Fungi play a vital role in the environment, and are economically important as a food source, as destructive pests, and as agents of disease. The study of fungi continues to produce important advancements in science.
What is a Fungus?
To be considered a fungus, an organism must (1) be eukaryotic; (2) possess cell walls; (3) grow by extending filamentous cells called hyphae, or by budding; (4) obtain nutrients by releasing digestive enzymes into the environment to break down organic molecules, which are then absorbed; and (5), have no chlorophyll.
The cells of fungi are quite similar to those of plants, protists, and animals. A lack of chlorophyll separates them from plants. Since they do not ingest their food (by eating), they are not animals. At first look, their separation from the protists is a bit less clear, but a filamentous or budding growth habit, along with the presence of cells walls, is good enough for most scientists.
Most fungi reproduce through the generation of spores (Mycelia sterilia being an exception). Fungal spores are non-motile (meaning they cannot move of their own accord). This further separates fungi from protists, which also can produce spores, but if they do, they are often motile.
Fungi are mostly saprophytes, meaning they obtain their nutrients from dead organic matter. They play an important role in the environment by decomposing and recycling organic matter. A few are parasitic on plants causing major losses of crops. Still others find their way into humans or other animals where they do their best to survive and cause serious infections.
The Structure of Fungi
The basic cell type of the majority of fungi is the hypha (pl hyphae), which is thin and tubular in shape. Hyphae are well designed for ranging into the environment to locate and exploit new resources. When many hyphae grow together in a mass, this forms the basic “body” of a fungus called a mycelium. When you see a bit of white fuzzy fungus growing on a piece of decaying wood or on a fruit that was left in the refrigerator for too long, you are looking at a mycelium. Not all fungi have hyphae. Yeasts are essentially unicellular, producing round or oblong cells that bud from a mother cell to produce daughter cells.
As mentioned, spores are a common cell type in fungi. Spores are specialized for dispersal and survival (in some species only for a short time, in others for much longer). They are produced in great numbers, and even though very few will find themselves in a place suitable for growth, enough do to keep the species going.
Spores can be produced asexually via mitosis, in which case they are called conidia. Spores may also be produced sexually and are then given a name reflective of the division to which the particular species belongs.
Fungal Classification
The fungi can be divided into four or five major divisions. There is little disagreement about the "big four" Basidiomycota, Ascomycoya, Zygomycota and Dueteromycota. However, the Oomycota (water molds) are considered by some to be “fungal protists” because they can exhibit a motile phase. Fungal classification is based on the morphology of sexually produced spores.
The Zygomycota can produce numerous conidia or reproduce sexually to form microscopic zygospores. Members of Ascomycota may form conidia as well as larger fruit bodies that contain sexually derived ascospores inside a cell type called the ascus. Basidiomycota more rarely form conidia, and are characterized by sexually produced basidiospores that form at the ends of cells called basidia. Some fungi have no known sexual phase and these are put in the Deuteromycota.
 |
