(a) Define hormone.

(b) "Hormones should be secreted in precise quantities. We have a feedback mechanism through which this is done." With the help of an example justify the statement.

This question directly draws on section 6.3 (Hormones in Animals) of the chapter.

(a) Definition: The chapter describes hormones as chemical compounds released by stimulated cells (in animals, secreted by endocrine glands directly into the blood) that diffuse or are carried to other cells/target organs, where they are detected by special molecules and bring about a response — 'Hormones produced in one part of an organism move to another part to achieve the desired effect.' This is chemical communication, as opposed to the electrical impulses used by the nervous system.

(b) Justification with example: The chapter states, 'If it is so important that hormones should be secreted in precise quantities, we need a mechanism through which this is done. The timing and amount of hormone released are regulated by feedback mechanisms.' It gives the concrete example: 'if the sugar levels in blood rise, they are detected by the cells of the pancreas which respond by producing more insulin. As the blood sugar level falls, insulin secretion is reduced.'

This shows a self-regulating loop: rise in blood sugar (stimulus) $\rightarrow$ pancreas detects it $\rightarrow$ more insulin secreted (response) $\rightarrow$ blood sugar falls $\rightarrow$ less insulin secreted. The output of the system (blood sugar level) feeds back to control the input (rate of insulin secretion), ensuring the hormone is neither over- nor under-secreted. The chapter also notes the danger of this mechanism failing, as in diabetes, where insufficient insulin secretion causes the sugar level in the blood to rise, causing many harmful effects — underscoring why precise, feedback-regulated secretion is essential.