BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. DIVERSITY OF LIFE ON EARTH

2.5. The Kingdom Fungi

2.5.2. Structure

The body Structure OF Fungi is unique. It consists of a mass of fine, branching, tubular filaments called hyphae (singular: hypha), and this entire network of hyphae is collectively referred to as the mycelium. Each hypha is enclosed by a thin, rigid Cell wall, the primary component of which is Chitin—a nitrogen-containing polysaccharide. Chitin is also a structural component of the arthropod exoskeleton (Sec. 2.8.6). Hyphae lack true Cellular Organization. The protoplasm of the hyphae is either entirely continuous or divided by transverse cross-walls known as septa (singular: septum). Septa partition the Contents of the hyphae into discrete compartments that superficially resemble Cells. Unlike true cell walls, The formation of septa is not associated with nuclear division. Typically, a small pore remains in the center of each septum, allowing protoplasm to flow freely from one compartment to another.

Each compartment may contain one, two, or multiple nuclei, which are distributed along the hypha at more or less regular intervals. Hyphae with cross-walls are termed septate, as seen, for example, in Penicillium (Fig. 2.25). Hyphae lacking cross-walls are termed aseptate (or non-septate), as found in Mucor (Fig. 2.26).

The Cytoplasm of hyphae contains typical eukaryotic Organelles: Mitochondria, the Golgi apparatus, The Endoplasmic reticulum, Ribosomes, and vacuoles. In older Regions of the mycelium, vacuoles are larger, and the cytoplasm occupies only a narrow peripheral zone. Periodically, hyphae aggregate to form denser structures known as fruiting bodies, which produce spores. Yeasts form a distinct group in that they are unicellular organisms and therefore lack hyphal structures, as exemplified by Saccharomyces (Fig. 2.21).

Penicillium, Mucor, and Rhizopus are commonly known as Molds. These are widespread saprotrophs, meaning they feed on dead organic matter. They are highly convenient for laboratory study because they are easily cultivated and readily form typical fungal hyphae.

Species of Penicillium form blue, green, and occasionally yellow molds on bread and decaying fruit. The mycelium forms small, circular colonies; the hyphae are septate, and the characteristic coloration of the colonies is due to the spores (Fig. 2.25, A). Reproduction in Penicillium is asexual, occurring via spores called conidia. Conidia are borne at the tips of specialized hyphae known as conidiophores (Fig. 2.25, B and C). Unlike sporangiospores, conidia are not enclosed within a sporangium; instead, they are naked and readily dispersed upon maturation. The structure of the hyphae is illustrated in Fig. 2.25, D. The economic importance of Penicillium is discussed in Section 12.11.1.

Mucor is a genus comprising several well-known molds. This fungus is widely distributed in soil and also grows on bread. When cultured on Agar, it forms more or less circular colonies. The hyphae are aseptate and extensively branched (Fig. 2.26, B). Spores develop within spherical sporangia borne on very tall, vertically growing hyphae called sporangiophores (Fig. 2.26, A and B). In the older regions of the mycelium, where sporangiophores are particularly abundant, they resemble a collection of pins; hence, members of the genus Mucor are often referred to as pin molds. Sporangia are easily visible under a Microscope at low magnification. Mucor grows rapidly, capable of covering an entire Petri dish within three days at 20 °C. The Internal Structure of the hyphae is typical of eukaryotes, similar to that of Penicillium (Fig. 2.25, D), with the sole exception that Mucor hyphae lack cross-walls. Rhizopus is very similar to Mucor. Certain hyphae, known as stolons, are somewhat curved in shape. At the tips of the stolons, clusters of short, ROOT-like hyphae develop. Unlike Mucor, whose sporangiophores arise singly, Rhizopus produces two or more sporangiophores from the same point.

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Fig. 2.25. A. Penicillium growing on nutrient agar in a Petri dish. Typically, it forms relatively small, circular colonies. The periphery of the colony, consisting of younger mycelium, appears white, while the mature central region, where spores have already formed, is darker. B. Asexual reproduction in Penicillium. Conidia are clustered in characteristic brush-like structures. C. Scanning electron micrograph of a conidiophore and conidia. D. Diagrammatic Cytology/practical/54.html">Longitudinal section of a hypha showing its internal ultrastructure.

Fig. 2.26. A. Scanning electron micrograph of a portion of the mycelium of Mucor hiemalis. Sporangia are clearly visible (x85). B. Diagrammatic representation of a Mucor mycelium as viewed under a Light Microscope at low magnification.

2.3. What is the function of sporangiophores?

Yeasts are unicellular saprotrophic fungi. They are widely distributed in nature and are particularly abundant on the sugary surfaces of fruits, often forming a powdery, sugary film on grapes. The Fermentation (Anaerobic Respiration) of sugars by yeasts produces alcohol—a process utilized by humans for millennia and the foundation of the wine and brewing industries. Under favorable conditions, yeasts reproduce rapidly by budding (a form of asexual reproduction; Fig. 2.27, A). Structurally, Yeast cells are entirely typical of eukaryotes (Fig. 2.27, B and C).

Fig. 2.27. Structure of yeast (Saccharomyces). A. Budding yeast cells as viewed under a light microscope (x400). B. Transmission electron micrograph of a yeast cell (x10,000). C. Diagrammatic representation of the ultrastructure of a yeast cell as revealed by Electron Microscopy.