Protozoa can be broadly defined as single-celled organisms which have animal-like nutrition (i.e. they do not contain photosynthetic pigment). Structurally, protozoa are equivalent to a single animal cell; functionally, they are equivalent to a whole animal. Each protozoan cell potentially possesses all the normal metazoan cellular organelles: nucleus, mitochondria, ribosomes, Golgi apparatus, etc. although some may have been lost in certain specialized organisms. Many protozoa also possess unique organelles not found in metazoa.

Parasites can, equally broadly, be defined as organisms which live within organs or tissues of other organisms during some or all stages of their life cycle, and obtain nutrients either from the host organism's food supply or from its tissues. The processes by which these nutrients are digested, and the processes of respiration by which protozoa obtain their energy, do not differ fundamentally from those operating in …