Membrane (Contact) Digestion

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Abstract
There is a very efficient mechanism ensuring hydrolysis on the surface of the small intestine. Due to membrane digestion, the function of the gastrointestinal tract in higher and probably lower animals is fairly specific. Cavital digestion is produced by enzymes secreted into the cavity of the gastrointestinal tract. Membrane digestion is due to enzymes adsorbed from chyme and enzymes structurally associated with the membrane of the intestinal cells. The initial stages of hydrolysis occur mainly in the cavity and are characterized by an orientation of enzyme relative to the substrate. The final stages of hydrolysis occur on the surface of the gut during membrane digestion when the substrate is oriented to the fixed enzymes. While membrane digestion changes the conditions of the treatment and absorption of foods making the functioning of the gastrointestinal tract more rational and more efficient, both membrane and cavital digestion are interacting mechanisms, ensuring high rates of assimilation of alimentary substances. Neither cavital nor membrane digestion plays a practical role in the oral cavity. Peristalsis produces continuous mixing of the alimentary substances, allowing closer contact with the surface of the intestine. There is a continuous sorting of foods[long dash]those treated in the cavity those hydrolyzed in the brush border. Interaction between membrane digestion and absorption occurs on the surface of the brush border. Thus the 2-link scheme of digestion (cavital hydrolysis-absorption) should be replaced by a 3-link system[long dash]cavital hydrolysis-membrane digestion-absorption. Both types of digestion exist in highly organized animals. The relative role of these processes varies with the quality of the nutritional substances appropriate to the given species and individual. The more dispersed the food substances the less the preliminary treatment, thereby lessening the role of cavital digestion. Apparently the suckling period in mammals is characterized by predominately membrane digestion and poorly developed cavital digestion. Numerous indirect data demonstrate that membrane digestion exists in low multicellular animals, while direct observations prove that the enzymatic splitting of substrates used for nourishment occur on the external surface of cytomembranes in yeasts and in some bacteria.