2024 VJC H2 Bio P3 MS

1 2024 H2 Biology Prelim Paper 3 Answer 1 Thyroid hormones are produced and released by the thyroid gland. In humans, the thyroid gland is found in the neck below the Adam’s apple. It is lined with thyroid follicular cells, also called thyrocytes, that surround a lumen. Fig. 1.1 shows the thyroid follicular cells. Fig. 1.1 Unlike most hormones with specific target cells or tissues, the thyroid hormones act on nearly every cell in the body and are important in regulating the basal metabolic rate of the body. They regulate the metabolism of proteins, fat and carbohydrates in cells, affecting protein synthesis and cellular respiration. They are also involved in the control of heat generation in the body and are essential for proper cell development and differentiation. There are two types of thyroid hormones, namely triiodothyronine (T3) and thyroxine (T4). Their structures differ by the number of iodine present. Fig. 1.2 shows the structure of T3 and T4. Fig. 1.2 (a) With reference to Fig. 1.1 and 1.2, (i) suggest how thyroid hormones are able to act on nearly every cell in the body. [1]

2 • Thyroid hormones are secreted into the capillaries and carried throughout the body by the circulatory system / via the blood; (ii) compare the structure of thyroid hormones to an amino acid. [2] Similarity • Both thyroid hormones and amino acid have a central carbon bonded to a hydrogen atom, an amino group, a carboxyl group and a variable R group; (accept mention of just 1 of the groups) Difference • Thyroid hormones contain iodine atoms but not amino acids; (b) The release of thyroid hormones from the thyroid gland is regulated by other hormones released from the brain. Neurones in the hypothalamus in the brain release thyrotropin - releasing hormone (TRH), which stimulates the release of thyroid -stimulating hormone (TSH) from cells in the anterior pituitary. TSH then stimulates the release of T3 and T4 from the thyroid gland. Fig. 1.3 shows the relationship between different tissues in the control of the release of thyroid hormones. Fig. 1.3 Research was conducted to study the effect of TSH on the release of thyroid hormones from the thyroid gland. In this research, pluripotent stem cells were induced to differentiate into a culture of thyroid follicular cells. TSH was then introduced into the culture medium

3 of this cell culture, and the amount of T 3 and T 4 released into the culture medium was measured for a period of one week. The result of this experiment is shown in Fig. 1.4. Fig. 1.4 With reference to Fig. 1.4, (i) compare the changes in the levels of T3 and T4 in the cell culture medium during the week. [2] Similarity • The levels of both T3 and T4 decreased in general; Difference • The starting level of T3 was lower at 8.8 µg/mL than that of T3 at 2.8 mg/mL; • The level of T 3 decreases at a faster rate , from 8.8 µg/mL in day 1 to 1