In the past, the true fungi, slime molds, and bacteria were all placed in a single division of the Plant Kingdom. Once the fundamental differences between prokaryotic and eukaryotic cells became known, however, the bacteria were placed in the prokaryotic Kingdom Monera. Then it became increasingly apparent that the metabolism, reproduction, and general lines
of diversity of fungi were different from those of members of the Plant Kingdom, with the fungi evidently having been independently derived from ancestral unicellular organisms. Accordingly, fungi were placed in their own kingdom.
All true fungi are filamentous or unicellular het-erotrophs, most of which absorb their food in solution through their cell walls. Some are saprobes (organisms that live on dead organic matter); others are parasitic decomposers; still others (mycorrhizal fungi) have a mutualistic relationship with plants. The slime molds engulf their food and were discussed in Chapter 18; they appear to be related to protozoa, which are single-celled protists included in Kingdom Protista. Chytrids and water molds are also funguslike, but their classification is controversial because, like most protists, their reproductive cells have flagella. However, chytrids, like other fungi, have chitin in their cell walls, and recent molecular research suggests the chytrids are probably more closely related to the true fungi than they are to members of Kingdom Protista; therefore, they are discussed here under Kingdom Fungi. The water molds, however, which have cellulose but no chitin in their cells walls, continue to be more appropriately included in Kingdom Protista.
The members of Kingdom Fungi are placed in five phyla. With the exception of some chytrids and all yeasts, they are filamentous. Most, but not all, fungi lack motile cells. Filamentous fungi produce hyphae that grow at their tips. Structures such as mushrooms are formed from hyphae tightly interwoven and packed together. The cell walls of true fungi consist primarily of chitin, a material also found in the shells of arthropods (e.g., insects, crabs). Fungi exhibit a variety of forms of sexual reproduction. The food substances that most fungi absorb through their cell walls are often broken down with the aid of enzymes secreted to the outside by the cells. Because of the great variety of form and reproduction throughout Kingdom Fungi, a neat pigeonholing of all the members into distinct groups is difficult. Broad groups can, however, be recognized.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.