Animal Ingestion and Digestion

All animals need to break down food molecules into smaller pieces so they can circulate them around their bodies to all their cells. Their cells take in small food molecules and use them as material for growth or as a source of energy.

Four main events occur from the moment food enters an animal’s body until the time the animal releases its wastes:

  • Ingestion occurs when an animal takes food into its digestive tract.

  • Digestion occurs when the animal’s body gets busy breaking down the food. Two types of digestion exist in all animals:

    • Mechanical digestion physically breaks down food into smaller and smaller pieces. It begins when an animal consumes the food and continues until the food enters the animal’s stomach.

    • Chemical digestion uses enzymes and acids to break down chewed or ground-up food into even smaller pieces. It also begins as soon as an animal consumes the food as the mouth’s enzymes go to work. Chemical digestion continues as the food moves through the stomach and small intestine and encounters enzymes and acids in the stomach and enzymes in the small intestine.

  • Absorption occurs when cells within the animal move small food molecules from the digestive system to the insides of the cells.

  • Elimination occurs when material that the animal can’t digest passes out of its digestive tract.

The basic way an animal’s digestive system works has a great deal to do with whether it can spend a few hours between meals or whether it has to keep consuming food constantly just to stay alive.

Animals with incomplete digestive tracts have the most primitive digestive systems. These animals have a gut with just one opening that serves as both mouth and anus. Animals like humans have more complex systems that scientists call a complete digestive tract. Complete digestive tracts have a mouth at one end and an anus at the other.

Complete digestive tracts are more efficient than incomplete digestive tracts because they allow thorough digestion of food before excretion occurs. Organisms with incomplete digestive tracts release undigested food along with their wastes, so they often have to take in food constantly to replace food that’s excreted before they extract all the nutrients:

  • Animals that must consume constantly because they take food in and then push it out soon afterward are called continuous feeders. Most of these animals are either permanently attached to something (think clams or mussels) or incredibly slow movers.

  • Animals that are discontinuous feeders consume larger meals and store the ingested food for later digestion. These animals are generally more active and somewhat nomadic.

You and all the other animals that are discontinuous feeders must have a place in the body to store food as it slowly digests. In humans, this organ is the stomach.

For questions 1–6, use the following terms to identify which step of digestion is occurring in each example.

a. Ingestion

b. Digestion

c. Absorption

d. Elimination

For questions 7–9, use the following terms to identify the type of digestive system described in each example.

a. Incomplete digestive system

b. Complete digestive system

  1. The enzyme salivary amylase breaks down starch molecules in the mouth into smaller simple sugars.

  2. A sea gull releases feces as it flies over a dock.

  3. A ball python swallows a mouse.

  4. A giraffe pulls some leaves from a tall tree with its tongue.

  5. The cells that line your small intestine transport amino acids from the solution in your intestines into their cytoplasm.

  6. Your pancreas releases lipases, enzymes that break down fat, into your small intestine where they break down lipids into smaller molecules.

  7. An anemone catches some food particles with its sticky tentacles and places them in its mouth. The food travels down into the anemone’s body cavity, where digestion occurs. The anemone releases indigestible bits of food from its mouth.

  8. A hamster nibbles on sunflower seeds. After swallowing and digesting the seeds, the hamster releases fecal pellets from its anus.

  9. A grasshopper chews some wheat with its mouth parts and then swallows the wheat. The grasshopper digests the wheat in its stomach and then absorbs small food molecules using the cells that line the stomach. In the hind gut, the grasshopper reabsorbs water from the remains of the food, concentrating the indigestible parts into fecal pellets that it releases from its anus.

    The following are answers to the practice questions presented.

  1. The answer is b. Digestion.

  2. The answer is d. Elimination.

  3. The answer is a. Ingestion.

  4. The answer is a. Ingestion.

  5. The answer is c. Absorption.

  6. The answer is b. Digestion.

  7. The answer is a. Incomplete digestive system.

  8. The answer is b. Complete digestive system.

  9. The answer is b. Complete digestive system.

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