Fungi are a separate kingdom of living things. For a long time they were placed among plants, but today they form their own kingdom, because they show features of both plants and animals. In nature fungi act as decomposers, form mycorrhiza with trees, and give people food (mushrooms, yeast) and medicine (penicillin). At the same time they damage crops, spoil food and cause disease. This topic also covers lichens — organisms formed by a fungus and an alga together. Exams ask about the structure, nutrition and reproduction of fungi and their main representatives.
§1. General features of fungi
Fungi are eukaryotes: their cells have a nucleus, mitochondria and other membrane-bound organelles. They are heterotrophs — they have no chlorophyll and take organic matter ready-made.
The body is made of thin branching threads — hyphae. All the hyphae together form the mycelium. The fluffy coating on mouldy bread is mycelium.
In most fungi the hyphae are divided by cross-walls into cells (multicellular mycelium); in some there are no cross-walls, and the whole mycelium is one huge cell with many nuclei (non-septate mycelium, for example in Mucor).
They absorb food through the whole body surface: hyphae release enzymes into the substrate and take up the digested substances.
Animal-like features:
the cell wall is made of chitin (as in the arthropod skeleton);
the storage carbohydrate is glycogen;
the product of nitrogen metabolism is urea;
heterotrophic nutrition.
Plant-like features:
they do not move and grow without limit throughout life;
they have a cell wall;
they feed by absorption;
they reproduce by spores.
A fungus: eukaryote, heterotroph, chitin wall, glycogen store, body made of mycelium.
§2. Nutrition and reproduction
By mode of nutrition there are three groups:
saprotrophs live on dead organic matter (humus, leaf litter, bread, fruit); in an ecosystem they are decomposers;
parasites live on living plants, animals or people and cause disease;
symbionts live in a mutually beneficial partnership with another organism (mycorrhiza, lichens).
Ways of reproducing:
spores (asexual) — the main method. Spores form inside sporangia (Mucor) or openly at the tips of hyphae as conidia (Penicillium, Aspergillus). One fungus releases millions of spores, carried by wind;
vegetatively — by pieces of mycelium;
budding — in yeasts: a bulge grows on the cell and separates;
a sexual process — fusion of two hyphae or cells.
§3. Moulds and yeasts
Mucor (white mould) grows on bread, vegetables and juicy fruit. Its mycelium is non-septate, with many nuclei. First it forms a white coating, then round sporangia ripen at the hyphal tips and turn black.
Penicillium (blue-green mould) has a multicellular mycelium; chains of conidia form on brush-like spore-bearing branches at the hyphal tips. In 1928 A. Fleming found that Penicillium stops bacteria growing — this is how the first antibiotic, penicillin, was discovered.
Aspergillus is a multicellular mould; some species build up dangerous toxins in food, so mouldy bread and nuts must not be eaten.
Yeasts are unicellular fungi; they form no mycelium and reproduce mainly by budding. Without oxygen they carry out alcoholic fermentation:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + energy
The CO₂ released makes dough rise (bread making), and the alcohol is obtained in wine and beer making. Yeasts are rich in B vitamins.
§4. Cap mushrooms
What we call a mushroom is the fruiting body: a stalk and a cap. The main part of the fungus is the mycelium in the soil; the fruiting body grows only to produce spores. It, too, is built of tightly interwoven hyphae.
Under the cap lies the spore-bearing layer — the hymenophore:
gilled — thin plates under the cap (field mushroom, fly agaric, death cap, honey fungus);
tubular (pored) — tiny tubes that look like a sponge (porcini, birch bolete, slippery jack).
Mycorrhiza (fungus-root) is a symbiosis in which fungal hyphae wrap around tree roots. The fungus supplies the tree with water and mineral salts; the tree gives the fungus carbohydrates made in photosynthesis. That is why many mushrooms grow only under certain trees (the birch bolete under birch).
Poisonous fungi: the death cap (the most dangerous; its poison is not destroyed by cooking), the red fly agaric, false honey fungi. Never pick or eat unknown mushrooms.
Field mushrooms and some others are also grown artificially.
§5. Parasitic fungi
In plants:
smut turns cereal ears into a black dusty mass of spores, so the harvest is lost;
rust fungi — rust-brown spots on leaves and stems;
ergot forms black horn-like bodies in rye ears, which are poisonous;
powdery mildew — a white coating on leaves;
wilt of cotton: the fungus blocks the water-conducting vessels;
bracket fungi live on tree trunks and destroy the wood.
In people and animals: ringworm and other fungal infections of skin, hair and nails; candidiasis (thrush) of mucous membranes. Prevention: personal hygiene, not using other people’s clothes and shoes.
§6. Lichens
A lichen is a complex organism formed by the symbiosis of a fungus and an alga (or a cyanobacterium). Its body is a thallus: mostly fungal hyphae with algal cells among them.
the fungus absorbs water and mineral salts and protects the alga from drying out;
the alga photosynthesises and makes organic substances.
They reproduce by pieces of thallus and by tiny bodies containing both hyphae and algal cells.
By form:
crustose — firmly attached to rock and bark;
foliose — leaf-like, attached by threads (Xanthoria);
fruticose — upright or hanging shrubby forms (reindeer moss, beard lichen).
Importance: they are the first to settle on bare rock and begin soil formation (pioneer organisms); the main food of reindeer in the tundra; indicators of air purity — lichens disappear in air polluted with sulfur dioxide.
Worked example
Compare the field mushroom, yeast and Penicillium by nutrition and reproduction.
All three are heterotrophic saprotrophs: they absorb ready-made organic matter.
Field mushroom — multicellular mycelium; spores ripen on the gills under the cap.
Yeast — unicellular, no mycelium, reproduces by budding.
Penicillium — multicellular mycelium, forms conidia on brush-like branches.
Answer: the mode of nutrition is the same (saprotrophic); structure and reproduction differ.
Worked example
Yeast fermented 2 mol of glucose. How many moles of ethanol and CO₂ are formed?
Equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
1 mol of glucose gives 2 mol of ethanol and 2 mol of CO₂.
2 mol of glucose give 2 × 2 = 4 mol of ethanol and 2 × 2 = 4 mol of CO₂.
Answer: 4 mol of ethanol and 4 mol of CO₂.
Worked example
Why are lichens almost absent from tree trunks in the centre of an industrial city but common in woodland on its edge?
Lichens absorb water and substances from air and rain through their whole surface.
Toxic gases in the air (sulfur dioxide) enter the body with them.
They have no protective covering, the algal cells are damaged and the symbiosis breaks down.
Answer: the city centre air is polluted — lichens are indicators of air pollution.
Check yourself
What substance makes up the fungal cell wall?
What is the storage carbohydrate of fungi?
What is special about the mycelium of Mucor?
Who discovered penicillin and when?
What is the symbiosis of fungal mycelium with tree roots called?
How do yeasts mainly reproduce?
How many moles of CO₂ are released when 3 mol of glucose undergo alcoholic fermentation?
A lichen is a symbiosis of which organisms?
Common mistakes
Treating fungi as autotrophs or plants — they have no chlorophyll.
Writing that the storage substance is starch; in fungi it is glycogen.
Thinking the fruiting body is the whole fungus — the main part is the mycelium in the soil.
Mixing up the mycelium of Mucor (non-septate) and Penicillium (multicellular).
Regarding a lichen as one simple organism.
Calling yeast a bacterium — it is a eukaryote, a unicellular fungus.