More recently, the push to analyze physiological processes, like digestion, at a cellular and molecular level has restricted our focus towards a few experimentally tractable model systems, which include vertebrates, as well as a few invertebrates (the insect Drosophila melanogaster or the nematode Caenorhabditis elegans, where food assimilation also occurs mainly by extracellular digestion (see Holtof et al. Historically, the widespread occurrence of phagocytosis and intracellular digestion among invertebrates was clearly recognized by pioneers of cell biology, such as Metschnikoff ( 1884). It is well known that animals belonging to more early diversified clades, such as Porifera or Cnidaria, ingest food mainly in the form of single cells (e.g., bacteria, microalgae, protists) that are taken up phagocytotically and degraded subsequently by intracellular digestion (Lunger 1963 Afzelius and Rosén 1965 Weissenfels 1976 Imsiecke 1993). The complex mammalian digestive system is a highly derived character among animals. Numerous other cell types, such as smooth muscle cells, neurons and enteroendocrine cells, are associated with glandular cells and intestinal cells to promote the digestive process. In mammals, for example, the digestive system is comprised of glandular organs with classes of cells specialized in the secretion of enzymes for the extracellular digestion of food particles (e.g., exocrine cells of the salivary gland, pancreas), as well as other organs with absorptive function (e.g., small intestine). Whereas all cells are capable of transporting molecules through the membrane, the efficient procurement, digestion and uptake of nutrients have become the function of specialized cell types and organs, forming the digestive system in multicellular animals. Small molecules pass into the interior of the cell via simple diffusion or active transport carried out by membrane bound transporters macromolecules, or entire cells, are taken up by endocytosis/phagocytosis, and are degraded intracellularly in specialized membrane bound compartments (lysosomes). Living cells depend on a constant supply of energy-rich organic molecules from the environment.
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