holozoic

Holozoic nutrition is a mode of heterotrophic nutrition in which an organism ingests complex organic matter and digests it to obtain usable nutrients. The term is commonly used in zoology and comparative physiology to distinguish animals and some protists that feed on solid or particulate food from organisms that rely on external digestion or from autotrophs that synthesize their own nutrients.

In most multicellular animals, holozoic feeding involves ingestion of food through an oral opening, followed by mechanical processing and extracellular digestion in a digestive tract with enzymes. The resulting monomers and peptides are absorbed across the gut lining and transported to cells where they are assimilated for growth and maintenance. Indigestible residues are eliminated as waste. In some unicellular holozoans, such as Amoeba, feeding occurs by phagocytosis, with digestion taking place inside food vacuoles.

Holozoic nutrition is often contrasted with saprotrophic nutrition, in which organisms secrete enzymes to break down external matter and absorb soluble nutrients, and with holophytic (photosynthetic) nutrition, in which organisms produce organic compounds from inorganic sources. The term is sometimes encountered in discussions of the broader evolutionary relationships among heterotrophs, and it highlights a feeding mode that emphasizes ingestion of intact food followed by digestion and absorption.

Etymology: from Greek holos, meaning whole, and phagein, meaning to eat. This reflects the idea of consuming and assimilating whole food items rather than absorbing dissolved nutrients directly.