Saprophytes are living organisms that feed on dead organic matter. They are considered extremely important in soil biology, as they break down dead and decaying organic matter into simple substances that can be taken up and recycled by plants. The term is usually used to refer to saprophytic fungi or bacteria.
In the strict botanical definition, the term "saprophyte" is something of a misnomer. "Phyte" means a plant, and bacteria and fungi are not classified as plants. Some higher plants such as certain types of orchids and a family of flowering plants called monotropes were once included in this category, because they do not use photosynthesis to make nutrients, so it was believed that they extracted nutrients from dead organic matter. It is now known that these types of plants are actually parasites that obtain their food by growing on living fungi. As such, there are no known true saprophytic plants.
Saprophytes are characterized by their use of a particular kind of digestion mechanism, called extra-cellular digestion. This process involves the secretion of digestive substances into the surrounding environment, where they break down organic matter into simple substances. The resulting nutrients are then absorbed directly through the membranes of the organism's cells, and metabolized.
In saprophytic nutrition, the main classes of matter that are broken down are proteins, fats, and starches. Proteins are digested into amino acids. Fats are broken down into glycerol and fatty acids. Starches are digested into simple sugars. All of the resulting substances are then of a small enough molecular size that they can be transported across the cell membranes.
Suitable conditions are needed for the optimum growth of the common types of saprophytes. There must be sufficient water in the soil or surrounding environment. There must usually be oxygen present, as the majority cannot grow under anaerobic conditions. The acidity of the soil or environment usually needs to be neutral, or slightly acidic, as most of these organisms do not thrive under alkaline conditions.
Some of the most common include certain saprophytic fungus types, such as those in the families of Rhizopus and Mucor. These fungi typically have an extensive network of hyphae, similar to tiny roots, which grow through the soil or through dead wood or other organic matter. They grow in a network called a mycelium. This enables the fungus to thoroughly penetrate the local organic matter, within which the hyphae secrete digestive enzymes and absorb the resulting nutrients.