1. AP Biology
Feedback Mechanisms

2. external environments.
Organisms also have feedback mechanisms that maintain dynamic homeostasis by allowing them to respond to changes in their internal and
external environments.
Negative feedback loops maintain optimal internal environments,
and positive feedback mechanisms amplify responses.
Animation
Bozeman video 14 min

3. • Operons in gene regulation • Temperature regulation in animals
Negative feedback mechanisms maintain dynamic homeostasis for a particular condition (variable) by regulating physiological processes, returning the changing condition back to its target set point.
• Operons in gene regulation
• Temperature regulation in animals
• Plant responses to water limitations
The trp Operon
Repressible operons are the opposite of inducible operons. Transcription occurs continuously and the repressor protein must be activated to stop transcription.
Tryptophan is an amino acid needed by E. coli and the genes that code for proteins that produce tryptophan are continuously transcribed as shown below.
If tryptophan is present in the environment, however, E. coli does not need to synthesize it and the tryptophan-synthesizing genes should be turned off. This occurs when tryptophan binds with the repressor protein, activating it. Unlike the repressor discussed with the lac operon, this repressor will not bind to the DNA unless it is activated by binding with tryptophan.. Tryptophan is therefore a corepressor.
The trp operon is an example of a repressible operon because the structural genes are active and are inactivated when tryptophan is present.

4. Plant responses to water limitations
Stomatal control of water losses has been identified as an early event in plant response to water deficit under field conditions, leading to a limitation of carbon uptake by the leaves
Stomata close in response to either a decline in leaf turgor and/or water potential

5. Positive feedback mechanisms amplify responses and processes in biological organisms. The variable initiating the response is moved farther away from the initial set-point. Amplification occurs when the stimulus is further activated which, in turn, initiates an additional response that produces system change.
• Lactation in mammals
• Onset of labor in childbirth
• Ripening of fruit

6. Lactation in mammals
Milk removal from the breast is accomplished by the contraction of myoepithelial cells, whose processes form a basket-like network around the alveoli where milk is stored, in concert with sucking by the infant.
When the infant is suckled, impulses from sensory stimulation of nerve terminals in the areolus travel to the central nervous system where they promote the release of oxytocin.
In the woman oxytocin release is often associated with such stimuli as the sight or sound or even the thought of the infant indicating a large cerebral component in this "neuroendocrine reflex".
The oxytocin is carried through the blood stream to the mammary gland where it interacts, initiating their contraction and expelling milk from the alveoli into the ducts and sub-areolar sinuses.
The passage of milk through the ducts is facilitated by longitudinally arranged myoepithelial cell processes whose contraction shortens and widens the ducts, allowing free flow of milk to the nipple. The process by which milk is forceably moved out of the alveoli is called milk ejection or let-down and is essential to milk removal from the lactating breast.

7. Onset of labor in childbirth
After labor begins the baby's head puts pressure on the cervix, causing the cervix to stretch. This stimulates stretch receptors the trigger the release of oxytocin, which increases the uterine contractions and pushes the baby against the cervix. This causes the cervix to stretch farther and the cycle continues.
Now, a positive feedback loop, by definition, is basically a cycle of increasing response. An external factor or event is required for the cycle to stop. In this example, the delivery of the baby is the external event. Once the baby is delivered, the uterine contractions stop, which halts the stretching of the cervix, and oxytocin is no longer released.
If the external factor is not introduced, the cycle continues. The duration of the cycle is what we refer to as labor.

8. Alteration in the mechanisms of feedback often results in deleterious consequences.
• Diabetes mellitus in response to decreased insulin
• Dehydration in response to decreased antidiuretic hormone (ADH)
• Graves’ disease (hyperthyroidism)
• Blood clotting

9. Dehydration in response to decreased antidiuretic hormone (ADH)
Antidiuretic hormone (ADH) is a protein hormone responsible for conservation of water in the body. This is done by decreasing the release of water from the kidneys into the urine.

10. Graves’ disease (hyperthyroidism)
The hypothalamus senses low circulating levels of thyroid hormone (T3 and T4) and responds by releasing thyrotropin-releasing hormone (TRH). The TRH stimulates the pituitary to produce thyroid-stimulating hormone (TSH). The TSH, in turn, stimulates the thyroid to produce thyroid hormone until levels in the blood return to normal. Thyroid hormone exerts negative feedback control over the hypothalamus as well as anterior pituitary, thus controlling the release of both TRH from hypothalamus and TSH from anterior pituitary gland .

11. Blood clotting
It is a positive feedback mechanism because the mechanism takes blood from a nonclotting state to a clotting state for as long as is needed for the clot to form. This is a change in one direction. When the clot has formed, the mechanism for clotting just stops, there is no reverse mechanism activated to slow and stop clotting. A mechanism that keeps growing until stopped is a positive feedback mechanism.