Fungi are a diverse group of eukaryotic organisms that usually grow best in dark, moist habitats.  They receive their energy from outside sources by secreting, into the environment, enzymes that digest organic matter.  Most fungi are saprophytes; the organic matter from which they take their nutrients is dead.  Thus these organisms play an essential role in decomposition.  They are also economically important in food, beverage and antibiotic production.  Conversely, many fungal species are pathogenic, causing diseases that range from athletes foot to ergot.

Fungi include both the unicellular, non-filamentous yeasts and the multicellular, filamentous molds. Yeasts are typically spherical or oval in shape and are widely distributed in nature (found on fruits, leaves of trees, etc).  Yeasts reproduce asexually by budding, a process by which a new cell forms as a protuberance (bud) from the parent cell.  Molds come in many sizes, shapes, and forms, ranging from mushrooms, puffballs and large bracket fungi found on trees to the small colonies often found as contaminants on moldy fruits and cheeses. 

The body of a fungus is called the thallus.  It can range in size and can be either a single cell or multicellular.  The cells within the thallus are coated with a cell wall made of a strong polysaccharide called chitin.  Long, branched filaments called hyphae further characterize a mold.  These hyphae form a tangled web that is referred to as mycelium.  Hyphae are composed of an outer cell wall and an inner lumen and are either septate (have cross walls) or coenocytic (protoplasm streams throughout).  

Most molds are capable of both sexual and asexual reproduction.  Asexual reproduction can occur either by central constriction of a parent cell to form two daughter cells or by spore formation.  There are several types of spore formation.  When hyphal fragementation occurs, the resulting spores are termed arthrospores.  If the cells are surrounded by a thick wall before hyphal fragmentation, the spores are instead called chlamydospores. Spores that develop within a sac (sporangium) at a hyphal tip are referred to as sporangiospores.  If spores are not produced in a sac, but do develop at a hyphal tip or sidewall they are called conidiospores.  Finally, spores produced from a vegetative cell by budding are blastosporesSexual reproduction involves the union of two nuclei.  Sexual life cycles vary from one fungal species to another.  However, most have a diploid stage, which involves the formation of a spore that can survive in harsh external conditions.

Lab 13 / Fungi / Classification of Fungi / Procedure / Results / Lab 13 Organisms

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