Fungi-Mycelium, Cell structure, Reproduction, Nutrition

Fungi-General Account

Mycologists, scientists who study fungi as well as fungi-like protists, believe there may be as many as 1.5 million fungal species. Fungi exist either as single-celled yeasts or in multicellular form with several different cell types.

Their reproduction may be either sexual or asexual, and they exhibit an unusual form of mitosis. The fungi, an often overlooked group of unicellular and multicellular organisms, have a profound influence on ecology and human health.

Along with bacteria, they are important decomposers and disease-causing organisms. Fungi are found everywhere from the tropics to the tundra and in both terrestrial and aquatic environments. Fungi made it possible for plants to colonize land by associating with rootless stems and aiding in the uptake of nutrients and water.

Mushrooms and toadstools are the multicellular spore-producing part of fungi that grow rapidly under proper conditions. A single Armillaria fungus can cover 15 hectares underground and weigh 100 tons, making it the largest organism in the world based on area.

Some puffball fungi are almost a meter in diameter and may contain 7 trillion spores enough to circle the Earth’s equator!

Yeasts are used to make bread and beer, but other fungi cause disease in plants and animals. These fungal killers are particularly problematic because fungi are animals’ closest relatives. Drugs that can kill fungi often have toxic effects on animals, including humans. In this chapter, we present the major groups of this intriguing life form.

The body of a fungus is a mass of connected hyphae

Some hyphae are continuous or branching tubes filled with cytoplasm and multiple nuclei. Other hyphae are typically made up of long chains of cells joined end-to-end and divided by cross-walls called septa (singular, septum). The septa rarely form a complete barrier, except when they separate the reproductive
cells. Even fungi with septa can be considered one long cell.

Cytoplasm characteristically flows or streams freely throughout the hyphae, passing through major pores in the septa (figure ). Because of this streaming, proteins synthesized throughout the hyphae may be carried to their actively growing tips.

As a result, fungal hyphae may grow very rapidly when food and water are abundant and the temperature is optimum. For example, you may have seen mushrooms suddenly appear in a lawn overnight after a rain in summer.

The mycelium

A mass of connected hyphae is called a mycelium (plural, mycelia). (This word and the term mycology, the study of fungi, are both derived from the Greek word mykes, meaning fungi.) The mycelium of a fungus (figure ) constitutes a system that may, in the aggregate, be many meters long. This mycelium grows into the soil, wood, or other material, and digestion of the material begins quickly.

In two of the four major groups of fungi, reproductive structures formed of interwoven hyphae, such as mushrooms, puffballs, and morels, are produced at certain stages of the life cycle. These structures expand rapidly because of the rapid inflation of the hyphae.

Cell walls with chitin

The cell walls of fungi are formed of polysaccharides, including chitin. In contrast, the cell walls of plants and many protists contain cellulose, not chitin. Chitin is modified cellulose consisting of linked glucose units to which nitrogen groups have been added; this polymer is then cross-linked with proteins.

Chitin is the same material that makes up the major portion of the hard shells, or exoskeletons, of arthropods, a group of animals that includes insects and crustaceans. Chitin is one of the shared traits that has led scientists to believe that fungi and animals are more closely related than fungi and plants.

Cell Structure

Fungi are different from most animals and plants in that each cell (or hypha) can house one, two, or more nuclei. A hypha that has only one nucleus is called monokaryotic; a cell with two nuclei is dikaryotic. In a dikaryotic cell, the two haploid nuclei exist independently. Dikaryotic hyphae have some of the genetic properties of diploids because both genomes are transcribed.

Sometimes, many nuclei intermingle in the common cytoplasm of a fungal mycelium, which can lack distinct cells. If a dikaryotic or multinucleate hypha has nuclei that are derived from two genetically distinct individuals, the hypha is called heterokaryotic. Hyphae whose nuclei are genetically similar
to one another are called homokaryotic.


Many fungi are capable of producing both sexual and asexual spores. When a fungus reproduces sexually, two haploid hyphae of compatible mating types may come together and fuse.

The dikaryon stage

In animals, plants, and some fungi, the fusion of two haploid cells during reproduction immediately results in a diploid cell (2n). But in other fungi, namely basidiomycetes and ascomycetes, an intervening dikaryotic stage (1n + 1n) occurs before the parental nuclei fuse and form a diploid nucleus.

In ascomycetes, this dikaryon stage is brief, occurring in only a few cells of the sexual reproductive structure. In basidiomycetes, however, it can last for most of the life of the fungus, including both the feeding and sexual spore-producing structures.

Reproductive structures

Some fungal species produce specialized mycelial structures to house the production of spores. Examples are the mushrooms we see above ground, the “shelf ” fungus that appears on the trunks of dead trees, and puffballs, which can house billions of spores.

As noted previously, the cytoplasm in fungal hyphae normally flows through perforated septa or moves freely in their absence.

Reproductive structures are an important exception to this general pattern. When reproductive structures form, they are cut off by complete septa that lack perforations or that have perforations that soon become blocked.


Spores are the most common means of reproduction among fungi. They may form as a result of either asexual or sexual processes, and they are often dispersed by the wind. When spores land in a suitable place, they germinate, giving rise to a new fungal mycelium.

Because the spores are very small, between 2 and 75 μm in diameter (figure ), they can remain suspended in the air for a long time.

Unfortunately, many of the fungi that cause diseases in plants and animals are spread rapidly by such means. The spores of other fungi are routinely dispersed by insects or other small animals. A few fungal phyla retain the ancestral flagella and have motile zoospores.

Biologists had believed for a long time that the worldwide presence of fungal species could be accounted for, on an evolutionary timescale, by the almost limitless, long-distance dispersal of fungal spores. Recent biogeographic studies, however, have examined the phylogenetic relationships among fungi in distant parts of the world and disproved this long-held assumption.


All fungi obtain their food by secreting digestive enzymes into their surroundings and then absorbing the organic molecules produced by this external digestion. The fungal body plan reflects this approach. Unicellular fungi have the greatest surface area-to-volume ratio of any fungus, maximizing the surface area for absorption.

Extensive networks of hyphae also provide an enormous surface area for absorptive nutrition in the fungal mycelium. Many fungi are able to break down the cellulose in wood, cleaving the linkages between glucose subunits and then absorbing the glucose molecules as food. Most fungi also digest lignin, an insoluble organic compound that strengthens plant cell walls.

The specialized metabolic pathways of fungi allow them to obtain nutrients from dead trees and from an extraordinary range of organic compounds, including tiny roundworms called nematodes.

The mycelium of the edible oyster mushroom Pleurotus ostreatus (figure ) excretes a substance that paralyzes nematodes that feed on the fungus. When the worms become slug-gish and inactive, the fungal hyphae envelop and penetrate their bodies. Then the fungus secretes digestive juices and absorbs the nematode’s nutritious contents, just like it would from a plant source.

This fungus usually grows within living trees or on old stumps, obtaining the bulk of its glucose through the enzymatic digestion of cellulose and lignin from plant cell walls. The nematodes it consumes apparently serve mainly as a source of nitrogen—a substance almost always in short supply in biological systems.

Other fungi are even more active predators than Pleurotus, snaring, trapping, or firing projectiles into nematodes, rotifers, and other small animals on which they prey. Because of their ability to break down almost any carbon-containing compound even jet fuel, fungi are of interest for use in bioremediation, using organisms to clean up soil or water that is environmentally contaminated.

As one example, some fungal species can remove selenium, an element that is toxic in high accumulations, from soils by combining it with other harmless volatile compounds.


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