A-Level Biology October/November 2024 Q10(a): Blood glucose concentration is maintained around a set point by homeostasis. Explain th…

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  Homeostasis is the maintenance of a constant internal environment. The principles involve a negative feedback loop.

  Any change or deviation in an internal factor from its set point acts as a stimulus. This change is detected by specialised receptors, which then send information to a central coordinator, such as the brain or spinal cord (CNS). The coordinator processes this information and sends signals, via nerve impulses or hormones, to specific effectors. Effectors, which are typically muscles or glands, carry out a corrective response. This response counteracts the initial change, returning the factor to its normal set point. This mechanism, where the response opposes the stimulus, is known as negative feedback.

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  How the marks are awarded

  • B1 — Stating that a change in a factor (stimulus) is detected by a receptor. The answer states: 'This change is detected by specialised receptors...'
  • B1 — Referencing a central coordinator, such as the CNS or brain. The answer states: '...send information to a central coordinator, such as the brain or spinal cord (CNS).'
  • B1 — Describing the transmission of signals from the coordinator to an effector, specifying the method. The answer states: '...sends signals, via nerve impulses or hormones, to specific effectors.'
  • B1 — Stating that the mechanism is negative feedback and/or that it returns the factor to a set point. The answer states: 'This response counteracts the initial change, returning the factor to its normal set point. This mechanism... is known as negative feedback.'

  Common mistakes

  • Only providing a specific example (e.g., blood glucose regulation with insulin/glucagon) without explaining the general principles of receptor, coordinator, and effector.
  • Confusing negative feedback with positive feedback, or omitting the term 'negative feedback' entirely.
  • Using vague language like 'messages are sent' instead of specifying 'nerve impulses' or 'hormones'.
  • Forgetting to mention that the purpose of the response is to return the internal condition to a 'set point' or 'norm'.

  Examiner tip: For questions about physiological processes, always learn the sequence of events as a flowchart: Stimulus → Receptor → Coordinator → Effector → Response.

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