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Summing up

Digestion

1- The mechanical digestion

In the mouth: The mechanical digestion has a role of reducing the size of the ingested food as
well as mixing it with saliva. Digestion begins in the mouth by the chewing action (mastication):
the incisors cut the foodstuffs, the canines tear them, the premolars and the molars crush and
grind them.
The chewed food impregnated with saliva passes towards the esophagus. In the esophagus, the
food progresses gradually to the stomach due to the peristaltic movements. The transfer of food
from the mouth to the esophagus is called the swallowing.

In the stomach: The muscles that form the gastric wall contract (peristaltic movement) and
create movements that mix the food with the gastric juice. Gradually the content of the stomach
passes to the small intestine.

In the small intestine: The small intestine exerts the same mechanical action. The food is mixed
to the different liquids secreted or released in the small intestine such as the bile, the pancreatic
juice and the intestinal juice. The food progresses through the small intestine because of the
peristaltic movement. In the large intestine, the wastes and cellulose progress due to the peristaltic
movements and are eventually eliminated through the anus.

2- The chemical digestion

The chemical digestion occurs at the same time with the mechanical one. The complex
carbohydrates, proteins and lipids that form our food are decomposed into simpler substances
called nutrients.

3- Action and properties of enzymes

The chemical transformation begins by the action of saliva: liquid secreted by the salivary glands.
Saliva contains an enzyme (chemical substance that activates and accelerates a chemical
reaction): salivary amylase. This substance transforms starch into simpler molecules of maltose
(two subunits of glucose).
An enzyme acts:
- In small doses
- At a temperature similar to that of the body
- The gastric enzyme act in an acidic pH (pH < 7)
- The intestinal and pancreatic enzymes are active in a basic pH (pH > 7)
- The enzymes are specific: pepsin acts only on proteins
- Enzymes are catalysts that accelerate the chemical reactions.

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