Positive & Negative Feedback (College Board AP® Biology): Study Guide

Feedback mechanisms

• Living organisms need to detect and respond to changes in their external and internal environment

  • This allows maintenance of constant internal conditions within suitable limits, also known as homeostasis

• The mechanisms by which organisms detect and respond to change are known as feedback mechanisms; these mechanisms involve:

  • receiving information about the environment

  • initiating an internal mechanism to allow an appropriate response to the change

• Feedback mechanisms can be either:

  • negative mechanisms that act to reverse the effects of change

  • positive mechanisms that act to amplify the effects of change

Negative feedback

• Negative feedback mechanisms allow organisms to maintain a constant internal environment by regulating physiological processes

• When any deviation from normal levels occurs, negative feedback mechanisms act to return systems to a target set point; this is achieved by reversing the direction of change, e.g.:

  • if body temperature goes up, negative feedback mechanisms activate processes that bring temperature back down

  • if body water content decreases, negative feedback mechanisms activate processes that increase water retention

• Negative feedback processes operate at the molecular and cellular levels

Example: blood sugar regulation

• Regulation of blood sugar concentration is an example of a negative feedback control mechanism

• Blood sugar levels are regulated by the hormones insulin and glucagon

  • A rise in blood glucose causes insulin to be released by the pancreas; insulin binds to receptors on target cells, resulting in, e.g.:

    • an increase in the uptake of glucose from the blood

    • conversion of glucose into glycogen

    • increased glucose metabolism

  • A fall in blood glucose causes glucagon to be released by the pancreas; glucagon binds to receptors on target cells, resulting in, e.g.:

    • conversion of glycogen to glucose

    • conversion of other biological molecules into glucose

Positive feedback

• Positive feedback mechanisms amplify change in living organisms as follows:

  • a change occurs during which levels move away from a set point

  • this change initiates further change, resulting in levels moving further from the set point

  • this process repeats

Example: fruit ripening

• Ethylene is a gas produced by fruit during the later stages of fruit ripening

• The gas can diffuse from one fruit to neighboring fruits, where it triggers further release of ethylene

• The ripening of one fruit therefore encourages the ripening of others

Example: labor in childbirth

• The hormone oxytocin is released by the pituitary gland in the brain

• Oxytocin stimulates contractions of the muscles in the uterus

• Stretch receptors in the cervix detect the contractions and signal the pituitary gland to increase oxytocin secretion

• More oxytocin creates further contractions, which in turn signal for further release of oxytocin in this positive feedback loop

• This process increases the contractions and results in childbirth

Example: lactation in mammals

• When a young mammal feeds from a mammary gland the suckling action results in production of the hormone prolactin

• Prolactin leads to milk production, which results in increased suckling action, etc.