THE LIVING WORLD
Unit Six. Animal Life
25. The Path of Food Through the Animal Body
Heterotrophs are divided into three groups on the basis of their food sources. Animals that eat plants exclusively are classified as herbivores; common examples include cows, horses, rabbits, and sparrows. Animals that are meat eaters, such as cats, eagles, trout, and frogs, are carnivores. Omnivores are animals that eat both plants and other animals. We humans are omnivores, as are pigs, bears, and crows.
Single-celled organisms (as well as sponges) digest their food intracellularly, breaking down food particles with digestive enzymes inside their cells. Other animals digest their food extracellularly, within a digestive cavity. In this case, the digestive enzymes are released into a cavity that is continuous with the animal’s external environment. In flatworms (such as Planaria) and cnidarians, like the hydra in figure 25.3, the digestive cavity in the center of the body has only one opening at the top that serves as both mouth (the red arrow bringing food in) and anus (the blue arrow passing waste out). There can be no specialization within this type of digestive system, called a gastrovascular cavity, because every cell is exposed to all stages of food digestion.
Figure 25.3. Two-way digestive tract.
Food particles enter and leave the gastrovascular cavity of Hydra through the same opening.
Specialization occurs when the digestive tract, or alimentary canal, has a separate mouth and anus, so that transport of food is one way. Three examples are shown in figure 25.4. The most primitive digestive tract is seen in nematodes (phylum Nematoda), where it is simply a tubular gut lined by an epithelial membrane. Earthworms (phylum Annelida) have a digestive tract specialized in different regions for the ingestion, storage (crop), fragmentation (gizzard), digestion, and absorption of food (intestine). All higher animals, like the salamander, show similar specializations.
Figure 25.4. One-way digestive tracts.
One-way movement through the digestive tract allows different regions of the digestive system to become specialized for different functions.
The ingested food may be stored in a specialized region of the digestive tract or may first be subjected to physical fragmentation through the chewing action of teeth (in the mouth of many vertebrates) or the grinding action of pebbles (in the gizzard of earthworms and birds). Chemical digestion then occurs primarily in the intestine, breaking down the larger food molecules of polysaccharides, fats, and proteins into smaller subunits. Carbohydrate digestion begins in the mouth of some animals, and protein digestion begins in the stomach in some animals. Chemical digestion involves hydrolysis reactions that liberate the subunits—primarily monosaccharides, amino acids, and fatty acids—from the food. These products of chemical digestion pass through the epithelial lining of the gut and ultimately into the blood, in a process known as absorption. Any molecules in the food that are not absorbed cannot be used by the animal. These wastes are excreted through the anus.
Key Learning Outcome 25.2. Most animals digest their food extracellularly. A digestive tract with a one-way transport of food allows specialization of regions for different functions.