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Viruses are quite different from the other members of this list. They are protein based but are not really considered to be living organisms. Viruses have no metabolism whatsoever, and are only able to reproduce by commandeering the interal mechanism of a specific, living host cell.
Bacteria are prokaryotes, which means they have a simplified cell structure with no membrane-bound organelles. Many bacteria are free-living and get their energy by breaking down organic materials. Other species live upon or within other living organisms, including our bodies. The majority of these seem to be relatively benign, but some species of bacteria can cause tissue damage, or can secrete substances that are toxic to the host cells. Bacteria are found in virtually every environment on Earth.
Molds and yeast, are classified as fungi. Fungi, like plants, have cells walls, but in fungi the walls are made of chitin, whereas plant cell walls are made of cellulose. While this group is highly variable, fungi generally are saprobes, breaking down dead organic matter, or else they are parasites on living cells. In general fungi do best in damp locations with a steady food supply. Many of them are intolerant of direct sunlight.
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