Fungi Are Heterotrophs Too!

The fungi constitute a large and diverse group of sedentary organisms; they are all absorptive heterotrophs that live on or in their food supply and digest and absorb nutrients from these immediate sources. Fungi lack internal digestive systems and depend mainly on absorption as their mode of feeding. They are usually either saprophytic (decomposing) or parasitic.

Fungi are primarily filamentous and multicellular. The fungal body usually consists of microscopic, tubular threads that branch and spread over the surface of the food supply. Each thread is known as a hypha; A mass of hyphae is called a mycelium. The hyphal threads have definite cell walls, which are often composed of chitin, a complex polysaccharide containing nitrogen. In most fungi the hyphae are divided by cross-walls (septa), forming compartments or cells. Sometimes, however, the partitions between the cells are incomplete or absent, so the cytoplasm is continuous. The individual “cells” of fungi, unlike those of plants and animals, often have more than one nucleus. Fungi lack an internal cavity where bulk food can be digested. Fungi simply absorb digested organic nutrients across their body surface much as plant roots absorb inorganic nutrients.

Somatic hyphae. A) Portion of a hypha having more than one nucleus (nonseptate); B) Portion of a septate hypha.

Bread mold is a familiar example of a fungus. The bread on which the fungus grows is composed mostly of starch, a rich source of energy. But starch is a polysaccharide, whose very large and insoluble molecules cannot move across the cell membranes of the mold. Before absorption can take place, the starch must be broken down to its constituent building-block compounds, the simple sugars; in short, the starch must be digested. Digestion is simply another name for enzymatic hydrolysis, which involves the addition of water to break the bonds between the simple sugars. In bread mold and other fungi, the hydrolysis takes place outside the cells, and the process is called extracellular digestion. Digestive enzymes synthesized inside the cells of the mold are released from the fungal cells into the bread and hydrolyze the starch into simple sugars, which are then absorbed, often by rootlike structures called rhizoids (See Fig. 1).

Mold living on bread exemplifies a saprophytic (decomposing) way of life, but many fungi are parasitic (see Figure). Indeed, bread mold itself is not restricted to saprophytic (decomposer) nutrition; it is one of the most common destructive fungi on fresh fruits and vegetables. Parasitic fungi employ basically the same mode of nutrition as saprophytes (decomposers): enzymes are secreted onto the food supply; digestion takes place extracellularly, and the products of digestion can be absorbed by the cells of the fungus. The various pathogenic fungi differ in their relationship with the hosts. Some small fungi grow between the cells of their host but send out rootlike structures, called haustoria (See Fig. 1), that make deep invaginations in the host's cell membranes, through which they absorb nutrients from the host cells. The intense itching caused by the athlete's foot fungus is produced when the haustoria grow past sensory endings in the skin.

A few fungi supplement their diets by trapping small animals such a nematode worms (Fig. 2). When the prey has been trapped, branches of the fungus penetrate the victim's body and release digestive enzymes; extracellular digestion takes place, and the products are absorbed.