| Themes > Science > Life Sciences > General Biology > Physiology > The Digestive System > Plans and Locations |
The digestive system uses mechanical and chemical methods to break food down into nutrient molecules that can be absorbed into the blood. There are two types of plans and two locations of digestion. Sac-like plans are found in many invertebrates, who have a single opening for food intake and the discharge of wastes. Vertebrates use the more efficient tube-within-a-tube plan with food entering through one opening (the mouth) and wastes leaving through another (the anus). Intracellular digestion: food is taken into cells by phagocytosis with digestive enzymes being secreted into the phagocytic vesicles; occurs in sponges, coelenterates and most protozoans. Extracellular digestion: digestion occurs in the lumen (opening) of the digestive system, with the nutrient molecules being transferred to the blood or body fluid; occurs in chordates, annelids, and crustaceans. |
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