From Cathy Wong,
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Mouth

Digestion of food begins in the mouth. Chewing breaks food down into smaller particles so it can be digested. If people do not chew food thoroughly because they eat quickly or have tooth problems, they increase the burden on the digestive organs. Saliva contains the digestive enzyme amylase, which begins breaking down starchy foods as soon as they enter the mouth.

Stomach

The stomach mechanically mixes food. It also releases substances that chemically break down food, such as hydrochloric acid. Hydrochloric acid, often called stomach acid, plays a key role in the stomach. It helps digest proteins, fat, vitamins, and minerals, maintains the acidity of the stomach, and helps kill bacteria, viruses, and parasites.

Small Intestine

Enzymes from the pancreas and small intestine are released into the small intestine to digest and absorb carbohydrates, fat, and protein. In addition, bile salts secreted from the gallbladder help with the digestion and absorption of fats and the fat soluble nutrients vitamin A, D, E, and K.

The small intestine is the primary organ involved in the absorption of nutrients. Anything that interferes with the secretion of enzymes or bile salts, or disrupts the absorptive walls of the small intestine, such as inflammatory bowel syndrome, chronic antacid use, chronic diarrhea, pancreatic insufficiency, or celiac disease, may result in vitamin deficiencies and fat malabsorption.

Large Intestine

By the time food reaches the colon, most of the nutrients have already been absorbed, leaving indigestible fiber and water. The large intestine absorbs water, electrolytes, and a few vitamins. The length of time taken for food to pass through the colon largely depends on fiber intake. Mucus is secreted to protect the cells lining the colon from physical trauma and bacterial toxins.