Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes.

 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.

 is the mechanism by which the body maintains conditions within particular limits. The body will do this by opposing a change that deviates from the normal.  Example 1: Body Temperature Regulation  Body temperature in mammals is regulated by a sensor that consists of cells within the hypothalamus of the brain.

 The amount of glucose in your blood is carefully controlled.  After you have eaten a meal, the blood glucose levels will begin to rise because the carbohydrates in the food are digested and absorbed. This rise is detected by beta cells, which then will produce more insulin. This insulin then binds to receptor proteins in cell membranes (particularly in the liver). This causes more protein channels to open so that more glucose can enter the cell.

 Also, insulin encourages enzymes to convert glucose to glycogen for storage.  If however, you have been doing a lot of exercise, and glucose is being used up, then alpha cells will produce glucagon, this causes the release of an enzyme that breaks glycogen to glucose

 The trp operon is a repressible operon  A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription

 b. 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 involves positive feedback in that as the baby suckles on the nipple there is a nerve response into the spinal cord and up into the hypothalamus of the brain, which then stimulates the pituitary gland to produce more prolactin to produce more milk.

 when a contraction occurs, the hormone oxytocin causes a nerve stimulus, which stimulates the hypothalamus to produce more oxytocin, which increases uterine contractions. This results in contractions increasing in amplitude and frequency.

 the first fruit that begins to ripen emits ethylene triggering the surrounding fruit to ripen- ”One bad apple…”

 Alteration in the mechanisms of feedback often results in deleterious consequences.  Example 1: Diabetes: a person has high blood sugar, either because the pancreas does not produce enough insulin, or because cells do not respond to the insulin that is produced

 Example 2: Dehydration in response to decreased antidiuretic hormone (ADH) In the kidneys, water is first filtered from the blood (along with waste products). Much of this water is reabsorbed and delivered back to the blood. ADH stimulates this reabsorption of water.  alcohol inhibits the release of ADH. This is why excess alcohol consumption can lead to frequent urination and dehydration.

 Example 3: Graves’ disease (hyperthyroidism)  affects the thyroid causing it to grow 2 to 3X its size (goiter); leads to increased heartbeat, muscle weakness, disturbed sleep, and irritability

Organisms respond to changes in their external environments.

 Photoperiodism

 Phototropism  Gravitropism  Thigmotropism

 Kinesis – random movement of organisms  Taxix – movement toward or away from a stimulus Chemotaxis in bacteria, sexual reproduction in fungi

 Hibernation and migration in animals  Nocturnal and diurnal activity: circadian rhythms (roughly 24 hour cycle in the physiological processes of living beings, including plants, animals, fungi and cyanobacteria)  Shivering and sweating in humans