The ability to ingest or absorb food is relatively common in nature; only the Kingdom Plantae is completely devoid of organisms that do not ingest or absorb their food, as they make their food internally through the process of photosynthesis. All other organisms rely on external sources of food, with some simply absorbing their food (e.g. fungi and monera) and others having developed complex systems to digest their food (e.g. Animalia). According to the Linnaean classifications of kingdoms, there are four kingdoms of animals that either ingest or absorb their food.

The Kingdom Animalia comprises multicelluar organisms that digest their food. The digestion system ranges in complexity. For example, a nematode has only a mouth (where food is initially ingested), an intestine (for extracting nutrients) and an anus (for expelling waste products). In contrast, human beings have highly evolved digestive systems that include the same three basic parts as the nematode, but have several additional features. Food sources for animals vary, as some are carnivorous (eating only other animals), some are herbivores (eating only plants) and others are omnivorous (which means they eat both).

Kingdom Protista is the only other kingdom that includes some organisms which ingest their food. Protists are single-celled organisms that have their nucleus contained in a membrane. Protists that ingest their food do so through a process known as "phagocytosis," in which animal-like protists (known as "protozoa") envelop their food through a mouth-like structure. All other protists absorb their food, as opposed to ingesting it, with examples being the plant-like protists (such as algae).

The Fungi Kingdom comprises multicellular organisms similar to the Plantae Kingdom. However, the main difference between fungi and plants is that fungi must absorb their food. Absorbing food is technically different from ingesting it because absorption merely involves the organism being placed on top of its food source and obtaining nutrients directly, where as ingesting food must involve a mouth and a system for breaking down nutrients within the organism. Common examples of fungi include mushrooms, moss and molds.

The Monera Kingdom comprises single-cell organisms that do not have their nucleus contained in a membrane. Some Monera make their food through photosynthesis (for example, there are some algae classified as Monera), however, others absorb their nutrients directly. Most bacteria are classified in the Monera Kingdom, and often gain their nutrients by living parasitically within a multicellular organism. This is why bacteria often gets people sick. They are tiny organisms which can draw nutrients away from their host.