Digestive System Introduction Single-celled organisms can directly take in nutrients from their outside environment. Multicellular animals, with most of their cells removed from contact directly with the outside environment, have developed specialized structures for obtaining and breaking down their food. Animals depend on two processes: feeding and digestion. Animals are heterotrophs, they must absorb nutrients or ingest food sources. 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.
Digestive System Introduction Single-celled organisms can directly take in nutrients from their outside environment. Multicellular animals, with most of their cells removed from contact directly with the outside environment, have developed specialized structures for obtaining and breaking down their food. Animals depend on two processes: feeding and digestion. Animals are heterotrophs, they must absorb nutrients or ingest food sources. 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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