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Which of the following pairs of chemical substances is correctly categorized?
A. Pepsin and prolactin – two digestive enzymes secreted in the stomach
B. Troponin and myosin – complex proteins in striated muscles
C. Secretin and rhodopsin – polypeptide hormones
D. Calcitonin and thymosin – protease enzyme

Last updated date: 13th Jun 2024
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Hint: Digestive enzymes are those enzymes that are released in the digestive system and help in the breakdown of complex substances into simpler ones. Striated muscles are muscle tissue having actin and myosin fibers that form cardiac and skeletal muscle. Polypeptide hormones are a class of hormones that have endocrine effects. Protease enzymes are employed in the breakdown of the proteins.

Complete answer: The body is a complex system of various physiological activities that are regulated by different metabolic processes guided by chemical substances called enzymes or hormones.
Now, let us first understand each given term and then relate them with correct pairing.
-The digestive enzymes are used to break down complex substances in the process of digestion. Pepsin is a digestive enzyme that helps in the digestion of proteins by breaking complex protein molecules into simple peptides. It is released in the stomach during the digestion process. Prolactin on the other hand is a hormone that stimulates milk secretion from mammary glands. Also, it helps in breast development in females. It is not released in the stomach. So, this match is wrong.
-The striated muscles are formed of intercrossed fibers of actin and myosin that help to make cardiac and muscle fibers. Troponin is a protein that is found in actin filaments. Myosin is also a protein that is found in myosin filaments. Both Troponin and myosin are complex proteins of striated muscles. So, this match is right.
-Polypeptide hormones are chemical messengers that stimulate the production of endocrine hormones. Secretin is a peptide that helps in balancing $pH$ of digestive juices during digestion in the duodenum. It secretes bicarbonate to balance acidic conditions. It is not a polypeptide hormone. Rhodopsin is not a hormone. It is a pigment present in the photoreceptor cells of the retina.
-Protease enzymes are protein-digesting enzymes. Calcitonin is a peptide hormone secreted by the thyroid gland. It results in reducing blood calcium levels. Thymosin is also a hormone that is produced by the thymus. It stimulates the development of T cells. So, both the calcitonin and thymosin are not protease enzymes.
Therefore, we can conclude from the above discussion that the right answer is option B.

Note: The troponin is a complex regulatory protein. It is used to regulate non-smooth muscle contraction in cardiac muscle. The troponin levels increase during the myocardial injuries. Thus, it is part of the contractile mechanisms of cardiac muscle. Myosin is also a functioning contractile filament regulatory protein. It is involved in the motion of other types of cells.