AP Biology · Lesson 4 of 6 · Unit 4

Positive and negative feedback in homeostasis

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Living systems cannot survive in a static state; they constantly detect internal and external changes and adjust their physiology using feedback loops (Topic 4.4). On the AP Biology exam, you will rarely be asked to simply define homeostasis—instead, you must interpret pathway diagrams, classify unfamiliar molecular or organismal mechanisms as positive or negative feedback, and predict what happens when a step in the loop is disrupted.

By the end you’ll be able to

Explain how negative feedback mechanisms maintain homeostasis by reducing the initial stimulus and returning a perturbed system to its target set point.
Explain how positive feedback mechanisms amplify responses and move a variable further from its initial set point.
Classify a physiological example as positive or negative feedback and justify the classification.
Predict the effects of a disruption to a feedback mechanism on an organism.

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Core ideas (continued)
Worked examples
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What the exam asks

Section I (Multiple Choice): Expect stimulus sets featuring signaling pathway diagrams (with $\rightarrow$ for activation and $\dashv$ for inhibition) or time-course graphs of hormone and metabolite levels. You will be asked to classify the feedback loop or predict the consequence of a receptor mutation, toxin, or inhibitor.
Section II (Free Response): Frequently tested in Question 4 (Conceptual Analysis) and Question 5 (Analyze Model or Visual Representation). Typical prompts ask you to describe how a feedback loop maintains a set point, predict the effect of a disruption (such as an autoimmune attack on endocrine cells), and justify your prediction using evidence from the provided model.

Core ideas

Negative feedback: stabilizing at a set point

Homeostasis is the maintenance of a stable internal environment around a target set point despite external fluctuations. Negative feedback is the primary mechanism that maintains homeostasis: the response of the system counteracts (reduces or dampens) the initial stimulus, returning the variable back toward its set point.

Stimulus: A physiological variable deviates above or below its target set point.
Sensor / Receptor: Detects the deviation and sends a signal.
Effector / Response: Activates a process that pushes the variable in the opposite direction of the initial deviation.
Return to set point: Once the variable returns to the target range, the initial stimulus is removed and the response shuts off.

Classic example: Blood glucose regulation

High blood glucose (after a meal): Pancreatic beta ( $\beta$ ) cells secrete insulin into the blood $\rightarrow$ insulin binds to receptors on liver and muscle cells $\rightarrow$ cells take up glucose and store it as glycogen $\rightarrow$ blood glucose decreases back to the set point.
Low blood glucose (fasting): Pancreatic alpha ( $\alpha$ ) cells secrete glucagon liver cells break down glycogen (glycogenolysis) and release glucose into the blood blood glucose increases back to the set point.

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