycelium and hyphae from
the same fungus don’t fuse.
• They want to sexually reproduce
with other fungi.
Fungi:
Sexual Reproduction
• Plasmogamy is the union of the
cytoplasm of two parent mycelia.
• Even though their cytoplasm is joined,
the haploid nuclei don’t fuse right away.
• The mycelia are said to be
heterokaryotic.
• Sometimes, haploid nuclei pair off two
to a cell and the mycelia are said to be
dikaryotic.
Fungi:
Sexual Reproduction
• Karyogamy is the next stage of the
sexual life cycle. In this stage, the
two haploid nuclei fuse and a
diploid cell is formed.
• This is usually the only diploid
stage in most fungi.
• Meiosis restores the haploid
condition and the mycelium
produces specialized reproductive
structures to disperse the spores.
Fungi:
Sexual Reproduction
• The processes of karyogamy and
meiosis generate a lot of genetic
variation within fungi.
The Asexual Life Cycle
• Many fungi reproduce asexually through
the production of a large number of
spores.
• The spores are clones and spread by air
and/or water.
• When they land in a suitable place, they
grow.
– Molds are examples of asexually reproduced
fungi--spores.
– Yeasts are another example of asexual
reproduction by a fungi-budding.
The Importance of Fungi
• Fungi are important decomposers.
• They are responsible for recycling
the organic matter in an
ecosystem.
• They make inorganic nutrients
available for plant growth.
The Importance of Fungi:
Symbiosis
• Mycorrhizae are common symbionts of plants
that are often essential for vigorous growth.
• Some animals make use of fungi in their guts
to help them break down food.
• Some insects actually “raise” fungi to assist
them in the breakdown of certain plants.
Lichens
• Lichens are symbiotic associations
of millions of photosynthetic
microorganisms held in a mass of
fungal hyphae.
• They form a surface-hugging
carpet often seen on logs and
rocks.
Lichens
• The photosynthetic portion is often a
unicellular or filamentous green algae
or cyanobacteria.
• The fungus is usually an ascomycete;
but some are basiodiomycetes.
• The algae or cyanobacteria usually
occupies the inner layer below the
lichen’s surface.
Lichens
• In most lichens, each partner
supplies something the other
couldn’t obtain on its own.
– Algae provide carbon compounds
– Cyanobacterial provide nitrogen
fixation and organic nitrogen
– Fungi provide a suitable environment-gas exchange, water, minerals.
– Fungi secrete acids that assist in the
uptake of minerals
Lichens
• Many of the fungi of lichens
reproduce sexually by forming
ascocarps or basidiocarps.
• Lichen algae reproduce asexually
by cell division.
Lichens
• Lichens are a good indicator of air
pollution because they don’t stand
up well to poor air quality.
Fungi: The Bad…
• About 30,000 fungi species are
parasites and they cause a variety of
ailments in humans and problems with
crops.
• They cause disease
– Ringworm, etc…
• They are the black mold that can cause
abandonment of buildings.
• Some fungal biproducts are highly toxic
to humans.
The Bad
Fungi: The Good…
• Fungi are also important in
agriculture.
• Mushrooms, morels, and truffels
are delicacies in the food industry.
• Yeasts are used in the alcoholic
beverage industry and for baking.
• Many medicines are derived from
them--antibiotics.
Fungi: The Good…
• Fungi have numerous practical
uses.
• They are decomposers of
ecosystems.
The Good