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Subject: General Science

Topic: The Digestive System

INTRODUCTION

Animals, for the most part, ingest their food as large, complex molecules that must be broken down into smaller molecules (monomers) that can then be distributed throughout the body of every cell. This vital function is accpomplished by a series of specialized organs that comprise the digestive system.

DIGESTIVE SYSTEM IN VARIOUS ORGANISM:

Single-celled organisms can directly take in nutrients from their outside environment. Multicellular animals, with most of their cells removed from direct contact 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.
• Ingestive eaters, majority of animals, use a mouth to ingest food.
• Absorptive feeders, such as tapeworms, live in a digestive system of another animal and absorb nutrients from that animal directly through their body wall.
• Filter feeders, such as oysters and mussels, collect small organisms and particles from the surrounding water Substrate feeders, such as earthworms and termites, eat the material (dirt or wood) they burrow through.
• Fluid feeders, such as aphids, pierce the body of a plant or animal and withdraw fluids.

LOCATIONS OF DIGESTIVE SYSTEM IN VARIOUS ANIMALS:

• The digestive system uses mechanical and chemical methods to break food down into nutrient molecules that can be absorbed into the blood. Once in the blood, the food molecules are routed to every cell in the animal’s body. There are two types of animal body plans as well as two locations for digestion to occur.
• Sac-like plans are found in many invertebrates, who have a single opening for food intake and the discharge of wastes.
• Vertebrates, the animal group humans belong to, use the more efficient tube-within-a-tube plan with food entering through one opening (the mouth) and wastes leaving through another (the anus).
• Where the digestion of the food happens is also variable.
• Some animals use intracellular digestion, where food is taken into cells by phagocytosis with digestive enzymes being secreted into the phagocytic vesicles. This type of digestion occurs in sponges, coelenterates (corals, hydras and their relatives) and most protozoans.
• Extracellular digestion occurs in the lumen (or opening) of a digestive system, with the nutrient molecules being transferred to the blood or some other body fluid. This more advanced type of digestion occurs in chordates, annelids, and crustaceans.