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  1. 1-biological-molecules

    1-1-biological-molecules-carbohydrates
    11 主题
  2. 1-2-biological-molecules-lipids
    3 主题
  3. 1-3-biological-molecules-proteins
    5 主题
  4. 1-4-proteins-enzymes
    12 主题
  5. 1-5-nucleic-acids-structure-and-dna-replication
    8 主题
  6. 1-6-atp-water-and-inorganic-ions
    4 主题
  7. 2-cell-structure
    2-1-cell-structure
    7 主题
  8. 2-2-the-microscope-in-cell-studies
    4 主题
  9. 2-3-cell-division-in-eukaryotic-and-prokaryotic-cells
    8 主题
  10. 2-4-cell-membranes-and-transport
    7 主题
  11. 2-5-cell-recognition-and-the-immune-system
    7 主题
  12. 2-6-vaccines-disease-and-monoclonal-antibodies
    6 主题
  13. 3-exchange-and-transport
    3-1-adaptations-for-gas-exchange
    6 主题
  14. 3-2-human-gas-exchange
    10 主题
  15. 3-3-digestion-and-absorption
    5 主题
  16. 3-4-mass-transport-in-animals
    6 主题
  17. 3-5-the-circulatory-system-in-animals
    8 主题
  18. 3-6-mass-transport-in-plants
    6 主题
  19. 4-genetics-variation-and-interdependence
    4-1-dna-genes-and-chromosomes
    7 主题
  20. 4-2-dna-and-protein-synthesis
    6 主题
  21. 4-3-genetic-diversity-mutations-and-meiosis
    7 主题
  22. 4-4-genetic-diversity-and-adaptation
    6 主题
  23. 4-5-species-and-taxonomy
    4 主题
  24. 4-6-biodiversity
    9 主题
  25. 5-energy-transfers-in-and-between-organisms-a-level-only
    5-1-photosynthesis-a-level-only
    5 主题
  26. 5-2-respiration-a-level-only
    7 主题
  27. 5-3-energy-and-ecosystems-a-level-only
    9 主题
  28. 5-4-nutrient-cycles-a-level-only
    4 主题
  29. 6-organisms-respond-to-changes-in-their-environments-a-level-only
    6-1-response-to-stimuli-a-level-only
    12 主题
  30. 6-2-nervous-coordination-a-level-only
    10 主题
  31. 6-3-skeletal-muscles-a-level-only
    6 主题
  32. 6-4-homeostasis-a-level-only
    11 主题
  33. 7-genetics-populations-evolution-and-ecosystems-a-level-only
    7-1-inheritance-a-level-only
    6 主题
  34. 7-2-populations-a-level-only
    3 主题
  35. 7-3-evolution-a-level-only
    5 主题
  36. 7-4-populations-in-ecosystems-a-level-only
    7 主题
  37. 8-the-control-of-gene-expression-a-level-only
    8-1-genetic-mutations-a-level-only
    2 主题
  38. 8-2-regulation-of-gene-expression-a-level-only
    11 主题
  39. 8-3-using-genome-projects-a-level-only
    4 主题
  40. 8-4-gene-technologies-a-level-only
    13 主题
  41. exam-guidance-and-skills
    essay-guidance
    3 主题
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Negative feedback

  • Most homeostatic control mechanisms use negative feedback to maintain internal conditions

  • Negative feedback can be described as:

a system in which any change is restored to original levels

  • Negative feedback loops involve:

    1. a receptor detects a stimulus, i.e. a change from normal levels

    2. signals are sent to a coordination system, e.g. in:

      • the nervous system

      • the hormone system

    3. signals are sent to an effector to carry out a response that reverses the change

Flowchart illustrating a homeostasis feedback loop: conditions change, detected, mechanisms activated, conditions return, mechanisms switched off.
Physiological control systems ensure that internal conditions within the body do not stray beyond the narrow limits required for survival

Examiner Tips and Tricks

While you may be very familiar with the concept of homeostasis from GCSE, it is essential that you use A-level language when describing it in exams, e.g.:

  • instead of ‘sensors detect change’ you could say that receptors detect a stimulus

  • instead of ‘messages are sent to the brain’ you could say that the brain acts as a coordinator

Negative feedback mechanisms

  • Changes to the internal conditions of an organism can occur in either direction, meaning that negative feedback systems have more than one corrective mechanism:

    • one for when the factor becomes too low

    • one for when the factor becomes too high

  • This increases the level of control that can be achieved by negative feedback

Flowchart illustrating homeostasis with corrective mechanisms A and B for factors outside the normal range, returning them to equilibrium.
Having two corrective mechanisms allows negative feedback systems to reverse changes in either direction

Interpreting information on negative & positive feedback

  • Exams may contain questions that provide information about negative or positive feedback systems

    • Negative feedback reduces the effect of the original stimulus to restore the original levels

    • Positive feedback enhances the effect of the original stimulus and causes conditions to deviate further from a normal range

Worked Example

The repair of broken bones is carried out by cells called osteoblasts and osteoclasts. The process by which these two cells work together is shown in the image below.

Positive feedback bone repair, downloadable AS & A Level Biology revision notes

Identify the type of feedback mechanism represented in the image and explain your answer.

[3]

Answer:

The type of feedback mechanism shown is:

  • Positive feedback; [1 mark]

We know this because:

  • Insulin stimulates osteoblasts to secrete osteocalcin AND (when activated) osteocalcin stimulates β cells to secrete more insulin; [1 mark]

  • The system will cause the secretion of osteocalcin to increase more and more / will enhance/amplify the change in osteocalcin secretion; [1 mark]