Exercises: Feedback in Biological Systems

Define negative feedback and give one example from the human body. State in your answer whether the response reduces or increases the original change.

Classify each process as negative feedback or positive feedback:

(a) insulin lowers blood glucose after a meal;

(b) oxytocin strengthens uterine contractions during childbirth;

(c) ethylene released by a ripening apple speeds up the ripening of nearby fruit;

(d) sweating cools the body when core temperature rises.

A home thermostat is set to 68 °F. When the room gets colder than the set point, it turns the furnace on, and when the room warms to the set point, it turns the furnace off. Match the parts of this system to the parts of human body temperature regulation (set point near 37 °C): the sensor, the control center, and the effector. Then state whether this is positive or negative feedback.

After a meal, a student's blood glucose rises from 90 mg/dL to 160 mg/dL. Over the next two hours it returns to about 90 mg/dL. Using the roles of insulin and the cells of the liver and muscle, explain how negative feedback produces this return. Then predict what would happen to blood glucose after a meal in a person whose cells no longer respond to insulin.

Thyroid hormone (T4) normally inhibits the release of TSH from the pituitary gland, and TSH stimulates the thyroid to release T4. Two patients have blood tests:

For patient A, suppose the thyroid gland itself is damaged. Use negative feedback to explain why TSH is high. Then suggest a possible explanation for patient B's results.

During labor, the time between contractions drops from about 30 minutes early in labor to about 3 minutes near delivery. Stretching of the cervix triggers the release of oxytocin, which strengthens contractions, which stretch the cervix further. (a) Describe this loop and explain why it is positive feedback. (b) Explain why positive feedback does not maintain homeostasis. (c) State what event ends the loop.

In a metabolic pathway, enzyme 1 converts compound A into B, enzyme 2 converts B into C, and enzyme 3 converts C into the end product D. D binds to an allosteric site on enzyme 1 and reduces its activity. (a) Explain why this is negative feedback and what it accomplishes for the cell. (b) A mutation changes the allosteric site of enzyme 1 so D can no longer bind. Predict and justify what happens to the concentration of D over time and to the supply of compound A. (c) Sketch in words what a graph of D concentration over time would look like for the normal cell compared with the mutant.

A hiker's core body temperature and sweat rate are measured during a hike on a hot day in Arizona.

(a) Identify the set point, the stimulus, and the effector response in this system. (b) Explain why the sweat rate lags behind the temperature change and why it falls after minute 30. (c) In heat stroke, high temperature can damage cells, which can further impair temperature regulation. Explain how this could turn a negative feedback loop into a dangerous positive feedback loop.