Which of the following biological substances is a hormone and not a digestive enzyme?

Introduction / Context:
Enzymes and hormones are both important biological molecules, but they have very different roles. Enzymes act as catalysts for chemical reactions, while hormones act as chemical messengers that regulate physiological processes. In many exam questions, you are asked to distinguish between hormones and enzymes based on examples. This question checks whether you can identify which substance is a hormone rather than a digestive enzyme.

Given Data / Assumptions:

• The list includes oxytocin, trypsin, pepsin, ptyalin, and amylase.
• Trypsin, pepsin, ptyalin, and amylase are well known digestive enzymes.
• Oxytocin is commonly covered in endocrinology topics.
• The task is to choose the option that is not an enzyme.

Concept / Approach:
Digestive enzymes such as pepsin, trypsin, ptyalin, and amylase help break down food into smaller molecules. Pepsin works in the stomach on proteins, trypsin acts in the small intestine on polypeptides, ptyalin (salivary amylase) begins starch digestion in the mouth, and amylase in general breaks down starch to simpler sugars. Oxytocin, on the other hand, is a peptide hormone produced by the hypothalamus and released by the posterior pituitary. It plays important roles in childbirth by stimulating uterine contractions and in lactation by aiding milk ejection. Because it regulates physiological processes rather than catalysing digestion, oxytocin is a hormone and not an enzyme.

Step-by-Step Solution:
Step 1: Recall that enzymes are catalysts that speed up specific chemical reactions, often related to digestion. Step 2: Identify pepsin as a gastric enzyme that digests proteins in the stomach. Step 3: Recognise trypsin as a pancreatic enzyme that continues protein digestion in the small intestine. Step 4: Remember that ptyalin is salivary amylase, an enzyme that begins the breakdown of starch in the mouth. Step 5: Note that amylase in general is a starch digesting enzyme secreted by salivary glands and pancreas. Step 6: Understand that oxytocin is a hormone involved in uterine contractions and milk ejection, not a digestive catalyst. Step 7: Therefore, oxytocin is the only option that is not a digestive enzyme.

Verification / Alternative check:
Endocrinology chapters describe oxytocin as a hormone released from the posterior pituitary and explain its action on smooth muscles of the uterus and mammary glands. In contrast, chapters on digestion list pepsin, trypsin, ptyalin, and amylase among key digestive enzymes. No textbook lists oxytocin in enzyme tables, which confirms that it is not an enzyme.

Why Other Options Are Wrong:
Trypsin: A pancreatic proteolytic enzyme that digests proteins and polypeptides. Pepsin: A gastric enzyme in the stomach responsible for initial protein digestion. Ptyalin: Another name for salivary amylase, which digests starch in the mouth. Amylase: A carbohydrate digesting enzyme present in saliva and pancreatic juice.

Common Pitfalls:
Students sometimes assume that any biological substance ending in the suffix “in” is an enzyme, but many hormones also share this ending. It is safer to learn specific examples of hormones and enzymes rather than relying only on name endings. Oxytocin, insulin, and adrenaline are hormones, whereas pepsin, trypsin, and amylase are classic enzymes.

Final Answer:
The correct answer is Oxytocin.