Intracellular digestion is a process where cells intake materials and break them down within the cell membrane, as opposed to extracellular digestion, where cells secrete enzymes to break down components outside the cell wall. Examples of both can be seen in some organisms. This process allows individual cells to derive nutrients from their environment, break down threats, and process their own waste products, as well as supporting themselves during periods of starvation.
In intracellular digestion, the cell is able to engulf materials from the outside environment. For unicellular organisms, this provides a method of obtaining nutrition to feed the cell. Another use of intracellular digestion is in the immune system, where cells can engulf bacteria, viruses, and other foreign particles and break them down to neutralize them, eliminating potential sources of infection.
There are two kinds of intracellular digestion. Heterophagic digestion involves breaking down objects outside the cell, while autophagic intracellular digestion involves consuming components from within the cell. This may occur when a cell is starving, and can lead to lysis, also known as cell death, if the cell digests too much of itself while seeking energy to function. Cells can also be programmed or triggered to start self digesting when they have outlived their usefulness, or when they are infected with pathogens.
In heterophagic digestion, once the cell has engulfed a target, structures inside the cell called lysosomes release enzymes to break up the target. The enzymes act like shears, cutting through the key proteins of the object the cell has engulfed. Depending on what it is, it may be broken into usable components the cell can recycle, or it can be cut into waste products for elimination. In the case of digestion for nutrition, usually some wastes are generated along the way and they can be passed back through the cell wall to be eliminated.
Intracellular digestion takes place at all levels of the food chain, from cells in a blue whale working to break apart bacteria to unicellular organisms in soil breaking down food sources into useful components. In multicellular animals, there is also a digestive tract for extracellular digestion, where food is broken into usable components outside the cell walls with the use of digestive juices, and these components can pass into the bloodstream to be distributed throughout the body. They will pass through the cell walls of individual cells to provide them with energy and other needed supplies for their survival.