How does the food we eat become energy and materials for our bodies? We will learn about the functions of the digestive system in animals.
Basic Knowledge
Digestion is the process of breaking down the large molecules in food into small molecules that can be absorbed by the body. Digestive enzymes act as catalysts.
Correspondence between the three major nutrients and digestive enzymes:
① Carbohydrates (starch) → glucose / enzyme: amylase (saliva and pancreatic juice)
② Proteins → amino acids / enzymes: pepsin (stomach), trypsin (pancreatic juice)
③ Fats → fatty acids and monoglycerides / enzyme: lipase (pancreatic juice), bile (produced by the liver; emulsifies fat; not an enzyme)
Digested nutrients are absorbed mainly through the villi of the small intestine (villi). Glucose and amino acids enter the capillaries, while fatty acids and monoglycerides are reassembled and enter the lymphatic vessels.
Digestive enzyme (a protein that acts as a catalyst to break down food)
Amylase (breaks down starch; contained in saliva and pancreatic juice)
Pepsin (breaks down proteins; contained in gastric juice; works in strongly acidic conditions)
Lipase (breaks down fats; contained in pancreatic juice)
Bile (made in the liver and stored in the gallbladder; emulsifies fats; not an enzyme)
Villus (a projection on the inner wall of the small intestine; absorbs nutrients)
Further Study (Background and Significance)
The digestive tract is one tube: mouth → esophagus → stomach → small intestine → large intestine → anus. Its accessory organs are the liver, gallbladder, and pancreas.
Gastric juice is strongly acidic, with a pH of 1〜2. It sterilizes food while activating pepsin. The stomach wall protects itself with mucus, but if mucus decreases because of stress or medicine, the stomach can be damaged and develop a stomach ulcer.
The small intestine is about 6〜7m long. Its inner wall has folds, villi, and microvilli, giving it a total surface area as large as one tennis court (about 200m²). This allows nutrients to be absorbed efficiently.
Water and some minerals are absorbed in the large intestine, and the remaining food waste is excreted as feces. About 1,000 species and 100 trillion bacteria (intestinal microbiota) live in the intestines and are involved in vitamin synthesis and immunity.
The optimum pH differs among digestive enzymes. Pepsin works best in the strongly acidic stomach (pH2), while trypsin and lipase work best in the weakly basic pancreatic juice (pH8).
- Different digestive enzymes act on each of the three major nutrients
- Starch→glucose (amylase)
- Protein→amino acids (pepsin and trypsin)
- Fat→fatty acids+monoglycerides (lipase)
- Bile emulsifies fats (it is not an enzyme)
- Site of absorption = villi of the small intestine
- Sugars and amino acids→capillaries/fats→lymphatic vessels
Points of Caution (Easily Confused)
① The correspondence between digestive enzymes and nutrients is essential to memorize. Amylase=starch, pepsin=protein, lipase=fat. ② Bile is not an enzyme. It only breaks fat into small droplets (emulsification). ③ Glucose and amino acids enter the capillaries, while fats enter the lymphatic vessels. Pay attention to the difference in pathways. ④ Villi are in the small intestine. The terms use different Chinese characters, but refer to the same structures.
Practice
- Name one digestive enzyme that breaks down starch.
- State the role of bile.
- What do fats absorbed in the small intestine enter to be transported?