The Role of Feedback Mechanisms (College Board AP Biology)

• The process of maintaining a constant internal environment is known as homeostasis
• Homeostasis relies on monitoring information received back from the body, known as feedback
• Acting on feedback information ensures that conditions inside the organism are kept within preset limits
• Homeostasis is critically important for organisms as it ensures the maintenance of optimal conditions for enzyme action and cell function
• Sensory cells can detect information about the conditions inside and outside the organism; if conditions have changed then the organism can respond to keep conditions constant
• Examples of physiological factors that are controlled by homeostatic feedback in mammals include
  • Core body temperature
  • Blood pH
  • Concentration of glucose in the blood
  • Osmotic concentration of the blood
• Feedback (positive) can also act to bring about a specific event; Examples include:
  • Ovulation
  • Labor in childbirth
  • Lactation in mammals
  • Ripening of fruit

• The majority of homeostatic control mechanisms in organisms use negative feedback loops to achieve homeostasis
• Negative feedback mechanisms work to return values to a set point; they reverse the effects of any change within a system
  • Negative feedback loops are essential for maintaining conditions within set limits; this is not the case in positive feedback mechanisms which instead amplify any change
• Negative feedback control loops involve:
  • A receptor – receptor cells detect change in a physiological factor
  • A coordination system – the brain and nervous system transfer information between different parts of the body
  • An effector – the muscles and glands bring about 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

A Generalized Feedback Loop Flowchart

An example of a generalized (negative) feedback loop

Regulation of Blood Glucose

• It is essential that blood glucose concentration is kept within narrow limits
  • Glucose is an essential fuel for respiration, so it is important that blood glucose levels do not drop too low
  • Glucose is soluble, so blood glucose concentration affects the osmotic balance between the cells and the blood
• The control of blood glucose concentration is a key part of homeostasis
• Blood glucose concentration is controlled by two hormones which are secreted into the blood by specialized tissue in the pancreas
• This tissue is made up of groups of cells known as the islets of Langerhans
  • The islets of Langerhans contain two cell types:
    • α cells that secrete the hormone glucagon
    • β cells that secrete the hormone insulin
  • These α and β cells are involved with monitoring and responding to blood glucose levels

Insulin Secretion by the Pancreas Diagram

The islets of Langerhans form the endocrine tissue of the pancreas, while the exocrine tissue is involved with the production of digestive enzymes

The Effects of Insulin

• Blood glucose concentration increases after a meal that contains carbohydrate
• This increase in blood glucose is detected by the β cells in the pancreas, which synthesize and secrete insulin
• Insulin is transported in the blood to target cells all over the body
  • Insulin's main target cells are in the liver and muscles
• The effects of insulin include: 
  • Glucose channels in cell surface membranes open, and glucose moves out of the blood and into the body cells by facilitated diffusion
  • Liver and muscle cells convert excess glucose into glycogen to be stored; this is glycogenesis
  • An increase in the rate of respiration, using up glucose
  • Conversion of glucose to fatty acids, resulting in fat storage
• Insulin lowers blood glucose concentration

 The Effects of Glucagon

• Glucagon is synthesized and secreted by α cells when blood glucose falls
  • Blood glucose could fall after a period of fasting, or after exercise
• Glucagon is transported in the blood to target cells
• The effects of glucagon include:
• • The activation of enzymes that enable the hydrolysis of glycogen in liver and muscle cells, releasing glucose that enters the blood; this is glycogenolysis
  • A decrease in the rate of respiration
  • Amino acids are converted to glucose; this is gluconeogenesis
• Glucagon increases blood glucose concentration

Regulation of Blood Glucose Diagram

Blood glucose is regulated by insulin and glucagon

Positive Feedback in Fruit Ripening

• The production of ethylene in fruits is an example of a positive feedback loop
  • 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 generally culminates in a singular event
• Ethylene (named ethene by International Union of Pure and Applied Chemistry, IUPAC) is a gas produced by fruit during the later stages of fruit ripening
• The gas can diffuse from one fruit to neighboring fruit which triggers further release of ethylene
• The effect is that all fruit ripens at the same time

Ethylene Positive Feedback Loop Diagram

The production of ethylene is an example of a positive feedback loop

Positive Feedback in Labor

• Oxytocin (secreted in the pituitary gland) stimulates contractions of the muscles in the myometrium
• Oxytocin is released by the pituitary gland in the brain
• 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 slowly and rhythmically
• The process leads to the singular event of childbirth

Positive Feedback in Labor & Childbirth Diagram

The positive feedback loop stimulates the release of oxytocin and causes the contraction of the uterus wall