Hydrochloric acid: produced by parietal cells (cells in the stomach lining that make hydrochloric acid and intrinsic factor in response to nervous or hormonal stimulation), acidifies stomach content, as a result kills most bacteria in food. Pepsinogen: produced by gastric glands, an inactive, protein digesting enzyme. It is activated to pepsin by the acid in the stomach. Pepsin breaks protein into shorter chains of amino acid called polypeptides. The acidic environment in the stomach stops the function of salivary amylase. This is why the digestion of starch stops in the stomach. However digestion of protein begins in the stomach. A thick layer of mucus protects the protein of the stomach wall from acid and pepsin. Peptic ulcer is the erosion of the stomach wall or another region of the gi tract, which happens when this layer of mucus is penetrated and pepsin and acid is able to damage the underlying tissue.