3. Animals eat diverse foods, but break them down into the same types of nutrients.
37.2 Digestive Diversity
4. Protista and sponges have intracellular digestion, in which cells engulf food and digest it in food vacuoles. More complex animals have extracellular digestion in a cavity outside cells. The cavity plus accessory organs form the digestive system.
5. Digestive cavities can have one opening or two. In a system with two openings, food enters through the mouth and is digested and absorbed; undigested material leaves through the second opening, the anus.
6. The length of the digestive tract, number of stomachs, and presence of a cecum are adaptations to particular diets. Plant matter is the hardest to digest.
7. In humans, digestion begins in the mouth, where teeth break food into smaller pieces. Salivary glands produce saliva, which moistens food and begins starch digestion. Swallowed food moves through the esophagus to the stomach. Waves of contraction called peristalsis move food along the digestive tract.
8. The stomach stores food, churns it until it liquefies into chyme, and mixes it with gastric juice. Hydrochloric acid in the gastric juice activates pepsinogen, forming the protein-splitting enzyme pepsin to begin protein digestion.
9. In the small intestine, trypsin and chymotrypsin break polypeptides into peptides, and peptidases break these down further. Pancreatic amylase and carbohydrases continue carbohydrate digestion. Nucleases and nucleosidases break down nucleic acids. The small intestine is the main site of nutrient absorption.
10. Material remaining after absorption in the small intestine passes to the colon, which absorbs water, minerals, and salts, leaving feces. Many bacteria digest remaining nutrients and produce useful vitamins that are then absorbed. Feces exit the body through the anus.
11. The pancreas, liver, and gallbladder aid digestion. The pancreas supplies pancreatic amylase, trypsin, chymotrypsin, lipases, and nucleases. The liver supplies bile, which emulsifies fat, and the gallbladder stores bile. The nervous and endocrine systems regulate digestive secretion.
12. Nutrients promote growth, maintenance, and repair of body tissues. Macronutrients, or energy nutrients, include carbohydrates, proteins, and fats. Micronutrients include vitamins and minerals. Water is also vital.