Body, nutrition and reproduction of fungi

Biology · Kingdom Fungi. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Fungi are eukaryotes with cell walls of chitin. They have no chlorophyll, so they are heterotrophs. They do not swallow food. They release enzymes onto it and absorb the small molecules, which is called absorptive nutrition. Surplus food is stored as glycogen and oil.

The body is made of thin threads called hyphae. All the hyphae together form the mycelium, which gives a huge surface for absorption. Yeasts are the exception: they are single cells.

Fungi live as saprotrophs, parasites, predators or mutualists. They reproduce by spores, both asexually and sexually. In sexual reproduction the cytoplasm fuses first and the nuclei fuse later.

Rules to remember

Septate hyphae have cross-walls (septa) with pores; non-septate (coenocytic) hyphae have none and hold many nuclei in one cytoplasm.
Saprotrophs anchor with rhizoids; parasites absorb from living host cells through haustoria; Arthrobotrys is a predator that traps nematodes.
Lichen = fungus + alga or cyanobacterium. The alga makes food; the fungus gives protection, water and minerals. Lichens are indicators of air pollution.
Mycorrhiza = fungus + plant roots. The fungus supplies minerals such as phosphate; the plant supplies sugars.
Asexual reproduction: spores in sporangia, conidia cut off from the tips of conidiophores, budding in yeast, fragmentation.
Sexual reproduction: plasmogamy (fusion of cytoplasm) → dikaryotic stage (n + n) → karyogamy (fusion of nuclei, 2n) → meiosis → haploid spores.

Common mistake

Fungi are not plants. Never give them cellulose walls, starch or photosynthesis: the wall is chitin, the store is glycogen and the nutrition is absorptive.

Worked example

A grey-green crust grows on a bare rock where there is no soil and no dead matter. It is made of fungal hyphae wrapped around green algal cells. How does the fungus get its food, and what does it give in return?

A fungus cannot photosynthesise and there is no organic matter on bare rock to absorb.
The algal cells have chlorophyll, so they make sugars by photosynthesis and pass some to the fungus.
The hyphae hold water and minerals and protect the alga from strong light and drying.

Answer: It is a lichen, a mutualistic association: the alga supplies food and the fungus supplies water, minerals and protection.

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Chapter 8: Kingdom Fungi

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