Fungi are heterotrophic in their mode of nutrition that is they require an organic source of carbon. In addition they require a source of nitrogen, usually organic substances such as amino acids. The nutrition of fungi can be described as absorptive because they absorb nutrients directly from outside their bodies. This is in contrast to animals which normally ingest food and then digest it within their bodies before absorption takes place. With fungi, digestion is external using extra-cellular enzymes. Fungi obtain their nutrients as saprotrophs,parasites or symbionts.
A saprotroph is an organism that obtains its food from dead and decaying matter. It secretes enzymes on to the organic matter so that digestion is outside the organism. Soluble products of digestion are absorbed and assimilated within the body of the saprotroph.
Saprotrophic fungi and bacteria constitute the decomposers and are essential in bringing about decay and recycling of nutrients. They produce humus from animal and plant remains. Humus, a part of soil, is a layer of decayed organic matter containing many nutrients. Some important fungi are the few species that secrete cellulase which breaks down cellulose. Cellulose being an important structural component of plant cell walls, rotting of wood and other plant remains is achieved by these decomposers secreting cellulases.
A parasite is an organism that lives in or on another organism, the host from which it obtains its food and shelter. The host usually belongs to a different pecies and suffers harm from the parasite. Parasites which cause diseases are called pathogens. Some parasites can survive and grow only in living cells and are called biotrophs or Obligate Parasites. Others can infect their host and bring about it’s death and then live saprotrophically on the remains, they are called necrotrophs or facultative parasites. Fungal parasites may be facultative or obligate and more commonly attack plants than animals. The hyphae penetrate through stomata, or enter directly through the cuticle or epidermis or through wounds of plants. Inside the plant hosts, the hyphae branch profusely between cells, sometimes producing pectinases(enzymes) which digest the middle lamellae of the cell walls. Subsequently the cells may be killed with the aid of toxins and cellulases which digest the cell walls. Cell constituents may be absorbed directly or digested by the secretion of further fungal enzymes.
Obligate parasites often possess specialized penetration and absorption devices called haustoria. Each haustorium is a modified hyphal outgrowth with a large surface area which pushes into living cells without breaking their plasma membrane and without killing them. Haustoria are rarely produced by facultative parasites.
They are symbiotic association found between algae and fungi. The alga is usually a green alga or blue green alga. The fungus is an ascomycete or basidiomycete. It is believed that the alga contributes organic food from photosynthesis and the fungus is able to absorb water and mineral salts. The fungus can also conserve water and this enables some lichens to grow in extreme dry conditions where no other plants can exist.
These are symbiotic associations between a fungus partner and roots of higher plants. Most land plants enter into this kind of relationship with soil fungi. The fungus may form a sheath around the center of the root (an ectotrophic mycorrhiza) or may penetrate the host tissue (an endotrophic mycorrhiza). The former type is found in many forest trees such as conifers,beech and oak and involve the fungi of the division basidiomycetes. The fungus receives carbohydrates and vitamins from the tree and in return breaks down proteins of the soil humus to amino acids which can be absorbed and utilized by the plant. In addition the fungus provides a greater surface area for absorption of ions such as phosphates.