A-Level Biology May/June 2025 Q7(a): Insulin is an example of a cell-signalling molecule of the endocrine system. Outline wh…

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  Insulin is a hormone, which is a chemical messenger used for cell signalling. It is part of the endocrine system as it is secreted by an endocrine gland, the β-cells in the islets of Langerhans within the pancreas, in response to a stimulus of high blood glucose concentration.

  As a hormone, insulin is transported throughout the body via the bloodstream to reach distant cells. It acts on specific target cells, such as liver and muscle cells, by binding to complementary glycoprotein receptors on their cell surface membranes. This binding triggers a specific response within the target cell, such as increasing the number of GLUT4 glucose transporter proteins in the membrane, which leads to increased uptake of glucose from the blood.

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  How the marks are awarded

  • M1 — The answer correctly identifies insulin as a hormone, which is a fundamental aspect of its role in the endocrine system.
  • M1 — The answer states that a stimulus (high blood glucose) causes insulin to be secreted from a named endocrine tissue (β-cells of the pancreas).
  • M1 — The answer correctly describes the mode of transport for an endocrine signal, stating that insulin travels in the blood.
  • M1 — The answer explains the mechanism of action on a target cell, which involves insulin binding to specific receptors on the cell surface.

  Common mistakes

  • Confusing endocrine with exocrine secretion, for example by stating insulin is released into a duct.
  • Being too vague, such as saying insulin 'travels around the body' instead of specifying 'in the blood', or 'affects cells' instead of 'binds to specific receptors'.
  • Confusing the roles of insulin and glucagon, for example stating that insulin is released in response to low blood glucose.
  • Incorrectly stating that insulin enters the target cell to cause an effect, rather than binding to a surface receptor to trigger a secondary messenger system.

  Examiner tip: For questions on cell signalling, always structure your answer by following the signal's journey: stimulus -> secreting gland -> transport medium -> target cell/receptor -> cellular response.

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