In the human digestive system, the saliva secreted into the mouth contains which enzyme that begins the chemical digestion of starch into simpler sugars?

Introduction / Context:
Digestion of food begins in the mouth, not only through chewing but also through the action of enzymes present in saliva. This question focuses on the specific enzyme in human saliva that acts on starch. Knowing that salivary amylase, also known as ptyalin, starts the chemical breakdown of starch into maltose and other simple sugars is a fundamental concept in human physiology and nutrition.

Given Data / Assumptions:
- Saliva is secreted by salivary glands into the oral cavity.- The options list several digestive enzymes: pepsin, salivary amylase, rennin, trypsin and lipase.- The question asks which enzyme is actually present in human saliva.- Basic knowledge of where each enzyme acts is assumed.

Concept / Approach:
Salivary amylase is produced by salivary glands and released into the mouth with saliva. It hydrolyses starch, a complex carbohydrate, into maltose and shorter polysaccharides. Pepsin is secreted in the stomach and digests proteins. Rennin is important mainly in the stomach of some young mammals for milk protein coagulation. Trypsin comes from the pancreas and acts in the small intestine to further digest proteins. Lipase digests fats and acts mainly in the small intestine. Therefore, the enzyme specifically found in human saliva is salivary amylase, also called ptyalin.

Step-by-Step Solution:
Step 1: Recall that chemical digestion of carbohydrates starts in the mouth, not in the stomach.Step 2: Remember that the enzyme responsible for this initial starch digestion is salivary amylase.Step 3: Recognise that pepsin and rennin are gastric enzymes and trypsin and lipase act in the intestine, not in saliva.Step 4: Examine the options to find the enzyme that matches the description of a salivary enzyme acting on starch.Step 5: Select option B, salivary amylase, also called ptyalin, as the correct answer.

Verification / Alternative check:
Demonstration experiments in school laboratories often show starch solution turning blue with iodine, then losing the blue colour when saliva is added, indicating starch breakdown by salivary amylase.Textbooks specify that this enzyme works best at near neutral pH found in the mouth and becomes inactive in the acidic environment of the stomach.These observations confirm that salivary amylase is the digestive enzyme present in saliva, not pepsin or trypsin.

Why Other Options Are Wrong:
Option A is wrong because pepsin is secreted by the gastric glands of the stomach and digests proteins, not present in saliva.Option C is wrong because rennin is mainly found in the stomach of young mammals and is not a major human adult salivary enzyme.Option D is wrong because trypsin is secreted by the pancreas into the small intestine and also digests proteins.Option E is wrong because lipase digests fats and is mostly active in the intestinal lumen, not in the mouth.

Common Pitfalls:
Some learners confuse the locations where different enzymes act and may wrongly associate pepsin with the mouth simply because it is one of the first digestive enzymes they learn.Another pitfall is to think that all digestion starts in the stomach and to overlook the role of saliva.Remember that salivary amylase is the enzyme in saliva that begins starch digestion, making it the correct answer here.

Final Answer:
The saliva in human beings contains the enzyme salivary amylase, also called ptyalin, which begins the digestion of starch.