Explain the difference between a positive and negative feedback loop. Describe at least one example of each.

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selmasharafaz | College Teacher | (Level 3) Adjunct Educator

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The homeostatic and physiological processes in living beings are controlled by feedback mechanisms.

In a feedback loop, the product of a biochemical reaction, regulates the reaction itself. The rate influencing product may be the immediate product of a reaction or product formed after subsequent reactions.

The two types of feedback loops are positive and negative feedback loops.

  1. Positive feedback loop - In this type of feedback regulation, the increased synthesis of a product acts as a stimuli to amplify the response. This results in a cascade of reactions, that causes significant changes.
    An example of positive feeback loop regulation is the accumulation of blood platelets at wound sites for blood clotting. Tissue factors released from platelets is the initiating stimuli for activation of various coagulation factors. Many factors in the cascade act as positive regulators of previous reactions thus resulting in an amplified response with a small amount of initial stimuli. (see the attached image for coagulation pathway)

  2. Negative feedback loop - In this type of feedback regulation, the product of a reaction pathway acts as an inhibitor of one of the initial reactions of the pathway. Thus, the response of a reaction pathway is not extended beyond a threshold value and gets arrested by the final product which is known as the effector.
    An example of this kind of regulation is the regulation of homeostasis. ie., controlling blood sugar level, regulation of body heat, production of hormones etc.
    For eg., in case of glucocorticoid hormones production from the adrenal cortex, once their level in blood goes higher than the required concentration, the hormone itself inhibits the production of ACTH from the Pituitary gland and CRH from Hypothalamus. Ultimately, there will be no ACTH to stimulate the adrenal gland to produce the corticoids.
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