Negative Feedback (AQA A Level Biology): Revision Note
Exam code: 7402
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:
a receptor detects a stimulus, i.e. a change from normal levels
signals are sent to a coordination system, e.g. in:
the nervous system
the hormone system
signals are sent to an effector to carry out a response that reverses the change
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
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 in order to restore 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.
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 oesteoblasts 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]
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