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All digestive fluid of the human body contain enzymes that degrade food into simple soluble substances. These substances called digestive enzymes can be divided into three types: amylase, protease and lipase.

Amylase promotes the degradation of carbohydrates, and the most complex carbohydrate is a polysaccharide. Each polysaccharide is composed of monosaccharide molecular chains, and most of the carbohydrates absorbed by the human body are polysaccharides, such as starch and glycogen. Starch is a plant-derived polysaccharide, glycogen or animal starch, a complex polysaccharide stored in animal liver or muscle. In the degradation of polysaccharides, the compounds sucrose and lactose composed of two monosaccharide molecules in the intermediate state are first formed. Sucrose is found in sugar beets and sugar cane, and lactose is found in milk. Monosaccharides are the simplest sugars, all due to the degradation of polysaccharides. Although some of the fructose absorbed by the body comes from fruit juice, some come from the degradation of sucrose. Glucose is the final result of all sugars, and both fructose and galactose are converted into glucose in the liver.

Proteases attack peptide bonds and promote the degradation of proteins into amino acids. Most protein molecules contain hundreds of amino acids, which are connected by substances called peptide bonds, called peptides. The dipeptide is a chain formed by two amino acid molecules and is an intermediate state formed by the degradation of a polypeptide.

The digestive system is the oral cavity first, its digestive juice is saliva, and the digestive enzyme is salivary amylase, which degrades the starch and glycogen of sugars to produce a shorter polysaccharide-dextrin.

Then the food enters the stomach. The digestive juice in the stomach is gastric juice, which is produced by the gastric glands of the gastric mucosa and secreted into the stomach. Digestive juice includes digestive enzymes and other components, including pepsin, hypertensive proteinogenase (existing only in young people), hydrochloric acid, and gastric lipase (mainly present in young people).

The role of gastric digestive juice is as follows: pepsin starts to degrade proteins (polypeptides). The hypertensive proteinogen enzyme interacts with calcium to form a viscous milky, that is, it interacts with protein (casein). Hydrochloric acid activates pepsin, which becomes thick and milky in adults and kills bacteria. Stomach lipase starts to degrade fat molecules in milk. The result is the formation of shorter peptide bonds, viscous milky solids and intermediate compounds.

Pancreatic juice is produced by the pancreas and secreted into the duodenum in the small intestine. Its digestive enzymes are trypsin, chymotrypsin, carboxypeptidase, pancreatic amylase, and pancrelipase. The first three enzymes continuously degrade proteins, from long peptides to short peptides. Pancreatic amylase continuously degrades sugars. Pancrelipase degrades fat into particles. The resulting products are dipeptides and certain amino acids, maltose (disaccharides), glycerol and fatty acids.

Bile is produced by the liver stored in the gallbladder and secreted into the duodenum in the small intestine. Its components are bile salts and bile acid. Break down fat (and intermediate compounds) into smaller particles, this process is called emulsification.

Intestinal juice is produced by intestinal glands in the lining of the small intestine. Eventually secreted into the small intestine. Its digestive enzymes include maltase, sucrase, lactase, and intestinal juice promoting hormone. They function to degrade maltose, degrade sucrose, degrade lactose, and complete protein degradation. The resulting products are glucose or dextrose, glucose and fructose, glucose and galactose, and amino acids.

The small intestine is the main place to absorb nutrients. All glucose, amino acids, glycerol, fatty acids and part of water, inorganic salts and vitamins are absorbed in the small intestine. After the blood flows into the wall of the small intestine, due to physiological activities, oxygen decreases and the absorbed nutrients increase to supply nutrients to the various organs of the body.