Difficulty: Intermediate | Prerequisites: Basic cell biology, Part 1 (land plant adaptations)
Fungi are not plants. They are a separate kingdom of eukaryotes that co-evolved alongside land plants and play essential roles in nearly every terrestrial ecosystem. This section covers what fungi are made of, how they reproduce, and the symbiotic relationships they form with plants, particularly mycorrhizae. Understanding fungi is critical because plant colonisation of land probably depended on fungal partnerships from the start. You should be comfortable with haploid and diploid cells and the basics of plant structure before tackling this material.
Fungi are heterotrophs built from networks of tiny filaments (hyphae) strengthened by chitin. They reproduce through a two-stage sexual process (plasmogamy then karyogamy) and form vital symbiotic relationships with plants, especially mycorrhizae. Lichens and endophytes are other important fungal partnerships.
Chitin
A strong but flexible nitrogen-containing polysaccharide that strengthens fungal cell walls. Think of it as the fungal equivalent of cellulose in plants (or the material in insect exoskeletons).
Hyphae
A network of tiny filaments that make up the body of a fungus. Each hypha is a thread-like tube, often just one cell wide.
Mycelium
The interwoven mass of hyphae that infiltrates the material the fungus feeds on. In simple terms, the mycelium is the main "body" of a fungus, usually hidden underground or inside its food source.
Haustoria
Specialised hyphae used to extract nutrients from, or exchange nutrients with, a plant host. They penetrate plant cells without killing them.
Mycorrhizae
Mutually beneficial relationships between fungi and plant roots. The fungus extends the root's reach for water and minerals; the plant supplies the fungus with sugars.
Ectomycorrhizal fungi
Fungi that form a sheath of hyphae over the surface of a root and grow into the extracellular spaces of the root cortex. They do not penetrate root cell walls.
Arbuscular mycorrhizal fungi (AMF)
Fungi that extend branching hyphae through the root cell wall and into tubes formed by invagination (pushing inward) of the root cell plasma membrane. They are the most widespread type of mycorrhizae.
Plasmogamy
The first stage of fungal sexual reproduction, in which the cytoplasms of two parent mycelia fuse. The nuclei remain separate at this point.
Karyogamy
The second stage of fungal sexual reproduction, in which the haploid nuclei contributed by the two parents fuse, producing diploid cells.
Nucleariids
Unicellular protists consisting of amoebas that feed on algae and bacteria. They are thought to be among the closest single-celled relatives of fungi.
Lichen
A symbiotic association between a fungus and a photosynthetic microorganism (usually a green alga or cyanobacterium). The fungus provides structure and protection; the photosynthetic partner provides sugars.
Endophytes
Fungi that live inside leaves or other plant parts without causing harm. Some may even benefit the plant by deterring herbivores.
Fungi are eukaryotic heterotrophs. They absorb nutrients from their surroundings rather than ingesting food.
Their cell walls contain chitin, a nitrogen-containing polysaccharide (plants use cellulose instead).
The basic building unit is the hypha, a thread-like filament. Masses of hyphae form the mycelium, which infiltrates soil, decaying matter or a host organism.
The mycelium's enormous surface-area-to-volume ratio makes fungi extremely efficient at absorbing nutrients.
Haustoria are specialised hyphae that penetrate host plant cells to exchange nutrients, found in parasitic and mutualistic fungi.
Fungi reproduce both sexually and asexually. Sexual reproduction involves two distinct stages:
Plasmogamy: the cytoplasms of two parent mycelia fuse. At this point, two genetically different haploid nuclei coexist in the same cell (a dikaryotic state).
Karyogamy: the two haploid nuclei fuse to produce a diploid nucleus. This is followed by meiosis, which produces haploid spores.
The time between plasmogamy and karyogamy varies enormously across fungal groups. In some, the dikaryotic stage persists for a long period.
Asexual reproduction occurs through budding, fragmentation or production of asexual spores.
Mycorrhizae are mutualistic associations between fungi and plant roots, found in roughly 90% of plant species.
Ectomycorrhizal fungi wrap a sheath of hyphae around the root surface and grow into extracellular spaces in the root cortex. They do not enter root cells.
Arbuscular mycorrhizal fungi (AMF) penetrate through the root cell wall and form branching structures inside the root cell (within tubes of invaginated plasma membrane). AMF are the most common type.
In both types, the fungus extends the plant's absorptive surface area for water and mineral uptake (especially phosphorus). The plant provides the fungus with photosynthetic sugars.
Lichens are symbiotic associations between a fungus and a photosynthetic partner (green alga or cyanobacterium). They colonise bare rock, tree bark and other surfaces.
Endophytes are fungi that live inside plant tissues without causing disease. Some produce compounds that deter herbivores, benefiting the host.
Students often assume fungi are plants. They are not. Fungi are heterotrophs (they absorb nutrients), they have chitin in their cell walls (not cellulose) and they are more closely related to animals than to plants.
Plasmogamy and karyogamy are frequently confused. Plasmogamy is cytoplasm fusion (nuclei stay separate); karyogamy is nuclear fusion (producing a diploid cell). They are two separate steps, sometimes separated by a long dikaryotic phase.
Students sometimes think mycorrhizae are parasitic. They are mutualistic: both the fungus and the plant benefit.
Lichens are sometimes mistaken for a single organism. A lichen is a composite of a fungus and a photosynthetic partner.
⚠️ Be able to distinguish ectomycorrhizal fungi from arbuscular mycorrhizal fungi by where their hyphae grow relative to the root cells.
⚠️ Know the two stages of fungal sexual reproduction (plasmogamy, then karyogamy) and what happens at each.
⚠️ Understand why mycorrhizae are considered mutualistic, and be able to state what each partner gives and receives.
⚠️ Expect questions on what chitin is and how it differs from cellulose (fungal cell walls vs plant cell walls).
True or False: Fungi have cellulose in their cell walls.
The interwoven mass of hyphae that makes up the main body of a fungus is called the ________.
True or False: Plasmogamy produces a diploid cell.
In a mycorrhizal relationship, the plant provides ________ and the fungus provides ________.
True or False: Lichens are a single organism.
Answers: 1. False (fungi have chitin, not cellulose). 2. Mycelium. 3. False (plasmogamy fuses cytoplasms but the nuclei remain separate; karyogamy produces the diploid cell). 4. Sugars; water and minerals (especially phosphorus). 5. False (a lichen is a composite of a fungus and a photosynthetic partner).
Q: Describe the basic structure of a fungus, from hyphae to mycelium.
A: The basic unit is the hypha, a thread-like filament with walls strengthened by chitin. Masses of hyphae interweave to form the mycelium, which infiltrates the fungus's food source (soil, decaying matter or a host organism). The mycelium's high surface-area-to-volume ratio makes nutrient absorption efficient.
Q: What is the difference between plasmogamy and karyogamy?
A: Plasmogamy is the fusion of the cytoplasms of two parent mycelia; the nuclei remain separate (creating a dikaryotic cell). Karyogamy is the fusion of those haploid nuclei to produce a diploid cell. These two events can be separated by a significant time gap.
Q: Compare ectomycorrhizal fungi and arbuscular mycorrhizal fungi.
A: Ectomycorrhizal fungi form a sheath over the root surface and grow into the extracellular spaces of the root cortex without entering cells. Arbuscular mycorrhizal fungi penetrate the root cell wall and extend branching hyphae into tubes formed by invagination of the root cell's plasma membrane, so they are in much closer contact with the plant cell.
Q: Why are mycorrhizae considered mutualistic rather than parasitic?
A: Both partners benefit. The fungus gains photosynthetic sugars from the plant. The plant gains improved access to water and minerals (especially phosphorus) because the fungal hyphae extend far beyond the root's own reach.
Q: What is a lichen, and what roles do its two partners play?
A: A lichen is a symbiotic association between a fungus and a photosynthetic microorganism (green alga or cyanobacterium). The fungus provides physical structure and protection. The photosynthetic partner produces sugars through photosynthesis.
Fungi connect directly to ecology (decomposition, nutrient cycling), to plant biology (mycorrhizae are essential for most land plants) and to evolution (nucleariids suggest fungi and animals share a more recent common ancestor than either does with plants). Lichens come up again in ecology when discussing pioneer species and succession on bare rock.
fungi kingdom, chitin cell wall, hyphae mycelium, haustoria, mycorrhizae mutualism, ectomycorrhizal fungi, arbuscular mycorrhizal fungi AMF, plasmogamy karyogamy dikaryotic, fungal reproduction, nucleariids, lichen symbiosis, endophytes, fungi and plants co-evolution, fungal structure, BIO chapter 26 fungi