Once an organism — a plant or animal — ingests (or absorbs) food, the organism’s digestive system immediately starts breaking down the food to release the nutrients. The smallest digestive system can be found in organisms made up of several cells. However, the system, in which specialized cells surround a digestive cavity, gets the job done. The cells acquire nutrients that the other cells in the organism can use for processes that keep the organism alive. As organisms increase in complexity, they expend more energy and require more nutrients. In keeping with their more complex metabolism, their digestive systems are more complex. The process of getting nutrients into the organism happens at the cellular level; that is, the cells lining the digestive tract take in the nutrients and pass them on to the rest of the body.
