What chemical transformations take place in the stomach?
The stomach wall has integrated gastric glands that secrete gastric juices. The hydrochloric acid in these juices acidifies food and destroys some of the bacteria in the stomach. Gastric juices also contain numerous enzymes, which trigger chemical reactions. Pepsin, for example, breaks down proteins into small chains of amino acids called peptides.

The main role of gastric lipase is to attack complex lipids and transform them into simple lipids.

The stomach wall is lined with a gel-like substance called mucus. This gel protects the stomach against the acidity of the gastric juices, as without it the stomach could digest itself!

What mechanical transformations take place in the stomach?
The stomach wall can contract. This mechanical movement ensures that the bolus of food is mixed thoroughly with the gastric juices.

The bolus of food is transformed into a semi-fluid mass called chyme. This is highly acidic and is where some of the complex nutrients are transformed into simpler ones. The contents of the stomach are continuously emptied into the duodenum. In some cases, chyme can remain in the stomach for up to 3 or 4 hours.