In general, prokaryotic cells are about 10 times smaller than eukaryotic cells, and they lack many of the eukaryotic cell structures such as organelles. Organelles are small, double membrane-bound structures in the eukaryotic cell that perform specific functions and include the nucleus, mitochondria, and chloroplasts.
All prokaryotes are microorganisms, but only some eukaryotes are microorganisms. The majority of microorganisms are single-celled, but some consist of a few cells. Certain invertebrate animals—such as helminths, many of which can be seen with the naked eye, are also included in the study of infectious diseases because of the way they are transmitted and the way the body responds to them, though they are not microorganisms.
Lifestyles of Microorganisms
The majority of microorganisms live a free existence in habitats such as soil and water, where they are relatively harmless and often beneficial. A free-living organism can derive all required foods and other factors directly from the nonliving environment. Some microorganisms require interactions with other organisms. Sometimes these microbes are termed parasites. They are harbored and nourished by other living organisms called hosts. A parasite’s actions cause damage to its host through infection and disease. Although parasites cause important diseases, they make up only a small proportion of microbes.