Extracellular digestion is the breakdown of food into nutritional components with the use of secreted enzymes. The organism digests the food, transforming it into compounds it can actually use, in an environment outside the cell. This contrasts with intracellular digestion, where cells engulf food and digest it within their walls. Both approaches to digestion can be seen at a variety of levels of the animal kingdom.
One common example can be seen in mushrooms. Mushroom species secrete enzymes to break down their substrate, and absorb the usable components as they grow. Humans also use extracellular digestion; in the gastrointestinal tract, cells secrete digestive enzymes and absorb components of food as it moves through. This contrasts with amoebae, organisms that engulf their food for digestion.
Multi-cellular animals use this process to metabolize their food and distribute it throughout the body via the bloodstream and other structures. It can be very efficient, as the body will be able to reuse some components to make more enzymes, even as it breaks cells down into chains of molecules it finds useful for various biological functions. Any waste products can be eliminated.
The level of complexity in an organism's digestive tract can vary. Some have very complicated digestive systems, while others may have a very simple alimentary canal. All food moves in one direction down the alimentary canal so the organism can bombard it with enzymes and pull out the necessary nutrients, disposing of waste material at the end opposite the mouth.
Extracellular digestion makes it possible for organisms to grow quite large, with differentiated cells, since each cell does not need to fend for itself. Instead, the cells bathe in a steady supply of nutrients to meet their needs. These nutrients move through blood, lymph, and other fluids, and the same fluids can also carry away waste products for elimination. Specialized cells like muscle tissue can form, as all cells in the body do not need to be capable of digesting food independently.
There may be limits on the kind of foods an organism can digest. A wide variety of enzymes would be necessary to digest every potentially edible thing on Earth, and most organisms do not have so many. Individuals can also be born with enzyme deficiencies; some humans, for example, lack the ability to digest milk. In some cases, enzyme deficiencies can be fatal, as the body may develop toxicity in response to the consumption of certain food items when it cannot break them down successfully.