Final answer:
The liver does not produce an alkaline mucus for the protection of the digestive tract, unlike the gastric glands, pancreas, and duodenal glands, which do produce protective alkaline secretions.
Step-by-step explanation:
The structures that participate in the production of an alkaline mucus that protects cells of the digestive tract and promotes optimal enzyme function include the gastric glands, which produce mucus to protect the lining of the stomach, as well as the pancreas and the glands in the submucosa of the duodenum, such as Brunner's glands that secrete a bicarbonate-rich alkaline mucus. The salivary glands begin chemical digestion in the mouth but do not specifically produce an alkaline mucus for protection of the digestive tract. The liver produces bile that emulsifies fats but is not responsible for the secretion of protective alkaline mucus. Therefore, the correct answer is that the liver does not participate in the production of an alkaline mucus for protective purposes in the context provided.