FUNGAL PHYSIOLOGY While the common fungus such as the mushroom is readily visible and synonymous with the term “fungus”, many fungi do not form mushrooms at all. Fungal cells are more similar to animal cells than they are to plant cells. There are about 100,000 identified species of the 1.5 million probable species of fungi. Typical fungi include the yeasts, molds, and edible or inedible mushrooms. Fungi are not capable of photosynthesis. Instead, they are heterotrophic, getting their food from complex organic compounds. As mentioned, many are spore-forming, with spores being haploid cells that can undergo mitosis to form multicellular haploid structures. They are essential to the ecosystem because they are decomposers that break down complex organic molecules for fuel. Most land plants have symbiotic relationships with fungi. Roots of many plants will form what are called mycorrhizae, in which the fungus and plant exchange water and nutrients. Another symbiotic relationship is that with algae or other photosynthetic organisms, forming lichens. Infections by fungal organisms can cause human, animal, and plant diseases. Dutch elm disease is a fungal infection of the elm trees that infect the tree’s vascular system with the elm bark beetle being a vector. Fungi are eukaryotic single or multicellular organisms that have the typical cell structure we’ve already discussed. They do not have chloroplasts because they don’t undergo photosynthesis. The bright colors seen in fungal organisms come from cell wall pigments that protect the fungus against ultraviolet radiation. The cell wall consists of both chitin and glucans. Chitin is the same polysaccharide that is found as part of the exoskeleton of insects. It protects the cell from predators and from drying out. The cell membranes are different from animals in that they do not have cholesterol in them but have ergosterol, another steroid molecule. As mentioned, the vegetative body of the fungus is called a thallus (whether it be unicellular or multicellular). Many dimorphic fungi can go from unicellular to multicellular to form two shapes. The two main morphologies seen in multicellular fungi are called vegetative and reproductive morphologies. The vegetative state is the hyphal
194
