Campbell Biology Chapter 31 Review: Fungi

31.1 How fungi feed and grow

pp. 636–638

On the AP exam? Background

Fungal anatomy isn't on the exam, but this section is a handy example of surface area-to-volume ratio (Topic 2.2), hydrolysis (Topic 1.3) and symbiosis (Topic 8.5).

In the course: Topic 1.3 Introduction to Macromolecules, Topic 1.4 Carbohydrates, Topic 2.2 Cell Size, Topic 4.5 Cell Cycle, Topic 8.5 Community Ecology (notes, videos and more questions)

Key points

Fungi are heterotrophs like you, but they don't swallow food. They pour digestive enzymes onto it, break big molecules into small ones outside their bodies, and then absorb the pieces.
That one way of eating supports three lifestyles. Decomposers live on dead matter, parasites feed on living hosts and harm them, and mutualists trade with a living partner so that both come out ahead.
Most fungi are built from hyphae, thin tubes of cytoplasm inside a cell wall. A tangled mass of hyphae is a mycelium, and it threads through soil, wood or a host. A mushroom is only the spore-making part; most of the fungus is the hidden mycelium.
A fungal cell wall gets its strength from chitin, the same nitrogen-bearing carbohydrate that makes up crab shells and beetle armor. Plants build their walls from cellulose instead.
Because hyphae are so thin, a mycelium has an enormous surface area for its volume, which makes absorbing food efficient. Growth happens at the very tips of the threads, and new material goes into making them longer, not wider, so the mycelium pushes into fresh food quickly.
In septate hyphae, cross-walls called septa divide each thread into cells, but holes in the septa are wide enough for organelles, and sometimes whole nuclei, to pass. Coenocytic hyphae skip the cross-walls: their nuclei kept dividing by mitosis while the cytoplasm never split, so one long cell holds many nuclei. Yeasts are single-celled fungi that live where sugars are dissolved and easy to absorb, like nectar or fruit juice.
Mycorrhizae are partnerships between fungi and plant roots. The fungus gathers phosphate and other minerals from a wide area of soil; the plant pays in sugars (and, for arbuscular fungi, fats too). Ectomycorrhizal fungi wrap the root and grow between its cells, while arbuscular mycorrhizal fungi push branched hyphae inside root cells without breaking their membranes.

Key terms (13)

heterotroph: An organism that can't make its own food and must get carbon compounds from other living or once-living things. Fungi and animals are both heterotrophs.
absorptive nutrition: Feeding by releasing enzymes onto food, letting them break it down outside the body, and then taking in the small molecules. It's how every fungus eats.
decomposer: An organism that feeds on dead material and wastes, breaking them down and releasing the nutrients inside.
hypha: One thin, threadlike filament of a fungus: a tube of cell wall with cytoplasm and nuclei inside. Plural: hyphae.
mycelium: The whole branching network of hyphae that makes up a fungus's body, usually hidden in soil, wood or a host.
chitin: The sugar-based polymer that gives fungal walls their strength. Its sugar units carry nitrogen, and arthropods such as crabs and beetles build their outer shells from it too.
septum: A cross-wall that divides a hypha into cells. Pores in it still let cytoplasm and organelles flow through. Plural: septa.
coenocytic: Describes a hypha with no cross-walls, so it's one long cell holding many nuclei.
yeast: A fungus that lives as single cells rather than hyphae, often where dissolved sugars are plentiful.
haustorium: A specialized hypha that pushes into a plant cell, without breaking its membrane, to take in or trade nutrients.
mycorrhiza: A partnership between a fungus and plant roots in which the fungus supplies minerals like phosphate and the plant supplies sugars. Plural: mycorrhizae.
ectomycorrhizal fungus: A mycorrhizal fungus that coats the outside of a root in a mat of hyphae and threads into the spaces between root cells, never entering the cells themselves.
arbuscular mycorrhizal fungus: A mycorrhizal fungus whose hyphae enter root cells and branch into tiny trees (arbuscules), pressing into the cell membrane without breaking it.

Check yourself: 31.1 How fungi feed and grow

4 questions on 31.1 How fungi feed and grow. Pick an answer to see if you got it, and why.

Question 1 of 4

A mold is grown on an agar plate made cloudy by added milk protein. Three days later, a clear ring surrounds the colony and extends a few millimeters beyond the farthest hyphae. What does the clear ring show most directly?

Question 2 of 4

Many plants make the enzyme chitinase and step up production when a fungus attacks their leaves. Why can this enzyme damage the fungus without harming the plant's own cells?

Question 3 of 4Calculator allowed

A fungal hypha is a cylinder 5 µm wide and 1,000 µm long. Its volume is about 19,600 µm³ and its surface area about 15,700 µm². A spherical cell with the same volume would have a radius of about 16.7 µm, and for a sphere, SA:V = 3 ÷ r. How does the hypha's surface area-to-volume ratio compare with the sphere's?

Question 4 of 4

Under a microscope, a hypha shows dozens of nuclei scattered through one continuous stretch of cytoplasm, with no cross-walls anywhere along it. Which process best explains how such a hypha forms?

0 of 4 answered

31.2 Spores and fungal life cycles

pp. 638–640

On the AP exam? Background

The exam won't ask about plasmogamy or dikaryons, but fungal life cycles run on the same cell signaling, mitosis and meiosis ideas you're tested on in Topics 4.1, 4.5 and 5.1–5.3.

In the course: Topic 4.1 Cell Communication, Topic 4.5 Cell Cycle, Topic 5.1 Meiosis, Topic 5.2 Meiosis and Genetic Diversity, Topic 5.3 Mendelian Genetics (notes, videos and more questions)

Key points

Most fungi spread by making huge numbers of tiny spores, sexually or asexually. Wind and water carry spores far and wide, and the few that settle somewhere damp with something to eat sprout into fresh mycelia.
Fungi spend most of their lives haploid: hyphae and spores carry one set of chromosomes. In most fungi, the only diploid stage is a short-lived zygote nucleus.
Sex usually starts with chemical signals. In species with mating types, each type releases a pheromone that only the other type can detect, and partners grow toward each other. Because cells ignore their own type, a mycelium can't mate with itself or a clone, so matings join two different genomes. (Some fungi are self-fertile, but many aren't.)
Fungal sex happens in two separate fusions. Plasmogamy joins the cytoplasm of two parents; karyogamy joins their nuclei, and the wait in between can be short or can stretch across years. Meanwhile, cells hold nuclei from both parents: a heterokaryon, or a dikaryon (n + n) when each cell has exactly one nucleus from each.
Karyogamy makes a diploid nucleus that soon goes through meiosis. Meiosis reshuffles the two parents' alleles, so sexual spores are genetically varied, which gives natural selection more to work with.
Asexual spores are made by mitosis, so they're genetic copies of the parent. Molds churn them out to grab new food fast, and yeasts reproduce asexually by budding or splitting in two.
Many fungi have never been seen reproducing sexually. They used to be dumped in a catch-all group called deuteromycetes ("imperfect fungi"), but DNA sequencing now places each one on the fungal tree, and that old group is no longer used.

Key terms (11)

spore: A tiny reproductive cell that can grow into a new organism without fusing with another cell. Fungi release spores by the millions.
pheromone: A chemical signal one individual releases to affect others of its species. In fungi, pheromones tell cells of the opposite mating type that a partner is near.
mating type: A genetically set category of fungal cell. Only cells of different mating types can mate, a bit like sexes but without male and female.
plasmogamy: The fusing of the cytoplasm of two parent cells or hyphae, while their nuclei stay separate.
karyogamy: The fusing of two haploid nuclei from different parents into one diploid nucleus.
heterokaryon: A fungal mycelium that holds genetically different nuclei from two parents in a shared cytoplasm.
dikaryotic: Describes a cell with two separate haploid nuclei, one from each parent, written n + n.
zygote: In fungi, the diploid nucleus or cell formed by karyogamy. It's usually the fungus's one diploid stage, and it soon goes into meiosis.
mold: Informal name for a fast-growing fungus that forms a visible fuzzy mycelium and makes many asexual spores.
budding: Asexual reproduction in which a small new cell swells off a parent cell and pinches free, as yeasts do.
deuteromycetes: An old catch-all name for fungi with no known sexual stage. DNA data now place them in real groups, so the term is out of date.

Check yourself: 31.2 Spores and fungal life cycles

4 questions on 31.2 Spores and fungal life cycles. Pick an answer to see if you got it, and why.

Question 1 of 4

In the life cycle of most fungi, which cells or nuclei are diploid?

Question 2 of 4

Haploid baker's yeast cells come in two mating types, a and α. Each type releases a pheromone that binds a receptor on cells of the other type, which then stop dividing and grow toward the signal before fusing. A mutant α cell lacks the receptor for the a-type pheromone. What will most likely happen when it's mixed with normal a cells?

Question 3 of 4

Two strains of a mold each carry one mutation: strain 1 can't make arginine, and strain 2 can't make histidine. Neither grows on a medium lacking both amino acids. When their hyphae fuse, the new mycelium keeps both kinds of nuclei separate, yet it grows well on that medium. What best explains this?

Question 4 of 4

A haploid mold strain carries an allele that makes it resistant to a fungicide; a second haploid strain carries the sensitive allele of the same gene. Which prediction is correct?

0 of 4 answered

31.3 Where fungi came from

pp. 640–641

On the AP exam? Background

Fungal origins aren't a topic of their own, but this section is good practice for reading trees and molecular clocks (Topic 7.9) and weighing fossil against DNA evidence (Topic 7.6).

In the course: Topic 1.4 Carbohydrates, Topic 7.6 Evidence of Evolution, Topic 7.7 Common Ancestry, Topic 7.9 Phylogeny (notes, videos and more questions)

Key points

DNA comparisons put fungi and animals on the same branch of the eukaryote tree, the opisthokonts, along with a few single-celled protists. Surprising but true: a mushroom is more closely related to you than to a fern.
Other clues fit. Fungi and animals both store glucose as glycogen, while plants store starch. And the flagellated cells of this branch, like chytrid spores and animal sperm, are pushed by a single flagellum at the back.
Fungi's nearest living kin are the nucleariids, single-celled amoebas that eat algae and bacteria, while animals' nearest kin are the choanoflagellates. Because each kingdom's nearest kin is single-celled, multicellular bodies most likely evolved separately in fungi and in animals.
The first fungi were most likely single-celled and lived in water with a flagellum. Most fungi later lost it, but chytrids and a few other early branches still make swimming spores.
Molecular clocks put the split between the fungal and animal lineages roughly a billion years ago. The oldest fossils that most experts accept as fungi date to roughly 460 million years ago; early fungi were tiny and soft and rarely fossilized, and some older fungus-like fossils are still debated.
Microsporidia are single-celled parasites that live inside animal cells. They lack normal mitochondria but keep shrunken organelles (mitosomes) built from mitochondrial parts, so their ancestors had mitochondria. DNA places them at or near the base of the fungal tree.
Fungi moved onto land along with plants. Fossils from Scotland's Rhynie chert, about 407 million years old, show branching hyphae inside the cells of early land plants, much like today's arbuscular mycorrhizae.

Key terms (8)

opisthokonts: The branch of eukaryotes that includes animals, fungi and their closest single-celled relatives. Their flagellated cells swim with one flagellum at the rear.
nucleariids: Single-celled amoebas that are the nearest living kin of fungi.
choanoflagellates: Single-celled (sometimes colonial) protists with a collar around one flagellum. They're the nearest living kin of animals.
glycogen: A branched polysaccharide that fungi and animals use to store glucose. Plants store starch instead.
molecular clock: A way to estimate when lineages split, using how many DNA differences have built up and a rate calibrated with dated fossils.
microsporidia: Single-celled parasites that live inside animal cells. They lost normal mitochondria and are now grouped at or near the base of the fungi.
mitosome: A tiny, stripped-down organelle that comes from a mitochondrion but no longer makes ATP using oxygen.
sister groups: Two lineages that share a more recent common ancestor with each other than with anything else on the tree.

Check yourself: 31.3 Where fungi came from

4 questions on 31.3 Where fungi came from. Pick an answer to see if you got it, and why.

Question 1 of 4

The table gives percent differences in the sequence of one shared gene among five eukaryotes (invented data). Compared with | Fungus | Nucleariid | Animal | Choanoflagellate Nucleariid | 18 | – | – | – Animal | 27 | 28 | – | – Choanoflagellate | 27 | 27 | 16 | – Plant | 39 | 40 | 38 | 39 Which tree best fits these data?

Question 2 of 4

Which observation adds support to the DNA evidence placing fungi nearer to animals than to plants on the tree?

Question 3 of 4

Molecular clocks suggest the fungal and animal lineages split about a billion years ago, but the oldest fossils most experts accept as fungi date to roughly 460 million years ago. Which new finding would most strongly support the molecular estimate?

Question 4 of 4

Microsporidia are single-celled parasites that live inside animal cells. They lack typical mitochondria, but they have tiny double-membrane organelles built with proteins related to mitochondrial ones. What is the best interpretation?

0 of 4 answered

31.4 The main fungal groups

pp. 641–648

On the AP exam? Not tested

The current course doesn't cover fungal groups or their life cycles. What carries over: counting the products of meiosis (Topic 5.1), reading trees and clades (Topic 7.9) and mycorrhizal symbiosis (Topic 8.5).

In the course: Topic 5.1 Meiosis, Topic 7.9 Phylogeny, Topic 8.5 Community Ecology (notes, videos and more questions)

Key points

The book sorts fungi into five groups: chytrids, zygomycetes, glomeromycetes, ascomycetes and basidiomycetes. They're still handy labels, but DNA has since redrawn the tree: the chytrids and the zygomycetes each turned out to be several separate lineages, not single clades.
Chytrids live in water and damp soil and make flagellated spores (zoospores), something almost no other fungi do; the few that also do are early-branching lineages once lumped in with chytrids. Some chytrids decompose, some are parasites (one, Batrachochytrium, has killed off frogs and other amphibians worldwide), and anaerobic relatives, now in their own phylum, help grazing mammals digest plants in their guts.
The old zygomycetes include fast molds that rot fruit and bread. Their hyphae are coenocytic, and when two mating types meet they form a tough, dormant zygosporangium that can wait out drought or cold before meiosis. These fungi are now split into two phyla, Mucoromycota and Zoopagomycota.
Glomeromycetes form arbuscular mycorrhizae, tree-shaped hyphae inside root cells, with most land plants. In nature they depend completely on a living plant partner; they can't even make their own fatty acids, so the plant supplies fats as well as sugars. (Only recently have lab cultures fed fatty acids made spores without a root.) Many mycologists now place them within Mucoromycota.
Ascomycetes (sac fungi) are the largest group. Their sexual spores form inside a sac called an ascus, where meiosis is usually followed by one mitosis to give eight ascospores, and they make asexual spores (conidia) in chains on hyphal tips. They include most yeasts, many molds and crop pathogens, and the fungi in most lichens.
Basidiomycetes (club fungi) include mushrooms, shelf fungi, rusts and smuts. Their sexual spores form on club-shaped basidia, four per basidium, and their mycelium spends most of its life dikaryotic. Certain club fungi outdo every other organism at breaking down lignin, the tough polymer in wood.
Ascomycetes and basidiomycetes together form a clade called the Dikarya, named for their shared n + n stage. A long dikaryotic stage means one mating can feed many separate rounds of karyogamy and meiosis, so a single pairing yields a huge variety of spores.

Key terms (15)

chytrid: A fungus, usually aquatic, that makes swimming spores with a flagellum. Chytrids belong to some of the earliest fungal branches.
zoospore: A spore that swims using a flagellum. Among fungi, only chytrids and their close kin make them.
zygomycetes: An older name for a group of fast-growing molds with coenocytic hyphae. DNA shows it isn't one clade, so it's now split into separate phyla.
zygosporangium: A tough, thick-walled structure that forms where two mating types of a mold fuse. It can stay dormant through bad conditions, then go through meiosis.
arbuscule: A tiny tree-shaped branching of a fungal hypha inside a root cell, where the fungus and plant trade minerals for food.
ascomycete: A sac fungus: a fungus whose sexual spores form inside sac-shaped cells. Yeasts, morels and many molds belong here.
ascus: The sac-shaped cell in which an ascomycete's nuclei fuse and go through meiosis, usually ending up with eight spores. Plural: asci.
ascospore: A sexual spore formed inside an ascus.
conidia: Asexual spores that form in chains or clusters at the tips of hyphae, typical of ascomycetes. Singular: conidium.
ascocarp: The fruiting body of an ascomycete, which holds the asci. It may be tiny or as big as a cup or a morel.
basidiomycete: A club fungus: a fungus whose sexual spores form on club-shaped cells. Mushrooms, shelf fungi and rusts belong here.
basidium: A club-shaped cell where two nuclei fuse and go through meiosis, usually producing four spores on its tip. Plural: basidia.
basidiospore: A sexual spore produced on a basidium.
basidiocarp: The fruiting body of a basidiomycete, such as a mushroom, built from tightly packed dikaryotic hyphae.
Dikarya: The clade made up of ascomycetes and basidiomycetes, named for the dikaryotic (n + n) stage they share.

Check yourself: 31.4 The main fungal groups

4 questions on 31.4 The main fungal groups. Pick an answer to see if you got it, and why.

Question 1 of 4

A student looks at a thin slice of a mushroom's gill under a microscope. Lining the gill are club-shaped cells, each topped by four spores on tiny stalks. Which group does this fungus belong to, and what happened in those club-shaped cells just before the spores formed?

Question 2 of 4

On the fungal tree, ascomycetes and basidiomycetes are each other's closest relatives, and the other fungal groups branch off earlier. Both groups have a long dikaryotic stage, but the earlier-branching groups don't. What is the simplest explanation for how this stage arose?

Question 3 of 4

Arbuscular mycorrhizal fungi are very hard to grow in the lab without a living plant root. Their genomes lack the genes for the enzyme complex that builds fatty acids. Which hypothesis best fits both facts?

Question 4 of 4

In a sac fungus, a young ascus contains a diploid nucleus that is heterozygous for mating type (one A allele and one a allele). The nucleus goes through meiosis, and then each product divides once by mitosis. How many spores form, and how many carry allele A?

0 of 4 answered

31.5 Fungi as recyclers, partners and pests

pp. 648–652

On the AP exam? Yes

These roles fit tested topics: decomposers recycling matter (Topic 8.2), partners and parasites (Topic 8.5), disease outbreaks (Topic 8.7) and fermentation (Topic 3.5). You won't need any fungus names.

In the course: Topic 3.5 Cellular Respiration, Topic 6.8 Biotechnology, Topic 8.2 Energy Flow Through Ecosystems, Topic 8.5 Community Ecology, Topic 8.7 Disruptions in Ecosystems (notes, videos and more questions)

Key points

Fungi and bacteria are the main decomposers. Fungi are especially good at breaking down cellulose and lignin in dead plants, which sends carbon back to the air as CO₂ and frees nitrogen, phosphorus and other elements in the soil for plants to reuse. If decomposition stopped, the nutrients plants need would pile up in dead leaves, logs and bodies, out of reach.
Endophytes are fungi that live inside healthy leaves and stems. Some make alkaloids that poison insects and grazing animals; others help plants survive stresses such as hot or dry soil.
Some insects farm fungi. Ambrosia beetles and leaf-cutting ants carry fungi into their nests, feed them plant material and eat the fungus, and the farmers and their crops have evolved together for millions of years.
A lichen is a fungus, usually an ascomycete, living with green algae or cyanobacteria. The partner inside does the photosynthesis (and a cyanobacterium can also fix nitrogen), while the fungus forms the lichen's body, shelters its partner and soaks up water. Lichens are early settlers on bare rock, which they slowly crumble. Having no roots or protective coating, they take in whatever falls on them, so polluted air kills them off. Newer dating suggests lichens arose after land plants, not before them.
Many fungi are parasites, mostly of plants: rusts, smuts, rice blast and Dutch elm disease wreck crops and trees. Fungal diseases have also devastated amphibians (chytridiomycosis) and hibernating bats (white-nose syndrome).
In people, fungal infections (mycoses) range from itchy skin infections to dangerous systemic ones that start with inhaled spores. Opportunistic yeasts like Candida overgrow when the immune system is weak or normal bacteria are wiped out, and drug-resistant Candida auris has become a hospital threat.
People use fungi for food (mushrooms, cheese ripening, soy sauce, tempeh), for fermentation (yeast turns sugar into ethanol and CO₂ in bread and drinks), for medicines (antibiotics like penicillin, drugs that stop the body rejecting a transplanted organ, and statins for cholesterol) and in biotechnology, where engineered yeast makes human insulin and the hepatitis B vaccine.

Key terms (10)

lignin: A tough, complex polymer that stiffens wood. Few organisms can break it down, and certain fungi do it best.
endophyte: A fungus (or other microbe) that lives inside a plant's tissues without making it sick, sometimes protecting it.
lichen: A partnership between a fungus and green algae or cyanobacteria that behaves like a single organism.
mutualism: A relationship between two species in which both benefit, like a plant trading sugar for minerals with a mycorrhizal fungus.
pathogen: An organism or virus that causes disease in its host.
mycosis: Any infection caused by a fungus, from athlete's-foot-style skin infections to serious lung infections.
opportunistic infection: An infection by a microbe that is usually harmless but grows out of control when the host's defenses or normal microbes are disturbed.
fermentation: Breaking down sugar without oxygen to make a little ATP. Yeast's version releases ethanol and CO₂.
antibiotic: A substance that kills or stops the growth of bacteria. Penicillin, made by a mold, was the first one found.
bioremediation: Using living things, such as fungi or bacteria, to break down or remove pollutants from soil or water.

Check yourself: 31.5 Fungi as recyclers, partners and pests

4 questions on 31.5 Fungi as recyclers, partners and pests. Pick an answer to see if you got it, and why.

Question 1 of 4

Mesh bags of fallen oak leaves were left on a forest floor for six months under four treatments (invented data). Treatment | Leaf mass remaining (%) No treatment | 45 Fungicide | 80 Antibacterial drug | 52 Fungicide + antibacterial drug | 94 Which conclusion is best supported?

Question 2 of 4

Yeast in a sealed jar of grape juice with no oxygen keeps making ATP by glycolysis for days, giving off CO₂ and ethanol. Why does making ethanol let glycolysis keep going?

Question 3 of 4

Tall fescue grass often carries a fungus that lives between its leaf cells. The fungus gets sugars from the grass, is passed to the next generation inside the grass's seeds, and makes alkaloids that poison insects and sicken cattle that graze on it. Which best describes the fungus–grass relationship?

Question 4 of 4Calculator allowed

White-nose syndrome is caused by a cold-loving fungus that grows on the skin of hibernating bats; within the range of cave temperatures, it grows faster where it's warmer. Bat counts at three caves before the fungus arrived and three winters later (invented data): Cave | Winter temperature (°C) | Bats before | Bats three winters later 1 | 4 | 5,000 | 2,900 2 | 7 | 8,000 | 2,400 3 | 10 | 12,000 | 1,200 Which statement do the data support?

0 of 4 answered