AP Biology Unit 4 Notes: Feedback Mechanisms in Cell Communication and the Cell Cycle

Feedback (biology)

A control strategy where the output of a biological process feeds back to influence earlier steps of the process, adjusting the pathway’s activity.

Controlled variable

The factor a system regulates to keep within a certain range (e.g., blood glucose concentration).

Sensor

A component that detects the state of the controlled variable and initiates signaling when the variable deviates from the desired range.

Signal

Information passed from sensor to effector (often a hormone or intracellular signaling molecule) to coordinate a response.

Effector

A component that carries out actions that change the controlled variable (helping restore or shift it).

Robustness

The ability of a biological system to keep functioning even when internal or external conditions vary; feedback helps create this stability.

Negative feedback

A feedback loop where the response reduces or reverses the initial change, bringing the variable back toward its normal range (stabilizing control).

Positive feedback

A feedback loop where the response amplifies the initial change, driving the system further in the same direction until an endpoint or limiting factor stops it.

Homeostasis

Maintenance of relatively stable internal conditions; commonly achieved through negative feedback mechanisms.

Set point (vs. range)

A target value for a regulated variable; many biological systems more accurately maintain a range rather than a single fixed set point.

Self-limiting response

A hallmark of negative feedback where the response diminishes as the variable returns toward normal, naturally turning the pathway down/off.

Signal amplitude

How strong a signaling response is; feedback helps prevent responses from being too weak or excessively strong.

Signal duration

How long a signaling pathway remains active; negative feedback commonly limits duration to prevent prolonged signaling.

Signal transduction

The process of converting an external signal (e.g., ligand binding a receptor) into an internal cellular response (e.g., gene expression changes).

Receptor desensitization

A negative-feedback mechanism where a receptor becomes less responsive after prolonged stimulation, reducing continued signaling despite the stimulus.

Receptor down-regulation

A negative-feedback mechanism where receptors are removed from the membrane (often via endocytosis), decreasing cellular sensitivity to a ligand.

Phosphatase

An enzyme that removes phosphate groups from proteins; often used in negative feedback to shut off phosphorylation-based signaling pathways.

Second messenger

An intracellular signaling molecule (e.g., cyclic AMP) whose levels/activities can be regulated to tune pathway strength and timing.

Gene-expression feedback (inducible inhibitor)

A slower negative-feedback mechanism where a signal induces transcription of a protein that inhibits part of the same pathway, providing longer-term control.

Amplification (in signaling)

When one activated molecule leads to activation of many downstream molecules; this is not feedback unless downstream activity loops back to regulate an earlier step.

Cyclin

A regulatory protein whose concentration rises and falls during the cell cycle; cyclins bind CDKs to control when CDKs are active.

Cyclin-dependent kinase (CDK)

A protein kinase that drives cell cycle events by phosphorylating targets when activated (typically by binding a cyclin).

Checkpoint (cell cycle)

A conditional control point that can pause or allow cell cycle progression based on internal/external conditions (e.g., damage, size, signals).

Cyclin degradation

An actively controlled “off switch” that reduces cyclin levels to lower CDK activity and help the cell exit a phase/reset timing (a negative-feedback-like control).

Knockout (of a feedback inhibitor)

An experimental removal of a gene; if the gene encodes an inhibitory feedback protein, knockout typically makes signaling stronger or last longer because shutoff is lost.