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 the enzyme 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 species 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.
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