Negative Feedback (AQA A Level Biology)

• 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

• 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

• 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