AQA A Level Biology复习笔记6.4.2 Negative Feedback

Negative Feedback


  • The majority of homeostatic control mechanisms in organisms use negative feedback to maintain homeostatic balance (ie. to keep certain physiological factors, such as blood glucose concentration, within certain limits)
  • Negative feedback control loops involve:
    • A receptor (or sensor) – to detect a stimulus that is involved with a condition / physiological factor
    • A coordination system (nervous system and endocrine system) – to transfer information between different parts of the body
    • An effector (muscles and glands) – to carry out a response


  • Outcome of a negative feedback loop:
    • The factor/stimulus is continuously monitored
    • If there is an increase in the factor, the body responds to make the factor decrease
    • If there is a decrease in the factor, the body responds to make the factor increase
    • The system is restored to its original level



A negative feedback control loop


Exam Tip

Negative feedback could come up as an essay question in the exam so make sure you know several examples and can explain how each work!

Control of Negative Feedback

  • Negative feedback loops help maintain a normal range or balance within an organism
    • They reduce the initial effect of the stimulus


  •  Receptors detect any deviations from the normal range (stimuli) which results in a corrective mechanism to return the factor back to its normal range
  • In a negative feedback loop there are usually two corrective mechanisms:
    • One for when the factor becomes too low
    • One for when the factor becomes too high


  • The corrective mechanisms may involve the nervous system or the endocrine system
  • The magnitude of the correction required to bring a factor back within its normal range is monitored and regulated by negative feedback
    • As the factor gets closer to its normal value the level of correction reduces



Two corrective mechanisms are involved in the negative feedback loop


Interpreting Information About Feedback Loops

  • In the exam you may be asked to identify whether a particular system is a negative or positive feedback loop
  • Negative feedback loops reduce the effect of the original stimulus to keep a factor within a normal range
    • There are two corrective mechanisms
    • One mechanism causes the factor to decrease
    • Another mechanism causes the factor to increase


  • In positive feedback loops, the original stimulus produces a response that causes the factor to deviate even more from the normal range
    • They enhance the effect of the original stimulus


Positive feedback loop in bone repair

  • The repair of broken bones is carried out via a positive feedback loop involving special cells called osteoblasts and osteoclasts
  • The osteoblasts secrete a hormone called osteocalcin
    • Osteocalcin is a protein


  • They secrete the osteocalcin in an inactive form
  • The osteoclasts secrete acid which lowers the pH and the acidic conditions cause the inactive form of the protein osteocalcin to change into the active form of osteocalcin
    • The low pH alters the hydrogen and ionic bonds in the protein which changes the tertiary structure


  • The active form of osteocalcin binds to a receptor on beta (β) cells in the pancreas which stimulates them to release insulin
  • Osteoblast cells possess insulin receptors which when stimulated causes them to release more inactive osteocalcin
  • The osteoblast cells enhance the effect of the original stimulus (insulin) - positive feedback



A positive feedback loop