Digestion and Absorption of the Major Organic Nutrients

1. Carbohydrates

①The mouth

When we chew some food manufactured by carbohydrates, we use mechanical digestion to break them into small pieces and mix it with saliva（include salivary amylase）, produced by several salivary glands in the oral cavity.

The action of the enzyme salivary amylasebreaks the long glucose chains of starch into shorter chains, some as small as maltose. Some changed into oligosaccharides (containing 3 to 10 glucose molecules).

The other carbohydrates don't undergo any enzymatic digestion in the mouth, such as sucrose and lactose.

②The stomach

The carbohydrates travel through our esophagus to the stomach. The low pH in the stomach inactivates salivary amylase, so it no longer works once it arrives at the stomach. Although there's more mechanical digestion in the stomach, there's little chemical digestion of carbohydrates here.

③The small intestine

The food(chyme) then goes from the stomach into the first part of the small intestine(duodenum, in lumen). This causes the pancreas to release pancreatic amylase. (Pancreatic amylase is secreted from the pancreas into the small intestine, and like salivary amylase, it breaks starch down to small oligosaccharides and maltose.) This enzyme breaks down the chyme(especially the oligosaccharides) into dextrin and maltose.

From there, the wall of the small intestine begins to make lactase, sucrase, dextrinase, glucoamylase, andmaltase. These enzymes break down the original carbohydrates even further into monosaccharides.

Maltoseis digested by maltase, forming 2 glucose molecules. Lactose is digested by lactase, forming glucose and galactose. Sucrose is digested by sucrase, forming glucose and fructose. Oligosaccharides are digested by dextrinase, glucoamylase, forming glucose etc.

④Circulation

By the end of this process of enzymatic digestion, we're left with three monosaccharides: glucose, fructose, and galactose. These can now be absorbed across the enterocytes of the small intestine(epithelium) and into the bloodstream (blood capillary) to be transported to the other part like the liver.

2.Proteins

①The mouth

The protein food pieces mashed by teeth enter the stomach through the esophageal sphincter.

②The stomach

The stomach releases gastric juices containing hydrochloric acid and the enzyme, pepsin, which initiate the breakdown of the protein.

Pepsin, which is secreted by the cells that line the stomach, dismantles the protein chains into small peptides (smaller and smaller fragments).

③The small intestine