Like plants, the cells have walls. But unlike plants, the walls are made of chitin rather than cellulose and cytoplasm flows freely throughout the organism. Like animals, the cells of fungi are heterotrophic . But unlike animals, digestion occurs outside the organism, with the digested foods then being absorbed. Unlike both plants and animals, fungi do not have organs nor true tissue systems.
Many fungi are parasitic, and some cause serious diseases in plants as well as animals.
Many more fungi, however, are saprophytes and valuable because of their role as decomposers. Yeast is a unicellular fungus important in baking and brewing. The kingdom Monera includes the bacteria and so called blue-green algae . These organisms are distinguished by lacking a true nucleus , having a nucleoid instead, and lacking organelles . The cells are tiny, often one to ten micrometers in length.