Extracellular digestion

Extracellular digestion is a process in which saprobionts feed by secreting enzymes through the cell membrane onto the food. The enzymes catalyze the digestion of the food into molecules small enough to be taken up by passive diffusion, transport or phagocytosis. Since digestion occurs outside the cell, it is said to be extracellular. It takes place either in the lumen of the digestive system, in a gastric cavity or other digestive organ, or completely outside the body.

The prefix 'extra' means outside, which tells us that extracellular digestion must occur outside the cell. During extracellular digestion, food is broken down outside the cell either mechanically or with acid by special molecules called enzymes. Then the newly broken down nutrients can be absorbed by the cells nearby. Humans use extracellular digestion when they eat. Their teeth grind the food, enzymes and acid in the stomach liquefy it, and additional enzymes in the small intestine break the food down into parts their cells can use.

Although fungi don't have a digestive tract like humans, they still use extracellular digestion. Fungi and other decomposers utilize nutrients derived from breaking down the substrate they grow on. Another example of extracellular digestion being used is in the hydra, or sea anemone. A large cavity, called the gastrovascular cavity, fills the center of the animal, with one opening for both food and waste. When unsuspecting prey swim into the opening, stinging cells paralyze the prey. The hydra uses its tentacles to push the prey further into the cavity, where enzymes are secreted to break down the food. Once the food is broken down extracellularly into nutrients, the cells of the hydra can absorb it for energy.

Extracellular digestion is a form of digestion found in all saprobiontic annelids, crustaceans, arthropods, lichens and chordates, including vertebrates.

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