Feedback loops let organisms stabilise internal conditions or rapidly complete key events by adjusting physiology based on a changing variable. AP Biology emphasises recognising whether a loop counteracts change or amplifies it, using classic examples.

Core idea: feedback loops in physiology and development

In AP Biology examples, focus on three recurring features:

• A regulated variable (e.g., blood glucose level)

• A sensor/control system (endocrine cells and hormones, or tissues producing signalling molecules)

• Effectors that change the variable (e.g., liver, muscle, mammary glands, uterine muscle, fruit tissues)

Negative vs positive feedback (as used in examples)

• Negative feedback: the response reduces the initial change, promoting stability.

• Positive feedback: the response increases the initial change, driving a process to completion.

Blood sugar regulation by insulin and glucagon (negative feedback)

Blood glucose is maintained within a functional range by two antagonistic hormones from the pancreas.

This diagram summarizes blood glucose homeostasis as a negative feedback system with two opposing hormonal pathways. It shows how elevated glucose stimulates insulin release to drive cellular uptake and glycogen storage, while low glucose stimulates glucagon release to promote glycogen breakdown and glucose release. The figure reinforces the idea of a regulated variable (blood glucose) maintained around a physiological set range. Source

• Stimulus (high blood glucose) after a meal

  • Pancreatic $\beta$ cells secrete insulin

  • Target cells increase glucose uptake and storage, lowering blood glucose

• Stimulus (low blood glucose) during fasting/exercise

  • Pancreatic $\alpha$ cells secrete glucagon

  • Targets raise blood glucose by releasing glucose into the blood

Key target-tissue responses that restore glucose balance:

• Liver (hepatocytes)

  • Insulin promotes glycogenesis (glucose $\rightarrow$ glycogen) and inhibits glucose release

  • Glucagon promotes glycogenolysis (glycogen $\rightarrow$ glucose) and gluconeogenesis (non-carbohydrates $\rightarrow$ glucose)

• Skeletal muscle and adipose

  • Insulin increases glucose uptake and storage (muscle glycogen; fat synthesis)

Why this is negative feedback:

• As blood glucose returns toward the physiological range, insulin secretion decreases; when glucose rises again from glucagon action, glucagon secretion decreases. The response opposes the original deviation.

Lactation (positive feedback during milk let-down)

Lactation includes a classic positive feedback loop tied to nursing.

• Stimulus: infant suckling activates mechanoreceptors in the nipple

• Control: nervous input to the hypothalamus triggers pituitary hormone release

• Effector/response: oxytocin causes myoepithelial cells around mammary alveoli to contract, ejecting milk (milk let-down)

Amplification logic (positive feedback):

• Milk ejection encourages continued suckling

• Continued suckling sustains oxytocin release

• Oxytocin sustains further milk ejection

The loop stops when the stimulus stops:

• When suckling decreases or ends, sensory input drops, oxytocin release falls, and milk let-down diminishes.

Childbirth contractions (positive feedback to complete delivery)

Childbirth is driven by an amplifying loop that increases uterine contractions until birth.

This figure depicts the positive feedback loop that intensifies labor: cervical stretch triggers oxytocin release, which increases uterine contractions, producing further stretch. The cyclical arrows make the amplification logic explicit and visually distinguish the stimulus, hormonal control signal, and effector response. It also supports the concept that the loop terminates when delivery removes the initiating stimulus (cervical stretch). Source

• Stimulus: the baby’s head stretches the cervix

• Control: neuroendocrine signalling increases oxytocin release (primarily from the posterior pituitary)

• Effector/response: oxytocin strengthens uterine smooth muscle contractions, pushing the baby harder against the cervix

Why this is positive feedback:

• Stronger contractions increase cervical stretch

• Increased stretch triggers more oxytocin release

• More oxytocin further strengthens contractions

Termination is built into the event:

• Once delivery occurs, cervical stretch is removed, breaking the loop.

Fruit ripening (positive feedback via ethylene)

Fruit ripening is often regulated by the plant hormone ethylene, which can stimulate its own production.

This figure contrasts climacteric and non-climacteric ripening patterns using respiration rate and ethylene production over time. For climacteric fruits, it highlights the characteristic ethylene burst associated with autocatalytic ethylene synthesis, which aligns with a positive feedback model. The plot helps connect the abstract “more ethylene → more ripening → more ethylene” idea to an observable time-course pattern. Source

• Initial trigger: developmental cues or minor ethylene exposure begins ripening

• Response: ripening tissues produce more ethylene

• Amplification: additional ethylene accelerates ripening-related changes, which further increases ethylene production

Ripening changes commonly promoted by ethylene (high-yield associations):

• Increased expression of enzymes that soften cell walls (texture changes)

• Conversion of starches to sugars (sweetness)

• Changes in pigments and volatile compounds (colour and aroma)

Why this is positive feedback:

• Ethylene acts as both signal and amplifier: more ethylene $\rightarrow$ more ripening $\rightarrow$ more ethylene, coordinating a rapid, whole-fruit transition.